A COLD and rainy summer has given seven bird species their worst-ever breeding season, with ornithologists warning they are finding it difficult to adapt to the effects of climate change.
Last night, experts said some of Britain's commonest birds could soon find themselves on the red list of those which are a real cause for concern.
According to the British Trust for Ornithology, blue tits were one of the worst-hit species this year, with volunteer ringers catching just over half the young birds they would expect.
Mark Grantham, who runs the trust's Constant Effort Scheme, which monitors bird numbers, said: "The cold, wet weather over the early summer will have made life incredibly tough for adults that still had hungry youngsters in boxes.
"Each blue tit chick will need around 100 caterpillars every day, and finding enough caterpillars in the poor weather we've seen is no mean feat."
Overall, there were 48 per cent fewer chicks than expected. The other six species for which 2007 was their worst breeding season were: great tit (33 per cent below normal level), reed warbler (27 per cent), whitethroat (25 per cent), willow warbler (19 per cent), treecreeper (55 per cent) and willow tit (63 per cent).
However, the researchers said that because only small numbers of the latter two were caught, the findings may not give an accurate picture.
Mr Grantham said: "For resident birds, such as blue and great tits, it will be interesting to see how they cope with the poor season. Most may well be able to bounce back next year. It is more serious for the migratory species - reed warbler, whitethroat and willow warbler.
"These are already suffering problems both on migration and in Africa, so a poor breeding season just adds to their plight."
Chicks have fewer feathers than their parents and are more susceptible to the wet and cold. Also, the rain washes caterpillars off the leaves, making it harder for juveniles to find food.
Paul Stancliffe, a spokesman for the Trust, said: "This is an alarm bell for these birds - these findings are significant. If there is repeated bad weather around spring and summer next year, some of these species may well go onto the red alert list, meaning we are extremely concerned about them.
"The unpredictability of the weather makes it difficult for birds to adapt. Birds are adaptable to the weather, but only over a long period. The weather is changing all of a sudden."
The BTO has been measuring bird productivity for 25 years, using the Constant Effort Sites Scheme. Trained volunteer bird ringers around the country put up the same nets in the same sites 12 times every summer.
The figures are for the UK as a whole, and RSPB Scotland said they were believed to apply equally to Scotland.
A spokesman said: "As far as we're aware, anecdotal evidence from Scotland supports these findings, especially for blue tits and great tits. Everyone will remember the disappointing weather in June and July, which unfortunately seems to have affected many birds that were nesting at the time.
"Some birds are more adaptable than others to having a failed brood, but blue tits put all their energy into breeding just as the caterpillars are out.
"However, blue tit numbers are generally up in Scotland since the mid 90s. We don't expect the odd bad breeding season to affect numbers hugely."
The researchers found one positive story - long-tailed tits had their most productive year yet, with an increase of 48 per cent on the long-term average. They are early nesters, and may have taken advantage of the better April weather.
SPECIES THAT BRED LESS
• Blue tits (-48%)
Nest in tree, wall or box - or more unusual place.
• Great tits (-33%)
Found in UK woods, parks, gardens, but not Northern/Western Isles.
• Reed warblers (-27%)
Visit UK in summer to breed. Few in Scotland.
• Whitethroats (-25%)
Breed widely, but avoid urban or mountain areas.
• Willow warblers (-19%)
Declined in past 25 years. On Amber List.
• Treecreepers (-55%)
Leave breeding territory in autumn but won't go beyond 20km.
• Willow tits (-63%)
During breeding season, feed mostly on insects. Like damp woodland or hedgerows.