Abstract
On their journey to wintering or breeding sites, migratory birds usually alternate between migratory flights and stopovers where they rest and refuel. Actually, migrating birds spend most of the time at stopovers. Consequently, selection to minimize total time spent on migration likely operates mainly on the effectiveness of stopover rest and refueling. Departure probability from stopover sites depends both on weather conditions and fuel stores, but their respective role has not been quantified. In the present study, we assess the relative contribution of factors driving the departure decision from a stopover site. As we cannot reliably characterize body condition and restness when capture probability is low, we propose to use the Time Since Arrival (TSA) as a proxy of the changes through days of the internal state of stopovering birds. We developed a specific capture-recapture model to quantify the relative contribution of TSA and climatic conditions on a 20-year capture-recapture dataset of a long-distance migratory songbird (Sedge warbler). The effect of TSA has yet the major contribution to departure probability compared to weather conditions. Here, low humidity and an increase of atmospheric pressure in the days preceding departure are the weather conditions associated with a higher departure probability but remain secondary compared to the time the individual has already spent at the site. The probability to depart from a stopover site is therefore largely determined by the time that a bird has already spent at the site. Whether this Time Since Arrival is rather a proxy of resting, feeding or fattening efficiency remains to be explored.