BIO 554/754

Bird Biogeography II


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):

Location of continents in the early Cretaceous
Location of continents in the early TertiaryLocation of continents in the Miocene

How the Andes were formed

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.

Photo of 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

Tectonic plate collision  


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. 
Colored maps showing variation in avian species richness in South America
 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).

Map of South America showing number of bird species present in different areas

Geographic variation in the richness of breeding terrestrial bird species in South America (Hawkins et al. 2003).

Colored maps showing variation in global species richness
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.

Maps showing processes involved in evolution of Trumpeters
A palaeobiogeographic model for terrestrial environments of Amazonia for the last 3 million years based on the evolutionary history of trumpeters (Psophia) and geological data. Rivers and their tributaries are depicted as in the present, but their palaeopositions may have differed. (a) About 3.0–2.7 million years ago (Ma): western lowland Amazonia is a large interconnected wetland/lake/river system. (b) About 2.7–2.0 Ma: the wetland system drained significantly and the lower Amazon River, which isolated northern and southern populations of Psophia, was established. As terra firme forests developed, populations expanded westward. (c) About 2.0–1.0 Ma: the Rio Madeira drainage was established, promoting speciation of P. leucoptera (Inambari area). (d) About 1.3–0.8 Ma: the Rio Tapajós drainage system developed, resulting in the differentiation of P. viridis (Rondônia area). At the same time, a barrier (red bar) is postulated to have isolated the Napo area of endemism within which P. napensis differentiated. (e) About 1.0–0.7 Ma: an isolating barrier associated with the lower Rio Negro formed, giving rise to P. ochroptera (Negro area) and P. crepitans (Guianan area). (f) About 0.8–0.3 Ma: two drainage systems on the Brazilian Shield, the Rio Tocantins and the Rio Xingu, were established as isolating barriers, creating three areas of endemism and their endemic species. AB, Amazon Basin; SB, Solimões Basin; P, location of Purus Arch.

River dynamics and Amazonia biodiversity -- Many hypotheses have been proposed to explain high species diversity in Amazonia, but few generalizations have emerged. In part, this has arisen from the scarcity of rigorous tests for mechanisms promoting speciation, and from major uncertainties about palaeogeographic events and their spatial and temporal associations with diversification. Ribas et al. (2011) examined the environmental history of Amazonia using a phylogenetic and biogeographic analysis of trumpeters (Aves: Psophia), which are represented by species in each of the vertebrate areas of endemism. Their relationships reveal an unforeseen ‘complete’ time-slice of Amazonian diversification over the past 3 million years. A temporally calibrated phylogeny was employed to test competing palaeogeographic hypotheses. Results were consistent with the establishment of the current Amazonian drainage system at approximately 3.0–2.0 million years ago and predict the temporal pattern of major river formation over Plio-Pleistocene times. Ribas et al. (2011) propose a palaeobiogeographic model for the last 3.0 Myr of Amazonian history that has implications for understanding patterns of endemism, the temporal history of Amazonian diversification and mechanisms promoting speciation. The history of Psophia, in combination with new geological evidence, provides the strongest direct evidence supporting a role for river dynamics in Amazonian diversification, and the absence of such a role for glacial climate cycles and refugia.

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

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

While historical factors such as refugia & topography have certainly contributed to the great diversity of birds in tropical rain forests, other factors are also involved. These include:


Drawing showing different strata or layers in a rainforest

Figure showing abundance of different foraging guilds in the tropics
Bird guilds and ecological specialization
(light bars for the tropics, dark bars for the temperate zone)

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 of a White-eyed Foliage-gleaner

Photo by Arthur Grosset

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

Mean degrees north latitude
Number of species
Ellesmere Island
Costa Rica
From Welty & Baptista (1988)

Map of North America showing number of different species of birds in different areas
Breeding bird species in North and Central America

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

Obviously, many factors can contribute to latitudinal gradients in diversity, but their importance varies among regions or continents. At a finer scale, diversity exhibits more complex patterns. For example, along the North American east coast, diversity actually decreases from north to south in both deciduous & coniferous woodlands. Why? Perhaps because of the high productivity of northern areas during the breeding season (and the resultant influx of migrants). The 'reverse gradient' disappears during the winter (Rabenold 1993).

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.

Figure showing mechanism for development of avian species richness in the tropics
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):

Animated gif of a foraging hummingbird

Maps showing species richness of ant-following birds in South America
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).

Map showing wintering and breeding ranges of Scarlet Tanagers

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).

Winged Migration

Sandhill Cranes  

The Great 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). 

Figure showing checklist of things that must be considered by a migrating bird
Checklist of things a migrant bird has to consider before departing on a long-distance flight (Piersma et al. 1990, Klaasen 1996).

