Influence of High Density and Parental Age on the Habitat Selection and Reproduction of Black-Billed Magpies

The habitats selected for nesting and the breeding biology of a dense population of Black-billed Magpies (Pica pica hudsonia) were studied over two years in northern Utah. Breeding density was at least twice that previously reported and first-year birds comprised approximately 40% of the breeding population. Nest territories were densely packed in all available habitats and had a mean size of 0.5 ha. Date of clutch initiation was negatively correlated with clutch size and number of fledglings. Earlier nests were more successful than later ones. First-year birds most often occupied marginal habitats and bred less successfully than older adults. Mixed-age pairs were intermediate in reproductive variables. Neither the timing, clutch size, nor success of the reproductive effort were affected by the high density or the large proportion of breeding first-year birds. Females sometimes had lower success, however, if they mated with inexperienced first-year males. Our results suggest that these magpies have a malleable territorial system in relation to density and that high density itself does not limit their production.