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A Lost Generation of Birds

News from the BTO
24th April 1997

Most bird watchers realised that 1996 was an appalling breeding season. Just how bad it was is only now apparent following the publication to-day of the first scientific appraisal of the situation last year. In BTO News the results from the Constant Effort Sites scheme, run by bird ringers of the British Trust for Ornithology, reveal the depth of the catastrophe for 24 common British Species.

This survey has been run every year since 1983. It provides information about many aspects of our common breeding birds. The report by Dawn Balmer and Will Peach concentrates on the results directly to do with the success - or rather failure - of the birds to raise young in 1996. On the same sites, where birds were caught in the same way in 1995 and 1996, the number of young birds went down for ALL 24 species. The percentage of young in relation to the number of adults caught was less for 23 of the species - only Song Thrush increased by a insignificant 2% from 34% to 36%.

The weather in spring and summer 1996 started of disastrously. With bitter cold conditions holding up the vital caterpillars and insect larvae that the chicks need in the nest. The worst affected species were mainly early breeders and the later breeding species did not suffer as badly. The lost opportunity to raise good numbers of young in just one year will certainly provide a glitch on the population graphs of many species but we are most worried by its effect on the farmland and other species that have been declining for a number of years.

The birds most affected, where the numbers or/or percentage of juveniles was significantly less in 1996 compared with 1995 were as follows:

Sedge Warbler-32&-11%
Lesser Whitethroat -39% ns
Whitethroat -44% -16%
Garden Warbler -23% ns
Blackcap -43% -15%
Chiffchaff -25% -9%
Willow Warbler -18% ns
Blue Tit -26% -8%
Great Tit -25% -11%
Treecreeper -27% ns
Chaffinch -26% ns
(ns - decrease but not significant)

For six species the breeding success, measured as the percentage of young caught compared with the adults was worse than for any year since the survey started in 1983. These were the Dunnock, Robin, Chiffchaff, Long-tailed Tit, Great Tit and Blue Tit. Overall the 1996 breeding season was a lost opportunity for many of our birds to breed. Let us hope that the weather does not become so cold in the next five or six weeks that the same thing happens in 1997.

More Information on the Constant Effort Sites (CES) Scheme

This innovative system of keeping tabs on the population and productivity of common birds from year to year was developed by British bird ringers during the early 1980s. The British Trust for Ornithology, who organise bird ringing in Great Britain and Ireland, run several other monitoring schemes but the CES is the only one which can readily tell us how well the birds have bred.

The concept behind the CES is simple. Bird ringers visit the same sites each summer and, using the same nets, catch and ring the local birds every ten days from May to August. The send details of both the new birds caught and the marked birds they recapture to HQ for analysis. The information gives a measure of population change, productivity and survival. The startling results documented overleaf and in BTO News are simply those relating to productivity.

There were five significant changes in adult numbers recorded from 1995 to 1996. Two of these were declines - Wrens down by 30% and Lesser Whitethroats also down by 30%. The latter is a long distance migrant and it is unclear where the birds were lost but the Wren decline was almost certainly driven by the cold weather early in 1996. Three species to increase were the migrant Chiffchaff (up 21%) and the two common tits (Blue up 15% and Great up 30%) which benefited from a very good breeding season in 1995.

During 1996 more than 110 sites were covered by British ringers and the results from 1995 to 1996 were based on 98 paired records. During 1966 there were 12,500 adult and 16,626 juveniles caught of the 24 main species. The pioneering work of the BTO's CES system has now spread to various other countries in Europe, to North America and even Australia is considering using it.

The Constant Effort Sites project is a part of the Integrated Monitoring Project of the British Trust for Ornithology and of the national Bird Ringing Scheme, also organised by the BTO. Part of the funding for the project is provided under contract with the Joint Nature Conservation Committee, on behalf of English Nature, Scottish Natural Heritage and the Countryside Council for Wales, and under a contract from the Department of the Environment for Northern Ireland. It provides an essential part of the unrivalled bird monitoring service which the BTO provides the Nation.

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