Using movement data from satellite telemetry to infer nesting status and breeding propensity of a colonial breeder, the American common eider (Somateria mollissima dresseri)

Background American common eiders ( Somateria mollissima dresseri ) are ecologically and culturally important sea ducks in northeastern North America. Female eiders may not breed every year, making breeding propensity (the proportion of sexually mature females breeding in a given year) a key demographic parameter. Methods We investigated regional variation in breeding propensity across the subspecies’ range using fine-scale movement data collected by satellite telemetry. Females were captured across six regions (Maine, New Brunswick, Nova Scotia, Québec, Newfoundland, Labrador) and fitted with platform terminal transmitters (PTT). After filtering, movement data from 123 individuals over three years (2022–2024), resulting in 197 unique bird-breeding seasons, were analyzed using a three-state hidden Markov model. Three behavioral movement states—limited, localized, and mobile —served as proxies for nesting, foraging/loafing, and transit, respectively. A decision tree classified breeding status into successful nesting, failed nesting, prospecting, or skipped breeding. Results Overall breeding propensity was only 53%, with regional variation ranging from 30 to 87%. Regional breeding propensity patterns did not fully align with expectations based on the most recent population trends. The discrepancy could be driven by localized factors such as predation pressure, disease outbreaks, and resource availability. Conclusions Our approach demonstrates the utility of movement-based behavioral classification for inferring timing of nest initiation and breeding status in a colonial waterbird using once-daily satellite PTT positional data. These findings reveal complex spatial patterns in breeding propensity, underscoring that region-specific monitoring can be used to inform management and conservation of this subspecies.