Site fidelity and territorial movements of males in a rapidly declining population of yellow-headed blackbirds

Abstract

We tested several hypotheses to explain low between-year territory fidelity in a breeding population of yellow-headed blackbirds (Xanthocephalus xanthocephalus). During a 5-year study the population of territorial males declined by two-thirds and some of the marshes that supported territories significantly deteriorated. Individual males held territories and bred for an average of 1.9 years. Of males that bred for at least 2 years, 30% skipped owning a territory in the study area during at least 1 year of their breeding lifetimes. Our information suggests that they may have bred outside of the area in those years. Of males with territories in two or more breeding seasons, 60% changed breeding marshes at least once. Males changed territories during 42.9% of between-year opportunities to do so. We found no support for the hypotheses that male yellow-headed blackbirds: (1) are more likely to move when territory density is low; (2) are likely to abandon territories that are deteriorating; or (3) change territories to improve their reproductive success. We suggest three non-mutually exclusive explanations for the yellow-headed blackbird's weak site fidelity: (1) it is a response to habitat deterioration and to other factors that may be causing the population's decline; (2) the males, being migratory, make fresh settlement decisions each year after they arrive on the breeding grounds in the general vicinity of their previous year's breeding; (3) yellow-headed blackbirds may have evolved in, and be adapted to, highly unstable habitats, moving frequently in response to changes in local breeding site conditions.