An allometric approach to seasonal changes in intake rates of migratory shorebirds in the Bijagós Archipelago, Guinea-Bissau

Abstract

Shorebirds migrating along the East Atlantic Flyway must travel long distances from their breeding sites in arctic and subarctic regions to wintering areas in Europe and Africa. Selecting a winter location is an important decision, as it can have both immediate and future consequences. Shorebirds must ensure they have enough resources to fulfil their energetic requirements during winter, as well as during the fuelling phase, when they must accumulate fat stores for the return migration. Migratory shorebirds are declining worldwide, with several populations wintering in tropical Africa facing steep declines, particularly when compared with their conspecifics wintering in temperate Europe. However, one of the most important wintering sites in Africa remains relatively unstudied, the Bijagós Archipelago, Guinea-Bissau. In this study, we quantify intake rates of eight migratory shorebird species wintering in the Bijagós and explore how these vary throughout their stay in the archipelago. Given the uncertainties associated with measurements of field intake rates, an allometric validation of our approach confirmed a slope comparable to that predicted by theory and verified in many situations. Contrary to our expectations, we found no evidence for an increase in intake rates in the fuelling period; in fact, decreases were noticeable in a few species over the non-breeding season. The allometric relationship across species also confirmed the seasonally decreasing daily energetic intakes, particularly by smaller species. The comparison between the intake rates in the Bijagós and those reported in other wintering sites along the flyway highlights the particularly low values for Ringed Plover Charadrius hiaticula, Curlew Sandpiper Calidris ferruginea and Bar-tailed Godwit Limosa lapponica. Limitations in intake rates and the associated energetic intake suggest that some species may be energetically constrained in the Bijagós, particularly during fuelling.