May, the Month of Dismay for Britain's Birds

News from the BTO
27th June 1996

A report on the early breeding season, by David Glue of the British Trust for Ornithology, pinpoints the cold weather of May as being the most important feature so far this year. It has caused the failure of millions of nests and will have a bad effect on the numbers of birds coming into Britain's gardens during the coming winter. This is a shame because survival through last winter was quite good for many species despite the cold spells.

The birds affected are mostly those that depend on caterpillars in the forest trees to feed their young. Blue and Great Tits and Chaffinches have been having a very hard time finding food as the cold May - the coldest for over 70 years - caused a delay in the leaves coming out. In turn this delayed the caterpillars and the birds, having already laid their eggs, could not delay their breeding season to coincide with their food supply. Many clutches were abandoned before they hatched and there are reports of many broods which failed to survive their first few days in the nest.

Aerial feeders like Swifts, Swallows and House Martins have also been hard hit. The well studied colony of Swifts in the University Tower in Oxford failed to lay any eggs at all in May for the first time wince they were first studied more than 40 years ago. Laying started in the first week of June and they should start to do well with the warm weather we are now enjoying. Many Swallows and House Martins have been very slow to start breeding but there is still time for them to have two successful broods if the weather stays warm.

The big winners during this spring have been the birds that feed on small mammals. Mouse and vole populations have been very high because of the good production of acorns and beech mast last autumn. Broods of Barn and Tawny Owls and of Kestrels have done very well. The survival of their chicks into the autumn will depend crucially on the ability of the mice and voles to maintain a high population - they show no signs of crashing yet.