PS 48-139 - Trading time for energy: Moulting Swainson’s thrushes’ search for good-quality habitats in Montreal

Background/Question/Methods

Moult and migration are energy-intensive, time-sensitive activities that can affect an animal’s fitness throughout its entire lifecycle. For some individuals in species like the Swainson’s thrush (Catharus ustulatus), these events overlap; migrants stop for long periods to moult their feathers. Moulting requires important changes in physiology and an incomplete moult can reduce flight efficiency and slow migration. The success of moult migrants depends on the food and protection provided by good quality habitats. Requiring areas with dense vegetation, Swainson’s thrushes are threatened by the lack of suitable stopover sites in cities. Moult migrants must therefore make trade-offs between (1) time lost by searching for good quality habitats within urban matrices and (2) energy gained by refueling at resource-abundant sites. This study evaluates what habitat quality thresholds influence Swainson’s thrushes’ decisions to stay or leave a stopover site. We captured 17 moult migrating Swainson’s thrushes during fall 2021 at the McGill Bird Observatory in Montreal. Each bird was translocated to one of seven possible sites of varying habitat quality and tracked using radiotelemetry. We compared the time spent in each translocation site with habitat quality, expecting birds to spend more time in areas with high food availability and concealment from predators.

Results/Conclusions

Of the seven translocation sites, three were dominated by mature forests, three by immature forests with dense forest margins, and one by field. Translocation sites characterized by field and mature forests had fewer berry-producing shrubs and less concealment from predators than those composed of dense forest margins. Moulting Swainson’s thrushes spent 1.7 ± 1.5 days in mature forests and fields compared to 39.7 ± 7.5 days spent in forest margins. Individuals spent less time at fields and mature forests compared to forest margins (p-values < 0.02). We were able to follow five individuals who moved from their original translocation site of mature forests and fields to surrounding forest margins. We conclude that moulting Swainson’s thrushes prefer stopover sites characterized by dense forest margins with abundant berry-producing shrubs and concealment from predators. This study demonstrates that (1) despite being a city, there are good-quality habitats in Montreal where migrating Swainson’s thrushes can successfully moult their feathers and (2) when selecting a stopover site, Swainson’s thrushes are willing to spend time searching for good quality habitats that can provide sufficient energy for their moult. Understanding birds’ behaviour during stopovers can inform the conservation of critical habitats for moult migrants in urban areas.