Phenology effects on productivity and hatching-asynchrony of American kestrels (Falco sparverius) across a continent

Abstract

Optimal reproductive performance occurs when birds time reproduction to coincide with peak food availability. Deviation from optimal timing, or mismatch, can affect productivity, though birds may mediate some mismatch effects by altering their incubation behavior. We studied the consequences of nesting timing (i.e., clutch initiation relative to an index of spring start) on productivity across the breeding range of American kestrels ( Falco sparverius ) in the United States and southern Canada, and associations between nesting timing, incubation behavior, and hatching asynchrony. We used observations from long-term nest box monitoring, remote trail cameras, and community-scientist-based programs to obtain data on clutch initiation, productivity, incubation, and hatching synchrony. Kestrels that initiated clutches after the extended spring index (SI-x, start of spring estimate) had higher rates of nest failure and fewer nestlings than earlier nesters, and effects of nesting timing on productivity were strongest in the Northeast. In contrast, kestrels in the Southwest experienced a more gradual decline in productivity across the season. Spatial effects may be the result of regional differences in growing seasons and temporal nesting windows (duration of nesting season). Specifically, resource availability in the Northeast was highly peaked during the breeding season, potentially resulting in shorter nesting windows. Conversely, resource curves were more prolonged in the Southwest, and growing seasons are becoming longer with climate change, potentially resulting in longer nesting windows. We found an inverse relationship between nesting timing and the onset of male incubation. Males from breeding pairs that initiated clutches after SI-x began incubation sooner than males from breeding pairs that initiated clutches before SI-x. Early-onset of male incubation was positively associated with hatching asynchrony, creating increased age variation in developing young. In sum, nesting phenology relative to the SI-x has consequences for American kestrels’ productivity, and these consequences vary across space. The early onset of incubation may act as a potential adaptive behavior to advance the average hatch date and spread out energetic demands. Given the effects of nesting timing on productivity, kestrels are likely to be sensitive to climate-driven advances in growing seasons and vulnerable to phenological mismatch, particularly in the Northeast. Top panel show availability of food (green polygon) in two different types of environments, a highly seasonal environment with peaked productivity and a less seasonal environment with prolonged growing seasons, and the outcome of two nesting attempts, one before the start of spring (yellow) and another after (purple). The later nesting attempt results in fewer young, especially in the seasonal environment. The bottom panel shows that in pairs that nest before the start of spring (yellow), males delay incubation and young hatching at the same time, whereas in later nesting pairs (purple), males may incubate earlier and young hatch asynchronously. • We studied the phenology and productivity of an avian predator across North America. • American kestrels nesting after the start of spring had lower productivity. • Effects of late nesting on productivity were strongest in the Northeast. • Later nesting kestrels altered incubation so that eggs hatched asynchronously. • Spatial patterns of phenology effects reveal populations sensitive to mismatch.