High fidelity to nest sites and partners in the Common Swift <i>Apus apus</i>

Capsule Individual Common Swifts show high fidelity to nest sites (94–95%), and lesser fidelity to partners (59%).Aims To determine the extent of individual fidelity to nest sites and partners by Common Swifts, and to identify any predictors of nest site fidelity.Methods We monitored nest boxes and captured adult Common Swifts over the 15-year period 2010–2024. Fidelity analyses were based on 243 breeding events and capture data from 190 adults matched to nest sites and pairings. Predictors of nest site fidelity were examined using logistic regression.Results From 243 breeding events, 91 individuals bred in multiple years. Across 216 breeding events involving at least one breeding individual captured in multiple years, 94.5% re-used a previously used nest site. Among 213 breeding events involving an individual captured in consecutive years, 94.3% re-used the previous year’s nest site. Nest site fidelity between consecutive years was significantly lower at higher colony densities, but was not predicted by individual age or previous breeding success. Of 36 consecutive year pairings involving the same two individuals, 89% re-used the same nest site. Among 168 breeding events where both pair members were identified, 39 unique pairings bred together in multiple years. Of 55 consecutive year pairings, 94.5% remained faithful and 5.5% divorced.Conclusions Common Swifts are declining in the UK, with nest site loss being a hypothesized contributory factor. We show that breeding swifts have very high rates of fidelity to nest sites and partners, and hence colonies, and the loss of nest sites is therefore likely to have impacts on individuals and colonies due to the need to find alternatives. We provide some evidence of local relocation by breeding individuals after nest site loss, suggesting that local provision of nest sites may help maintain local populations.