Abstract
When caring for their young, parents must compensate for threats to offspring survival in a manner that maximizes their lifetime reproductive success. In birds, parents respond to offspring threats by altering reproductive strategies throughout the breeding attempt. Because altered reproductive strategies are costly, when threats to offspring are limited, parents should exhibit a limited response. However, it is unclear if response to offspring threat is the result of an integrated set of correlated changes throughout the breeding attempt or if responses are a flexible set of dissociable changes that are stage‐specific. We test these hypotheses in a system where house wrens (Troglodytes aedon) compete for nesting cavities with Carolina chickadees (Poecile carolinensis) by usurping and destroying their nests during the early stage of the breeding attempt (the egg stage). Due to the specificity of the house wren threat, we can test whether parental responses to an offspring threat show flexibility and stage specificity or if parental strategies are an integrated and persistent response. We monitored nests in a natural population to compare life history traits of chickadees nesting in boxes that were in the presence of house wrens to chickadees nesting in boxes that did not overlap with house wrens. Carolina chickadees that nested near house wrens laid significantly smaller clutch sizes (early change in reproductive strategy) but did not alter nestling provisioning or nestling stage length (late change in reproductive strategy), suggesting that chickadees respond in a flexible and stage‐specific manner to the threat of house wrens. By responding only when a threat is highest, parents minimize the cost of antithreat responses. Our study suggests that parents can respond in subtle and nuanced ways to offspring threats in the environment and specifically alter reproductive behaviors at the appropriate stage.
Highlights
Parental investment is a complex suite of behaviors that are subject to competing demands on parents as they attempt to care for current offspring, conserve resources for future reproduction, and ensure their own health while dealing with environmental stresses
If chickadee strategies show flexibility and stage specificity, we expect to see that parents in the presence of house wrens will alter their reproductive strategy during the egg stage and not the nestling stage
Presence of a threat is often associated with changes in reproductive strategy throughout the nesting attempt including changes in clutch size, incubation rhythm, nestling provisioning, and nestling development (Chalfoun & Martin, 2010; Ferretti et al, 2005; Fontaine & Martin, 2006)
Summary
Parental investment is a complex suite of behaviors that are subject to competing demands on parents as they attempt to care for current offspring, conserve resources for future reproduction, and ensure their own health while dealing with environmental stresses. Threats from predation can result in costly alterations in parental behavior throughout nesting (i.e., both egg and nestling stages) because general offspring predators are often relevant throughout the entire breeding attempt (e.g., Fontaine & Martin, 2006; Zanette, White, Allen, & Clinchy, 2011). Nest competition between house wrens (Troglodytes aedon) and Carolina chickadees (Poecile carolinensis) is an ideal study system in which to test whether changes in reproductive strategy are an integrated or flexible set of traits because house wrens are a stage-specific threat to Carolina chickadee offspring (Figure 1a). If chickadee strategies show flexibility and stage specificity, we expect to see that parents in the presence of house wrens will alter their reproductive strategy during the egg stage (i.e., clutch size and incubation behavior) and not the nestling stage (i.e., nestling provisioning and nestling stage length; Figure 1b). We discriminate between the predictions of a dissociable response versus the predictions of an integrated response by measuring and comparing life history traits of free-living chickadees naturally nesting in boxes that are near nesting house wrens to chickadees nesting in boxes that are not
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