Ecology Of Birds In Natural Landscape

The approximately 9,800 species of living birds belong to what zoologists call the Class Aves of the Animal Kingdom. Modern birds are easily distinguished from their fellow vertebrates such as amphibians, reptiles, and mammals by a unique feature that they all share: feathers. Even penguins, which don't fly, but swim in the sea using their flippers to propel themselves, have feathers—although their coat may look superficially like fur.

Feathers are modified scales. Unlike scales, feathers are very flexible and extremely light, because of the air inside the rachis, or stem, and the loose organization of their component parts, called barbs and barbules. With the exception of penguins, ostriches, rheas, cassowaries, emus, and some island rails, birds use their feathers to fly. Feathers are also a marvelous insulating agent that permit their owners, whether flying or not, to regulate their body temperature. Hence even small and fragile-looking birds like juncos (belonging to the bunting family) can survive without problems in cold and snow—provided they have enough food to maintain their high

Turkeys, like other birds, use feathers to communicate among themselves. The colors and shapes of wing, tail, or body contour feathers, when exhibited in display, convey information about sexual readiness, social status within flocks, and aggression. (Swift/Corbis)

metabolism and internal temperature. Unlike whales and seals, birds do not have a thick layer of insulating blubber underneath their skin. However, birds can ruffle up their feathers by means of special muscles, creating an air layer between skin and feathers that isolates them from the low temperatures of the outside world. Eskimos living in the Arctic use a similar principle when they wear loose clothing of animal fur (caribou, polar bear, or seal) that leaves plenty of air circulating between their skin and their parkas and pants. Feathers also permit birds to communicate among themselves. The colors and shapes of wing, tail, or body contour feathers, when exhibited in display, convey information about sexual readiness, social status within flocks, and aggression.

The closest living relatives of modern birds are crocodiles and their kin. Modern birds are also related to several extinct groups of vertebrates, the dinosaurs, especially theropods. Recent research has suggested that although some of these dinosaurs had feathers, or featherlike structures, they did not fly. Paleontologists believe that they used their feathers for thermal insulation and regulation. As the possession of feathers is not always associated with flying, one may think that these feathered dinosaurs and modern birds are not only related but also belong in the same zoological group. Some paleontologists even argue that modern birds are descendants from dinosaurian ancestors, and that modern birds are indeed surviving dinosaurs. The hardy juncos all fluffed up against the cold at our winter feeders may be tiny flying dinosaurs.

Whether or not they are dinosaurs, and whether or not they fly, modern birds are probably the best known group of animals, vertebrates or invertebrates. For example, whereas zoologists describe dozens, sometimes hundreds, of new species of fish, beetles, or flies each year, ornithologists propose on average only about three to five new species of birds in the same period of time. Of course these "new" bird species did not just evolve. They were present all along, but it is only recently that ornithologists discovered them while exploring some of the most inaccessible areas in the world, such as remote forests in the Andes of South America or the mountains of Africa or Asia. Although remarkable, the discovery of as-yet undescribed species of birds increases their known biodiversity by no more than a fraction of 1 percent annually. Biodiversity increases only during the process known by evolutionary biologists as speciation, when one or more new species evolve from an ancestral one. The evolution of new species is a problem that intrigued Charles Darwin, but it was one that he did not solve. Ornithologists have contributed greatly to our understanding of the intricacies of the speciation process. This contribution has been made possible by our detailed knowledge of birds.

Not only are the great majority of bird species described and catalogued, they are also quite well known in term of their geographical distribution, behavior, voice, nesting habits, habitat preferences, and migration routes. Chiefly thanks to their amazing capacity to regulate their body temperature, birds are found just about everywhere over the oceans and on earth. Thus wandering albatrosses (Diomedea exulans) can be observed in the most isolated stretches of the turbulent Southern Ocean, bar-headed geese (Anser indicus) have been seen in the fall flying over some of the highest ranges of the Himalayas, above 20,000 feet, and white-winged Diuca finches (Diuca speculifera) have been detected roosting in glaciers at 16,000 feet in the Bolivian Andes. I have watched black-legged kitti-wakes (Rissa tridactyla) and ivory gulls (Pagophila eburnea) at the North Pole; in the Antarctic, south polar skuas (Catharacta mccormicki) have been sighted not far from the South Pole.

