Tropical rain forests (areas that receive more than 100 mm of rain/month) support the richest avifaunas in the world. Why are there so many bird species in rain forests & how (& why) do the avifaunas of rain forests in different parts of the world vary? To answer these questions requires a look at the history of rain forests & their avifaunas (Karr 1990):

• Gondwanaland began to break up by the mid-Cretaceous (125 mya; see map below), separating India & Africa. Eventually, Madagascar separated from Africa & South America lost its connection to Antarctica, setting the stage for the evolution of widely divergent avifaunas.

• By the early Cenozoic (60 mya; below left), the continents had reached their maximum isolation. Favorable tropical climates in the southern continents during the early & mid-Cenozoic favored diversification. By the Miocene (15 mya; below right), equatorial regions were dominated by modern rainforest floras.

• Frequent connections between Africa & southeast Asia produced avifaunal overlap between those tropical areas, but the isolation of South America generated a diverse & unique avifauna (only recently influenced by incursions of birds of Nearctic origin). Suboscines diversified in isolation from other continents in South America, while the oscines diversified and, to a great extent, mixed between African and Asian tropics.
• Wet & dry cycles generated many shifts in habitat distribution & fragmentation during the Pleistocene:
• South America had extensive forest &, apparently, a large number (14) of wet-forest refuges during the dry periods of the Pleistocene (see The Biogeographic Evidence Supporting the Pleistocene Forest Refuge Hypothesis; but also see Paleoclimatology:Amazon's ancient rainforest & this abstract)
• Africa had less forest & possibly only two or three wet-forest refuges during dry periods
• the Asian tropics had even less forest & is characterized by large islands & archipelagoes that have exhibited varying degrees of separation & fusion throughout the Pleistocene
• South & Central America have been well fragmented by numerous mountain ranges (e.g., Andes), while tropical Africa has been dominated by large plateaus & few mountain ranges.

Off the coast of South America, the oceanic Nazca Plate is pushing into & being subducted under the continental part of the South American Plate. In turn, the overriding South American Plate is being lifted up, creating the Andes mountains.

The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form the towering peaks of the Andes, as seen here in Peru (Source: pubs.usgs.gov/publications/text/understanding.html).

Tectonic plate collision  

 
Peru

Avian species richness -- Numerous hypotheses have been proposed to explain regional variability in species richness, and recent research efforts have winnowed the number of potential hypotheses to a credible few: (1) energy availability, (2) evolutionary time, (3) habitat heterogeneity, (4) area, and (5) geometric constraints. Rahbek and  Graves (2001) examined bird diversity in South America and found that the 1° quadrats (map a to the right) that exhibited the highest avian diversity (>650 species) were restricted to Andean Ecuador (peaking at 845 species) and in southeastern Peru (peaking at 782 species) and southern Bolivia (peaking at 698 species). These quadrats were physiographically complex (range = 1,700-5,700 m) and characterized by moderate precipitation (1,058-3,096 mm/yr) and maximum daily temperatures (16.9-25.3°C). Thus, neither area nor energy alone is sufficient to explain patterns of avian species richness in South America. If energy and biome area were the primary determinants of species richness, then species richness would be highest in central Amazonia, which was not the case.      Species richness in neotropical birds seems to be linked directly to habitat diversity, which is correlated with topographic heterogeneity. The number of different ecosystems found in 1° quadrats was correlated with topographic relief at tropical latitudes (<20°). Quadrats in the species-poor zone in central Amazonia overlapped 5-16 distinctive ecosystems, whereas species-rich quadrats (>650 species) overlapped 16-24 ecosystems.      The extraordinary abundance of species associated with humid montane regions at equatorial latitudes reflects the overwhelming influence of orography and climate on the generation and maintenance of species richness. Rahbek and  Graves' (2001) data reinforce the hypothesis that terrestrial species richness from the equator to the poles is governed by a synergism between climate and coarse-scale topographic heterogeneity.

Spatial variation in species richness of 2,869 breeding land and fresh- water birds (Aves) of South America compiled at 1° × 1°, 3° × 3°, 5° × 5°, and 10° × 10° scales. Note the loss of information and the spurious extrapolation of high species densities in species-poor localities at coarser spatial scales (From: Rahbek & Graves 2001).

---

Global maps of avian species richness at three sampling resolutions. An equal-area (Behrmann) map projection was used,
with grid cells having longitudinal cell resolutions of: (a) 1°; (b) 2° and (c) 4°.

Topography, energy and the global distribution of bird species richness -- A major goal of ecology is to determine the causes of the latitudinal gradient in global distribution of species richness. Current evidence points to either energy availability or habitat heterogeneity as the most likely environmental drivers in terrestrial systems, but their relative importance is controversial in the absence of analyses of global (rather than continental or regional) extent. Davies et al. (2007) used data on the global distribution of extant continental and continental island bird species to test the explanatory power of energy availability and habitat heterogeneity while simultaneously addressing issues of spatial resolution, spatial autocorrelation, geometric constraints upon species' range dynamics, and the impact of human populations and historical glacial ice-cover. Global maps of avian species richness data used in their analyses showed a consistent pattern across all three sampling resolutions (see Figure above), namely higher species richness within the tropics, with peaks coinciding with major mountain chains, most notably along the Andes and the southern slopes of the Himalayas and to a lesser extent the African Rift Valley. The best-fit multi-predictor global model accounting for glacial history and human impacts showed elevation range to be the strongest predictor of avian species richness, closely followed by temperature and then habitat diversity and productive energy. This global perspective confirms the primary importance of mountain ranges in high-energy areas.

As a result of this history (& other factors described below):

• South American forests (which, unfortunately, are being cut at a high rate, e.g., to grow soybeans) support about 1,300 species with many endemics (including many suboscines)

 
The Amazon Rainforest - Gone Within Our Children's Lifetime?

