The role of immigration in the decline of an isolated migratory bird population

Low immigration is rarely considered a potential causal factor in the population decline of migratory birds. Because migratory songbirds exhibit low natal philopatry, however, recruitment is from outside the population. Therefore, if isolation affects immigration the geographic fragmentation of a species' range may result in population declines. I tested four hypotheses on the possible causes of population decline of an isolated population of Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) in Illinois (US.A.): (1) local habitat loss, (2) poor reproductive success, (3) low adult survival, and (4) low immigration. Historical accounts indicate this population has declined and become more isolated over the last century. From 1998 to 2003, I determined the population size, the annual return rate, and mean reproductive success of Yellow-headed Blackbirds in Illinois. I also determined the amount of suitable habitat for the species in the area. Over the course of the study the population declined by 25%. Although habitat loss was likely to be responsible for the population's historical decline, currently the habitat is not saturated and many seemingly appropriate sites remain unoccupied. The reproductive success of the Illinois population is one of the highest recorded for this species. Adult survival was within the range of survival values found in other stable populations. Yellow-headed Blackbirds, like most migratory passerines, exhibit low natal philopatry; therefore, recruitment is primarily from outside the population. Recruitment was unusually low in this population, suggesting the population's isolation on the periphery of the species' range has reduced the number of immigrants. Although individuals in the population had high survival and reproduction, the population continued to decline because of low immigration. Conventional management (i.e., restoring habitat) may not mitigate a population's decline if low immigration is responsible for the decline. Greater connectivity at a large scale may be necessary to conserve isolated populations of migratory birds.