Whereas the above observations have been of nonbreeding birds, of course the fact remains that birds can breed in amazing and quite unexpected places, once again showing their extraordinary ability to exploit all available niches. For example, the gray gull (Larus mod-estus) nests in the middle of one of the most arid deserts in the world, the Atacama Desert of Chile. Some snow petrels (Pagodroma nivea) breed on nunataks, isolated bits of bare rock sticking out of the huge South Polar icecap. A hummingbird, the Andean hillstar (Oreotrochilus estella), places its nest along rocky overhangs in the treeless reaches of Peru's and Bolivia's Andes, at elevations reaching 15,000 feet. At such altitudes within the tropics, nocturnal temperatures regularly dip below freezing, and in order to save energy the tiny birds lower their body temperature and go into a sleeplike state called torpor. The great dusky swift (Cypseloides senex) of Brazil builds its conelike nest on wet rocky ledges behind some of the waterfalls in Iguagu National Park. And the nocturnal common potoo (Nyctibius griseus) of Central and South America's forests and brush lands makes no nest at all but simply lays its single egg on the top of a broken branch stub. When incubating, the adult bird looks like a branch herself. Perhaps the most amazing breeding behavior—and habitat— of all birds are those of the emperor penguin (Aptenodytes forsteri). In the early Antarctic winter the single egg is laid, then incubated on his feet in a pouch of skin and feathers by the male. Emperor penguin chicks hatch and grow in the middle of the long Antarctic winter night, in the coldest and harshest environment on earth, the Antarctic sea ice. No wonder that Apsley Cherry-Garrard, recounting the ordeals of the expedition that first found and collected an emperor penguin's egg, entitled his book The Worst Journey in the World. (The emperor penguin was then thought to be such a primitive bird that the embryology of its egg might give clues about the origin of birds. How wrong ornithologists were at that time: we now know that penguins are like all other birds, and no more primitive than crows or grackles.)

Our extensive knowledge of birds, together with their wide dispersion and the general ease with which they can be studied in the field, makes them very important biodiversity indicators. Indeed, we can now compile detailed lists of rare, at risk, or endangered species from all parts of the world. Of the nearly 9,800 species of birds, no fewer than 1,186 (or about 12 percent) are estimated to be at risk of becoming extinct in the next 100 years. Of these 1,186 species, 182 are considered to be critically endangered and 321 to be vulnerable (BirdLife International, 2000, p. 2). Hence a staggering 503 species (about 5 percent of the world total) could well disappear from the globe in the twenty-first century. If this projected extinction rate applies not only to birds but to other creatures as well, our planet is about to suffer a serious loss.

Avian biodiversity, as measured by numbers of species, is not evenly distributed among given areas, habitats, or biogeographical regions, but instead follows several broad patterns. Thus equatorial and tropical regions have more species than do temperate ones of the same surface area, and those, in turn, have more species than do boreal or arctic regions. Within the tropics, the Amazonian rain forest biome of South America has more species than the Congo forest biome in Africa. Some estimates suggest that as many as 500 bird species can be found in one square mile of Amazonian rain forest. In general, forests have more bird species than savannas or grasslands. A continental chunk of land has more species than an equiv alent area on an island. And within archipelagos, remote islands have fewer species than those of similar surface area closer to a mainland. Oceanic birds too have distributional and biodiversity patterns. Many more species of petrels and petrel-like birds are found in southern seas than in the north. Thus the Arctic Ocean has but one species of petrel, the northern fulmar (Fulmarus glacialis), whereas the Southern Ocean has about a dozen other species in addition to the southern fulmar (Fulmarus glacialoides). But biodiversity of birds expressed purely in terms of species numbers masks an important part of the picture.

Just as species are not distributed evenly or randomly over the globe, the distribution of each species, and of its constituent populations, follows patterns. In general, large species, such as eagles and hawks (family Accipitridae), have extensive territories and thus lower population densities than do smaller birds such as sparrows (Emberizidae) or wood warblers (Parulidae). Tropical birds, especially species living in rain forests, often have very patchy distributions and low population densities where they are found, even though the species' total distributional range might be very large. Thus a given species found through much of Amazonia's forests may actually be unaccountably absent from many areas where these forests look (to us) suitable for its presence. In addition, the population dynamics of many species fluctuate. In other words, the numbers of birds of any given species vary over space and time. Even without clear-cut influences from human agencies, some species decrease in their range or in their relative abundance, whereas others increase.

