Timing of attachment to wintering site as revealed by experimental displacements of Dunlins (Calidris alpina)

Two groups of adult and two groups of juvenile colour-marked Dunlins (Calid ris alpina) were moved from their wintering site in four successive years, in order to assess the existence of time- and age-dependent differences in homing behaviour and, in particular, to determine when attachment to winter ing grounds is achieved by juveniles. Each group was released on different dates at the beginning of the wintering season 133 km to the south of the c apture site. We expected that the behaviour of adults would not vary accord ing to date, whereas that of juveniles would. In ail four experiments the m ajority of birds left the release site. Juveniles displaced early in the se ason (5 Nov.) remained at the release site in higher numbers (34% vs 0-14%) and homed to a lesser extent (20% vs 57-62%) than juveniles displaced late in the season (13 Dec.) and both adult groups. The final return rates of a dults displaced early and late in the season (8 and 24 Nov.) were Similar. Most birds that left the release site departed within 20 days of displaceme nt, whereas observations of colour-marked Dunlins at the capture site indic ated a more gradual pattern of return. The first homed birds were observed three days after release, but new birds continued arriving back at the rele ase site for at least 40 days longer. Most juvenile Dunlins seem to become attached to a wintering site during November. Recovery rates of displaced a nd non-displaced birds retrapped in following winters suggested that year-t o-year site fidelity is developed after a longer period.