• African forests support about 400 species with few endemics
• Southeast Asia forests support about 380 species with few endemics

• habitat diversity
• e.g., Manu National Park (Peru) includes a variety of forest habitats, including transition forest, high ground forest, upland forest, swamp forest, & bamboo forest. Each has a characteristic set of bird species.

 
Manu

Versicolored Barbet (Eubucco versicolor)

Gray-breasted Mountain-Toucan (Andigena hypoglauca)

• patchiness in both horizontal (e.g., habitat diversity) & vertical dimensions

• MacArthur et al. (1966) noted the importance of vertical stratification (see diagram below) in the avifaunas of tropical forests, with many species having narrow vertical foraging zones. The various layers of a rainforest include:
• The emergents. Widely spaced trees 100 to 120 feet tall and with umbrella-shaped canopies extend above the general canopy of the forest. Since they must contend with drying winds, they tend to have small leaves and some species are deciduous during the brief dry season.
• A closed canopy of 80 foot trees. Light is readily available at the top of this layer, but greatly reduced below it.
• A closed canopy of 60 foot trees. There is little air movement in this zone and consequently humidity is constantly high.
• Shrub/sapling layer: Less than 3% of the light intercepted at the top of the forest canopy passes to this layer. Arrested growth is characteristic of young trees capable of a rapid surge of growth when a gap in canopy above them opens.
• Ground layer: sparse plant growth. Less than 1 percent of the light that strikes the top of the forest penetrates to the forest floor. In such darkness few green plants grow. Moisture is also reduced by the canopy above: one third of the precipitation is intercepted before it reaches the ground.

• Mist nets operated in the undergrowth of a tropical forest results in captures of fewer than half the local species of birds (in a temperate deciduous forest such netting would result in the capture of more than 90% of the species present) (Karr 1990).

• trophic structure - comparisons between temperate & tropical forest avifaunas indicate that:
• many tropical species use resources not available (or as available) in temperate forests (e.g., fruit, nectar, large insects, & arthropods flushed by army ants)
• large numbers of similar species can coexist in the same areas of tropical forest (especially insectivores)
• many species have narrowly defined niches (e.g., raptors specializing on reptiles or lichen-feeding parrots)
• the floristic & structural diversity of tropical forests provides a greater variety of microhabitats (e.g., dead-leaf foraging)

Arboreal dead-leaf-searching birds of the Neotropics -- Remsen and Parker (1984) reported that at least 11 species of birds in northern Bolivia and southern Peru are dead-leaf-searching “specialists”: more than 75% of their foraging observations of these species involved individuals searching for insects in dead, curled leaves suspended above ground in the vegetation. All known specialists of this kind belong to the families Furnariidae and Formicariidae. An additional six species exhibited dead-leaf-searching behavior in 25% to 75% of their foraging records. The number of specialists and regular users decreases with rising elevation in the Andes. Specialists disappear from the gradient between 2,000 m and 2,575 m, but regular users occur as high as 3,300 m, near timberline. As many as eight species of dead-leaf-searching specialists coexist in western Amazonia.

Photo by Arthur Grosset

Why, in general, does bird species diversity increase from the poles to the equator?
 

A number of possible explanations for these latitudinal diversity gradients have been suggested (Wiens 1991):

• There has been more time without geological disruptions in the tropics, & a richer fauna has evolved.
• Habitat complexity is greater in the tropics, providing greater opportunity for ecological specialization of species.
• Competition among species is more intense in the tropics, restricting species to using a narrow range of resources and thereby permitting more species to coexist.
• Predation and parasitism are more intense in the tropics, reducing population sizes of competitors; though allowing them to overlap more in the use of resources.
• Climatic stability is greater in tropical than in temperate latitudes, so enabling species to specialize on a narrower range of resources and leading in turn to lower extinction rates.
• The tropics have greater biological productivity than the temperate zones, and the greater amount of useable energy permits more species to coexist.
• There are distinctive resources such as fruits or nectar-producing plants in the tropics that support categories of species not found in many temperate locations.

Tropical Conservatism Hypothesis -- Wiens and Donoghue (2004) have proposed a 'tropical conservatism hypothesis' to help explain the the tendency for species richness to increase from poles to equator. This hypothesis or model combines three basic ideas: • Many groups of organism that have high tropical species richness originated in the tropics & have spread to temperate regions either more recently or not at all. If a clade originated in the tropics then (all other things being equal) it should have more tropical species because of the greater time available for speciation in tropical regions to occur (i.e. the time-for-speciation effect). • One reason that many extant clades of organism originated in the tropics is that tropical regions had a greater geographical extent until relatively recently (30–40 million years ago, when temperate zones increased in size). If much of the world was tropical for a long period before the present, then (all other things being equal) more extant clades should have originated in the tropics than in temperate regions. • Many species and clades are specialized for tropical climates, and the adaptations necessary to invade and persist in regions that experience freezing temperatures have evolved in only some. Tropical niche conservatism has helped maintain the disparity in species richness over time. At least two lines of evidence support this tropical conservatism hypothesis. First, many groups of organisms that show the expected gradient in species richness also appear to show the predicted pattern of historical biogeography, with an origin in the tropics and more recent dispersal to temperate regions. For example, analyses of New World birds reveal older average divergences among tropical taxa than among temperate ones, as predicted by this hypothesis. Second, many distantly related groups show similar northern range limits, in spite of the lack of an obvious geographical barrier, suggesting that cold climate and niche conservatism act as barriers to the invasion of temperate zones by tropical clades. Thus, many neotropical clades currently have their northern range limits in the tropical lowlands of Mexico (e.g., tinamous), whereas many other groups have their northern range limits in southern China and Vietnam (e.g., broadbills). These regions of biotic turnover in Mexico and Asia have not gone unnoticed; in fact, they correspond to borders between the global zoogeographical realms recognized by Wallace. Further, many of the groups involved are old, suggesting that there has been ample time for invasion of temperate regions, but that their northward dispersion was limited by their inability to adapt to colder climates.

