Wednesday, 15 October 2008

On the move

When you think of bird migration you tend to think of summer or winter visitors, such as Swallow, House Martin or Redwing; birds that are only with us for a small part of the year. This means that many of the birds that are on the move during autumn or spring go unnoticed, simply because they happen to belong to species that you would normally see throughout the year. For instance, birds as familiar to us as Song Thrush, Chaffinch and Goldfinch all either have a migratory component to their British breeding population or to a population elsewhere within wider European range.

While most of our breeding Song Thrushes and Chaffinches are rather sedentary in their habits, some winter abroad in France, Spain, Ireland or Portugal. At the same time, Britain also receives Song Thrushes and Chaffinches arriving here to overwinter, or passing through en route further south during the autumn months. A visit to the Norfolk coast soon after dawn may well reveal some of these visitors. On Sunday morning, for example, I saw and heard good numbers of Chaffinches and smaller numbers of Song Thrushes passing overhead at Kelling Quags, together with other autumn migrants – including Brambling, Siskin and Meadow Pipit.

Both Meadow Pipit and Chaffinch are diurnal migrants (migrating during the day), and the birds I saw may well have been continuing journeys initiated in Scandinavia over recent days. Some of these birds, notably the Brambling, will have made a direct crossing of the North Sea but others, like the Chaffinch, will have avoided the crossing altogether by taking a longer route around the North Sea coast, through the Low Countries and then making a short hop across the English Channel. Such a convoluted journey will have required some nifty navigational skills along the way, given the change in compass direction needed at different stages of the journey.

Another interesting aspect of Chaffinch migration (at least for the birds leaving the northernmost part of the breeding range) is the difference in timing between males and females, and adults and young. Females Chaffinches tend to move before males (and migrate further) and the same is true of young birds, when compared with adults. In the case of the Song Thrush, individuals from different parts of the breeding range show variation in their movements; those from the northernmost populations tend to move the furthest south, with some birds even reaching North Africa. What amazes me about all these movements is not simply that such small birds can undertake such large movements but how individuals of the same species can behave so differently depending upon where they were born, what sex they are or whether they are a young bird or not.

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