A Taste for Squid: Red-Billed Tropicbird (Phaethon aethereus) Diet in the South Atlantic

Examining prey choice is important for understanding the drivers of survival, breeding success and population change of tropical seabirds. We analysed spontaneous regurgitations from Red-billed Tropicbirds (Phaethon aethereus), at St Helena, South Atlantic between 2013 to 2018. Squid from four families, eight fish families, and one octopus species were identified. Dietary composition and diversity revealed Red-billed Tropicbirds fed predominantly on epipelagic juvenile squid, especially neon flying squid (Ommastrephes cylindraceus). Commercially exploited fish and species commonly used as bait and/or discarded, had a low frequency of occurrence (8.8%) and by number (3.5%) in regurgitations, indicating tropicbirds may have a low risk of mortality by bycatch. The consumption and size of squid varied among years; 2017 had significantly smaller squid consumed in greater proportions than in the other years observed. Chicks had larger meals (by ∼41 g ± 39 g SD) and higher diversity of prey compared to adults, suggesting adults favoured selecting higher quality, heavier prey as a provisioning strategy for their chicks. The high frequency of occurrence of juvenile squid, and reliance on neon flying squid, suggests future dietary measures may provide valuable information about juvenile squid abundance and development, before they are mature and become commercially important to fisheries.