Assessment of Trophic Positions for the Seabirds of South Florida using C and N Stable Isotopes

Seabirds outside the NE Pacific and NE Atlantic regions are poorly understood from a trophic perspective. To elucidate trophic level relationships, we examined the concentration of ∂ ¹³ C and ∂ ¹⁵ N in the blood, breast muscle, and feathers of nine species of adult marine-associated birds common in South Florida: osprey Pandion haliaetus , black skimmer Rynchops niger , brown pelican Pelecanus occidentalis , northern gannet Morus bassanus , double-crested cormorant Phalacrocorax auritus , royal tern Thalasseus maximus , herring gull Larus argentatus , laughing gull Leucophaeus atricilla , and ring-billed gull Larus delawarensis . We collected specimens from four wildlife rehabilitation centers after death; three centers were in mainland-coastal areas, while one was coastal-island. Calculated mean trophic levels (TLs) ranged from 5.15 (osprey and northern gannet) to 3.64 (ring-billed gull), findings consistent with existing diet information, and all species exhibited expected ∂ ¹³ C:∂ ¹⁵ N ratios. Herring and laughing gulls (TL 4.59 and 3.90, respectively) appear to have a straight trophic enrichment based on coastal POM. Northern gannets (TL 5.15) are likely consuming a limited diet in offshore areas consistent with POM-based enrichment patterns and feeding strategies. Brown pelicans, osprey, and royal terns (TLs 4.78, 5.15, and 4.11, respectively) overlap diets when in South Florida, likely reflecting feeding areas; consistent with field observations, when onshore winds are strong, these species feed inshore on carbon-enriched prey, but when winds are weak, they feed near/offshore on carbon-depleted prey, such as lipid-rich fishes. However, double-crested cormorants (TLs 3.40 to 5.59) appear to have an unusually bifurcated ∂ ¹³ C and ∂ ¹⁵ N signal between specimens, apparently representing the two habitats inhabited by individual birds in South Florida: one along the coastal/nearshore ecotone and one along the inland/coastal ecotone.