Some bird species have benefited from our activities and clearly enjoy human environments. Well-known among these are domestic pigeons and house (or English) sparrows in cities and towns all over the world, European

Shorebirds fly over tide flats along Cumberland Sound on Cumberland Island National Seashore, Georgia. (Raymond Gehman/Corbis)

starlings in the North American countryside, and black kites and house crows in Asian cities. Unfortunately, we also know that human-induced changes in our environment as well as direct human pressures such as habitat destruction (logging, paving over agricultural land, filling marshes and swamps) and indiscriminate hunting have resulted in the extinction of several bird species. Examples include the great auk (Alca impennis), the emblem of the most important ornithological society in North America; the passenger pigeon (Ectopistes migratorius); and several species of beautiful parrots in the West Indies. Other species, said by some ornithologists to be still living, are believed by others to be already extinct. The Eskimo curlew (Numenius borealis), which used to be called the swit-

fwing (because of its flying ability) by native Americans from the Ungava Peninsula of Labrador, was once an abundant bird, whose hordes bred in northern Canada and migrated thousands of miles southward across the equator to spend the winter on the pampas of Argentina before returning to their far northern nesting grounds. Relentless hunting along its migration routes in North America and in its southern haunts in South America, combined with the complete modification of its prairie habitat in the north and of the pampas in the south, decimated the species. Although very small numbers of Eskimo curlews might still breed in the remote Canadian Arctic, none of the sightings made in the last two decades have been confirmed by reliable authorities.

There is a net (and of course irreversible) loss of biodiversity when species become extinct, whether they do so because of natural agencies (as was the case in the extinction of most dinosaurs at the Cretaceous-Tertiary boundary following the catastrophic crash of an asteroid) or man-induced changes (such as the case of the great auk). These losses, when natural, take place at slow average rates (in terms of human generation times), perhaps of the order of 1 species of bird per 100 years, according to some estimates (ibid., p. 2); more likely is 1 species every 10,000 or even 100,000 years. But more than 100 species of birds have become extinct because of man-related or man-induced causes in the last 200 hundred years, a rate of about 1 species every 2 years. If we were to continue losing bird species at the same rate, avian biodiversity would inexorably plummet toward a biodiversity desert. This loss would be evident not only in terms of how many species become extinct but also which species remain. Predictably, the survivors would be the common and ubiquitous species, as well as man's commensals, such as house sparrows, European starlings, and house crows.

Can natural evolutionary processes that result in an increase in biodiversity—namely, speciation—make up for these losses? That is unfortunately not possible. Perhaps as much as 30 percent of the enormous bird diversity of the Amazon basin, with as many as 2,000 species (about 20 percent of the world's total), has been hypothesized by some ornithologists to have originated through natural spe-ciation events in the last million years or so. Thus about 600 new species of birds may have evolved naturally in 1,000,000 years, a rate of about 0.06 species per 100 years, or six new species per 10,000 years. Although other ornithologists have disputed these estimates, independent evidence suggests that a speci-ation rate of 1 new species per 10,000 years is not so far-fetched. The Antarctic island of South Georgia, in the South Atlantic Ocean, is home to only one species of land bird, the Antarctic pipit (Anthus antarcticus), a small and rather fragile-looking passerine (or songbird). Even though South Georgia is largely covered with snow and ice in the winter, the pipit there, which eats chiefly small invertebrates living in the intertidal zone, is resident and evidently finds enough food along the shores to sustain it during winter blizzards and cold. South Georgia's pipit is closely related to a widespread South American mainland relative, the Correndera pipit (Anthus correndera), which also lives on Cape Horn and the Falkland Islands. All available evidence (morphology, behavior, voice, DNA) indicates that the ancestor of the South Georgia species was carried there from South America by the strong westerly winds that prevail in the high latitudes of the Southern Ocean. Such colonization events, followed by residency in the colony and their subsequent differentiation into new species, have been documented in many other bird groups, such as the famous Darwin's finches and mockingbirds of the Galapagos Islands. Because South Georgia was entirely covered by an icecap during the latest phases of glacial advance in southern latitudes during the last 500,000 years before the present, no land bird like a pipit could have survived there. The icecap retreated from the shores and lower altitudes of South Georgia only about 10,000 years ago. Hence successful colonization of this island by a South American pipit could only have occurred 10,000 years ago or even later. The resident Antarctic pipit of South Georgia cannot therefore be more than 10,000 years old. Most ornithologists, and many other zoologists, would consider such a speciation rate very rapid. It is clear that species losses of about 1 species every 2 years

