Over the past few weeks I have spent a number of very pleasant evenings out on the fen. Working with a team of bird ringers, I have been involved in an important international project tracking the autumn swallow migration. The British Trust for Ornithology’s ‘Swallow Roost Project’ sets out to assess the pre-migratory strategies of different populations of swallows by collaborating with teams of bird ringers in other parts of Europe. Our own efforts on the fen over the last few years have varied in their success. On some evenings the swallows play ball and fill our nets. On other evenings, the nets remain empty.
The swallows come into roost just before dusk, favouring the edge of a reedbed where, with water beneath them, they feel safe from predators. Small parties of swallows, moving through the river valley, are drawn down to the reedbed by the swallow calls emanating from our tape-lures but it requires knowledge of where they prefer to roost to make sure that we place our nets correctly. If we get it right then the birds are caught in the nets as they pass low over the reeds. Once all the birds have been safely removed from the nets, we begin the process of ringing the birds and taking the all-important measurements that will help us establish how these long-distance travels prepare for their epic journey. After we have processed each swallow it is placed in a roosting box where it will spend the night, before being released early the next morning. Preliminary examination of the information that has been collected suggests that British swallows, in contrast to many other small migratory birds, do not put on much weight before they depart our shores. Instead, they appear to fatten up on the Continent before making the journey across the Mediterranean, into Africa and across the great expanse of the Sahara.
Sometimes our efforts do not succeed because the swallows choose to roost in a different part of the reedbed. On other occasions, the presence of a hobby may frighten the swallows off, such that they choose to roost somewhere else entirely. The hobby is a superbly adapted bird of prey, with an explosive burst of speed and well-able to catch a swallow. As the season progresses, hobbies seem to appear more frequently and the number of swallows using the roost becomes more erratic. Just last week we had at least two hobbies over the fen and the 60 or so swallows remained high, refusing to venture down into the reedbed roost. With the hobbies present the swallows eventually moved off elsewhere. Even on those evenings when we fail in our task, there is always plenty to see, making the effort worthwhile.