Two approaches to the problem of explaining global patterns of species richness. Standard ecological approaches (a) seek correlations between the numbers of species of a given group at a given location (numbers along the edge of the globe) & environmental variables (e.g., temperature, indicated here by different shades of  red). By contrast (b), Wiens & Donoghue (2004) advocate considering the biogeographical history of the species and clades that makes up these differences in species richness between regions, & understanding how ecology, phylogeny & microevolution (e.g. adaptation) have combined to shape that biogeographical history. Each dot in the above diagram represents a species & its geographical location, and the lines connecting them represent both their evolutionary relationships and the simplified paths of dispersal. (b) also illustrates the tropical conservatism hypothesis, i.e., there are more species in tropical regions because most groups originated in the tropics & are specialized for a tropical climatic regime, that most species and clades have been unable to disperse out of the tropics (because of niche conservatism), and that the greater time and area available or speciation in the tropics has led to higher species richness in the tropics for most taxa. As shown here, the tropical conservatism hypothesis predicts that temperate lineages are often recently derived from clades in tropical regions, leading to (on average) shallower phylogenetic divergences among temperate lineages than among tropical lineages. Although not illustrated here, an important part of the tropical conservatism  hypothesis is the idea that tropical regions were more extensive until the mid-Tertiary, which might help explain the greater number of extant clades originating in these areas.

Some terrestrial birds can be found almost everywhere. For example, Ospreys, Barn Owls, & swifts  occupy every continent except Antarctica. However, although most can fly, most species of birds do not occur everywhere on earth where conditions appear to be favorable for them. Barriers to dispersal for birds include (Welty & Baptista 1988):

• physical barriers
• mountain ranges
• deserts
• oceans
• biological barriers
• plant distribution, e.g.:
• hummingbirds restricted to feeding on certain flowers

• several species (such as crossbills) are restricted to coniferous forests
• animal distribution
• e.g., cuckoos may be limited by the distribution of their hosts
• e.g., antbirds are limited by the distribution of army ants
• 'psychological' barriers
• some land birds, even very good fliers, are reluctant to cross even relatively small bodies of water, e.g.:
• the Amazon & its larger tributaries serve as effective barriers for many species
• Of the more than 300 species of lowland birds in New Guinea, 134 have failed to disperse to even the closest nearby islands

Species-richness maps of (a) occasional/non-following species that are sister to ant-following clades, (b) obligate
army-ant-followers [with distributions of Myrmeciza fortis (obligate) and M. melanoceps (occasional) outlined],
(c) regular army-ant-followers (thamnophilid and furnariid species combined), and (d) regular thamnophilid army-ant-followers.

Army-ant-following birds -- One of the most novel foraging strategies in Neotropical birds is army-ant-following, in which birds prey upon arthropods and small vertebrates flushed from the forest floor by swarm raids of the army-ant Eciton burchellii. This specialization is most developed in the typical antbirds (Thamnophilidae) which are divisible into three specialization categories: (1) those that forage at swarms opportunistically as army-ants move through their territories (occasional followers), (2) those that follow swarms beyond their territories but also forage independently of swarms (regular followers), and (3) those that appear incapable of foraging independently of swarms (obligate followers). Brumfield et al. (2007) found that regular following evolved only three times, and that the most likely evolutionary progression was from least (occasional) to more (regular) to most (obligate) specialized, with no reversals from the obligate state. Despite the dependence of the specialists on a single ant species, molecular dating indicates that army-ant-following has persisted in antbirds since the late Miocene.

Ant-following species density maps (above) illustrate clear differences between obligate and regular followers. Obligate followers are found almost exclusively in Amazonia, with increasing densities moving west toward the Andes. This same pattern is observed in occasional/non-followers. In contrast, regular followers were most abundant where obligates were least prevalent or absent (Atlantic coastal forests of Brazil, the Guyanan shield, and the forested lowlands west of the Andes). These results suggest than competition could have played a role in the evolution of army-ant-following species and in shaping their distribution.

Migration & Bird Biogeography

The avifauna of most regions (Nearctic, Palearctic, Neotropical, Ethiopian, & Oriental) includes numerous migratory species; species that spend only part of the year in those regions. About 400 species have breeding ranges in the Holarctic and winter in the tropics (primarily Central & South America, Africa, & Indomalaysia)(Lovei 1989).

• Central & South America are obviously important wintering areas for Nearctic migrants, while Africa is more important for Palearctic migrants than India or southeast Asia (Lovei 1989).
• About 1/3 of all Palearctic species migrate to Africa (Ethiopian region) for the winter.
• In the Nearctic, 62% of bird species that breed in deciduous forests are migratory, 80% that breed in coniferous forests are migratory, & 76% of grassland species are migratory (Willson 1976). Of course, not all Nearctic migrants migrate to the Neotropics, but many do (e.g., the Scarlet Tanager).

What is the ancestral 'home' of migrants? For example, are most migrants in this hemisphere Nearctic species that move south to avoid winter conditions or Neotropical species that move north to take advantage of temporarily favorable conditions for breeding? Evidence suggests that many northern latitude migrants are tropical birds exploiting the long days and abundant insects of high-latitude summers rather than temperate birds escaping northern winters (Levey and Stiles 1992).

