Year-round tracking reveals multiple migratory tactics in a sentinel North Pacific seabird, Cassin’s auklet

Nonbreeding distributions and migratory phenology can vary within and among populations of many taxa, including seabirds. Such differences can drive variation in survival and reproduction. Knowledge of where, when, and why individuals move is therefore crucial to understanding and predicting population-level processes, particularly in the context of oceanographic change. Here we present the first year-round tracking data for Cassin’s auklet Ptychoramphus aleuticus, a small, zooplanktivorous North Pacific seabird, revealing the nonbreeding behavior of birds from colonies in British Columbia, Canada, across 3 years of variable oceanographic conditions. These birds moved both north (summer, fall) and south (fall, winter) from the colony, using 1 of 4 migratory tactics: North, South, North-South, or Local. Distributions ranged from the Aleutian Islands to Baja California, to maximum distances of 2757 and 3110 km, respectively. Within tactics there was additional spatiotemporal variation among individuals, resulting in diverse nonbreeding experiences. Some variability was explained by year and sex. Cassin’s auklets appear to target distinct, ecologically differentiated areas within their nonbreeding range which may have good quality habitat. Variation in these target regions across years suggests somewhat flexible migratory behavior; however, consistent migration to the Southern California Bight despite very low productivity in one study year suggests that fixed tactics may exist. This work contributes to our understanding of predator movement in the North Pacific and highlights the importance of individual-level spatial data for studies of carryover effects and conservation planning.