# Individual Variation in Migration and Wintering Patterns of Long‐Tailed Ducks Clangula hyemalis From a Population in Decline

## TL;DR

Long-tailed ducks in the Baltic Sea are declining, but tracking shows most still winter there, suggesting a real population drop rather than just moving north.

## Contribution

High individual repeatability in migration patterns suggests the decline is not due to shifting wintering ranges.

## Key findings

- 94% of tracked ducks overwinter in the Baltic Sea, not Arctic waters.
- Individuals show high repeatability in wintering site longitude and migration timing.
- Population decline is likely real, not just a range shift.

## Abstract

The population of long‐tailed ducks
Clangula hyemalis
has declined dramatically since the 1990s at the species' most important wintering area, the Baltic Sea. It is unclear if this represents a real population decline at the flyway level or merely a northward shift in the wintering range, with part of the population moving from the Baltic Sea to rarelysurveyed ice‐free Arctic waters. To investigate wintering area choice and individual repeatability, we deployed light‐level loggers on female long‐tailed ducks at three breeding sites in the Western Russian Arctic across two annual cycles, from 2017 to 2019. We obtained data from 94 year‐round migration tracks (78, 14 and 2 from each breeding site) from 65 females. Females moved from freshwater breeding sites to mostly marine post‐breeding sites after wing moult. For wintering, the majority of the birds (94%) migrated to the Baltic Sea, while the rest overwintered in the White and Barents Seas. Spring migration involved staging at marine sites in the Arctic Ocean for most birds. Individual repeatability scores were high for longitudes of wintering sites, departure dates from breeding and wintering sites, and low for arrival dates at breeding and wintering sites. Therefore, our results suggest that the observed decline in the long‐tailed duck wintering population in the Baltic Sea is unlikely the result of a shift in wintering range within individuals, so that a real decline in the population size remains the most parsimonious explanation. High repeatability values indicate that the substantial variation in wintering sites throughout the Baltic Sea is clearly attributable to between‐individual variation rather than within‐individual variation across years. Still, addressing the underlying causes of population decline remains a challenge for this Arctic‐breeding species.

The population of the long‐tailed duck (
Clangula hyemalis
) in its primary wintering area, the Baltic Sea, has significantly declined since the 1990s, potentially due to a northward shift in wintering range to ice‐free Arctic waters. We tracked female ducks with light‐level loggers from their breeding sites, discovering that most birds (94%) still overwinter in the Baltic Sea, with high individual repeatability. This consistency in behaviour suggests the population decline is likely due to a real reduction in numbers rather than individual range shifts, indicating the need for further investigation into the causes of this decline.