I have snaffled the following from the BTO press release.
The British Trust for Ornithology has been measuring bird productivity for 25 years, using the Constant Effort Sites Scheme, funded under the BTO/JNCC partnership (Note 2). Trained, volunteer bird ringers around the country put up the same nets in the same sites on twelve occasions every summer. By looking at how many adult and how many juvenile birds are caught, they can get a very good idea of how successful the breeding season has been.
Mark Grantham, who runs the Constant Effort Scheme, 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, but 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.”
Some Blue Tits in southern England fledged before the bad weather started but, from stories told to BTO staff by birdwatchers from around the country, it seems as if most Blue Tits were not quite this early and that many died in nest boxes. As Mark explains:
“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 (up to 12 in a box) will need around 100 caterpillars every day, and finding enough caterpillars in the poor weather we’ve seen is no mean feat.”
Newly-fledged, fluffy juveniles experienced problems too:
* Young birds have far fewer* feathers than their parents and get wet and cold much more easily
* In wet weather, caterpillars are washed off leaves, making finding food much harder – especially as youngsters have little idea of how to fend for themselves.
(* important to get out of the nest as quickly as possible, to avoid predation. Youngsters grow extra feathers a few weeks after they leave the nest)