The Breeding Biology of an Isolated Bobolink Population in Oregon

Abstract

Breeding biology and population dynamics are described for the isolated Bobolink [Dolichonyx oryzivorus] population at Malheur National Wildlife Refuge in Oregon [USA]. Population size varied from 200-350 individuals, with slightly more females than males. Long-term changes in population size were not found despite large year-to-year variability. Males arrived in mid-May, and females arrived about a week later. Site fidelity was high for adults in optimal habitat but probably not in marginal habitats. The return rate of banded adults averaged nearly 60% annually. About 30% of males in the population were polygynous. Fledging success of hatched eggs averaged about 70% for first mates of mated males and over 50% for second mates of polygynous males. The higher success for first mates stemmed largely from lower nestling starvation, even though exposure was a more important cause of mortality. Predation rates were unusually low, apparently because predator densities were low. Recruitment of yearlings was about 20-30% of fledgling production in good years but only 5% in 1 dry year. The nestling period coincided with peak abundance of nestling food resources. The most important factors regulating population density are apparently survival of newly fledged young and mortality of adults and juveniles during migration. The study population has not expanded into other areas, probably because site fidelity is high and because females faced with habitat shortages can become polygynous mates. In addition, further range expansion is made less likely because higher adult mortality caused by a longer annual migration route reduces chances for a population surplus.