Friday, 19 August 2011

Swallows fatten up for the journey ahead

At this time of the year, many small birds will be getting ready to migrate south for the winter. Preparation is largely centred on the need to build up sufficient fat reserves to fuel the journey ahead, the amount of fat deposited varying in relation to the length of the distance to be covered. Thanks to the efforts of bird ringers we know a lot about the different strategies adopted by migrating birds. Some, for example, complete their journey in a single hop, while others break it up into a series of smaller flights each broken by a stopover to take on fuel. Many small birds will, for example, stop before crossing the Mediterranean, fattening up on its northern shores.

Now you might think that Swallows, being aerial feeders, would not need to fatten up. They can, after all, feed as they go, snacking on the aerial plankton of flies and other invertebrates with which they share the sky.  It seems, however, that this is not the case and that our Swallows also fatten up ahead of their journey. The latest research on this subject is based on some work with which I, as a bird ringer, was involved a few summers ago – you may even remember me discussing it in one of my previous columns. We’d spent a run of summer evenings netting Swallows at South Lopham, as the birds came into roost each evening in the reed beds there. It was a wonderful way to spend the evening, hearing the birds approach and then watching them drop down to roost. There was even the occasional sighting of a Hobby, it too fattening up ahead of its migration – though this time fattening up on the Swallows and martins! We were not working alone and it is only now, seeing the report in which the research was published, that you realise just how many other bird ringers were spending their summer evenings in a similar fashion, at other reed beds up and down the country.

As well as ringing the birds, we also took various measurements, some of which revealed the quantity of fat reserves being laid down. The results show that our Swallows fatten up in southern Britain, taking on sufficient reserves to allow them to travel to the Continent where, according to the work of colleagues overseas, they then take on more reserves in preparation for crossing the Mediterranean and the Sahara of northern Africa. It is amazing to think that our efforts have contributed, in some small way, to increasing our understanding of how these wonderful birds migrate, and the strategies that they use to undertake such an amazing journey to their wintering grounds in South Africa.

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