Marine Ornithology

Common Murres Uria aalge are upper trophic-level consumers and useful indicators of the impacts of anomalous climatic events and other perturbations, such as marine heatwaves, long-term change in ocean ecosystems, and changes in predator populations. We used an existing archive of aerial photographs of murre colonies along the coast of Oregon to expand available data on the number of murres nesting in that portion of the California Current ecosystem during 1988-2014. We completed counts of all murre colonies in Oregon ca. every 10 years, developed a stratified random sample of colonies representative of total counts, and completed sample counts ca. every three years. We documented a nearly 25% decline in the number of nesting murres in Oregon, with ca. 97,000 fewer counted in 2014 compared with 1988. Murre numbers declined by 74% (ca. −157,000 murres) along the northern coast of Oregon, increased by 105% (ca. +60,000 murres) along the central coast, and were nearly stable along the southern coast. The centroid of the Oregon breeding population shifted 84.4 km to the south during the study period. Most Oregon breeding colonies experienced growth rates that exceeded the maximum intrinsic growth rate and/or new breeding activity, indicating that philopatry was not strong and large numbers of murres immigrated or recruited from other colonies. Frequent shifts in colony sizes strongly implied substantial movement of murres among colonies throughout the study period, including those colonies on the southern coast where total numbers were stable. Predation and disturbance by Bald Eagles Haliaeetus leucocephalus are known drivers of higher murre mortality rates and lower reproductive success at colonies on the northern and central coasts. The recovering eagle population along the Oregon coast is likely the main factor causing high movement rates of murres among breeding sites, through the “ecology of fear.”