Comparative hatching characteristics of nonparasitic and parasitic icterids: is the hatching of cowbird young constrained by an unusually thick eggshell?

Abstract

Studies of avian brood parasitism have provided some of the best evidence of coevolutionary arms races. One of the parasitic adaptations exhibited by cuckoos (family Cuculidae) and cowbirds (family Icteridae) is the development of a thick eggshell, presumably to avoid damage from puncture ejection by some hosts and/or accidental damage during egg-laying in the host nest. However, it is unknown whether this trait constrains the hatching of parasitic young. The differences in hatching characteristics between the host red-winged blackbird (“redwing;” Agelaius phoeniceus) and the parasitic brown-headed cowbird (Molothrus ater) were examined. Prehatched cowbird young were found to spend more time hatching than pre-hatched redwing young and to emit click sounds at a greater rate in relation to pulmonary respiration than pre-hatched redwing young. However, cowbird hatchlings appear to have evolved other hatching-related traits that may compensate for the greater hatching effort, such as body parts and an egg tooth that are relatively large compared with those of redwing hatchlings.