Responses of parent birds to changes in demands by their offspring have typically been investigated by manipulating brood size and testing for altered patterns of provisioning. We applied an alternative method to breeding pairs of American Kestrels (Falco sparverius) by modifying the duration of their nesting periods. Rather than focusing on how hard parents can work on any given day, this novel approach tests how long they can sustain the stress of provisioning and the associated behavioral consequences. We manipulated nesting periods by exchanging nearly fledged young at the age of 20 days with offspring from other nests that were 10 days old. We found that males provisioned their second foster broods at rates similar to those for their original broods. By contrast, females brooded offspring in second foster nests less and also delivered less food to them. As a consequence, male offspring in second nests gained less mass than those in first nests, and nestlings of both sexes had reduced feather growth in second nests. Our results, together with those from previous investigations on parental behavior of American Kestrels, suggest that investment strategies of males and females are determined by different proximate mechanisms. Males appear to invest according to the number of young in their nests, whereas females appear to fine-tune their investment according to variation in attributes of individual offspring, such as nutritional condition or reproductive value. We further suggest that female parents invested less in second foster broods because they are more susceptible to costs of reproduction than males.