MEPS 419:267-282 (2010)  -  DOI: https://doi.org/10.3354/meps08860

ABSTRACT: During nesting periods, seabirds are known to exhibit considerable inter-annual variability in diets, yet little is known about the diets of pelagic seabirds during non-breeding periods. Over 5 yr (2005 to 2009), we studied dietary partitioning between sympatric greater and sooty shearwaters, Puffinus gravis and P. griseus, during migratory staging periods in the Northwest Atlantic. Stable-isotope (SI; n = 253) and fatty-acid (FA; n = 127) signatures from blood samples were used to assess inter-annual patterns in diet and quantify prey choices. In addition to significant effects of year, capture site, and body condition, SI signatures revealed subtle, but consistent, dietary partitioning between species. In all years, greater shearwaters fed at slightly higher trophic levels (overall mean δ¹⁵N = 13.6‰) and lower δ¹³C (–19.1‰) than sooty shearwaters (δ¹⁵N = 13.3‰, δ¹³C = –18.9‰). SI mixing models revealed that sooty shearwaters relied more heavily on euphausiids Meganyctiphanes norvegica, while greater shearwaters consumed more herring Clupea harengus, squid Illex illecebrosus, and, in some years, mackerel Scomber scombrus. In 2005/2006, bird diets consisted primarily of herring and krill, but demonstrated a shift towards krill and squid during 2007–2009. FA from bird plasma showed little inter-specific partitioning but a strong signal of annual variation for both species. We used a subset of prey FA and a modified multivariate approach to model bird diets and obtained dietary preferences broadly in agreement with SI results. The present study revealed inter-annual variability and dietary partitioning in sympatric species of pelagic seabirds, and highlights potential shifts in prey availability to predators in the Bay of Fundy.

KEY WORDS: Foraging ecology · Shearwaters · Stable isotopes · Fatty acids · Bay of Fundy