Comparison of regional eggshell porosity between the brood parasitic Brown-headed Cowbird (Molothrus ater) and its hosts: the Dickcissel (Spiza americana), and two non-parasitic relatives, the Red-winged Blackbird (Agelaius phoeniceus) and the Common Grackle (Quiscalus quiscula).

Abstract

The Brown-headed Cowbird (Molothrus ater) is a generalist brood parasite that lays eggs in the nests of many host species, including the Dickcissel (Spiza americana) and two non-parasitic relatives: the Red-winged Blackbird (Agelaius phoeniceus) and the Common Grackle (Quiscalus quiscula). Cowbird eggs reportedly hatch sooner than equivalently-sized host eggs, presumably via accelerated embryonic development enabled by a greater eggshell porosity and consequently greater gas exchange. However, the distribution of pores among apical, equatorial and basal eggshell regions within cowbirds and host species is undetermined. I tested the hypothesis that equatorial porosity would be greatest because respiratory gases primarily cross the eggshell pores and enter or exit the embryo’s circulatory system via the chorioallantoic membrane located in that region. I found that the equatorial region of eggs within species had significantly greater pore density, pore area, and porosity than the basal or apical regions of eggshells from cowbirds (P ≤ 0.006), Red-winged Blackbirds (P ≤ 0.002), and Dickcissels (P ≤ 0.005). Grackle eggshells did not follow this regional pattern, and porosity characteristics did not differ significantly between the equator and base (P \u3e 0.05). Notably, cowbird eggshells had significantly greater pore area and porosity in equatorial regions compared to its three hosts (P ≤ 0.012). Cowbird eggshells had a greater apical pore area than that of the Dickcissel (P \u3c 0.001) and grackle (P = 0.003), and did not have significantly greater eggshell basal pore area or porosity compared to either of its relatives. These observations demonstrate region-specific rather than global increases in eggshell porosity, which may further explain the accelerated embryonic development of cowbirds compared to host species