Effects of food supplements on eastern bluebird nestling growth and survival.

Abstract

The number of young that avian parents attempt to raise during a breeding attempt is hypothetically constrained by the amount of food available in the environment. Costs of reproduction to parents, as is evident by declining physical condition of avian parents during nestling provision, are associated with insufficient procurement of food to maintain physical condition. Additionally, low body mass and decreased survival of underfed nestlings lends support to food limitation theories. However, equivocal results obtained in studies examining the effects of differential food availability on parents and nestlings provided the impetus for my study. The effects of increased food availability on Eastern Bluebird (Sialia sialis) nestling growth and survival were examined in east central Kansas during the 1996 and 1997 breeding seasons. Nestling growth and survival characteristics, including morphological measurements, nestling survival, fledging age of nestlings, and the number of nestlings to fledge were measured and compared between unsupplemented and food supplemented groups. No significant differences existed between any variables measured; thus, the Eastern Bluebird population in my study was not food limited. Other variables that might influence the degree of food limitation, such as weather and nest site competition, are discussed. Evidence suggests that food might be less limiting to secondary cavity nesting species compared to open-cup and primary cavity nesting species because of competition for limited numbers of nest sites. Simultaneous investigation of breeding bird density and food limitation theory is needed to better understand the relative effect that food limitation has on regulating the number of young that adults can successfully raise per breeding attempt.