Swimming modes affect dive durations in diving birds

Dive duration and body mass are positively correlated across air-breathing vertebrate species, but other factors affecting the dive duration are inadequately studied. Here, we examined the effect of a previously overlooked factor, swimming mode, using phylogenetically informed statistics with data on the diving capabilities of 53 species of avian divers. We predicted that wing-propelled birds would dive for longer durations than similar-sized foot-propelled birds because of their lower locomotion costs. The maximum dive duration was 1.1 times longer for the wing-propelled birds than for the foot-propelled birds for a given body mass and dive depth. In contrast, the mean dive duration was 1.1 times longer for foot-propelled birds than for wing-propelled birds. Our results indicate that swimming mode may be one of the factors influencing the physiological limits on dive duration. Foraging style, which potentially explains the effect of swimming mode, may also determine the maximum and mean dive duration. For example, most foot-propelled birds are benthic foragers and may routinely dive longer than expected from their higher locomotion costs. In particular, blue-eyed shags have greater diving capabilities than wing-propelled birds with similar body masses, despite being foot-propelled birds. Physiological studies of their unusual diving capabilities would provide important insights into the evolution of diving behavior.