Cooperative breeding in Lanius Shrikes. II: Maintenance of group-living in a nonsaturated habitat

Abstract

We studied the cooperatively breeding Gray-backed Fiscal Shrike (Lanius excubitorius) near Naivasha, Kenya, to explore the roles of social and ecological factors in the maintenance of group-living.Gray-backs occurred in groups of 2-9 or more individuals, with only one breeding pair per group.Supernumeraries act as helpers to the breeders.Habitat of this species varied in the amount of perennial shrub cover.In the dry months high-cover areas had significantly more prey items than did low-cover areas.Group size was significantly positively correlated with shrub cover at 3 of the 4 times tested during the 3-yr study, and mean group size over an 18-month period also was correlated with shrub cover.Both individual survivorship and production of young increased with increasing cover.Group size also was correlated with survivorship and production of young.Partial correlation analysis could not separate the influences of group size and cover on survivorship.However, cover was significantly associated with the production of young when group size was held constant, and not vice versa.Pairs or small groups occasionally colonized lowcover areas; typically, however, the birds immigrating into these areas disappeared without having bred successfully.In contrast, high-cover areas held a constant number of breeding pairs and a varying number of supernumeraries.Supernumerary birds appeared to have a choice between dispersal and natal-territory fidelity, with the decision to disperse apparently weighed against the probability of eventual attainment of breeding status within or near the natal territory.