Stopover behaviour and departure decision of northern wheatears, Oenanthe oenanthe, facing different onward non-stop flight distances

On the small North Sea island Helgoland (54 degrees 11' N, 07 degrees 55' E ) we studied the stopover ecology of two subspecies of northern wheatear, O enanthe oenanthe, during spring migration. Birds heading for Scandinavia (O . o. oenanthe) face only short flights across an ecological barrier (50-500 km) whereas those originating from Greenland and Iceland (O. o. leucorhoa) have to cover between 1,000 and 2,500 km in the impending flight. Colour-r inged individuals showed that 90% of Scandinavian birds left on the day of ringing while 40% (males) and 30% (females) of Greenland/Icelandic birds st ayed at least I night. The birds who remained were thus mostly O. o. leucor hoa. They often established desirable feeding territories on the beach and had a high rate of body mass increase (1.7 g/day). However, subspecies did not differ in habitat choice and in foraging effort, but O. o. leucorhoa ha d a higher success rate in pecking. Departure decisions were analysed by co mparing (a) conditions on the day of ringing between departing and staying birds and (b) for birds staying between the day of departure and the preced ing day. The factors that were probably important in the decision to depart differed between subspecies. In O. o. leucorhoa, few birds departed with b ad or deteriorating weather conditions (tailwind component, cloud cover), w hereas departures of O. o. oenanthe seemed to be little affected by those f actors. A few O. o. oenanthe stayed early in the spring migration season an d/or had low fat reserves. Interference during foraging seemed to play a ro le because both subspecies tended to leave when the densities of northern w heatears were high. Other factors related to refuelling conditions (food su pply, foraging effort, predation risk) failed to show differences between s taying and departing individuals. In summary, almost all Scandinavian birds departed quickly and irrespective of refuelling and weather conditions, wh ereas many (but not all) Greenland/Icelandic birds seemed to prepare for a long-distance flight and carefully adjusted departure to weather conditions . The observed differences in stopover behaviour and departure decisions in the two subspecies of northern wheatear indicate that the distance to the next stopover site or to the goal area has to be considered when applying o ptimal migration models.