Abstract
Trophic habits are a central component of a species' life history with important conservation implications. Evaluating variation in trophic habits across ecological contexts provides key insights into species plasticity and conservation management, particularly when predator-prey interactions involve invasive species. The Striated Caracara (Phalcoboenus australis), endemic to islands in southern Patagonia, is a raptor heavily associated with seabird colonies during the breeding season, where it obtains abundant food resources. Generally, seabird availability is positively associated with Striated Caracara breeding density and success, but this is mostly based on populations that rely exclusively on native prey. To evaluate variation in trophic habits among sites, we examined the trophic ecology of Striated Caracaras co-occurring with exotic prey during the breeding season at Observatorio Island (Argentina) and compared it with a nearby breeding site of similar density—Franklin Bay—where caracaras feed exclusively on native prey despite the presence of invasive species. To characterize caracara's diet, we combined pellet and stable isotope analysis. Native prey (i.e., seabirds) were the main food item for caracaras at Observatorio Island; however, invasive prey remains—European Rabbit (Oryctolagus cuniculus) and Black Rat (Rattus rattus)—were detected in at least one pellet in 69% (95% CI 42%–88%) of the nests (pellets = 102; nests = 13). The isotopic niche of chicks from Observatorio Island was four times larger (mode: 4.22‰2, 95% CI 2.41‰2–7.28‰2) than those from Franklin Bay even in its most variable year (mode: 0.81‰2, 95% CI 0.38‰2–1.69‰2), consistent with the incorporation of prey with differing isotopic signatures. At Observatorio Island, invasive prey consumption was relatively spread across breeding pairs (rabbits: 61%, 95% CI 35%–83%; rats: 38%, 95% CI 16%–65%), in contrast with Franklin Bay, where the isotopic niche was smaller and no invasive prey were recorded. Our study highlights contrasting patterns of resource use between sites and underscores the importance of understanding the role of invasive species in the diet of this species in a site-specific framework to inform conservation strategies, especially when considering eradication efforts and their potential ecological impacts.