Understand how birds get ready to migrate

BBC Wildlife features editor Ben Hoare explains how birds get ready to migrate. 

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Willow warbler
Illustrations by Mike Langman

 

Before setting off, migratory birds prepare intensively for several weeks. Now is a great time to spot the changes in their behaviour and appearance, because we’re in the middle of the autumn migration season that lasts from late July to November.

The first easily observable change is moult. Every species has its own annual moulting cycle, but in willow warblers, whitethroats and many other birds the adults replace worn feathers in summer after the exertions of breeding. Young birds also moult, obtaining their first set of adult feathers, so in late summer and autumn they often look different to their parents (the three birds shown below are all juveniles).

Another change you may notice is foraging behaviour. Birds feed more frequently as migration nears, and switch to high-energy foods. Many insect-eating warblers start eating fruit, packed with carbohydrates and fat, while sedge warblers target aphids, which are rich in fat and protein. Plumage hides this weight gain, but researchers gently blow on the feathers of mist-netted birds to check how much subcutaneous fat is lining the flanks and base of the neck.

Whitethroat

In autumn whitethroats gorge on blackberries and elderberries. The juveniles have extensive rufous in their wings, and most look brighter than the adult birds.

Sedge warbler

Masses of aphids are devoured by sedge warblers before they migrate. Juvenile birds have freckle-like spots on the breast.

Willow warbler

Juvenile willow warblers are yellower than adults. This species migrates in August; the similar-looking chiffchaff flies south in September or October.

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