Check out 'A Year on the Wing' - a year in the life (including migration) of Eastern Curlews (Photograph by Clive Minton)

Winged Migration

Evolution of migratory behavior -- Migratory behavior can evolve when a resident species expands its range due to intraspecific competition into an area that is seasonally variable, providing greater resources for reproduction but harsher climactic stress and reduced food availability in the non-breeding season. Individuals breeding in these new regions at the fringe of the species' distribution are more productive, but to increase non-breeding survival they return to the ancestral range. This results, however, in even greater intraspecific competition because of their higher productivity, so that survival is enhanced for individuals that winter in areas not inhabited by the resident population. The Common Yellowthroat (Geothlypis trichas) of the Atlantic coast of the U.S. is a good example. Birds occupying the most southern part of the species' range in Florida are largely nonmigratory, whereas populations that breed as far north as Newfoundland migrate to the West Indies in the winter, well removed from the resident population in Florida. Because a migrant population gains an advantage on both its breeding and wintering range, it becomes more abundant, while the resident, non-migratory population becomes proportionately smaller and smaller in numbers. If changing environmental conditions become increasingly disadvantageous for the resident population or interspecific competition becomes more severe, the resident population could eventually disappear, leaving the migrant population as characteristic of the species. These stages in the evolution of migration are represented today by permanent resident populations, partial migrants, and fully migratory species. As for all adaptations, natural selection continues to mold and modify the migratory behavior of birds as environmental conditions perpetually change and species expand or retract their geographic ranges. Hence, the migratory patterns that we observe today will not be the migratory patterns of the future (From: Lincoln et al. 1998).

Checklist of things a migrant bird has to consider before departing on a long-distance flight (Piersma et al. 1990, Klaasen 1996).

Swainson's Thrush

To view this graphic full-size, go to www.jsonline.com

Sense of direction -- Cochran et al. (2004) found that migrating thrushes rely on a built-in magnetic compass that they recalibrate each evening based on the direction of the setting sun. The research, that involved attaching radio transmitters to birds and following them by truck is the first extensive study of bird navigation in the wild. The results appear to resolve conflicts between earlier laboratory-based studies, which had identified several possible navigational mechanisms, but produced no consensus. Previous theories suggested that birds use some combination of magnetism, stars, landmarks, smells and other mechanisms as navigational aides. Cochran et al. (2004) caught birds just before they left for their overnight migratory flights and placed them in an artificial magnetic field. They then released the birds and followed them throughout the night. Birds that had been in the artificial magnetic field flew in the wrong direction, but recovered their orientation the next night. 

"In the morning, shortly before they land, they see the sun and realize they have made a mistake," said Martin Wikelski, a co-author. "You can see them turn around 90 degrees." Cochran et al. (2004) concluded that birds rely on the location of the sunset to determine which way to fly. To maintain that heading throughout the night, they sense the Earth's magnetic field, just like a pilot uses a compass at night or in bad weather. "It is the simplest and most foolproof orientation mechanism we can imagine," Wikelski said. The combination of cues sun and magnetic field neatly correct each other for possible problems. Migrating at night, birds cannot maintain a fix on the sun and cannot rely on seeing stars because of clouds. A bird's magnetic compass also is not sufficient. The location of the magnetic north pole the spot on the globe where a compass points is not stable (it is currently in Canada, hundreds of miles from the geographic north pole). The magnetic poles also completely reverse locations every few thousand years, so the north arrow on a compass would suddenly point south. It makes sense then that birds use the magnetic field only as a guide to keep them on a path that is determined primarily by the sun, Wikelski said. It doesn't matter which way the magnetic field points, so long as it stays steady through the night.

In one set of experiments, the researchers tracked Gray-cheeked Thrushes, which all tend to migrate in the same direction. Seven of eight birds exposed to an artificial magnetic field flew in a significantly different direction than 14 that were not. The researchers did similar experiments with Swainson's Thrushes (pictured above), whose headings vary considerably from one bird to another. They exposed the Swainson's thrushes to artificial magnetic fields and followed them for at least two days. All the treated birds picked a significantly different direction on their second day, whereas unmanipulated birds kept flying the same direction both days. Wikelski believes the results, while based on just two species, are likely to apply to most migratory birds. "It's such a simple and elegant mechanism that I would say it is widespread," he said. -- Princeton Weekly Bulletin

Greater noctule bat

Bats vs. migrating Palearctic passerines -- Along food chains, i.e., at different trophic levels, the most abundant taxa often represent exceptional food reservoirs, and are hence the main target of consumers and predators. The capacity of an individual consumer to opportunistically switch towards an abundant food source, for instance, a prey that suddenly becomes available in its environment, may offer such strong selective advantages that ecological innovations may appear and spread rapidly. New predator-prey relationships are likely to evolve even faster when a diet switch involves the exploitation of an unsaturated resource for which few or no other species compete. Using stable isotopes of carbon and nitrogen as dietary tracers, Popa-Lisseanu et al. (2007) reported support for the controversial hypothesis that the greater noctule bat (Nyctalus lasiopterus), a rare Mediterranean aerial-hawking bat, feeds on the wing upon the multitude of flying passerines during their nocturnal migratory journeys, a resource that, although showing a predictable distribution in space and time, is only seasonally available. No predator had previously been reported to exploit this extraordinarily diverse and abundant food reservoir represented by nocturnally migrating passerines.

Shearwater migrations originating from breeding colonies in New Zealand. (a) Interpolated geolocation tracks of 19 Sooty Shearwaters
during breeding (light blue) and subsequent migration pathways (yellow, start of migration and northward transit; orange, wintering grounds and
southward transit). The 30° parallels, equator, and international dateline are indicted by dashed lines. (b–d) Representative figure-eight movement
patterns of individual shearwaters traveling to one of three "winter" destinations in the North Pacific. These tracks also represent those of three breeding
pairs to reveal the dispersion and extent of each pair.

