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Food and the final push
Last modified: 11 January 2005
The fat forms a layer beneath its skin, which is converted into energy as the bird flies. Smaller species can gain 3-4% of their body weight a day during this time; a sedge warbler almost doubles its weight from 10 g to 20 g in just three weeks. This extra load gives it enough energy to fly all the way to Africa.
Once the migrants are loaded with food and ready to go, they wait for the weather to fire the starting gun. A spell of calm weather with clear skies is usually the signal. This weather is caused by high atmospheric pressure (an anticyclone), and is typical for the UK during late summer and autumn. Low atmospheric pressure (a depression) causes wind, cloud and rain. This is bad for migration, so birds sit tight until it has passed.
Many birds start their southerly migration in short feeding hops, moving to reedbeds and other favoured areas on the south coast. As they fatten up, bird ringers record the amount of fat birds are carrying before their migration. Depending on the summer, birds can find plenty or very little food at this time, so the ringers' activity helps to show how healthy the countryside is for birds.
Last modified: 11 January 2005