In 1983, a population of dark-eyed junco birds became established on the campus of the University of California, San Diego (UCSD), which is located many miles from the junco's normal habitat in the mixed-coniferous temperate forests in the mountains. Juncos have white outer tail feathers that the males display during aggressive interactions and during courtship displays. Males with more white in their tail are more likely to win aggressive interactions, and females prefer to mate with males with more white in their tails. Females have less white in their tails than do males and display it less often. Refer to the paragraph on dark-eyed junco birds. The UCSD campus male junco population tails were, on average, 36 percent white, whereas the tails of males from nearby mountain populations averaged 40—45 percent white. If this observed trait difference were due to a difference in the original colonizing population, it would most likely be due to .