Bird Biogeography II
An updated version of these notes can be accessed from a new "Avian Biology' page
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):
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
|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.
As a result of this history (& other factors described below):
|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?
Breeding bird species in North and Central America
A number of possible explanations for these latitudinal diversity gradients have been suggested (Wiens 1991):
|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.
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
emperate 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 regionswere 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):
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).
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).
|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).|
of direction -- Cochran et al. (2004) found that migrating thrushes
rely on a built-in magnetic compass, which they recalibrate each evening
based on the direction of the setting sun. The research, which 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.
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
Recent changes in Bird Biogeography - Extinction:
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:
"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
|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:
|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.||
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
1. Large cactus finch (Geospiza conirostris), 2.
Large ground finch (G.
Recent changes in Bird Biogeography - Introductions:
|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?:
|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.
(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).
Digestive System: Food & Feeding Habits
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Atlas of the Ice Age Earth
Birds sing rainforest history
Geography and Ecology of Species Distributions
Geology: Plate Tectonics
Glaciers may not have driven modern bird evolution
and the Sea
to BIO 554/754 Syllabus