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rapid junco evolution

Dark-eyed Juncos (Junco hyemalis), are common little sparrows throughout much of North America.  This species generally breeds at high latitudes (e.g., across Canada), or further south at higher altitudes (usually over 1500 feet/460 m).  There it nests in coniferous forests, typically on the ground (as in the photo).

Last week I read an interesting paper from Trends in Ecology and Evolution, on urban ecological processes [1].  One example in the paper was the rapid adaptation of juncos to an urban environment, in San Diego, CA. Normally juncos are winter visitors to coastal parts of San Diego County, but retreat into coniferous or live oak woodlands in the mountains during the nesting season.  In the early 1980s, juncos began sticking around the University of California-San Diego campus (actually in La Jolla, altitude 13 feet/4 m).  There is now an established resident breeding population of about 70 pairs of juncos in a one square mile (2.5 sq km) area centered on campus, about 43 miles (70 km) from the nearest nesting population.

Behavioral changes
Ground nesting birds by necessity need to be secretive and conceal their nests.  On the 19,000-student UCSD campus, juncos also construct nests on the ivy growing on campus buildings, in light fixtures, in enclosed courtyards, in flower pots, on trellises, and, in the remarkable photo in the article, at least once in a bicycle helmet sitting on the back of a bike in a garage!

The behavior of the birds has changed in other ways. In the mountains, snow may postpone the breeding season until as late as June. Thus, the birds at the higher altitudes raise one or rarely two broods a year. Juncos on the UCSD campus have taken advantage of the mild coastal climate, and raise up to four broods a year [2]. These juncos are also less aggressive than their mountain counterparts.

Morphological changes
The UCSD juncos have shorter wings and shorter tails than the montane source population, and have about 22% less white in their outer tail feathers.  Migratory populations of songbirds tend to have longer wings, so the shortened wing length is likely an adaptation the sedentary nature of the UCSD junco population [3].

The white in the outer tail feathers of juncos serves several purposes.  One is that it is a status signal.  More white both signals sexual fitness in males, and social dominance in both sexes.  It is believed that the reduction in the white is a result of sexual selection, in that females of the UCSD population value a studly male less than they do a male that helps raise all those young. And signalling dominance may be not be very important to these less aggressive birds, contributing to the reduction in the amount of white in the tail [4,5].

Banding and genetic studies have examined immigration rates, survival rates, and other demographic features of this population. The juncos on the UCSD campus are not just a set of random wintering birds that stays each year to nest. The immigration rate is too low to maintain the population at the 140 or so birds at which it has stabilized. The UCSD population relies on the extra chicks produced by the longer breeding season.

DNA tests indicate that these physical traits — size and reduced white in the tail — are unlikely to be due to genetic drift or “founder effect,” where there is a loss of genetic diversity in a population founded by a few individuals.  Instead, these unique characteristics are adaptations to their new, urban environment, and they evolved in fewer than 20 years.

[1] Shocat, E., P. S. Warren, S. H. Faeth, N. E. McIntyre, and D. Hope. 2006. From patterns to emerging processes in mechanic urban ecology. Trends in Ecology and Evolution 21:186-191.

[2] Yeh, P.J. and T. D. Price. 2004. Adaptive pheontypic plasticity and the successful colonization of a novel environment. American Naturalist 164:531-542.

[3] Rasner, C. A., P. Yeh, L. S. Eggert, K. E. Hunt, D. S. Woodruff, and T. D. Price. 2004.  Genetic and morphological evolution following a founder event in the dark-eyed junco, Junco hyemalis thurberi. Molecular Ecology 13:671-681.

[4] Balph, M. H., D. F. Balph, and H. C. Romesburg. 1979. Social status signalling in winter flocking birds: an examination of a current hypothesis.  Auk 96:78-93.

[5] Yeh, P. J. 2004.  Rapid evolution of a sexually selected trait following population establishment in a novel habitat. Evolution 58:166-174.

Filed in Birds, Natural history

Comments on this entry are closed.

  • pascal April 19, 2006, 7:52 pm

    Cool – micropaedomorphosis!

  • Home Bird April 19, 2006, 10:09 pm

    A really interesting post about one of my favorite little birds. Thanks, Nuthatch: your blog is one of my favorite learning spots on the web.

  • John April 20, 2006, 11:05 am

    That is quite remarkable. Thanks for posting this story. I wonder if the reduced white in the tail could possibly be an adaptation to make the urban birds less obvious to cats and other predators?

  • Nuthatch April 21, 2006, 9:20 pm

    I've been made aware that the elevation of the UCSD campus is actually about 400 feet (~122 m). I got the quick and dirty elevation in the post from Weather Underground for San Diego.

  • Rob Miller April 27, 2006, 9:56 am

    Great story. I identified one of these birds for the first time last year. I am always interested in how birds adapt to urban settings. I've tracked the story of Pale Male in New York. Here in Boise, we also have a Perigrine Falcon pair that nests downtown, when the wide open foothills are just 1/2 mile away.