Investigating maternal effects: enhanced immunity in the eggs of an obligate avian brood parasite, the brown-headed cowbird
Brown-headed cowbirds are obligate brood parasites that parasitize nests of a wide variety of other species. As such, young cowbirds are exposed to a variety of diseases and parasites from their host species. Thus, cowbirds are an ideal species to examine how evolution shapes the design and function of the immune system in response to elevated exposure to microbes and parasites. We hypothesized that selection favored the evolution of enhanced immune defenses in the cowbird. We compared the type and amount of maternal antibodies in the eggs of the cowbird and a preferred host, the red-winged blackbird. In 2009, we collected host and cowbird eggs from blackbird nests in the cowbird’s ancestral range in North and South Dakota. Eggs were collected before incubation to measure levels of maternal antibodies before any metabolism due to development had occurred. We measured immunoglobulins (IgA and IgY) and lysozymes in the eggs. Immunoglobulins are proteins that protect against viruses and bacteria. IgA protects the gut of the nestling, and IgY circulates systemically in the blood. Lysozyme is an enzyme that protects against gram-positive bacteria (e.g.,
Bacillus, Staphylococcus, and
Streptococcus). Although both species provided their offspring with adaptive passive immunity to protect their young from systemic infections by pathogens, cowbird eggs had higher lysozyme and IgA content in albumen than blackbird eggs. IgY in the yolk did not differ between species. The results are in agreement that cowbirds show selectively enhanced immune defenses. In a follow-up study (2010-present), we collected host and cowbird eggs from nests of eight hosts that receive different levels of brood parasitism in the northern Great Plains. These eggs currently are undergoing laboratory analysis.
Lawrence D Igl