Tropical Seabird Response to Record Ocean Heat at Culebra, Puerto Rico

Record ocean heat occurred near Culebra, Puerto Rico in June 2024. Shifts in consumption patterns and condition were observed during and after the Marine Heat Wave (MHW). Nesting seabirds had lower δ¹⁵N in red blood cells (RBC), suggesting a shift towards lower trophic levels. A reduction in sardines may have contributed to reduced body condition in white-tailed tropicbirds (Phaethon lepturus) and brown noddies (Anous stolidus), and breeding phenology was markedly later for noddies in 2024. Primary feathers replaced prior to the MHW were higher in δ¹³C and δ¹⁵N compared to red blood cells (rbc) from sooty terns (Onychoprion fuscatus) sampled in June 2024, and differences were significant even after accounting for differences in tissue fractionation. Elevated ocean heat probably contributed to a sharp increase in sargassum in 2025, and this represented a secondary advected food web. In 2025, sooty terns had lower δ¹³C in rbc and breast feathers compared to 2024, and this mirrored lower δ¹³C in sargassum associated prey. During 2024 and 2025 δ¹⁵N was negatively correlated with δ¹³C in sooty tern breast feathers (-.82, -.72) and rbc (-.77, -.58) in 2024. This pattern may be related to the occurrence of small fish fry in sooty tern diets. Sooty terns benefit from facilitated foraging, but this may have occurred less if predatory fish were displaced by ocean heat. Total antioxidant capacity was higher in 2024, and oxidative stress (TBARS) was lower, while the inverse pattern was manifest in sooty terns in 2025. Baseline corticosterone was relatively high in both years. In breast feathers, which are replaced between breeding seasons, corticosterone was higher in 2025 for sooty terns. In conclusion, we found that the MHW altered marine food webs, and we hypothesize that the foraging niches of seabirds shifted away from optimums.