Coyotes ( Canis latrans) are known to consume marine foods, but the importance and persistence of marine subsidies to coyotes is unknown. Recent access to a marine subsidy, especially if gained following apex predator loss, may facilitate coyote expansion along coastal routes and amplify the effects of mesopredator release. Our goal was to quantify and contextualize past and present marine resource use by coyotes on the central coast of California via stable isotope analysis. We measured δ13C and δ15N values in coyotes, their competitors, and their food resources at two modern sites, seven archaeological sites spanning in age from ~3000 to 750 BP, and from historical (AD 1893–1992) coyote and grizzly bear hair and bone sourced from coastal counties. We found evidence for marine resource use by modern coastal California coyotes at one site, Año Nuevo, which hosts a mainland northern elephant seal ( Mirounga angustirostris) breeding colony. Seals and sea lions account for ~20% of Año Nuevo coyote diet throughout the year and this marine subsidy likely positively impacts coyote population size. Isotopic data suggest that neither historic nor prehistoric coyotes consumed marine-derived foods, even at sites near ancient mainland seal rookeries. Marine resource use by some contemporary California coyotes is a novel behavior relative to their recent ancestors. We hypothesize that human alteration of the environment through extirpation of the California grizzly bear and the more recent protection of marine mammals likely enabled this behavioral shift.

Scat is frequently used to study animal diets because it is easy to find and collect, but one concern is that gross fecal analysis (GFA) techniques exaggerate the importance of small-bodied prey to mammalian mesopredator diets. To capitalize on the benefits of scat, we suggest the analysis of scat carbon and nitrogen isotope values (δ13C and δ15N). This technique offers researchers a non-invasive method to gather short-term dietary information. We conducted three interrelated studies to validate the use of isotopic values from coyote scat: 1) we determined tissue-to-tissue apparent C and N isotope enrichment factors (ε13* and ε15*) for coyotes from road kill animals (n = 4); 2) we derived diet-to-scat isotope discrimination factors for coyotes; and 3) we used field collected coyote scats (n = 12) to compare estimates of coyote dietary proportions from stable isotope mixing models with estimates from two GFA techniques. Scat consistently had the lowest δ13C and δ15N values among the tissues sampled. We derived a diet-to-scat Δ13C value of -1.5‰ ± 1.6‰ and Δ15N value of 2.3‰ ± 1.3‰ for coyotes. Coyote scat δ13C and δ15N values adjusted for discrimination consistently plot within the isotopic mixing space created by known dietary items. In comparison with GFA results, we found that mixing model estimates of coyote dietary proportions de-emphasize the importance of small-bodied prey. Coyote scat δ13C and δ15N values therefore offer a relatively quick and non-invasive way to gain accurate dietary information.