Clutch size and the rejection of parasitic eggs: a comparative test of the maternal investment hypothesis

Obligate brood parasitic birds lay their eggs in the nests of other species, reducing the host’s own reproductive output. To circumvent these fitness costs, many—but not all—host species have evolved the ability to recognize and reject brood parasitic eggs. What factors constrain egg rejection, and why do host species vary in their likelihood of rejection? Previous comparative studies have found that egg rejection rates covary with several biotic factors (including larger body size, smaller relative brain size, and more northerly breeding latitudes), but much behavioral variation in the occurrence of egg rejection remains unexplained. In this study, we test a corollary of the maternal investment hypothesis, by assessing whether species with higher clutch sizes are more likely to eliminate parasitic eggs. We examined two published data sets comprising over 200 unique bird species, controlling for phylogeny and other known interspecific correlates of egg rejection rates. Contrary to the prediction, we found no evidence for a positive relationship between clutch size and egg rejection rate. Rather, our analyses suggest a weak but consistent negative relationship between absolute and relative metrics of clutch size versus egg rejection rate across species. These results are instead consistent with two previously proposed alternative hypotheses: that egg rejection is constrained by a trade-off between maternal investment and anti-parasitic defenses, possibly mediated by endocrine mechanisms linked to parental care, and/or that cognitive decision rules facilitate the detection of dissimilar eggs in smaller clutches.