Breeding season forest fragment size does not create negative carry-over for adult Wood Thrushes on fall migration timing or apparent annual survival

Although carry-over effects related to wintering habitat quality are known
to influence population dynamics of migratory songbirds, the presence of
breeding season carry-over is under-studied in full annual cycle models.
To test whether forest fragment size on the breeding grounds can impose
negative carry-over effects on a migratory songbird, we fitted adult Wood
Thrushes with one-year coded radio-tags in forest fragments ranging from
11­–499 ha in southwestern Ontario during the 2016–2019 breeding seasons
and utilized automated telemetry via the Motus Wildlife Tracking System to
record fall migration timing and returns the following spring (apparent
annual survival). To examine short-term effects of fragment size on
breeding females, during the 2018 and 2019 breeding seasons, we collected
blood samples during incubation to measure corticosterone levels and
tracked complete reproductive success and nest timing. We found that Wood
Thrushes breeding in small forest fragments were not subject to strong
negative effects on body condition (mass, corticosterone), reproductive
success, or timing of the last nest of the season. Next, we found that the
onset of fall migration departure was not delayed for birds nesting in
small fragments and that apparent annual survival was not linked to
breeding fragment size. This suggests that habitat differences linked to
fragment size were not strong enough to trigger the kinds of negative
carry-over effects (delayed migration, lower reproductive success) that
have been documented in other species as a result of poor wintering ground
habitat quality. The strength of breeding fragment size-induced seasonal
carry-over remains a critical gap in full annual cycle models for other
declining migratory songbirds. Our findings suggest that while the
importance of preserving large forested areas is often prioritized in
conservation projects, small forest fragments can also have high
conservation value.