The migration cycle of Semipalmated Sandpipers
Semipalmated Sandpipers (Calidris pusilla) stop in the Bay of Fundy (New Brunswick, Canada) during their fall migration from breeding areas in the Arctic. This crucial stopover allows the sandpipers to store large lipid reserves by eating seasonally abundant amphipods (Corophium volutator) buried in the mudflats. An estimated 75% (about 1 million individuals) of the world population of Semipalmated Sandpipers stops at this location. The Corophium mudshrimp contains unusually large amounts of n-3 polyunsaturated fatty aids (45% of total lipids) and is found only in the Bay of Fundy and along the coast of Maine. Dietary n-3 fatty acids are not only used as an energy source, but they also act as performance-enhancing substances to increase the capacity for endurance exercise, just before the sandpipers cross the Atlantic ocean to South America. The remainder of the annual migration cycle appears to be achieved through multiple short flights over land and along coastal areas. Pollution and rising sea levels caused by global warming are major threats to strategic stopover sites such as the Bay of Fundy and to the future of these sandpipers (Weber 2009).

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

MIgration of Gray-cheeked Thrushes

Photo of a Greater Noctule bat
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.

Map showing migration pathways of 19 Sooty Shearwaters
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. (bd) 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:Photo of a Carolina Parakeet

Source: American Bird Conservancy

Figure showing main threats to bird habitat in the United States
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). 

Declining bird populations  

Tourism and Lesser Prairie Chicken conservation 


Delaware Bay - A critical stopover point for shorebirds

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.

Colored map showing distribution of threatened species of birds in different parts of the world
Distribution of threatened species of birds. Dark red = highest number of
threatened species; dark blue = lowest number (Source: IUCN).

Figure showing changes in estimated abundance of all birds from 1700 to 1990
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 Drawing of two Passenger Pigeonswill 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

Figure showing main threats to birds

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

Amazon drought and deforestation

Figure showing timing of bird extinctions in Hawaii
Extinction of native breeding birds since 1778. Steps mark the decade of the last record for each form considered extinct (A.O.U. 1983).
The 70 forms shown as currently existing include 13 in peril, with steps marking the decades of their last known records.
Yellow represents prehistoric forms. (Source:

Hawaiian birds  

Photo of a brown tree snake

Population trends of 9 species of birds on Guam
Population trends for Guam birds as indicated by roadside surveys, 1976–1998 (Wiles et al. 2003).

An endemic radiation of Malagasy songbirds -- The bird fauna of Madagascar includes a high proportion of endemic Map showing Africa and Madagascarspecies, 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:

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. 
Drawing of two Stephens Islands Wrens

Map showing location of Easter Island 
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

Figure showing changes in population of Puerto Rican parrots since 1500

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

Drawing of Darwin's Finches

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), and 13. Mangrove finch (Cactospiza heliobates)

Recent changes in Bird Biogeography - Introductions:

House Sparrows

Map showing breeding range of House Finches
House Finch Range based on Breeding Bird Survey data

Big brains & novel environments -- The widely held hypothesis that enlarged brains have evolved as an adaptation to cope with novel or altered environmental conditions lacks firm empirical support. Sol et al. (2005) tested this hypothesis for birds by examining whether large-brained species show higher survival than small-brained species when introduced to nonnative locations. Using a global database documenting the outcome of >600 introduction events, they confirmed that avian species with larger brains, relative to their body mass, tend to be more successful at establishing themselves in novel environments. Moreover, Sol et al. (2005) provided evidence that larger brains help birds respond to novel conditions by enhancing their innovation propensity rather than indirectly through noncognitive mechanisms. These findings provide strong evidence for the hypothesis that enlarged brains function, and hence may have evolved, to deal with changes in the environment.

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

Map showing breeding range of Blue Grobeaks

Map showing year round range of Tufted Titmice


Colored map showing population trends for Blue-gray Gnatcatchers
Blue-gray Gnatcatcher population trends (1966-2003). Note the increase in the northern portion of its breeding range.
Colored map showing population trends for Inca Doves
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.


Birds wintering further north

Migratory restlessness in a non-migratory bird - The urge of captive birds to migrate manifests itself in seasonally occurring restlessness, termed “Zugunruhe.” Key insights into migration and an endogenous basis of behavior are based on Zugunruhe of migrants but have scarcely been tested in nonmigratory birds. Helm and Gwinner (2006) recorded Zugunruhe in African Stonechats, small passerine birds that defend year-round territories and that diverged from northern migrants at least 1 million years ago. Such results demonstrate that Zugunruhe is a regular feature of their endogenous program, and is precisely timed by photoperiod. Such programs could be activated when movements by non-migratory birds become necessary. Helm and Gwinner (2006) propose that low-level Zugunruhe may be common in birds, including residents, and could underlie recent rapid changes in movement and range patterns attributed to global change and other human interventions. Attention to Zugunruhe of resident birds promises new insights into diverse and dynamic migration systems and enhances predictions of avian responses to global change.

Photograph of a Stonechat

Stonechats (Saxicola torquata) were ideal subjects for this study, with two subspecies: a north temperate migrant (S. t. rubicola) and an equatorial resident (S. t. axillaris). They display persistent circannual rhythms of molt and reproduction under constant conditions. Migrants show distinct Zugunruhe, timed by precise photoperiodic programs. In contrast to northern obligatory migrants, stonechats from equatorial Kenya defended their breeding territories throughout the year. Genetic distances between the two disjunctly distributed subspecies are large.

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. 


Colored maps showing current and future breeding ranges of Bobolinks
(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 - birds nesting earlier

Emperor Penguins and Global Warming

Global warming

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