Poor ocean conditions reduce chick provisioning in a threatened seabird

Abstract Determining variation in behavioral traits is critical for predicting how animals cope with anthropogenic climate change, including those that buffer reproductive output against changing environmental conditions. We investigated how contrasting ocean conditions influenced parental care in the Marbled Murrelet (Brachyramphus marmoratus), a secretive seabird that nests in coastal coniferous forests along the northeastern Pacific Ocean. Using >12,000 hr of video recordings, we quantified chick provisioning at 18 nests that represented 1,332 feeding events across 2 yr of poor (2018, 2019) and 2 yr of more favorable ocean conditions (2021, 2022). Adults provisioned chicks with a single forage fish during nest visits, which occurred primarily during early morning and late evening. Chick diet was consistent across ocean conditions: Pacific sand lance (Ammodytes personatus) and species in the smelt family (Osmeridae) comprised >90% of identified prey, with Pacific herring (Clupea pallasii) and Northern anchovy (Engraulis mordax) also present. Chicks received significantly fewer fish per day during poor ocean conditions (2.0 fish day−1; 95% CI: 1.6, 2.4) relative to more favorable ocean conditions (2.7 fish day−1; 95% CI: 2.4, 3.2). Fewer fish per day were also delivered as nest distance from the coast increased, independent of ocean conditions. However, estimated daily energy intake of chicks was slightly greater during more favorable ocean conditions, likely because Pacific sand lance delivered to chicks during poor ocean conditions were larger and estimated to have a greater energy density. Despite this partial compensation during poor ocean conditions, chicks took 25% longer (8 days) to fledge, increasing their risk of being depredated. Our study indicates that successful murrelet reproduction is dependent on a small number of prey species and that adults did not seek alternative prey to buffer the impacts of poor ocean conditions when rearing offspring. Therefore, climate-driven changes in prey availability are an important limitation on murrelet reproductive output.