Conference 94 Breeding Birds of East Anglia

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SUFFOLK NATURALISTS' SOCIETY CONFERENCE, 29TH OCTOBER 1994 BREEDING BIRDS OF EAST ANGLIA The Annual Conference was held on Saturday 29th October 1994, at Ipswich School Conference Centre. The conference was sponsored by Anglian Water, together with the British Trust for Ornithology, English Nature, Forest Enterprise, Ipswich Borough Council and Suffolk Wildlife Trust. A synopsis of the papers presented at the conference is given below.

DIGGING FOR BITTERNS - RE-SHAPING MINSMERE'S SCRAPE Geoff Welch As a British breeding bird, the Bittern has had a very chequered history. In the 17th century its range extended throughout England, Wales and southern Scotland and at this time shooting parties in the Fens would bag 20-30 birds in a morning and roast Bittern was the Fenman's traditional Sunday lunch! However, persecution and habitat loss through drainage reduced numbers and by the mid-1800s Bitterns were restricted to the Norfolk Broads. The last nest was found in 1868 and the last young bird killed in 1886. Birds continued to be recorded as passage or winter visitors but breeding was not proved again until 1911, at Hickling Broad. Thanks to enthusiastic protection, numbers began to increase, to reach a peak of around 70 pairs by the late 1960s, but since then there has been a steady decline to just 16 booming males in 1994. Research The RSPB has been aware of, and concerned by, this decline for some time, and in the early 1980s it carried out a great deal of work on its reedbed reserves, opening up ditch systems to provide more feeding edge. However, it was not until 1988 that a detailed investigation into the species' ecology and habitat requirements was started. One of the greatest problems proved to be how to study a bird which is hardly ever seen! This was tackled in two ways — radio tracking and 'voice fingerprinting'. By fitting small radio transmitters to the legs of three birds at the RSPB's Leighton Moss reserve in Lancashire, it was shown that male Bitterns move around a reedbed far more than was originally thought, and may have several booming sites within their territory. The latter fact is very significant, as in the past Bitterns were censused by mapping boom sites — but i f males are highly mobile there is a real risk of over-estimating the number of males present. Radio tracking also revealed that rather than feeding along ditch edges, Bitterns frequently feed a few metres in from the ditch, in the reedbed. But this is only possible when water levels in the ditches and reedbed are high enough for fish, eels and frogs to swim out of the ditches and through the reeds where the Bitterns can catch them. Basically, Bitterns need really wet reedbeds! Linking in with the radio tracking, the RSPB, working closely with Nottingham University, has found that the boom of each male is individually recognisable i f good quality tape recordings are used to produce a visual representation of the boom, known as a sonogram. Given recordings of all the booming birds, an accurate assessment of the number of males can be made and an annual census of booming males by this technique now forms a major part of the ongoing study of the species. What is not yet clear is whether all males keep the same boom pattern from year to 7


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