Sex-specific associations between reproductive output and hematozoan parasites of American kestrels

Parasites have the potential to decrease reproductive output of hosts by co mpeting for nutrients or forcing hosts to invest in immune function. Conver sely, reproductive output may affect parasite loads if hosts allocate resou rces to reproduction such that allocation to immune function is compromised . Both hypotheses implicitly have a temporal component, so we sampled paras ites both before and after egg laying to examine the relationship between r eproductive output (indexed using a combined measure of clutch size, egg vo lume, and initiation date) and blood parasite loads of American kestrels (F alco sparverius). Parasite loads measured prior to egg laying had no advers e effects on subsequent reproductive output. Females that previously had la rge reproductive outputs subsequently had lower parasite intensities than t hose whose outputs were smaller, suggesting that females were capable of al locating energy to both forming clutches and reducing parasite loads. Becau se male kestrels provide most of their mate's energetic needs before, durin g, and after egg laying, mate choice by females may have consequences for t heir parasite loads. Females choosing high-quality mates may not only have increased reproductive output, but may also obtain sufficient resources fro m their mates to enable them to reduce their parasite burdens. Males whose mates had large reproductive outputs were more likely to subsequently be pa rasitized and have more intense infections. For individual males sampled bo th before and after egg laying, those whose mates had larger reproductive o utputs were also more likely to become parasitized, or remain parasitized, between sampling periods. Increased parasite loads of males may be one mech anism by which the costs of reproduction are paid.