Temperature Regulation in Great Frigate Birds Fregata minor

Abstract

Great frigate birds are large black tropical seabirds. Those breeding on Midway Atoll absorb more than 90% of the incoming solar radiation and yet can remain on their nest exposed to intense solar radiation for up to 16 days. Despite experiencing air temperatures as high as 33.3 C and environmental temperatures ($T_{e}'s$, an index of total heat load) up to 51.7 C, the birds maintained body temperature within relatively narrow limits, averaging 39.3 ± 0.5 C. They reduce overheating by using gular fluttering and by assuming three distinct postures, which were used sequentially as $T_{e}$ increased. The most common posture was erected feathers, followed by open wing and droop wing. Despite sexual dimorphism in size and plumage, there were no sexual differences in posturing, dorsal absorptivity to solar radiation, or response to $T_{e}$. The postures evidently contribute to water conservation in the hot breeding habitat of frigate birds, as the birds appear to have low rates of water loss.