Table 1

The Major Families of Birds

Table 1

Number

Number

Non-Passeriformes

of Species Family

Passeriformes*

of Species

Family

Ostrich

1

Struthionidae

Miners and Earthcreepers

221

Furnariidae

Tinamous

46

Tinamidae

Antbirds and Antpittas

236

Formicariidae

Penguins

18

Spheniscidae

Tyrant Flycatchers

390

Tyrannidae

Loons

5

Gaviidae

Cotingas and Fruiteaters

65

Cotingidae

Grebes

21

Podicipedidae

Lyrebirds

2

Menuridae

Albatrosses

14

Diomedeidae

Larks

85

Alaudidae

Petrels and Shearwaters

72

Procellariidae

Swallows and Martins

80

Hirundinidae

Pelicans

8

Pelecanidae

Wagtails and Pipits

55

Motacillidae

Cormorants

33

Phalacrocoracidae

Bulbuls and Greenbuls

124

Pycnonotidae

Herons, Bitterns, and

Shrikes

69

Laniidae

Egrets

60

Ardeidae

Wrens

69

Troglodytidae

Storks

19

Ciconiidae

Mockingbirds and Thrashers 30

Mimidae

Flamingos

5

Phoenicopteridae

Thrushes and Robins

324

Turdidae

Ducks, Geese, and Swans

149

Anatidae

Babblers and Wren-Tit

256

Timaliidae

American Vultures and

Old World Warblers

376

Sylviidae

Condors

7

Cathartidae

Old World Flycatchers

147

Muscicapidae

Hawks, Eagles, and Kites

225

Accipitridae

Titmice and Chickadees

50

Paridae

Falcons and Caracaras

61

Falconidae

Nuthatches

25

Sittidae

Pheasants, Grouse, and

Sunbirds

130

Nectariniidae

Quail

211

Phasianidae

Honeyeaters

174

Meliphagidae

Cranes

15

Gruidae

Buntings, Juncos

321

Emberizidae

Rails and Coots

124

Rallidae

Cardinals

43

Cardinalidae

Plovers

66

Charadriidae

Tanagers

256

Thraupidae

Sandpipers and Snipe

86

Scolopacidae

New World Warblers

116

Parulidae

Skuas

7

Stercorariidae

Hawaiian Honeycreepers

23

Drepanididae

Gulls and Terns

91

Laridae

Vireos

52

Vireonidae

Auks, Puffins, and Auklets

22

Alcidae

Grackles, Cowbirds

97

Icteridae

Pigeons and Doves

309

Columbidae

Finches, Serins

134

Fringillidae

Parrots and Parakeets

353

Psittacidae

Weavers, House Sparrow

114

Ploceidae

Cuckoos

150

Cuculidae

Starlings, Mynas

108

Sturnidae

Owls

204

Strigidae

Birds of Paradise

42

Paradisaeidae

Potoos

5

Nyctibiidae

Ravens, Crows, and Jays

117

Corvidae

Nightjars and Nighthawks

83

Caprimulgidae

Swifts

92

Apodidae

Hummingbirds

332

Trochilidae

Kingfishers

92

Alcedinidae

Toucans and Toucanets

35

Ramphastidae

Woodpeckers and Flickers

200

Picidae

* Perching birds or songbirds

Sources: Burnie, David, and Don E. Wilson, eds. 2001. Animal. New York: DK; Howard, Richard, and Alick Moore. 1998. A Complete Checklist of the Birds of the World, 2d ed. New York: Academic.

* Perching birds or songbirds

Sources: Burnie, David, and Don E. Wilson, eds. 2001. Animal. New York: DK; Howard, Richard, and Alick Moore. 1998. A Complete Checklist of the Birds of the World, 2d ed. New York: Academic.

cannot be matched by species gains of even 1 species every 10,000 years.

—François Vuilleumier

See also: Biogeography; Evolutionary Biodiversity; Extinction, Direct Causes of; Galapagos Islands and Darwin's Finches; Speciation; Systematics

Bibliography

BirdLife International. 2000. Threatened Birds of the World. Barcelona: Lynx Edicions and BirdLife International; Burnie, David, and Don E. Wilson, eds. 2001. Animal. New York: DK; Howard, Richard, and Alick Moore. 1998. A Complete Checklist of the Birds of the World, 2d ed. New York: Academic; Howell,

Thomas R. 1983. "Desert-nesting Sea-gulls." Natural History 93: 52-59; Norell, Mark. 2001. "The Proof Is in the Plumage." Natural History 110: 58-63; Oeland, Glenn. 1996. "Emperors of the Ice." National Geographic 189, no. 3: 52-71; Stotz, Douglas F., John W. Fitzpatrick, Theodore A. Parker III, and Debra K. Moskovits. 1996. Neotropical Birds: Ecology and Conservation. Chicago: University of Chicago Press; Ter-borgh, John. 1989. Where Have All the Birds Gone? Princeton: Princeton University Press; Vuilleumier, François. 1996. "Birds Observed in the Arctic Ocean to the North Pole." Arctic and Alpine Research 28: 118-122; Vuilleumier, François, Ernst Mayr, and Mary LeCroy. 1992. "New Species of Birds Described from 1981 to 1990." Bulletin of the British Ornithologists' Club. Centenary Supplement 112A: 267-309.

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