Post-fledging survival of juvenile barn swallows

We use radio-telemetry to investigate the ecological bottlenecks in the breeding season of a double-brooded model species, the barn swallow.

Aims

The post-fledging period is a critical part of the life cycle in many bird species. However, the processes and mechanisms operating remain largely unexplored due to the difficulties in tracking juvenile birds and their parents in the post-fledging period. Using radio-telemetry, we aim to investigate the factors affecting post-fledging survival and range use up to migration.

Approach

Just after fledging, juvenile barn swallows were tagged with tiny radio-transmitters. This allowed continuous tracking of individual birds for 3–5 weeks to quantify habitat use, range use and post-fledging survival. We used mark-recapture models to quantify survival of juvenile birds in relation to body condition and parental behaviour. Using cross-fostering experiments, we studied the effects of parental behaviour such as timing of breeding or the duration of post-fledging care on the post-fledging survival of juvenile barn swallows. The causes of mortality were identified in juveniles found dead.

Significance

The post-fledging period of altricial birds is recognised as a main bottleneck in survival, and therefore many selective processes are likely to operate at this stage. However, quantitative studies on differential post-fledging survival hardly exist. Establishing the factors affecting post-fledging survival is therefore important for the understanding of avian life-history evolution and population dynamics. The study contributes to the knowledge on reproductive systems as well as the evolutionary processes shaping them.

Results

A long duration of parental post-fledging care considerably increased survival probability of juveniles after fledging. Fledglings of late second broods had markedly lower survival rates compared to fledglings of early second broods. Moreover, the duration of post-fledging care was significantly reduced in first broods compared to single and second broods. We therefore conclude that barn swallows face a strong intra-seasonal trade-off in the allocation of their efforts to their broods: parent barn swallows sacrifice part of the output of their first brood, thereby increasing the output of their subsequent breeding attempt by an earlier onset of breeding. Estimates of reproductive output demonstrate that double-brooded barn swallow pairs optimize the allocation of time to yield a net gain in the annual reproductive output. Hence, in multi-brooded altricial species, the timing of family break-up appears to be an adaptive reproductive decision maximizing lifetime reproductive output. The results demonstrate that extending investigations of avian reproductive systems beyond fledging reveals exciting insights into life history evolution and behavioural ecology, even in well-known model species.

Project management

Beat Naef-Daenzer
Martin Grüebler

Financial support

Swiss National Science Foundation
Karl Mayer Stiftung
Ella & J. Paul Schnorf Stiftung
Stiftung für Suchende

Publications