Drier winter habitat affects songbirds’ ability to survive migration

A new study led by researchers at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) shows that environmental conditions in the winter homes of migratory birds are prolonged and affect their ability to survive spring migration and the breeding season. reproduction.

While scientists have long known that the quality of winter, or non-breeding, habitat influences the timing of migration and the breeding success of migratory birds, the study, published today in Current biologymarks the first time researchers have linked winter conditions to migration survival.

Analysis of data on Kirtland’s Warbler and Black-throated Blue Warbler revealed reduced rainfall and decreased vegetative productivity in the non-breeding habitats of Caribbean birds, resulting in fewer birds surviving spring migration. For Kirtland’s warblers, poor winter habitat also reduced survival into the following breeding season.

As bird populations continue to decline, it is crucial to understand the factors that influence their survival throughout the year. The Caribbean is expected to become drier in coming decades due to climate change, suggesting that migratory bird species such as these warblers may face even greater challenges in the future.

“If winter habitat quality continues to deteriorate over the next half century due to climate change, we can now say that this will reduce the ability of birds to survive spring migration,” said Nathan Cooper, lead author of the study and a research ecologist at the NZCBI. “This knowledge can help us prioritize conservation measures on the most drought-tolerant breeding grounds.”

Starting in 2017, Cooper and his co-authors wanted to study how environmental conditions on the breeding grounds affect survival during spring migration and on the breeding grounds. However, GPS tags, which allow individual birds to be tracked continuously, are too heavy to attach to most songbirds.

Instead, songbird researchers must rely on other devices, like radio transmitters, which are lighter but only reveal their location when a receiver is within 6 to 10 miles . Many species breed across large swaths of North America, making it virtually impossible to correctly guess where an individual bird might end up breeding with enough precision to detect its radio transmitter.

But Kirtland’s warblers gave Cooper and his co-authors a unique opportunity, because almost the entire population breeds in a relatively small part of Michigan. This resulted in the same marked individual birds being found in their non-breeding habitat in the Bahamas.

The team analyzed three years of radio tracking data from 136 tagged Kirtland’s warblers to estimate their survival rate throughout their migration and its relationship to environmental factors.

In addition to directly estimating survival of Kirtland’s warblers, the team also indirectly estimated migratory survival rates of black-throated blue warblers using a statistical technique first developed by Scott Siillett, co-author of the study and wildlife ecologist from the NZCBI, in 2002. Researchers enhanced Sillett’s methods to analyze 14 years of capture-recapture data from the breeding grounds of the black-throated blue warbler in New Hampshire and its breeding grounds. non-breeding in Jamaica, then linking survival to environmental conditions.

Despite the two different methodologies, both species showed lower survival rates during migration compared to stationary periods. The study also confirmed that poor winter habitat conditions, such as drought or reduced vegetative productivity, led to lower survival rates during spring migration and, in some cases, during the breeding season that follow up.

This is the first direct evidence of this type of carryover effect on survival during migration. The approach facilitated by the restricted breeding ranges of the Kirtland’s warbler allowed the team to validate the more indirect approach to estimate migration survival.

“To effectively conserve migratory birds, we need to identify when and where in their annual cycle they are threatened,” Cooper said. “Being able to reliably estimate survival during migration compared to stationary periods is the first step in determining when, where and how birds die throughout the year.”

But Cooper said it was essential to take a more comprehensive approach to effectively protect migratory bird species. For example, it is unclear whether deaths during migration are the main source of mortality or just one of many factors.

In the future, Cooper and his co-authors will build population models of the full annual cycle for Kirtland’s warblers. This model will help scientists determine how the dangers of migration fit into a broader context for warblers, a strategy that could prove useful for the conservation of other declining migratory species.