Just hanging around

For 3-4 years now we've been checking the Catacombs for hibernating bats.

Bat team, winter 2015-2016

Gloomy, cold, dank, a little spooky, and just about perfect temperature and humidity conditions to keep bats in steady hibernation. There is high risk of serious disturbance at this crucial time, not just torchlight and noise, but also inadvertently raising the temperature a few degrees by our movement around inside. Hence our monitoring group is kept small, and monitors have a special bat licence for carrying out the checks. This being an urban park in the middle of the city, the risk assessment includes looking out for broken glass and discarded needles around the gates and grilles. 

The Batacombs: spot the bat ... (c) Greg Slack

The Catacombs have been here since the late 1830s, comprising a number of arched sub-chambers, very few of which have been filled, and in one spot a further walled enclosed alcove. On our first ever visit, this alcove was where we found some 16 horseshoes bats, mostly lesser horseshoe, but also a couple of rarer greater horseshoe bats, and, although numbers greatly fluctuate, this feature has been one of the more regularly tenanted locations ever since.

Fitting with the dripping, cave-like ambience, our torch beams pick out overwintering peacock butterflies, convincingly camouflaged as dead leaves, herald moths, snails bleached to a limestone white, and massed clusters of flared-skirted southern pill woodlouse. One fissure in a wall contained a huddle of Eristalis drone flies. Cave spiders guard their ping-pong egg sacs high in the corners. Among the darting flattened-plane, two dimensional shadows thrown about by the torch sweeps, suddenly there is a small, darker, more solid shape.

Just as they are meant to, the horseshoe bats hang upside down from walls and ceilings, though it is still somehow eerie to see them suspended impassively there, like strange dormant leathery fruits, or tiny thumb-sized folded-up umbrellas (our other UK, non-horseshoe, vesper bats do not generally do this, but tuck themselves tightly into nooks and crevices). This year curiously the horseshoes have been clinging to the open walls quite low down - something to do with unseasonal temperatures this winter? At the alcove, one almost had to climb in to the stone coffin to see an individual bat clinging to the underside of the lid.   

Strange umbrella fruit: lesser horseshoe bat (c) G. Slack

Cave spider, probably Meta menardi, guarding egg sac

Noting the popularity of this feature, last August we set up a little habitat enhancement, in the form of a free-cycled wardrobe, modified by part-cladding with roofing felt and roughened sarking board, to simulate a few extra secluded nooks and crannies with varying micro-climates. Archaeologists of the future must interpret what this incongruous, Tardis-like installation among the early Victorian masonry might signify.

Habitat enhancement, Narnia style

Over the last 18 months we've recorded bat droppings in the wardrobe, but no roosting or hibernating bats so far. This winter our hibernation counts were 8 lesser horseshoe bats in January (temperature 2-3 degrees C) and 4 lesser horseshoes in February (temp. 9-10 degrees C); also a single Natterer's bat, with its snowy white tummy fur. Our monitoring results will contribute to national bat recording schemes. These numbers sound low, but, perhaps surprisingly, this is the 9th highest known winter roost for lesser horseshoe bats in Devon; this, in a part of the country which is a stronghold for both greater and lesser horseshoes.

It reflects how much is unknown, and still to be discovered. Horseshoe bats are keenly light averse, and will avoid streetlit areas if they can - so why have they adopted this city centre, urban site? Usually at our annual Pips & Pints bat walks in May each year, we pick up occasional lesser horseshoe calls on the bat detectors, amongst the circling pipistrelles. When we carried out some harp trapping here at the park a couple of years ago with Dr Fiona Mathews from the University, we caught all three pipistrelle species (common/bandit, soprano, and the bigger, rarer Nathusius' pipistrelle), and a lesser horseshoe; yet why don't many of the crevice-roosting species, with the exception of our single Natterer's bat this year, seem to overwinter in the Catacombs?

