Life-history theory suggests that organisms face fitness trade-offs during reproduction. Using observational and experimental methods, I tested how variation in proximate factors influenced reproductive effort and success in the American kestrel ( Falco sparverius), a small falcon. Specifically, I investigated how parasites, food supply and brood size affected parental effort decisions in kestrels and the consequences of those decisions for both parents and offspring. Parasites may compete with hosts for nutrients or force hosts to invest in immune function, thereby decreasing host reproductive effort ('parasite cost' hypothesis). Conversely, reproductive effort may affect parasite loads if hosts allocate resources to reproduction such that allocation to immune function is compromised ('reproductive cost' hypothesis). I tested these hypotheses by examining the relationship between effort invested in eggs and blood parasite loads of kestrels. Parasite loads measured prior to egg laying had no detectable effects on reproductive effort. Females that made large investments in reproduction subsequently had lower parasite intensities than those whose reproductive efforts were smaller, suggesting that females were capable of allocating energy to both forming clutches and reducing parasite loads. Females choosing high quality mates may not only have large reproductive outputs, but may also obtain sufficient resources from their mates to enable females to reduce their parasite burdens. Males that made large investments in reproduction were more likely to subsequently be parasitized and have more intense infections. I investigated how natural variation in the main prey of kestrels, small mammals, influenced reproductive success. My results suggest that provisioning behaviour of kestrels raising offspring was not constrained by the ' abundance' of food, but rather the 'availability' of food, mediated through variation in weather. Moreover, nestlings exposed to inclement weather were in poorer condition and less likely to survive to fledging. I also performed a food supplementation experiment to test how food supply influenced reproduction. Results from both unmanipulated and supplemented nests suggests kestrels raising offspring are limited by food. In addition to food supply, parental effort often varies with brood size. However, it is unclear whether brood size is the proximate determinant of parental effort, or conversely whether parental effort is the proximate factor to which brood size is adjusted. To distinguish between these hypotheses, I experimentally reduced brood sizes of kestrels. My results suggested that males adjusted their provisioning rates accordingly. Conversely, female kestrels did not adjust provisioning behaviour to brood size. Quality and survival of offspring in reduced broods were similar to control young, which I speculate may be the result of larger food requirements because of potentially higher thermoregulatory costs compared to young in control nests. I conclude that brood size is a proximate determinant of parental effort in kestrels. (Abstract shortened by UMI.