Dry Winter Habitats: A Risk for Migratory Birds

A recent study by researchers at the Smithsonian's National Zoo and Conservation Biology Institute (NZCBI) highlights how environmental conditions in migratory birds' winter habitats impact their survival during spring migration and the breeding season. This research is the first to directly link winter conditions to migration survival.

Focusing on the Kirtland's warbler and the black-throated blue warbler, the study found that reduced rainfall and lower plant productivity in their Caribbean winter homes resulted in decreased survival rates during migration. Poor winter habitat also led to fewer Kirtland's warblers surviving the subsequent breeding season.

As bird populations decline, understanding the factors affecting their year-round survival is vital. Climate change is anticipated to make the Caribbean drier, which could further complicate challenges for migratory species like these warblers.

Lead author Nathan Cooper noted that continued degradation of winter habitat quality due to climate change will diminish birds' survival chances during migration. This insight can inform conservation strategies targeting the most resilient non-breeding habitats.

The investigation began in 2017, aiming to determine how non-breeding habitat influences survival during migration and breeding. Researchers faced challenges with tracking small songbirds, as GPS tags were too heavy. Instead, they utilized lighter radio transmitters with limited range, complicating the tracking of individual birds across extensive breeding territories.

The Kirtland's warbler offered a unique opportunity for study since nearly the entire population breeds in a small region of Michigan, enabling researchers to track tagged birds from their winter habitats in the Bahamas.

Analyzing three years of radio tracking data from 136 Kirtland's warblers, the team estimated survival rates during migration in relation to environmental conditions. They also used statistical techniques to estimate migratory survival rates for black-throated blue warblers, relying on capture-recapture data from their breeding grounds in New Hampshire and non-breeding areas in Jamaica. Both species showed lower survival rates during migration, confirming that adverse winter conditions negatively impacted their overall survival.