Bird Communities and Competition

photograph of a Red-winged Blackbird by Rohan Kamath
A biological community consists of all of the organisms -- microbes, plants, and animals -- that live in an area. A community, together with the physical environment to which it is tied by a series of processes (such as the production of oxygen by plants and its use by animals, and the reciprocal production of carbon dioxide by animals and its use by plants), is called an ecosystem. How the species composition of communities is determined, and how those species interact with each other and with their inanimate surroundings, are major foci of ecological research.

In order to simplify the investigation of communities, ecologists usually study some subset of the organisms present, either a functional complex such as a group of

herbivores and the plant species they feed on, or a taxonomic group, such as the resident insects or birds. Thus one may read about "mammal communities" or "bird communities." Unlike mammals (which are often nocturnal and secretive), birds are relatively easy to observe; hence, a great deal of research has been done on avian communities.

Much of that research has been focused on a seemingly simple question: Are the birds that compose a community merely a chance association of species that share similar tolerances to a physical environment, or do biological interactions among the birds determine which species are included in a community and which are excluded from it? In particular, interest has centered on whether competition -- two or more species using the same limited resource -- is responsible both for excluding some species from communities and for causing certain kinds of differences to evolve between the species that do live together. So far, no simple answers to these questions about species composition have emerged.

Many avian ecologists do believe that competition can be an important factor influencing both the composition of bird communities and the behavioral and morphological characteristics (structure, color, etc.) of the birds themselves. That belief is based on diverse lines of evidence, such as many observations of closely related birds apparently dividing up resources such as food or suitable space for territories, or one species excluding another from apparently desirable habitat. For example, in the northwestern United States males of both Red-winged and Yellow-headed Blackbirds set up territories in open marshes. The Redwings arrive earlier in the spring and occupy the entire marsh. When the Yellowheads fly in, they take over the best territories (areas of cattails and other plants in deep water that harbor the richest insect life) and force the Redwings into the shallower, drier, more marginal habitats. The Redwings are able to breed successfully in these areas, however, while the Yellowheads are unable to exploit the less productive sites successfully.

The Yellowheads are bigger, perhaps a result of natural selection favoring the ability to oust Redwings from high-quality areas, perhaps to eat larger seeds in the winter, or perhaps both -- or perhaps neither! Many such details remain uncertain, but it is clear that the Redwings have a broader niche (loosely, "way of life") than do the Yellowheads. Nevertheless, Yellowheads are better competitors within their own narrow niche, and thus are able to exclude the Redwings from it. Territorial habitat is a scarce resource; the Yellowheads take the richest and the Redwings get the rest.

Dividing up resources (resource partitioning) often takes the form of similar bird species either feeding in different parts of the same habitat (as do some closely related warblers that live together) or else taking food of different sizes. The former is evident from differences in foraging behavior; the latter is often inferred from differences in bill size of the species.

One approach to detecting the existence of competitive interactions in bird communities is to determine whether closely related species are distributed independently over a large sample of similar habitats, or whether the presence of one or more species has an influence on the others. For four years, ecologists Catherine Toft, David Trauger, and Horatio Murdy analyzed statistically the distribution of five duck species breeding in 236 ponds in the Northwest Territories of Canada. Three of the ducks were dabblers: Mallard, American Wigeon, and Green-winged Teal. Two were divers: Lesser Scaup and Ring-necked Duck. The species showed significant differences in both time of hatching and pond-size use. For instance, the dabblers nested early, running the risks of springtime inclement weather. The divers nested later, risking the loss of their broods with the arrival of the fall freeze. Mallards showed little preference for pond size, teal strongly preferred small ponds, and wigeon and divers preferred large ponds (and were almost never found in small ones). Detailed analysis of the results led to the conclusion that competition, past and present, was responsible for patterns of pond use, and partially responsible for the temporal differences in dabbler and diver breeding. The most dramatic current competition was between the two divers; the scaup and Ring-necks would often occupy the same pond in different years, but not occur there together.

Some of the most interesting studies of avian communities that consider the distributions of related species have been done not on North American birds, but on assemblages of birds found in different parts of New Guinea and on other islands of the southwest Pacific. The species compositions of these communities have been documented by ecologist Jared Diamond, who found many cases of "checkerboard" distributions, sort of an expanded version of the scaup and Ring-neck situation, with certain combinations of closely related species never found. For instance, Diamond searched for two species of Macropygia cuckoo doves on 33 islands of the Bismarck Archipelago. He found one of the species on 14 islands, the second on 6 other islands, and neither on 13 islands. The two kinds of cuckoo doves never lived together on the same island. Similarly, two small nectarivores, the Black Sunbird (Nectarinia sericea) and the Bismarck Black Honeyeater (Myzomela pammelaena) occupy 41 islands in the archipelago. The sunbird is found on 18, the honeyeater on 23, but no island plays host to both. The two birds, although they belong to different families, are quite similar in color, size, foraging techniques, and habitat preference. Diamond concluded from finding such patterns in several groups of ecologically similar species that certain combinations of species were "forbidden" by competition. That conclusion has been challenged, but Diamond's interpretation seems correct to us. His work provides some of the most persuasive evidence that competition can play a key role in determining which species are found in a given bird community.

So competition occurs, but how common it is, and the degree to which it is responsible for the composition of bird communities worldwide, remain unresolved. The arguments are complex and technical. They involve a wide range of questions, some of which reflect the difficulties of studying birds. How are the resources fed upon by birds best measured? How can accurate censuses of bird populations be obtained? Do spot observations of birds feeding provide unbiased information about foraging behavior? Might birds behave differently in relatively open situations where they are readily observed from the way they behave when out of sight in thick foliage? How does one interpret observed differences in bill size or shape among closely related species? Are these differences a result of natural selection produced by competition, by the need for members of the same species to recognize one another, or by other factors?

Other questions involve difficult statistical issues -- such as how one determines if an assembly of bird species is a "random" assortment, that is, a chance subset of the species that theoretically might be members of a community. How strongly do factors (such as predation and random disturbances) other than competition affect community composition? Careful observations and experiments on many more bird communities will be required before these questions can be answered with assurance.

SEE: Bird Guilds; Bills; Dabblers vs. Divers; MacArthur's Warblers Metabolism.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.