Spatial and temporal variation in prey use of common murres at two disjunct colonies in the California Current system

I compared prey use and reproduction of common murres (Uria aalge) at two spatially disjunct colonies during 2006 and 2007 to examine if prey use of murres differed between locations and investigate how foraging ecology affects murre productivity. Prey items brought to colonies were identified utilizing remote control video systems at Devil's Slide Rock in central California (2006-2007) and Castle Rock National Wildlife Refuge located in northern California (2007). Prey use differed significantly between colonies and between years at Devil's Slide Rock. Northern anchovy (Engraulis mordax) and clupeid species comprised the majority of prey items identified at Devil's Slide in 2006 (85% of prey) and 2007 (65% of prey). At Castle Rock in 2007 smelt species (Osmeridae) comprised 53% of prey identifications. Prey size was significantly larger at Devil's Slide than Castle Rock, but did not differ between years at Devil's Slide. Differences in prey composition between colonies were accompanied by a large difference in energetic content of predominate prey types, with Castle Rock murres obtaining less energy rich prey. Lacking independent information on prey availability, I inferred that prey composition reflected differences in distribution and abundance of fish between years and near each colony based on current literature. In 2006, common murres at Devil's Slide bred later, had lower fledging and lower breeding success than in 2007 in response to delayed coastal upwelling that reduced overall ocean productivity. In 2007, mean date that eggs were laid occurred one week earlier at Castle Rock than Devil's Slide. Castle Rock murres spent less time in co-attendance (2% of the sampling period) than murres at Devil's Slide (10-15% of the sampling period). Lower energetic content of prey items and distance of prey from the Castle Rock colony may have contributed to the observed difference in murre foraging effort between colonies. Fledging success was lower at Castle Rock than at Devil's Slide in 2007, but provisioning rates and breeding success were similar between colonies. My results support findings of others that co-attendance is a sensitive parameter to changes in the marine environment and that murres will adjust the amount of time spent at the nest site in order to maintain feeding rates. Comparing the biological response of murres between years and colonies with varied prey use may assist in identifying the causes to the observed differences in co-attendance and reproductive success.