Year-round dynamics of diving behaviour across life-stages and sexes in a generalist top predator, the great cormorant (Phalacrocorax carbo)

Variation in animal behaviour across life-stages and sexes has important ecological and demographic consequences, potentially influencing individual survival and population dynamics. Diving behaviour in generalist top predators provides insights into foraging strategies and resource use, yet how these patterns vary across life-stages and sexes throughout the annual cycle remains poorly understood. We investigated year-round diving behaviour in great cormorant (Phalacrocorax carbo), comparing adults, immatures, and juveniles, as well as adult males and females, between breeding and nonbreeding periods. Using 984,417 dives from 69 individuals equipped with GPS loggers with integrated time-depth recorders (TDRs) along the south-eastern Baltic Sea coast between May 2018 and March 2026, we quantified variation in maximum dive depth, proportion of benthic dives, daily diving time and daily number of bouts. The spatial extent of diving locations covered the entire Western Palearctic and varied by demographic group, expanding later in the annual cycle from the breeding grounds. Maximum diving depths were similar among life-stages (4.5 ± 3.3 m), with juveniles reaching dive depths comparable to adults. Diving performance varied seasonally across life-stages, particularly in juveniles, which exhibited approximately a 64% lower proportion of benthic dives and 59% lower daily diving time than adults. Overall, adult males dove deeper than females (6.1 ± 4.3 m versus 3.9 ± 2.6 m) across both periods. Sex-specific diving performance patterns observed during the breeding season persisted during the nonbreeding season. Both sexes might display relatively consistent diving strategies across the annual cycle rather than being influenced by reproductive demands alone, likely influenced by diverse diving ranges. Our findings show that life-stage and sex contribute to variation in great cormorant diving behaviour across the annual cycle. We highlight the value of year-round approaches in animal research for identifying areas of management interest and overlooked biological mechanisms that further influence population dynamics.