Foraging ecology and nutritional stress of tufted puffins (Fratercula cirrhata) inferred from stable isotopes, fatty acid signatures, and field endocrinology

Prey availability has a major impact on the reproductive output of seabirds, yet information on seabird diets throughout the breeding season is often lacking. Although reduced prey availability is known to affect the growth and survival of nestling seabirds, few studies have demonstrated similar effects on indices of adult body condition. I used stable isotopes and fatty acid (FA) signatures to investigate seasonal and age-related variation in the foraging niches of tufted puffins (Fratercula cirrhata). I conducted captive feeding experiments to determine whether inferences based on these techniques are affected by moderate food restriction during growth. I also examined how adult puffins prioritize the competing goals of maximizing the growth rate of their offspring and maintaining their own condition, as measured by body mass and by the stress hormone, corticosterone (CORT). Food restriction during nestling growth affected adipose tissue FA signatures and resulted in blood that was depleted in 15N and 13C relative to well-fed controls. However, effects of nutritional restriction on delta 15N, delta13C, and FA signatures were small compared to variability in prey, indicating physiological effects do not preclude use of these techniques as dietary tracers. Stable isotopes and FA signatures of free-living adults indicated foraging niches changed over the course of the breeding season. Stable isotopes suggest chick-rearing adults and nestlings feed at the same trophic level while FA signatures indicate that parents feed nestlings a diet different from their own. Body mass of adult puffins declined between incubation and chick rearing periods. For females the magnitude of mass decline did not differ between years, whereas for males the decline was greater in the year where young puffins fledged at a lower mass. In a separate analysis, baseline CORT values of adults of both sexes did not differ between years, but were lower than those observed in a separate study area during two consecutive years with low rates of nestling growth and survival. Assuming elevated CORT and reduced body mass impact survival and/or future fecundity, these results suggest the cost of reproduction may be higher for those adults able to fledge young in years characterized by low productivity.