The Brown-headed Cowbird (Molothrus ater) is a brood parasite that lays its eggs in nests of other species; the eggs are incubated and the offspring are then raised by the host. Grasslandnesting Dickcissels (Spiza americana) are commonly parasitized by Cowbirds. Cowbird eggs have been reported to hatch sooner than equivalently-sized host eggs, giving their hatchlings a competitive advantage over host offspring. Our study focused on the hypothesis that the apparent accelerated development of Cowbirds is caused by greater eggshell porosity which allows for increased availability of oxygen during incubation. The mean pore area of Cowbird eggshells (2.043 ± 0.674 µm2/mm2; mean ± SD) was 5 times greater than Dickcissel eggshells (0.383 ± 1.095 µm2/mm2; t = 5.772, df = 31.598, p \u3c 0.001). However, the mean number of pores per eggshell did not differ significantly between Cowbirds (0.263 ± 0.122 pores/mm2) and Dickcissels (0.229 ± 0.130 pores/mm2; t = 0.846, df = 38, p = 0.403). The data support our hypothesis that Cowbirds have greater eggshell porosity than their host, which could lead to a shorter incubation period
