When organisms that are ecologically intimate -- for example, predators and prey, or hosts and parasites -- influence each other's evolution, we say that coevolution is occurring. Birds are often important actors in coevolutionary systems. For example, predation by birds largely drives the coevolution of model and mimetic butterflies. Some butterflies have evolved the ability to store poisonous chemicals from the food plants they eat as caterpillars, thus becoming distasteful. This reduces their chances of being eaten, since birds, once they have tried to devour such butterflies, will avoid attacking them in the future. Other butterflies have gradually evolved color patterns that mimic those of the distasteful butterflies (called "models"). It is disadvantageous for the models to be mimicked, because if the mimics become common then most of the butterflies with the model's color pattern taste good, the birds may resume attacking the models. Being tasted and spit out by a bird is a most dangerous experience for a butterfly. Therefore, mimicry presumably leads to a coevolutionary race -- the mimics evolving toward the color patterns of the models, and the models evolving away from the converging mimics. The birds actually may be directly involved in the entire coevolutionary complex, since they may be under selection for better powers of discrimination. Individuals that can tell the mimetic butterflies from the models will gain more nourishment at less cost in time and effort.

Birds, of course, are presumed to be directly involved in many coevolutionary relationships with their competitors, predators, prey, and parasites. The relationship of seed-hoarding Clark's Nutcrackers and Pinyon jays with pinyon pines is a relatively well-studied example; and the evolution of long bills and sickle-shaped bills in some Latin American hummingbirds which match the long or sharply curved flowers from which they sip nectar (and which they pollinate) is another obvious case of coevolution.

Hermit hummingbirds and the curved flowers of the genus Heliconia (seen increasingly as horticultural cut flowers) provide widespread and conspicuous examples of the latter phenomenon throughout the lowland moist forests of Central and South America.

Many fruit-eating birds, especially in tropical rain forests are coevolving with the plants whose fruits they eat. The birds get nourishment, and in the process the plants get their digestion-resistant seeds dispersed by regurgitation or along with the birds' droppings. Many characteristics of the plants have evolved to facilitate dispersal, and the behavior and diets of the birds have responded to those changes. In particular, the plants have evolved conspicuously colored, relatively odorless fleshy fruits to attract the avian dispersers of their seeds. They are coevolving in response to the finely honed visual systems of the birds; plant species coevolving with color-blind mammalian seed-dispersers have, in contrast, dull-colored but smelly fruits. The bird-dispersed plants often have evolved fruits with giant seeds covered by a thin, highly nutritious layer of flesh. This forces the bird to swallow the fruit whole, since it is difficult or impossible just to nip off the flesh. In response, birds that are specialized frugivores (that is, that do not take other kinds of food) have evolved both bills with wide gapes (so they can swallow the fruit whole) and digestive tracts that can rapidly dissolve the flesh from the large impervious seed, which then can be regurgitated.

The most dramatic examples of avian coevolution are probably those involving brood parasites, such as cuckoos and cowbirds, and their hosts. The parasites have often evolved eggs that closely mimic those of the host, and young with characteristics that encourage the hosts to feed them. In response, some hosts have developed the ability to discriminate between their own and parasitic eggs, and various methods of destroying the latter. As one might expect, Brown-headed Cowbirds have their most serious impact on hosts, such as Kirtland's Warblers, that are thought to have only recently been subjected to cowbird attack and have not yet had time to evolve defensive reactions.

Many examples of coevolution in response to competition between bird species can be inferred from studies of dietary habits and bill structures in various guilds of birds. Here, as in the other cases mentioned, direct evidence of coevolution is lacking. It is lacking for the same reason that there are very few cases of plain old single-population evolution actually being observed in nature. The process occurs over hundreds or thousands of generations, and extraordinary circumstances are required for it to be "caught in the act."

SEE: Natural Selection; Hoarding Food; Bird Guilds; Bird Communities and Competition; Brood Parasitism; Cowbirds.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.