Many migratory bird populations are declining in the face of habitat degradation and climate change, making it important to identify which stages of their annual cycle are most affected in order to guide conservation measures. The Bewick's Swan Cygnus columbianus bewickii , an Arctic‐breeding waterfowl species, has suffered a dramatic population decline (from approximately 30 000 individuals in 1995 to around 13 000 in 2020) due to low reproductive output. However, it is unknown which component of reproduction (breeding propensity, nesting success and post‐hatching survival) is the underlying cause. We analysed GPS tracking and accelerometer data from over 60 adult female Bewick's Swans collected over 8 years, to derive breeding propensity (i.e. the decision to breed or not) and nesting success (i.e. completing incubation) at the individual level, and assess how these metrics are influenced by onset of spring and migration timing. The odds of breeding increased by 3.9% (estimate = −0.04, se = 0.02, P = 0.04) for each day that the swans arrived earlier at the breeding grounds. The odds of successfully nesting increased by 5.8% (estimate = −0.06, se = 0.03, P = 0.06) per day of earlier snowmelt anomaly, suggesting a slight positive effect of climate warming on nesting. However, the proportion of breeding females was consistently low throughout the study period, suggesting that other factors may be driving the long‐term population decline of the species by negatively impacting breeding propensity. Our study shows that distinguishing between different phases of the reproductive cycle is critical for a better understanding of the causes of migratory population declines in a changing world.
Tal et al. (Thu,) studied this question.