Reduced territorial responses in dark-eyed juncos following population establishment in a climatically mild environment

Colonization of novel environments may often lead to changes in socially and sexually selected traits, but there are few documented examples. We examined territorial responses of male dark-eyed juncos, Junco hyemalis thurberi, to song playback in association with colonization of an environment very different from that experienced by the ancestral population. A small population of dark-eyed juncos became established on the campus of the University of California at San Diego in the early 1980s. This population experiences a milder climate than the nearby mountain population from which it was probably derived. The San Diego juncos have a breeding season that is more than twice as long and rear about twice as many young per individual. When confronted with song playback, male responses in such traits as the total number of songs and total number of swoops in the San Diego population were about half those recorded in the mountain population. Previous work has shown that the amount of white in the tail of this species (a trait used in aggressive displays) has also decreased in the San Diego population, and lowered territorial response may be one reason favouring the evolution of less white.