Variation in laying date, clutch size, egg size, and egg composition of yellow-headed blackbirds (Xanthocephalus xanthocephalus): a supplemental feeding experiment

Mean nest initiation dates varied by up to 6 d among wetlands, and supplementally fed blackbirds initiated nests 2 d earlier than controls, on average (although this latter difference was not quite significant). Clutch size declined with laying date, but was unaffected by wetland location, food supplementation, or interactions between these two factors and laying date. Although egg size did not vary among wetlands or in relation to supplemental feeding, egg composition varied with both of these factors. In general, egg production was not greatly affected by food availability. This may have been due to any of the following factors: 1) inaccessibility of food supplements owing to competition between male and female blackbirds, 2) insufficient time for females to respond to food supplements, owing to rapid settlement and nest initiation, 3) a nutritionally inappropriate food supplement (ie protein availability may not have been enhanced among fed birds), or 4) superabundance of natural foods such that food availability was not limiting egg production. -from Author