Avian quick-change artists: exemplars of rapid adaptation, house finches show that mothers know best

Adaptation to the environment is the cornerstone of Darwinian natural selection. Among the most conspicuous consequences of this process are changes in the size and shape of animals in response to climate. Nearly 200 years ago, long before the publication of Darwin's Origin of Species, zoologists recognized that in wide-ranging species, individuals that inhabit the colder parts of the range tend to be larger and to have shorter limbs and appendages (black bears and white-tailed deer, for example, show this trend in North America). When one is considering species that have had stable ranges over thousands of years, such changes in body size and shape can be assumed to have evolved very slowly, by incremental stages, over many thousands of generations. Biologists only rarely have a chance to witness the pace of such changes when a population of vertebrates spreads into a very new environment. Over the past few years, however, we have documented rapid and adaptive changes in the size and shape of one vertebrate species--the house finch--and we have discovered a fascinating and unanticipated mechanism that allows this bird to adjust quickly to local environments.

House finches were originally found only in western North America. When northern Europeans first settled on the East Coast and the Spanish colonized the Southwest, these sparrowlike birds ranged from Oregon and Wyoming to southern Mexico and east to the foothills of the Rocky Mountains. Originally birds of open savannas, canyons, and deserts, house finches avoided both forested and treeless regions; the Great Plains and the dense forests of the Pacific Northwest were unsuitable habitat for them. Eventually, unbroken woodlands were felled to make way for farms and cities, and in the process, huge new areas suitable to house finches emerged. In response, they expanded their range, eventually spreading to British Columbia and western Montana by the 1980s. In the eastern United States, humans lent a hand in establishing house finch populations (see "Hollywood, Honolulu, and Hoboken," page 61). Today the song of the house finch can be heard from Ontario to Hawaii and from Florida to Oaxaca. House finches have not just colonized these new areas but have thrived, becoming among our most familiar year-round backyard birds. Their total number in North America was recently estimated at more than a billion birds, a significant portion of which live east of the Mississippi.

As they spread across the continent, house finches faced an array of diverse new climates and habitats. Consider the differences in temperature and humidity between California's coastal oak savanna (part of the species' historical range), the Hawaiian Islands (from suburban Honolulu to 6,000 feet up the slopes of Mauna Loa on the big island of Hawaii), southern Michigan, Long Island in New York State, the Rocky Mountains of western Montana, and southern Alabama. These sites range from tropical to cold-temperate, from arid to extremely humid, from high elevation to sea level, from windy to calm, and from having extreme seasonal changes to virtually no seasons at all.

Given the wide range of environments to which the house finch is now exposed, we wondered if the birds that had settled in different areas had become different in physical appearance. We chose seven populations for which we knew the history of colonization, and we measured the size and shape of individual birds. Surprisingly, given the brief time since some of the populations had diverged and had settled in their new environments, we found substantial--up to 10 percent--differences in the size and shape of individuals among populations. Moreover, the patterns of variation were complex. It was not simply that birds in the North were big and birds in the South were small. Within most populations, males and females differ in size and shape, that is, they are sexually dimorphic. We found, however, that male and female house finches were changing seemingly independently, resulting in differing degrees of dimorphism from population to population. In some populations, the males had longer tarsi (lower legs), whereas in others, the females had longer tarsi. The same held for body mass and bill size. And most surprisingly, house finch populations separated for only decades were as different as finch populations that we knew had been separated for hundreds or thousands of years.

Once we had documented that rapid change had occurred in the sizes and shapes of males and females, we wanted to find out what environmental pressures were responsible. Choosing two populations that live and breed at the climatic extremes of the species' range--hot, humid lowlands in Auburn, Alabama, and cold, arid mountains in Missoula, Montana--we monitored thousands of finches (color-banded so that we could distinguish individuals) for six years.

Our investigation required more than simply catching and measuring birds. We needed to follow individuals all year to gather data on how certain aspects of size and shape correlated with survival, success in attracting mates, and the number of young produced (fecundity). We found that populations differed in which one of these three factors had the greatest impact on the birds' size and shape. In Montana, for example, higher fecundity was most strongly affected by body size, whereas in Alabama it was survival that was strongly impacted by body size. The reasons for these differences are unknown, but it may be that the Montana population is still expanding rapidly, making fecundity particularly important, while in the warm, wet climate of Alabama, avoiding parasites and diseases may be the key to success (see "Backyard Epidemic," page 62). We also found that the specific size of a trait could be beneficial in one context but not necessarily in another. For instance, in Montana, males with longer wings were more successful in attracting mates. Conversely, smaller males survived better than large males in both populations, but the effect of size on survival was much greater in Alabama.