Low Initial Refueling Rate at Stopover Sites: A Methodological Effect?

For various reasons, migrating birds may not refuel and gain mass immediately after they have arrived at a stopover site. That led to the concept of a search-settling time after arrival with a low or negative initial refueling rate, but its existence has not been clearly demonstrated in field studies. Body-mass changes resulting from capture–recapture data can be misleading if used for the estimation of a natural low initial refueling rate because (1) it is usually unknown whether the day of first capture is also the first day of stopover, and (2) handling at first capture may have an adverse effect on subsequent body-mass development. To circumvent those problems we increased probability of catching birds shortly after arrival by inducing landfall by tape-luring, and we estimated body-mass change without previous handling effects from concentration of two metabolites in blood plasma. In the Eurasian Reed-Warbler (Acrocephalus scirpaceus) studied at a stopover site in Switzerland, there was no difference in plasma-metabolite concentrations between a group of mostly newly arrived individuals and a group with few newly arrived birds. Similarly, there was no difference in those parameters between birds that had been handled before and birds at first capture. However, the analysis of capture–recapture data from two other Swiss stopover sites with longer handling times indicated that mean body mass of Eurasian Reed-Warblers and European Robins (Erithacus rubecula) dropped after capture and reached initial values only after one to several days. We concluded that mass loss after capture depended mainly on lost foraging time and that natural low initial refueling rate after arrival at a stopover site is not detectable under the conditions of this study.