Effects of Egg Size, Hatching Asynchrony, Parental Care, and Environment on the Growth of Nestling Red-Winged Blackbirds (Agelaius phoeniceus) in Southerstern Michigan
Neonate birds hatching from large eggs tend to be larger than conspecifics hatching from smaller eggs. This difference may have an impact on survivorship immediately after hatching except, perhaps, in nests where eggs hatch asynchronously. Hatching asynchrony, most common in nidicolous birds, has a large influence on the size and possibly, competitive hierarchy of hatchlings in a nest. We examined the effects of egg size and hatching order on fledging success of Red-winged Blackbird (AgeZaius phoenicius) nestlings, an altricial, nidicolous songbird nesting in Southwestern Michigan. Egg size was measured to determine the effects it has on hatchling size between the time of hatching and fledging, while hatchlings were measured with sibling hierarchy and survivorship monitored during the nesting stage. We found that egg mass only influences size directly after hatching, and the hatching sequence has a large impact on the size of the hatchling and survivorship for the first few days of the nestling period. However, when nestlings are ready to fledge, size differences are greater among nests than within nests making size differences among siblings insignificant. While egg size and hatching order affect hatchling size and survivorship early in the nesting stage, it seems to be that the environment and parental care are responsible for the survival of the nestlings at the time they are ready to fledge.