We experimentally investigated the relative role of kinship and density on juvenile dispersal in the common lizard. A few days after birth, juveniles were introduced into seminatural enclosures, where they experienced different social environments: in the first experiment we varied the density of unrelated adults (males or females) within the enclosure (0, 1, or 2 adults), and in the second experiment, we varied the level of kinship and familiarity between juveniles and adults. Each enclosure was connected to a second enclosure by small holes which allowed only juveniles to move between enclosures. Juvenile movements were monitored during 14 days after birth, as juvenile dispersal is mainly completed within 10 days after birth under natural conditions. Most juveniles did not return to the first enclosure. Sex had no effect on juvenile dispersal. Adult density and kinship with adults both affected dispersal. Adult female density increased juvenile dispersal whatever the level of kinship and familiarity with the females. Dispersers had better body condition than nondispersers at high female density, and this difference was significantly greater when the mother and the familiar female were present in the enclosure. Furthermore, body condition of mothers and familiar females was positively correlated with juvenile dispersal, whereas there was no such correlation in the case of unfamiliar and unrelated females. These results strongly suggest that adult female density is a major factor promoting dispersal in this species and that both intraspecific and kin competition motivate dispersal.

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.

No abstract

On the journey to wintering sites, most migratory birds alternate between flights and stopovers, where they rest and refuel. In contrast to the time-minimization strategy commonly assumed to drive the pre-breeding migration, birds are rather expected to follow an energy minimization during post-breeding migration. It is the cumulative duration of flights and stopovers that determines the total energy requirements and duration of the journey. Since migrating birds actually spend most of the time at stopovers sites, selection to minimize the amount of energy or time spent on migration is likely to operate on the effectiveness of stopover rest and refueling. Here, we address the relative contribution of factors acting on departure decisions from a stopover site during the post-breeding migration in a long-distance migratory songbird. When capture probability is low, it is impossible to measure fattening over the entire duration of the stopover. To get around this limitation, we use time since arrival (TSA) as a proxy for the progressive temporal change occurring in the internal state of an individual (i.e. rest, physiological recovery, and fuel loading) during the stopover. We develop a capture–recapture model to address the respective effects of estimated TSA and of weather conditions on departure probability. Using a 20-year dataset for Sedge Warblers (Acrocephalus schoenobaenus), we show that TSA served as a surrogate of the most important information that birds use when deciding to depart from a stopover site, while low humidity and rising atmospheric pressure only slightly increase daily departure probability. Hence, a bird would resume migration mainly according to the time it had to rest and refuel, and then fine-tuning departure decision according to weather conditions. The generality of these results needs to be assessed by applying this modeling framework to other migratory species and at sites or times with greater weather variability.