Saturday, 17 September 2011

Tiny passage of globally threatened bird

Recent weeks have seen the now annual handful of records of the globally threatened Aquatic Warbler from sites along the south coast of England. This cracking little bird, which resembles the more familiar Sedge Warbler in its general appearance, is something of an autumn speciality, a rare passage visitor to these shores.

It has been several years since I saw my one and only Aquatic Warbler and I can still picture the bird sitting quietly in the bottom shelf of a mist net (used by licensed ringers to catch birds for ringing) in an East Sussex reedbed and the look of joy on my colleague’s face as he reached to extract it from the net. The entire world population of this warbler, estimated at just 18,000 pairs, breeds in central and Eastern Europe, favouring extensive areas of river valley marshland, sedge bed and damp hay meadow. Drainage and agricultural improvement have led to a significant decline in the breeding population, which is why it has been given such a high profile conservation status.

At the end of the breeding season Aquatic Warblers migrate southwest across Europe, heading for wintering grounds situated in West Africa, south of the Sahara. The precise wintering grounds are unknown and more work is needed to work out where these are in order to determine if conditions there are deteriorating for the birds. If, during the early stages of the autumn migration, the birds encounter winds from the east, then they may be displaced across the English Channel to reach sites spread along the south coast of England. In exceptional autumns they may be found further north, with several Norfolk records and birds reaching as far north as Northumberland. There are even a few spring records, though these are even less common than autumn ones.

One of the things that makes the Aquatic Warbler such an interesting bird, aside from its rarity here, is that the male Aquatic Warbler has taken promiscuity to unprecedented levels! Females tend to be rather elusive, living skulking lives within the complex structure of vegetation in which they make their home. When a male encounters a female he copulates with her, remaining in copula for up to thirty minutes; in most birds, by contrast, copulation lasts just a few seconds. The male does this to prevent other males from mating with the female at the critical time and to introduce sufficient sperm to swamp those of any previous matings. In order to deliver this quantity of sperm, the male Aquatic Warbler is particularly well-endowed. Having mated successfully he will then seek out other mates, leaving the female to rear the young all on her own.

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