Warbler watcher’s week

Sedge Warbler migration is at its peak; large numbers of this intricately-marked warbler are passing through watchpoints on the south coast right now. This was illustrated perfectly at a bird ringing site on the Pett Level in East Sussex this weekend - a team from BTO joined the regular ringers to help get a handle on the huge volume of birds leaving the country at the moment and of more than 2,000 birds caught and ringed, about 25% were Sedge Warblers. The Birdtrack reporting rate shows perfectly how this species is flooding out of the UK.


Other species that were well represented included Willow Warbler and Whitethroat, whilst Sand Martins - our earliest departing member of the swallow family - far outnumbered Swallows at the evening roosts.

Common Terns have become more obvious offshore as they make their way south in migrating flocks. 2,530 were counted past Spurn Point on 16 August. Flocks of migrating terns flying just above the waves determinedly heading south provide one of the greatest spectacles of autumn migration. However, this week the Terns at Spurn were been eclipsed by Swallows- 7,500 were counted heading south over there on the same day. Over the next few weeks the number of these two species should increase as more and more begin their migrations.



The first juvenile cuckoos are also beginning to appear at coastal watchpoints. It is interesting to think that some of these could be the youngsters of the BTO satellite-tagged cuckoos that are being followed to their wintering grounds, which are already south of the Sahara.

Question of the week - What triggers migration?

It is largely recognised that there are two types of migration. Obligate; controlled by genetics, and facultative; controlled by external factors such as local weather conditions. For birds such as Swallows, terns and cuckoos, it is obligate migration that we are interested in.

Change in day length is an important factor in the timing of migration for obligate migrants, and coupled with genetic influence, can give greater year-to-year consistency in the timing of migration in individual species. For example, British Swifts largely tend to leave the country during the first week of August.

Birds are already on the move

For those following our five satellite tagged cuckoos this will come as no surprise, with four of them already south of the Sahara desert. However, lots of other birds are also leaving the UK, probably the most noticeable being the Swift.

All summer they have been screaming around our streets and houses but have disappeared during the last week. Many of these could already be well south of the Sahara and close to their winter quarters.

Willow Warblers have also been flooding out of the country with numbers in excess of one hundred birds being counted at some south coast migration watchpoints; these have often been in the company of smaller numbers of Common Whitethroats.

Another warbler, the Grasshopper Warbler, probably sneaks out of the country largely unnoticed but at one ringing site in Hampshire, 267 have been trapped and ringed during July alone, all but one of them being young birds.

Ospreys are also on the move, and now is a good time to catch up with one of these impressive raptors. Although they often still associate with waterbodies, a bird on active migration could be seen anywhere as it makes it way south. During the last week, Ospreys have been seen in 24 British and Irish counties.

Question of the week.

Why do birds migrate?

This is a very difficult question and to answer this we probably have to look back to the last Ice Age, around 10,000 years ago. At this time, large parts of northern Europe were under ice, which will have retreated north during the summer months only to return during the colder winter months. As the ice retreated north, uncovered habitat will have been exploited by birds from further south that themselves have retreated south as the colder months returned.

As the planet warmed and the southern edge of the ice retreated further and further north, birds will have moved further north during the summer months and flown greater distances back during the winter months; migrating.

Today, the conditions during the winter months are still unsuitable for many of our summer migrants, although we are seeing evidence of more species attempting and succeeding to stay in northern Europe during the winter months. In recent times, several species of warblers, Swallows and a few Turtle Doves have all been recorded in the UK during the winter. These pioneers stay much closer to their breeding area than those that have left the country and, in theory will have first choice of prime territories come the breeding season, ensuring that their offspring get the best start.