2026 Undergraduate Research Showcase

Nesting Phenology and Migration Strategy of the American Kestrel (Falco sparverius)

Document Type

Student Presentation

Presentation Date

4-24-2026

Faculty Sponsor

Dr. Julie Heath

Abstract

Phenology, the seasonal timing of recurring events, is important because a mismatch of timing for energy-intensive activities (e.g. breeding) with periods of peak resources can negatively impact an individual’s fitness. This issue has been especially relevant recently as many species have been documented shifting their phenology in response to a warming climate. This has been the case for American kestrels (Falco sparverius). Longitudinal studies on nesting kestrels in SW Idaho have indicated they are advancing their average nesting date by 15 days from 1992 – 2015. Kestrels in this area are partial migrants with both year-round residents and short-distance migrants who are present only for the breeding season. We aimed to update previous research by looking at whether the average kestrel nesting date has continued advancing since 2015. Additionally, we used the novel method of measuring stable isotopes extracted from the claw to classify winter strategy (migrant versus resident) for each kestrel. We then looked at whether nesting timing was different for the two strategies. Results indicate that the average nesting date has plateaued in more recent years with the average nesting date stabilizing around mid to late April. There was a significant interaction between sex and wintering strategy with a significant difference found in nesting timing between female residents and migrants, and no significant difference between migrant males and resident males. Nesting date may no longer be advancing because earlier nesting is constrained by winter. Resident female kestrels may begin nesting earlier than migrant kestrels because they can better track the conditions of the breeding ground by staying through the winter. There may be no difference between migrant and resident males because all males compete for early access to females and nest sites. These findings help us understand what factors can impact timing and give us an idea into whether there may be any constraints in moving nesting times earlier.

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