Human-Induced Long-Term Shifts in Gull Diet from Marine to Terrestrial Sources in North America's Coastal Pacific: More Evidence from More Isotopes (δ2H, δ34S)

Measurements of naturally occurring stable isotopes in tissues of seabirds and their prey are a powerful tool for investigating long-term changes in marine foodwebs. Recent isotopic (delta N-15, delta C-13) evidence from feathers of Glaucous-winged Gulls (Larus glaucescens) has shown that over the last 150 years, this species shifted from a midtrophic marine diet to one including lower trophic marine prey and/or more terrestrial or freshwater foods. However, long-term isotopic patterns of delta N-15 and delta C-13 cannot distinguish between the relative importance of lower trophic-level marine foods and terrestrial sources. We examined 48 feather stable-hydrogen (delta H-2) and sulfur (delta S-34) isotope values from this same 150-year feather set and found additional isotopic evidence supporting the hypothesis that gulls shifted to terrestrial and/or freshwater prey. Mean feather delta H-2 and delta S-34 values (+/- SD) declined from the earliest period (1860-1915; n = 12) from -2.5 +/- 21.4 parts per thousand and 18.9 +/- 2.7 parts per thousand, respectively, to -35.5 +/- 15.5 parts per thousand and 14.8 +/- 2.4 parts per thousand, respectively, for the period 1980-2009 (n = 12). We estimated a shift of similar to 30% increase in dependence on terrestrial/freshwater sources. These results are consistent with the hypothesis that gulls increased terrestrial food inputs in response to declining forage fish availability.