Electronic tracking tags have revolutionized our understanding of broad-scale movements and habitat use of highly mobile marine animals, but a large gap in our knowledge still remains for a wide range of small species. Shaffer et al. (2006) reported the extraordinary transequatorial postbreeding migrations of a small seabird, the Sooty Shearwater (Puffinus griseus), obtained with miniature archival tags that log data for estimating position, dive depth, and ambient temperature. Tracks (262 ± 23 days) revealed that shearwaters fly across the entire Pacific Ocean in a figure-eight pattern while traveling 64,037 ± 9,779 km roundtrip, the longest animal migration ever recorded electronically. Each shearwater made a prolonged stopover in one of three discrete regions off Japan, Alaska, or California before returning to New Zealand through a relatively narrow corridor in the central Pacific Ocean. Transit rates as high as 910 ± 186 km·day^–1 were recorded, and shearwaters accessed prey resources in both the Northern and Southern Hemisphere’s most productive waters from the surface to 68.2 m depth. Their results indicate that Sooty Shearwaters integrate oceanic resources throughout the Pacific Basin on a yearly scale. Sooty Shearwater populations today are declining, and because they operate on a global scale, they may serve as an important indicator of climate change and ocean health.

Recent changes in Bird Biogeography - Extinction:

• At least 80 species of birds have become extinct since 1800. Extinct birds in the U.S. (excluding Hawaii) include:
• Heath Hen
• Passenger Pigeon
• Carolina Parakeet (museum specimen pictured to the right)
• Ivory-billed Woodpecker (probably) (check these videos)
• Bachman's Warbler

• According to Stuart Pimm (Columbia Univ.), 11% of birds, or 1,100 species out of the world’s nearly 10,000, are on the edge of extinction; it’s doubtful that the majority of these 1,100 will live much beyond the end of this century

• The five orders with the highest percent of threatened species are those that include:
• rails and cranes, 26%
• pheasants, and partridges, 25%
• parrots, 25%
• pigeons and doves, 17%
• kingfishers and bee-eaters, 12%
• Countries with the most threatened & endangered  birds are: Brazil (111); Indonesia (94); China (82) Colombia (81); Peru (79); India (70); Philippines (69); Australia (69); Ecuador (65); New Zealand (65); United States (65); Mexico (49); Argentina (42) Thailand (42); Myanmar (41); Vietnam (38); Russia (35); Japan (34); Papua New Guinea (32); Madagascar (30); & Tanzania (30).

Source: American Bird Conservancy

Threats (historical & current) that have been the primary causes of habitat loss in U.S. threatened habitats.
Threats that have only affected one each of the 20 most threatened habitats are combined as 'other',
and include forest succession, coastal engineering, deer, fisheries issues, recreation, and fire
(Source: American Bird Conservancy).

Tourism and Lesser Prairie Chicken conservation  

 
In October 1993, William Lishman took off from his Toronto farm in a microlight of his own design and construction.
Eighteen birds followed 'Goose Leader' in a V-formation all the way south to Virginia. The following April the banded
flock returned unaided to Lishman's farm to greet their surrogate father. Eventually the principles learned by working
with Canada Geese and Sandhill Cranes were applied to endangered species such as Whooping Cranes and Trumpeter Swans.

Trying to solve the mystery of the decline in American Black Duck populations

Distribution of threatened species of birds. Dark red = highest number of
threatened species; dark blue = lowest number (Source: IUCN).

Estimated global numbers of individual birds (in billions) over the past
several hundred years, based on low (bottom), medium (middle), and high (top)
densities, beginning with the pre-agricultural pattern of land use (Gaston et al. 2003).

Habitat conversion and global avian biodiversity loss -- The magnitude of the impacts of human activities on global biodiversity has been documented at several organizational levels. However, although there have been numerous studies of the effects of local-scale changes in land use (e.g. logging) on the abundance of groups of organisms, broader continental or global-scale analyses addressing the same basic issues remain largely wanting. Nonetheless, changing patterns of land use, associated with the appropriation of increasing proportions of net primary productivity by the human population, seem likely not simply to have reduced the diversity of life, but also to have reduced the carrying capacity of the environment in terms of the numbers of other organisms that it can sustain. Gaston et al. (2003) estimated the size of the existing global breeding bird population, and made a first approximation as to how much this has been modified as a consequence of land-use changes wrought by human activities. Summing numbers across different land-use classes gives a best current estimate of a global population of about 87 billion breeding bird individuals (with about 25% of these birds in tropical forest, 13% in tropical woodland, 11% in boreal forest, 8% in savannah habitat, and 19% in human-modified habitats like cropland and pasture). Applying the same methodology to estimates of original land-use distributions suggests that conservatively this may represent a loss of between 20-25% of pre-agricultural bird numbers. This loss is shared across a range of temperate and tropical land-use types.

