Foraging Behavior of Incubating and Chick-Rearing Thick-Billed Murres Uria Lomvia

The varying demands associated with egg incubation and chick-rearing are known to have a corresponding effect on the foraging behavior of seabirds. We deployed data loggers on incubating and chick-rearing thick-billed murres Uria lomvia to examine differences in their diving behavior and characteristics of habitats used for foraging. To compare diets of incubating and chick-rearing birds we collected their stomach contents using a water offloading technique. We found that incubating birds performed longer foraging trips than chick-rearing birds (incubating: 19.0 +/- 7.2 h; chick-rearing: 9.9 +/- 5.6 h). Incubating birds foraged in the offshore stratified water masses (sea surface temperature [SST] > 9 degrees C) and frequently dived to the depth of the thermocline (20 to 50 m). Chick-rearing birds spent more time foraging in the inshore, well-mixed water masses (SST < 8 degrees C), and at depths >60 m. Small juvenile walleye pollock Theragra chalcogramma, squid and euphausiids were the dominant prey of incubating and chick-rearing birds. Distributions of these small prey were commonly associated with the thermocline, while larger fish, which parents brought back to feed their chicks, were distributed below the thermocline. Results suggest that incubating murres mainly foraged at shallow depths near the thermocline with higher concentrations of small prey, while chick-rearing murres feed their chicks large prey caught on deep dives while feeding themselves on small prey caught on shallow dives.