Why do Gull-billed Terns Gelochelidon nilotica feed on fiddler crabs Uca tangeri in Guinea-Bissau?

Wintering Gull-billed Terns Gelochelidon nilotica in Guinea Bissau mainly fed on Fiddler Crabs Uca tangeri and were occasionally seen feeding on fish and locusts. Fiddler crabs have a low energy content, requiring a large gross intake to meet daily energy demands. Fiddler crabs also have a low ratio of digestible flesh to exoskeleton, and therefore tern food intake may be limited by gut capacity. Thus feeding on Fiddler Crabs may pose a problem to the terns. Activity budgets of Gull-billed Terns feeding on Fiddler Crabs show that a considerable part of the time is allocated to quiescence. The duration of resting intervals increased with energy intake and was positively correlated with the metabolisable energy content of the crab eaten, suggesting that resting periods are required for a proper digestion. However, the poor quality of Fiddler Crabs was offset by high capture rates. So daily energy expenditure of the terns could easily be met and feeding on Fiddler Crabs did not pose a problem to reach this goal. Even when resting pauses are included in foraging time, foraging for about 1.5 hours on Fiddler Crabs satisfies the terns’ daily energetic requirements, whereas feeding on energy rich fish would require about 2.5 hours to satisfy daily energetic requirements. Compared to the more specialised piscivorous Little Tern Sterna albifrons and Sandwich Tern S. sandvicensis , capture rate of fish was poor in Gull-billed Terns. From an energetic point of view, wintering Gull-billed Terns feeding on Fiddler Crabs seem to have an easy living in Guinea Bissau