Ten percent of all bird species are likely to disappear by the year 2100, and another 15% could be on the brink of extinction. This dramatic loss will have a negative impact on forest ecosystems and agriculture worldwide. Sekercioglu et al. (2004) estimated that, by 2100, as many as one out of four may be functionally extinct—that is, critically endangered or extinct in the wild. "Even though only 1.3% of bird species have gone extinct since 1500, the global number of individual birds is estimated to have experienced a 20-25% reduction during the same period," wrote Sekercioglu et al. (2004). "Given the momentum of climate change, widespread habitat loss and increasing numbers of invasive species, avian declines and extinctions are predicted to continue unabated in the near future." The study was based on an analysis of all 9,787 living and 129 extinct bird species. To forecast probable rates of extinction, the authors simulated best-case, intermediate-case and worst-case scenarios for the future: Best case - conservation measures in the next 100 years would prevent additional bird species from becoming threatened with extinction. Worst case - the number of threatened species will increase by about 1% per decade. "These assumptions are conservative, since it is estimated that, every year, natural habitats and dependent vertebrate populations decrease by an average of 1.1 percent," the authors wrote. Intermediate case - statistics from 1994 through 2003 were used as a basis for calculating the likelihood that a non-threatened species would become threatened after a decade. The results of the three future scenarios were dramatic. The computer forecast that between 6 and 14% of all bird species will be extinct by 2100, and that 700 to 2,500 species will be critically endangered or extinct in the wild. Even the middle-of-the-road intermediate scenario revealed that one in 10 species will disappear a century from now, and that approximately 1,200 species will be functionally extinct. Reasons for the expected decline in bird populations include habitat loss, disease, climate change, competition from introduced species, and exploitation for food or the pet trade. "It's hard to imagine the disappearance of a bird species making much difference to human well-being," said co-author Daily. "Yet consider the case of the Passenger Pigeon. Its loss is thought to have made Lyme disease the huge problem it is today. When Passenger Pigeons were abundant—and they used to occur in unimaginably large flocks of hundreds of millions of birds—the acorns on which they specialized would have been too scarce to support large populations of deer mice, the main reservoir of Lyme disease, that thrive on them today." The authors also found that numerous insect-eating species face extinction. "Exclusions of insectivorous birds from apple trees, coffee shrubs, oak trees and other plants have resulted in significant increases in insect pests and consequent plant damage," the authors wrote, adding that the extreme specializations of many insectivorous birds, especially in the tropics, make it unlikely that other organisms will be able to replace the birds' crucial role in controlling pests. "The societal importance of ecosystem services is often appreciated only upon their loss," the authors wrote. "Disconcertingly, avian declines may in fact portray a best-case scenario, since fish, amphibians, reptiles and mammals are 1.7 to 2.5 times more threatened [than birds]." Invertebrates, which may be even more ecologically significant than animals, also are disappearing, they noted. Therefore, "investments in understanding and preventing declines in populations of birds and other organisms will pay off only while there is still time to act," the authors concluded. -- Stanford News Service

Percent

Major threats to globally threatened bird species
(Source: IUCN)

 
Amazon drought and deforestation

• Avifaunas of islands are particularly threatened:

• Hawaii:
• 2000 years ago there were ~ 125-145 species
• Then the Polynesians arrived (~1500 years ago):
• hunting and overharvesting
• fire and other types of landscape change
• the Pacific rat
• domesticated dogs
• pigs
• jungle fowl
• introduced diseases (check this video: "Survivors in Paradise").

• Of the approximately 71 native land birds occurring in Hawaii when Captain Cook landed there, 23 are probably extinct. Now, even common native birds are declining.

• Guam:
• The Brown Tree Snake (Boiga irregularis; see photo below), a venomous predator that grows up to eight feet long, has spread through the island of Guam since arriving on U.S. military cargo ships from the Solomon Islands in the 1940's. Their density is up to 12,000 per square mile in some places. This introduced snake has caused the extinction on Guam of 12 of 14 forest bird species, two of three bat species, and at least six lizard species.

• Madagascar:
• Of the approximately 290 birds that occur in Madagascar, 107 are endemic. Three of the world's most endangered birds of prey exist on Madagascar: the Madagascar Serpent-Eagle, Madagascar Red Owl and Madagascar Fish-Eagle. Madagascar was home to gigantic flightless birds called elephant birds. These birds were larger than any other birds - living or extinct. They were heavier than the famous 10-foot-tall moas of New Zealand. The eggs of elephant birds could hold the fluid contents of about 180 chicken eggs!

 

An endemic radiation of Malagasy songbirds -- The bird fauna of Madagascar includes a high proportion of endemic species, particularly among the passerines. The endemic genera of Malagasy songbirds are not allied obviously with any African or Asiatic taxa, and their affinities have been debated since the birds first were described. Cibois et al. (2001) used mitochondrial sequence data to estimate the relationships of 13 species of endemic Malagasy songbirds, 17 additional songbird species, and one species of suboscine passerine. Most previous classifications of these endemic Malagasy songbirds suggested colonization of Madagascar by at least three different lineages of forest-dwelling birds (babblers,bulbuls, & warblers), but the phylogeny of Cibois et al. (2001) suggests a single colonization event. They suggest that a single colonization seems more likely, based on the observation that successful colonization of islands by forest-restricted birds is rare. The avifauna of islands typically is comprised of habitat generalists, while nine species in the endemic Malagasy clade are forest dwelling. Overall, the endemic Malagasy songbird clade rivals other island radiations, including the vangas of Madagascar and the finches of the Galapagos, in ecological diversity.

Why have so many endemic species been lost from islands? The main problems arise from exotic (introduced, non-native) plants, predators and herbivores.  They have caused island extinctions in the following ways:

• Predation. Introduced predators such as rats, cats, dogs, foxes and mongooses are thought to have been responsible for about half of island bird extinctions. Indigenous animals on islands are well known for their tame behavior, which makes them extremely vulnerable to predation by man and introduced predators.

• Rats. Introduced predators responsible for the largest number of extinctions are the Polynesian rat Rattus exulans carried across the Pacific by Polynesians and Micronesians and two other rat species, Rattus rattus and Rattus norvegicus, carried around the world by Europeans in the last 5 centuries. These rats often feed on eggs and chicks of ground-nesting birds. Introduced rats have exterminated bird species on 26 islands including catastrophic extinction waves that eliminated a large fraction of the native bird species within a few years on Hawaii, Midway Island, Lord Howe and the South Cape Island. On the Galapagos islands, black rats have reduced populations of the Dark-rumped Petrel by preying on eggs. Some islands have received introduced rats without consequent extinction waves. These include Fiji, Tonga, and Samoa. These are islands that have native rats or land crabs. Land crabs are nocturnal scavengers that climb trees, enter holes and are the invertebrate ecological equivalent of rats. Presumably, species that have evolved along with land crabs are well adapted to defend themselves against rats.

• Cats. The flightless Stephen Island wren, one of only a few species of flightless songbird ever to have lived, was driven to extinction in 1894 by loss of habitat and a lighthouse keeper's cat.

