Although seabird population biology is relatively well studied, little data exists on mortality at-sea. Beached bird surveys are used to identify patterns of seabird mortality, but resulting patterns are difficult to interpret without corresponding data on at-sea density. We examined seabird mortality relative to at-sea density in Monterey Bay, California over 10 yr by integrating data from monthly beachcast seabird and offshore seabird censuses. Beachcast seabird numbers were relatively constant (mean 2.82 ± 0.31 seabirds km -1 ) throughout the year. At-sea seabird density (mean 148.9 ± 16.12 seabirds km -2 ) peaked in the summer upwelling period and was least in the winter Davidson period. A principal components analysis of seasonal climatic, prey availability, and anthropogenic variables for Monterey Bay derived 3 significant principal components (PCs) (explaining 70% of variance) characterized by storm activity and low prey availability (PC1), river discharge and krill abundance (PC2), and oiling (PC3). These principal components were used in detailed analyses of the 2 most abundant seabird species and indicate that sooty shearwater Puffinus griseus relative mortality is greatest during decreased productivity and increased storm activity. While relative mortality of common murres Uria aalge cannot be explained by the derived principal components, relative mortality increased in late winter when prey availability decreased concurrent with the annual increase in reproductive stress. Beachcast seabird data is difficult to interpret in isolation; however, when linked to at-sea densities of seabirds, it becomes a powerful tool to examine the relative impacts of oceanography and direct human perturbations on seabird demography. KEY WORDS: Seabirds at-sea · Beached bird survey · Density · Mortality · Sooty shearwater · Common murre
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 392: [295][296][297][298][299][300][301][302][303][304][305] 2009 birds (Roletto et al. 2003, Fleet 2006, Harris et al. 2006, Heubeck 2006, Zydelis et al. 2006, Parrish et al. 2007). An advantage of this method over CMR studies is that beached bird surveys enable researchers to examine the seabird community as a whole, including residents and migrants, away from breeding colonies, and sampling can occur year-round. Few researchers, however, have studied the correlation of the number of birds at sea to the number of birds that wash ashore (Mason 1997, Camphuysen & Heubeck 2001. For example, in November 2003, Nevins & Harvey (2003 reported a mass mortality event of northern fulmars Fulmarus glacialis in Monterey Bay, California. Two possible scenarios explain the event: (1) it was a mass mortality event, or (2) mortality was normal and there was an extraordinary influx of northern fulmars into Monterey Bay. Scaling beachcast density to at-sea density as a per capita mortality rate allows us to discern between these 2 possible scenarios, which have different management implications,...