Intra- and interannual variation in the foraging behavior of common Murres (Uria aalge) in the Central California current

Abstract

Seabirds modify their behavior during the breeding season as they transition from incubation (self-feeding) to chick-rearing (self-feeding + chick provisioning) periods, which affects parental foraging patterns and distribution. However, it is less clear how much flexibility parents have to modulate their behavior when environmental conditions are unusually poor. Using GPS loggers and time depth recorders, we examined the foraging flexibility of common murres (Uria aalge) at Southeast Farallon Island as adjustments were made between breeding phases and across consecutive years (2019 and 2020) with contrasting oceanographic conditions (i.e. prominent marine heatwave in 2019). Foraging trips were longer during the incubation period (2019: 31.7 ± 20.1 h; 2020: 23.5 ± 7.2 h) compared to the chick provisioning period (2019: 12.2 ± 9.9 h; 2020: 7.5 ± 8.3 h). Moreover, parents generally ventured farther from the colony during incubation (2019: 34.7 ± 17.6 km; 2020: 30.1 ± 11.5 km) than when foraging to provision chicks (2019: 22.6 ± 17.1 km; 2020: 14.1 ± 15.0 km) in each year. Nevertheless, overall foraging ranges had a high degree of overlap (> 60%) within breeding phases and across years, suggesting that murres at this colony are constrained by the habitats primarily based on foraging range. Murres dove deeper during incubation compared to chick-rearing in 2020 (36.0 ± 24.4 vs 19.5 ± 15.2 m), but there were no other differences within or across years in diving behavior, suggesting that diving effort was less affected by breeding stage. Interannual differences in foraging behavior only occurred during chick-feeding trips and were likely driven by the prey species present. For example, rockfish were more common in chick diets in 2020, when ocean temperatures were cooler compared to 2019. Despite similarities in trip parameters between years, breeding productivity was much lower in 2019 compared to both long-term averages and the 2020 season, suggesting a potential constraint on murre behavioral flexibility when oceanographic variation occurs. These results emphasize the importance of examining behavior on multiple time scales to gain a better understanding of how predators respond to changes in their environment.