Evening locomotor activity during stopover differs on pre-departure and departure days in free-living songbirds

The length of time songbirds remain at a migratory stopover site is likely regulated by a daily stay/go decision informed by fat stores and weather conditions, but the finer-scale timing of this decision and associated pre-departure behaviours are still poorly understood. Using automated radiotelemetry of free-living songbirds captured at a migratory stopover site in spring, we tested whether individuals change their locomotor activity near sunset on their migratory departure day compared to their non-departure days. To do so, we extracted precise transition times between diurnal activity and nocturnal inactivity, which always precedes departure, using changepoint analysis of radio transmission signal strength. Among four warbler species, individuals extended diurnal activity by 8-19 minutes towards sunset on their departure day. In three species, this extension was significant. In contrast, white-throated sparrows significantly shortened diurnal activity on their departure day by 13 min, also towards sunset. This is the first study to detect and quantify a change in locomotor activity schedule on departure versus non-departure days in free-living songbirds, and is consistent with the hypothesis that birds engage in pre-departure preparatory behaviours near sunset.