Stephens Island Wren (Traversia lyalli) is only known from recent times from Stephen's Island, New Zealand, although it is common in fossil deposits from both of the main islands. The species was flightless and restricted to the rocky ground. Construction of a lighthouse on Stephens Island in 1894 led to the clearance of most of the island's forest, with predation by the lighthouse keeper's cat delivering the species' coup-de-grace.

Source: www.nzbirds.com/StephensWren.html

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• Habitat destruction. Introduction of goats and other herbivores on islands has often been devastating. Many plant species and half the endemic bird species on Guadalupe Island off Baja California have been lost due to destruction of most of the island's vegetation by introduced goats. Goats have also caused great damage on the California Channel Islands and on some of the Galapagos Islands. The goats are slowly being removed from some of these islands, but they breed very quickly and it is difficult to remove every one. Introduced rabbits removed vegetation from Laysan Island, leading to the extinction of an endemic rail and two of the island's three songbirds.

 
Easter Island

For birds, Easter Island ’s remoteness and lack of predators made it an ideal haven as a breeding site, at least until humans arrived. Among the prodigious numbers of seabirds that bred there were albatross, boobies, frigate birds, fulmars, petrels, prions, shearwaters, storm petrels, terns, and tropic birds. With at least 25 nesting species, Easter Island was the richest seabird breeding site in Polynesia and probably in the whole Pacific. Pollen records show that destruction of Easter Island’s forests was well under way by the year 800, just a few centuries after the start of human settlement. Not long after 1400, the palm became extinct, and soon thereafter the forest itself. Its doom had been approaching as people cleared land to plant gardens; as they felled trees to build canoes, to transport and erect statues, and to burn; and probably as the native birds died out that had pollinated the trees’ flowers and dispersed their fruit. The destruction of the island’s animals was as extreme as that of the forest: without exception, every species of native land bird became extinct. The colonies of more than half of the seabird species breeding on Easter or on its offshore islets were wiped out. -- Jared Diamond

• Competition for Food. When the introduced species has a similar diet to the indigenous species, food competition can be an important factor in the decline of the indigenous species. The decline of a rare forest bird in New Zealand, the kokako, is attributed at least in part to food competition since introduced herbivores, especially opossums, feed on the same parts of the same plants eaten by the kokako, e.g., the fruit.

• Competition for Space. Competition for space is an important factor leading to decline of indigenous animals and plants.  Many exotic plants crowd out or cover native species. Competition for nesting sites has been shown to be an important factor, along with habitat destruction, in the decline of the Puerto Rican Parrot (see figure below). The introduced honey bee uses the same kind of tree cavities and has reduced the availability of nest sites for the parrots.

• Diseases. Diseases introduced with exotic birds such as domestic chickens. Following the introduction of numerous types of birds from Asia into Hawaii in the 1920's, avian malaria spread throughout many species of native forest birds and sharply reduced their abundance and range. It was spread by mosquitoes, all of which are introduced species, Hawaii having no native ones.

Study helps explain island populations' susceptibility to exotic diseases - Lindström et al. (2004) have found that Darwin's finches on smaller islands in the Galapagos archipelago have weaker immune responses to disease and foreign pathogens—findings that could help explain why island populations worldwide are particularly susceptible to disease. Johannes Foufopoulos, one of the co-authors, noted that "The introduction of exotic parasites and diseases through travel, commerce and domestic animals and the resulting destruction in native wildlife populations is a worldwide problem, but it's even more serious for species that have evolved on islands. For example, in the Hawaiian islands, many native bird species have gone extinct after the introduction of avian malaria. The Galapagos authorities are now realizing that the greatest danger to the islands' wildlife comes from exotic species, such as invasive pathogens, accidentally introduced by humans." The investigators found that larger islands with larger bird populations harbor more native parasites and diseases, because the number of parasites is directly dependent on the size of the population. Island size and parasite richness then influenced the strength of the immune response of the hosts. By challenging the birds immune systems with foreign proteins, they measured the average immune response of each island population. Finches on smaller islands with fewer parasites had a weaker immune response. For these birds, Foufopoulos said, "maintaining a strong immune system is a little bit like house insurance: you don't want to spend too much on an expensive policy if you live in an area with no earthquakes, fires or floods." Similarly, if parasites are scarce, the birds don't need to invest in an "expensive" immune system, he said. When new parasites are then accidentally introduced by humans to these islands, the birds are ill-prepared to resist infection.  See also: Darwin's Finches At Risk

Source: users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html 1. Large cactus finch (Geospiza conirostris), 2. Large ground finch (G. magnirostris), 3. Medium ground finch (Geospiza fortis), 4. Cactus finch (G. scandens), 5. Sharp-beaked ground finch (G. difficilis), 6. Small ground finch (G. fuliginosa), 7. Woodpecker finch (Cactospiza pallida), 8. Vegetarian tree finch (Platyspiza crassirostris), 9. Medium tree finch (Camarhynchus pauper), 10. Large tree finch (Camarhynchus psittacula), 11. Small tree finch (C. parvulus), 12. Warbler finch (Certhidia olivacea) 13. Mangrove finch (Cactospiza heliobates)

Recent changes in Bird Biogeography - Introductions:

• Humans have introduced non-native species of birds on every continent & many islands for a variety of reasons (Lever 1987):
• sport
• nostalgia
• aesthetics
• potential source of food
• biological control of insect pests
• curiosity (what will become of them in a new environment?)
• Some introductions have been accidental (e.g., birds escaping from captivity)

 
House Sparrows

• As a result of such planned or unplanned introductions:
• Two of the three most common nesting species in North America today are birds whose ancestors were brought here from Europe (House Sparrows & European Starlings). Other introduced species include:
• Rock Dove (Columba livia),
• Ring-necked Pheasant (Phasianus colchicus),
• Mute Swan (Cygnus olor),
• Gray or Hungarian Partridge (Perdix perdix), & the
• Chukar (Alectoris chukar)
• Many 'tropical birds' in Florida (e.g., Budgerigar, Red-whiskered Bulbul, Spot-breasted Oriole, & several others)
• A recent addition to the avifauna of the eastern United States is the House Finch (Carpodacus mexicanus). These finches, native to the western U.S., were released (illegally) on Long Island, New York, in the early 1940's. They now breed in every eastern state.

