Basin Notes - August 2011

Migration Begins

There is depressing news for those who are still hoping that we might have a hint of summer. The autumn migration of birds is already under way.

The majority of swifts have already departed, heading for their winter quarters in Mozambique, Zimbabwe and the Republic of South Africa. The swift spends almost its entire year airborne, feeding, drinking and roosting on the wing. The only time it is not in the air is during the short nesting season.

Juvenile swifts who first took to the wing a few weeks ago will have commenced their migration almost immediately, often flying south before the adult birds start their journey. Adults and juveniles alike will remain in flight until at least next summer when those who successfully mate will often return to their previous nest sites.

For the next few weeks it is still possible to see large flocks of swifts, but since our local birds will already have departed, any such flocks are likely to be passage birds which have nested elsewhere.

Sand martins and house martins are already on the move but they will continue to be seen fairly regularly until mid-September. Most of the swallows will be with us until the end of September, and during August and September they will form large communal overnight roosts, especially in reed beds.

The reed beds at the west end of The Basin frequently hold several thousand swallows overnight, and in some larger reed beds, roosts of over 100,000 birds have been recorded.

No sooner have the birds departed than we begin to speculate on when they might return. Global temperature has increased rapidly since the 1970s and this has had an impact on the dates when birds undertake their northern migration and when resident birds start nesting.

Across the UK swallows are now arriving about 20 days earlier than they did in the 1970s with an even more marked change of 25 days for sand martins. It is a worldwide issue, with about half of the birds studied across the globe showing a statistically significant trend to earlier nesting.

The concern expressed in many of the studies is that although the timing of many biological events is occurring earlier in the year, different organisms have responded at different speeds. For example, although pied flycatchers in mainland Europe have begun to nest earlier, their main caterpillar prey is emerging earlier still, and is therefore not available for feeding chicks. This is having a major impact on breeding success.

The study of the timing of biological events is known as phenology and it is clear that we are going through a period of very substantial change. The effects of some of these changes are causing concern and we are likely to hear much more on the topic of phenological mismatch in future years.