Surplus yearlings and the regulation of breeding density in blue grouse

Abstract

An experiment in which yearlings of both sexes of blue grouse (Dendragapus obscurus fuliginosus) were removed from breeding range showed that about two times, or more, yearlings of each sex were identified here as compared with a nearby control area, though numbers of "surplus" yearlings varied among years. The removal of large numbers of yearlings from the experimental area did not reduce breeding density, except ephemerally in 1 year. Nor did the removal of these birds affect populations in the surrounding area. These data confirm results from earlier studies with blue grouse, that surplus yearlings of both sexes are present on the breeding range. Replacement yearlings of both sexes (those taking the place of birds removed) were able to breed, but those removed sequentially (1974 to 1976) were lighter in body weight than those captured on control areas or those that settled synchronously after a near-total removal experiment in the same area. I infer from these results that some form of behaviour keeps surplus yearlings from settling on the breeding range and that this behaviour is a key mechanism involved in the regulation of breeding density. The important behaviour may involve either yearling–yearling or adult–yearling interactions.