It does appear that UK bats are migratory to some extent. Recent work by Dr John
Altringham has shown that in some areas certain species of bats do travel to upland sites in Autumn to "swarming" sites. The area Dr Altringham studied was in North Yorkshire, and he has evidence of bats travelling 50km+ to get tothese sites.
Through my work on larger windfarm sites I have recorded massive increases in activity in other upland areas during late summer and early autumn. Unfortunately, at this point in time we do not know enough to make an informed decision on many of the possible impacts.
I do take your point reagrding the financial implications of bat surveys on smaller projects. However, why should turbines be exempt from the same scrutiny as other development projects. Even though I can see the benefit of such projects, at the same time I see the need for such schemes to be designed and placed properly.
The majority of turbine projects I have been involved with look to locate the turbine on the edge of a site, to minimise land loss elsewhere on site, usually in, or adjacent to hedgerows. If this hedgerow is in close proximity to a roosting site it could be used by a large percentage of bats present.
I always recommend the early involvement of an ecologist in a scheme, as it is considerably more cost effective, and sometimes a simple change of placement can be decisive. I know this can effect turbine effciency sometimes, but these things are always a trade off. I've also looked at habitat modfication as a method of mitigating.
I suppose my last point is this. How do you know bats will be unaffected by a scheme unless a survey is carried out by a suitably experienced individual ?
In this situation it may be necessary for wider research to be carried out on the impacts of turbines. The question is, who will pay for it?