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Cetti's WarblerThe Yare Valley colony of Cetti's warblers, centred on the Royal Society for the Protection of Birds' Strumpshaw and Surlingham reserves, attracts much interest. The bird is a notorious skulker, tending to show itself only momentarily, and is as difficult to see well as a nightingale. Fortunately, Cetti's warbler balances its skulking habits with what is surely a most astonishing song - at least in loudness - among small European birds. From the depths of a thick patch of cover, there is quite literally an explosion of clear, penetrating notes. Each performance lasts a few seconds, ceases as abruptly as it begins and may not be repeated for several minutes. In addition to singing in spring from dawn to dusk, ever-industrious male Cetti's also perform at night, possibly advertising vacant space in their territories for more hens. Nocturnal song may be heard almost continuously from the early hours until shortly after dawn, recommencing, in the breeding season, 20 minutes later by the start of the day-song. Many observers have heard Cetti's but seldom actually seen them. When the population is high, Cetti's may occupy young plantations and hedge-ringed cornfields well away from water. But this warbler is usually associated with tangled vegetation near water or with reedbeds containing alders and an undergrowth including bramble, nettle and willowherb. On occasions, when watching from a Strumpshaw hide, I have glimpsed Cetti's creeping mouse-like on the ground, continually flicking the tail downwards. Named after an 18th century Italian zoologist, the resident Cetti's warbler is badly hit in severe winters. A combination of heavy snow and low temperatures has interrupted the spread of European population more than once. Perhaps surprisingly, at the turn of the century Cetti's was almost confined in Europe to the Mediterranean. Yet this ever-restless and highly secretive warbler has made rapid northward advances through France, reaching the northern coast of Brittany and the Channel Isles in 1960. Then the species suffered a population crash following the severe 1986/87 winter, almost disappearing from the Camargue. England (Hampshire and Sussex) and Germany were both reached in 1961, Belgium in 1962 and Holland in 1968. By Michael J Seago
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