from the Field
A Mountain Bird Adapts to City Life
The Dark-eyed Junco is a common wintering bird in the city of San Diego, frequently seen in city parks, suburban yards, and university campuses. It typically arrives in San Diego in September and leaves in April, to return to its traditional breeding habitat of coniferous woodlands. In 1983, however, a small flock of juncos was discovered foraging and breeding on the campus of the University of California, San Diego--an urban habitat more than 60 km from the species' nearest natural breeding habitat in the mountains of the Cleveland National Forest.
Fast forward 17 years to the present. Now, there are many more individuals in the campus population, and juncos are one of the most commonly sighted birds on campus. At the Grove Caffé, a popular student hangout, juncos and students mix freely, with the eager birds hopping about, searching for muffin and croissant crumbs left on tables and on the ground. In other areas of campus, juncos have been known to boldly hop into and explore dorm rooms left open. Similarly, the junco's song--heard from January until August--is among the most commonly heard sounds on campus: long after the other breeding birds have stopped singing, it is easy to hear the high trill of a male junco, singing atop a building or on a low tree branch.
Contemplating this unusual population brings some questions to mind: what are the effects of this new environment on a species that typically breeds in montane habitats, a species that may not be well adapted to the pervasive trappings of human civilization, such as paved roads, buildings, and aggregated food sources like bird feeders and garbage dumps? How do these new inhabitants affect other species already in the urban habitat? What is it about the UCSD campus that enables the juncos to survive and breed? Why don't they breed in other city areas of San Diego?
I am a graduate student working in Dr. Trevor Price's lab in the UCSD biology department, and we have been studying the campus population of juncos for the last three years. We are interested in mechanisms of speciation, and this population provides a rare and ideal system for studying how populations diverge after natural invasion into a novel habitat, and how evolutionary factors such as natural selection, sexual selection, drift, and plasticity interact when a population invades a new habitat.
The first thing we did when we started our research was to determine some of the population's basic vital stats: How many birds are there? Are they year-round residents? When do they start pairing up? Where are the population's boundaries? Once we had answers to some of these more basic questions, we delved deeper into questions surrounding their origin, the effects of the urban habitat on the birds, the differences between the urban birds and the mountain birds, and some of the potential reasons for these differences. We have only three years of field data thus far, so our answers are quite preliminary, but they have proven to be both very intriguing biologically and very useful in shaping future questions we intend to address.
Briefly, we've discovered the following: There are approximately 130 juncos in the population, the population size remains fairly stable across the years, and the birds are indeed year-round residents of the campus. The population covers an area of slightly less than 1 square mile, and the boundary of the population barely extends off campus into surrounding La Jolla. These urban juncos have an extremely long breeding season in comparison to most juncos elsewhere--they start singing in January, build nests in February, and fledge their last chicks in August and September. The urban birds have shorter tails, shorter wings, and less white in their tail feathers than the mountain juncos. They also weigh less and are less aggressive than mountain juncos. We're currently trying to figure out why we see these traits differing in these directions. We anticipate having more solid answers to our questions in several years, and are very excited at the prospect of understanding a fascinating example of evolution in progress!I should add that the San Diego County Bird Atlas has been immensely helpful to our junco research, as we have been able to see with great detail the areas where juncos do and do not breed. We remain very interested in hearing about junco distribution in the city of San Diego, in both wintering and breeding seasons. I can be reached at email@example.com and would love to hear about any unusual junco sightings and behavior!