Seabird populations can be controlled by top-down mechanisms, such as predation or bottom-up mechanisms, such as prey availability. Populations of the tufted puffin (Fratercula cirrhata: TUPU) have significantly declined in the past decades in Washington and Oregon while those of the rhinoceros auklet (Cerorhinca monocerata: RHAU), a closely related sympatric species, have not experienced the stark levels of decline that TUPU have. TUPU are currently listed as Endangered by the state of Washington and are a candidate for listing at the federal level; however, drivers of their decline are poorly understood. Changes in diet quality and composition have been proposed as a mechanism of population decline in TUPU, yet we know nothing about their historic diets. To reconstruct historic diets for TUPU (n=49) and RHAU (n=64) in Washington and Oregon, we conducted stable isotope analyses of feathers from museum specimens spanning 1911 to 2015. Nitrogen ratios (δ15N) are used to identify trophic level and carbon ratios (δ13C) are used to distinguish between nearshore and offshore foraging. Trophic position and offshore feeding habits were quantified for both species over time.
Results/Conclusions
TUPU and RHAU in Washington and Oregon were found to have extensive trophic overlap as their δ15N values were similar. Their δ13C values were different, which indicates that these species are segregated spatially; TUPU tend to feed more offshore while RHAU are distributed between nearshore and offshore. There was very little change in their isotope ratios over time, which indicates that TUPU and RHAU maintained their respective spatial feeding habits and trophic positions over the last century. These results are unlike what has been documented for similar studies involving Marbled Murrelets (Brachyramphus marmoratus) and Common Murres (Uria aalge) that have shifted in δ15N over time. These preliminary results demonstrate that TUPU in Washington and Oregon have not experienced a trophic shift over the last century; however, a change in abundance and availability of prey species over the past century cannot be discounted as a potential mechanism of decline and should be investigated further.