And how common is this phenomenon of urban horseshoe bats, let alone urban overwintering bats, when built-up areas would be expected to be unsuitably warmer than surrounding countryside? (See Tania Esteban's blog for a discussion of urban bat ecology). There seem to be few examples from the UK: the only urban horseshoe bats we've come across up to now are here in Exeter, and in Taunton; any other records would be of great interest.



If you want to find out more about Exeter's bats, please join us for our annual Pips & Pints event this May, a joint event run by DWT Exeter Local Group, Exeter City & East Devon Bats, and Devon Mammal Group.
Exeter LG (link to event details will be here in due course)
Exeter City & East Devon Bats
Devon Mammal Group

The Watching Hour

Getting ready for the annual Big Garden Birdwatch this weekend Sat 30 - Sun 31 Jan.

I say 'getting ready', but a garden and a chair are all one needs, and even then the chair isn't essential. The study involves watching a garden (or other green space) for one hour and recording the birds you see, then sending these records to RSPB via the interweb link above. The scheme has run for over 36 years, with the value and statistical power to interpret results accumulating with each yearly repeat survey. This is very easy, rewarding fieldwork: you don't need to go anywhere near a field, plus can do it from an armchair with a cup of coffee. And still participate gathering nationally useful citizen science data, towards significant findings.

Over the last few years' Big Garden Birdwatching, it has mostly been house sparrows for me, but this is fine - good, in fact, that these seem to be surviving reasonably well in Exeter. Otherwise the house sparrow in the UK, traditionally ubiquitous, has declined overall 71% since 1977*, a proportion and rapidity enough for the species to crash on to the national Red List of birds of Conservation Concern.

Our tiny street-side city centre Exeter garden meanwhile, often hosts 10-15 at any one time. As sparrows live in colonial family groups, I'd assumed the same dozen or so individuals might be using several adjacent contiguous gardens, no doubt encouraged by birdfeeders and the safety of cover provided by a big old hedge. During 2015 hour's vigil, I counted 36 in the garden at the same time. A fleeting blue, great or coal tit was a source of moderate excitement. I suspect the collective sparrow gang does a good job of guarding and monopolising the feeder; ground feeding birds like dunnock, robin, and blackbirds put in a few more appearances, with a few more chances to pick up the feeder spillage jetsam underneath. There was a winter female blackcap once; crows, jackdaws and starlings are around but haven't come into the garden during the allotted hour; occasional pigeons and magpies usually add to the hour's species list.

Familiarity and local dominance doesn't preclude some sparrow subtleties to look out for. With the colder weather, fatballs seem to be preferred over seeds, grains or wholemeal bread, though the improvised bird baths are used just as much as usual, despite all the recent rain. As nesting season approaches, the black 'bib' patch on the male's chest becomes larger and glossier, indicating status among peers competing for mating opportunities. In breeding mode, the bill also darkens colour to shiny black from grey**. Such sparrow politics add interesting sub-plots to the garden observations.

UPDATE

Magnificent 7 sparrow posse

And results just in for my Garden Birdwatch hour, Sunday 31 January 2016:
Highest counts
Blackbird 2           House sparrow 16
Wood pigeon 2     Dunnock 1
Long-tailed tit 4    Wren 1


STOP PRESS: RSPB have now published the 2016 results - link here
There are downloadable spreadsheets to see county and national results. In Devon, there was an average of 4 house sparrows per site, present in 67.4% of participating gardens/open spaces. Most widespread species was blackbird, in 86.6% of gardens, followed by robin 84.5%. Some other interesting records were barn owl, redpoll and yellowhammer.


* from BTO and RSPB sparrow webpages. Various reasons have been suggested for the decline: a drop in survival rates of chicks; decline in invertebrate prey; concreting over of gardens; as yet no single cause is known
**as told by eminent sparrow expert Denis Summers-Smith, in Hugh Warwick's Beauty in the Beast book, and in a scientific paper from 2010.