Figure 1.

Western bluebird's range expansion. (a) Changes in western bluebirds' breeding range in the northwestern United States. Black circles indicate eight study populations where aggressive behaviour of western and mountain bluebirds was measured. Population numbers indicate the order of population colonization by western bluebirds. (b) Breeding distribution of western bluebirds based on Breeding Bird Survey (BBS) data from 1994 to 2003. Contours are based on the mean number of birds counted on BBS survey routes (see Sauer et al. 2008 for details).