It's probably about time for a blog update. We had a trapping break whilst female bats were heavily pregnant, givng birth and had newborns, but we are back out and have been very busy. The past month has been particularly intense, as we've been doing two to three surveys a week across the Bernwood area (to minimise disturbance we only visit individual woodlands a maximum of three times a season, usually less, and try and spread surveys around). All the late nights do take a toll, although in Toby’s case being a student he has very flexible working hours and so has slipped into his own personal time zone - a few hours behind everyone else in the country.
The reason for doing so many surveys - apart from having such a large area to cover, including some woods where we have not previously had access - is that we were planning to radio track Bechstein's this year, and we needed to do the surveys to catch them. For our current aims, a suitable individual to radio track would be a breeding female, with a healthy weight and condition, and in short we haven't done any radio tracking because we haven't caught a suitable individual.
We have seen males, non-breeding females and even a juvenile (male), however we took the decision not to try tracking those bats. While all individuals are vital parts of a healthy population, we are most interested in the movements of breeding females; maternity roosts are the most valuable to find as they are used by more individuals, and possibly for longer time periods compared to the sites used by lone individuals. In addition, breeding females seem to form the core of the population, so gathering information about their foraging movements is our priority. This is not to say that we are not interested in the movements of non-breeding females or males, and certainly plan to track these individuals in the future. We had planned to track some males this year, if we already had tags on breeding females, but for now they are not top priority.
We have had much discussion amongst ourselves, and with colleagues working on similar projects, as to the reason for the difficulties observed this year (which are not limited to the Bernwood Forest area). The primary suspect for most of our issues is the unusually wet - and often cold - weather that we have been seeing. Adverse conditions make it difficult for bats to feed, and also affects the numbers of their insect prey meaning the effect can continue even when conditions improve. This particularly impacts upon pregnant females, who need extra energy and may be able to forage only a limited distance from maternity roosts - a problem compounded by the greater density of females competing for food.
A general effect of wet years that we observe in bats is a reduced number of pregnant females in the population. If a female cannot find enough food to support a pregnancy, it makes sense to that individual to concentrate on its own survival - bats are generally quite long lived, and so it is a good long term strategy to survive to breed again in a better year. When females are not breeding, they can disperse away from maternity roosts, thereby avoiding competition in a densely populated area, and while they may end up in a less optimal habitat they are nonetheless better off. The theory as to why we are not catching so may bats with our primary survey method (using a sonic lure to call them in) is that with fewer females breeding and greater dispersal, there is a reduced density of bats in maternity woodlands, and reduced territoriality, and so individuals are less likely to respond to investigate the social calls emitted by the lure. It is hard to come up with hard evidence to support some of this idea, but nonetheless it is an effect that has been observed in wetter years in the past and makes things difficult.
Changing their behaviour in wet years makes sense as a long-term strategy from the bats perspective, but it's still quite frustrating for us when it means that we are unable to achieve things that we had planned for the season. What's that oft quoted phrase about working with children and animals...
We will continue surveys for next few weeks, as long as the weather allows, and while there has possibly been a certain air of pessimism in this post, I should finish by noting that this year has not been completely without success. We have added several new Bechstein's woodlands and had a great night last weekend when we were joined by a crew filming a segment for the One Show on BBC1 (watch out for a further blog post on this, and we'll share the broadcast date when it's confirmed).
September may be creeping into view already, but our season isn't quite over yet!