SWALLOWS are still around but most southern migrants have left our shores by now. The wheatears from Iceland, Greenland and Canada, now passing through Ireland on their way to Africa, are ‘the last of the summer wine’.
But do birds depart with the great energy and zeal they displayed when arriving here in the spring? According to a Swedish study, just published, they don’t. Cecilia Nilsson of Lund University claims that migrants arrive full of determination and enthusiasm but delay and dawdle when returning south in the autumn.
The breeding season over, a bird must moult. Then, equipped with a new set of pristine flight feathers, it’s ready to head southwards. Soon, it reaches the coast. Reluctant to venture out over the sea, a migrant will follow the coastline as far as it can. Scandinavian birds converge on the Falsterbo Peninsula at the south-west tip of Sweden, a famous bird-watching site, where the two coastlines of Scandinavia come together. There the travellers wait for favourable weather before crossing to the Danish islands and Germany. Most small birds migrate at night; they are safe from predators then and the daylight hours can be used to lay on extra fat for the journey. Radar allows scientists to ‘see’ flocks in the dark. Nilsson and her team used it to monitor bird movements at Falsterbo over a three year period.
Comparing flight-speeds recorded in different seasons, Nilsson found that birds heading south in autumn moved more slowly than those coming north in spring. Measurements made at other migration sites showed a similar pattern. Birds have urgent priorities when they arrive from the south; each male must find a territory. ‘Possession is nine points of the law’ if you’re a bird; a territory-holder will have the edge on an interloper. The male with a desirable property will be well-fed, rested and strong. A newly-arrived intruder might be tempted to try and evict him but it’s easier for the newcomer, tired and hungry, to move on, seeking a vacant property. Violent confrontation is best avoided.
‘It’s a truth universally acknowledged that a single man in possession of a good fortune must be in need of a wife’. Jane Austen could have been writing about birds. A single male with an up-market territory will do much better in the marriage stakes than a rival with an impoverished one. Breeding successfully and leaving behind viable offspring is what a bird’s life is all about. Being ahead of the posse, therefore, is advantageous for a new arrival in spring.
There are no such imperatives in autumn; a migrant does not benefit by reaching its winter destination ahead of everyone else. When to depart, and how quickly to travel, will depend on local food availability, the vagaries of weather and the direction of the wind. Nilsson discovered, however, that some species dawdle more than others.
Comparing the travel arrangements of birds bound for various destinations, she found differences between the departure speeds of short-distance migrants and those of birds heading to far-away destinations. The speeds of short-haul travellers, relative to the ground, tend to be higher than their speeds relative to the air. This means that they only move when the wind helps carry them along. Waiting a night or two for more favourable conditions won’t compromise short-term migrants’ survival prospects; they can linger on at places such as Falsterbo until the winds favour departure. Like business-class air travellers, they can afford flexible ticketing, changing and postponing flights without penalties.
The air-speeds of long-distance migrants, however, are often greater than their ground speeds. This means that they travel even when conditions are unfavourable; they are prepared to head into the wind. With huge distances to be covered, there is little time for dawdling; a bird facing a huge journey can’t always afford the luxury of waiting for the wind to change.
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