House Finch Range based on Breeding Bird Survey data
(Source: http://www.mbr-pwrc.usgs.gov/id/framlst/BBSMap/ra5190.gif)

• In the Hawaiian Islands, introduced songbird species far exceed native ones. Visitors to Honolulu see only exotic songbirds unless they hike mountain trails in search of the few remaining endemic species.
• Puerto Rico has breeding populations of about 40 kinds of exotic birds.
• Australia has at least 26 introduced species of birds (Lever 1987).
• Japan has at least 10 introduced species  (Lever 1987).

 

Recent changes in Bird Biogeography - Range Extensions and Shifts in Response to Climate Change?:

• Several species in the Holarctic have expanded ranges northward over the past several decades, perhaps in response to climate changes or human-caused changes in habitats. Examples include:
• Blue Grosbeak  (U.S.)

• Tufted Titmouse (U.S.)

• Chaffinch (Europe)
• European Blackbird (Europe)
• Crested Tit (Europe)

Blue-gray Gnatcatcher population trends (1966-2003). Note the increase in the northern portion of its breeding range.

Inca Dove population trends (1966-2003).

Breeding distributions of North American birds moving north -- Geographic changes in species distributions toward traditionally cooler climes is one hypothesized indicator of recent global climate change. Hitch and Leberg (2007) examined distribution data on 56 bird species. If global warming is affecting species distributions across the temperate northern hemisphere, data should show the same northward range expansions of birds that have been reported for Great Britain. Because a northward shift of distributions might be due to multidirectional range expansions for multiple species, the possibility that birds with northern distributions may be expanding their ranges southward was also examined. There was no southward expansion of birds with a northern distribution, indicating that there is no evidence of overall range expansion of insectivorous and granivorous birds in North America. As predicted, the northern limit of birds with a southern distribution showed a significant shift northward (2.35 km/year). Among the species showing the greatest shift northward were Inca Doves, Fish Crows, Blue-winged Warblers, Hooded Warblers, Blue-gray Gnatcatchers, Black-billed Cuckoos, and Golden-winged Warblers. This northward shift is similar to that observed in previous work conducted in Great Britain: the widespread nature of this shift in species distributions over two distinct geographical regions and its coincidence with a period of global warming suggests a connection with global climate change.

• Global warming could also impact bird migration - check this website and video
• As a result of global warming (Line 2003):
• the Great Lakes states may lose 53% of the Neotropical migrant songbirds that presently nest in the region.
• Other projected losses include 45 percent fewer Neotropical migrants in the Mid-Atlantic region, 44 percent fewer in the northern Great Plains and 32 percent fewer in the Pacific Northwest.
• Some losses could be offset by new species moving into an area, e.g., the Southwest may gain some birds now limited to Mexico
• Overall, avifaunal diversity is expected to decline  

 

Birds and Global Warming Scientists of the U.S. Geological Survey, in cooperation with Canadian scientists, conduct the annual North American Breeding Bird Survey, which provides distribution and abundance information for birds across the United States and Canada. From these data, collected by volunteers under strict guidance from the U.S. Geological Survey, shifts in bird ranges and abundances can be examined. Because these censuses were begun in the 1960's, these data can provide a wealth of baseline information. Price (1995) has used these data to examine the birds that breed in the Great Plains. By using the present-day ranges and abundances for each of the species (e.g., Bobolink, top map on the right), Price derived large-scale, empirical­statistical models based on various climate variables (for example, maximum temperature in the hottest month and total precipitation in the wettest month) that provided estimates of the current bird ranges and abundances (middle map on the right). Then, by using a general circulation model to forecast how doubling of CO2 would affect the climate variables in the models, he applied the statistical models to predict the possible shape and location of the birds' ranges and abundances (bottom map on the right).  Significant changes were found for nearly all birds examined. The ranges of most species moved north, up mountain slopes, or both. The empirical models assume that these species are capable of moving into these more northerly areas, that is, if habitat is available and no major barriers exist. Such shifting of ranges and abundances could cause local extinctions in the more southern portions of the birds' ranges, and, if movement to the north is impossible, extinctions of entire species could occur. We must bear in mind, however, that this empirical­statistical technique, which associates large-scale patterns of bird ranges with large-scale patterns of climate, does not explicitly represent the physical and biological mechanisms that could lead to changes in birds' ranges. Therefore, the detailed maps should be viewed only as illustrative of the potential for very significant shifts with different possible doubled CO2 climate change scenarios.  Source: http://biology.usgs.gov/s+t/SNT/noframe/cl110.htm

(a) Map of current range and abundance of the Bobolink as determined from actual observations during the U.S. Geological Survey Breeding Bird Survey and (b) map of current range and abundance of the bobolink as estimated from the empirical­statistical model. The high correspondence in patterns between maps a and b suggests that this model reliably captures many of the features of the actual observed range and abundance of this species as depicted in map a. (c) Map of the forecasted range and abundance of the bobolink for climate change response of a model with doubled CO2. This map illustrates the potential for very significant shifts that doubled CO2 could cause (Price 1995).

 
Global warming

Global warming in the UK by David Attenborough  

 
Mass Extinction

Digestive System: Food & Feeding Habits

Back to Bird Biogeography I

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Useful links:

Atlas of the Ice Age Earth

Birds sing rainforest history

Ecological Complexity

Geography and Ecology of Species Distributions

Geology: Plate Tectonics

Glaciers may not have driven modern bird evolution

Non-native Birds

Wallace's Line

Zoogeography and the Sea

Back to BIO 554/754 Syllabus