DAUBENTON’S BAT SURVEYS UPDATE

First of all a very, very big thank you to all the people who have turned out, in some cases, weekend after weekend, to take part in this increasingly interesting and important voyage of discovery.  If you remember, at the start I said no-one has surveyed like this before, and certainly no-one has surveyed the Lincolnshire waterways at all, apart from a few consultancy one-offs and surveys for the National Bat Monitoring Programme, which is simply recording presence or absence of Daubenton’s over the same 1 km of watercourse over many years, to see whether the population stays the same or shows an increase or decline.  There are only a handful of recorders across the county doing this, and not even evenly spaced, making for a very unrepresentative sample of Lincolnshire’s Daubernton’s population if we want to use the data at local level.

At the start the aims were all rather vague.  With the exception of Tattershall church and castle, as far as we know Daubenton’s don’t roost in buildings, so we had very few roost records for what is a common species.  Would recording what time Daubenton’s turned up at bridges tell us anything about how close to a roost they might be?  And if this worked then what else did we want to know?  Did we want to survey as many drains and rivers as possible (given that there are literally hundreds of miles of them in Lincs) – or did we want to home in on some of our findings?  The truth of the matter, of course, was that I wanted to do both!  But were people up for simply going to a bridge with a bat detector and a watch/ timing device and then sending the result back to me, several times over the summer?  And a lot of people have been.

So what have we learnt?  To begin with I assumed (always dangerous: golden rule for every naturalist: never assume anything!) that Daubenton’s were found everywhere over watercourses above a certain size (and I deliberately excluded ponds and lakes from the start, as too many access problems to get a clear picture of what they were doing – I wanted to find where they were roosting), but this turns out to be not the case, even when the water course looks tailor-made for abundant activity, based on its plant, and therefore likely insect, communities.  The bats are totally reliant on suitable roost sites, we’ve come to realise, so no suitable bridge, tree hole or culvert within a requisite distance and there are no bats.

But we have found roosts – and all, except one, so far in modern concrete bridges, not picturesque brick humpbacked ones (which may explain why there are relatively few Daubenton’s on the Grantham Canal, for those of you who live in that area), some confirmed, some strongly suspected from emergence times.  The exception is on the River Ancholme at South Ferriby Sluice, where Julie Ellison’s persistence paid off, after two years’ worth of timing surveys showed there had to be a roost there somewhere.  With part of the structure being off limits to the public she could get no further – until work was needed on the sluice and she was able to go out with the consultant ecologists, when they found the roost, located in the stone wall on the tidal side of the sluice, and counted out 42 bats.

This year we have started off teams on three new sites, with some already interesting results, which I hope they will be able to follow up with more surveys next year.  Down in the south, where we’ve just completed our fourth year, we’ve had a look at the River Glen (which is so far totally confusing!) and moved up a level with the site we started at, got the local drainage board involved – and have found something really interesting, which I’ll talk about at the Christmas meeting.  Do come along!

Annette Faulkner