A SUCCESSION of years without hordes of juicy caterpillars has been severely hitting the Forest's smaller nesting birds, with many having to abandon their young because they could not feed them.

One of the main theories about the lack of caterpillars is that mild winters are encouraging the eggs of various moth species to hatch before tree buds have opened, causing emerging caterpillars to starve.

"There has been a failure of the Forest's spring supply for the past four years," said bird expert Ivan Proctor. "We have had nothing like the number there is normally, with species like the pied flycatcher at Nagshead particularly badly hit.

"Last year, 1999, was particularly bad, and it causes a recruitment problem for nesting in the following years."

So far this year has seen a slight improvement in nesting successes but the oak-feeding caterpillars – which have sometimes occurred in plague proportions in the Forest in the past – have been absent again in any numbers.

"Climate warming being to blame is just a theory, however," Mr Proctor cautioned. "We always have a tendency to find a problem then look for a convenient cause, and it isn't necessarily the right one – there may be other reasons for the caterpillars not appearing."

Whatever the cause the population of pied flycatchers at Nagshead had slumped from 50 pairs a few years ago to around 30-plus this year.

But it is not all bad news – last year's breeding season saw fewer than 100 nestlings being produced, whereas this year around 150 have survived, which is encouraging for future nesting.

"The birds themselves may be having other problems, like the development of agriculture in Central Africa where the bird overwinter," said Mr Proctor. "They may be harming bird life much as we have already done over here with intensive agriculture."

•Details of the latest status of the Forest's birds, including sightings of rarities like the two-barred crossbill, are in the just-published Gloucestershire Bird Report 1998 – a fascinating record which this year for the first time includes some stunning colour photographs and is a must for bird enthusiasts. It is available at £7 with £1 for post and packing from GOCC, 2 Beech Close, Highnam, Gloucester GL2 8EG.