Bat activity in winter – what can it tell us?
Winter tends to be the off season for bat ecologists. And why not? By early October, most surveyors are pretty frazzled. In any case, bats hibernate in the winter don’t they? So why waste time and effort ‘looking’ for them? Any results are likely to be abnormal and therefore incapable of informing anything useful.
While there is some logic in this line of thinking, it does seem surprising that so little attention is paid to activity at a time when, arguably, bat mortality is likely to be highest. This is evident in the methodology for many forms of ecological assessment; these tend to follow the general guidance which says that activity surveys should be conducted during the ‘optimal’ period, May to September inclusive, and only when temperatures are at least 5 (preferably 10) Celsius.
Since 2009/10, I have been surveying winter bat activity at a single suburban site in North Somerset. The latest set of results, for January 2018, are shown in the table below, with December 2017 data for comparison.
Winter bat activity 2017/18
|SPECIES||December 2017||January 2018|
|Myotis sp.||1 (1)|
|Nyctalus sp.||2 (1)||1 (1)|
|17 (6)||16 (4)|
|3 (3)||5 (4)|
|14 (7)||14 (6)|
|Bat sp.||4 (4)|
|Social calls only - Pipistrelle spp.||7 (2)|
|TOTAL||58 (11)||36 (11)|
|Monthly Activity Score (%)||0.20||0.13|
Other species recorded at the study site since 2009/10 in winter (December to February) are: Lesser Horseshoe (Rhinolophus hipposideros), Greater Horseshoe (R. ferrumequinum) and Brown Long-Eared Bat (Plecotus auritus), bringing the total number of species recorded to at least 11. Incidentally, the lowest recorded temperature at which a bat has been recorded during this study so far is -5C, which occurred at 04:54 on 8th January, 2010. During 2010 (both winter periods combined), Pipistrelle spp. were active on eight nights in sub zero temperatures.
So is bat activity in winter new? Not really, although it still tends to be regarded as somewhat unusual. A number of studies in Europe and North America have highlighted winter activity in bats before and a range of explanations have been offered to account for this phenomenon, some of which are discussed below:.
- Disturbance – at roosting sites. This may cause bats to rouse from torpor and become active involuntarily while they attempt to replenish energy reserves and find a suitable new site to resume hibernation.
- Feeding – during mild spells. Bats may be ‘programmed’ to wake up and fly to feed and drink as part of their adaptive strategy for over-winter survival; in which case, proximity of high quality habitat to winter roost sites is likely to be very important.
- Temperature/humidity – extremes may result in freezing to death or starvation; bats caught in these situations must at some point take to the wing, to find food or a more suitable environment for hibernation
- Roost switching – for reasons unrelated to any of the above.
- Social behaviour – such as mating, exchanging information or even engaging in conflict!
All of these explanations are plausible (some more than others), but there are likely to be complex interactions going on which make it difficult to pin down any one reason in particular.
I would be interested to hear from anyone wanting to form an informal research group to look further at winter bat activity. Developing a network of sites (and data sharing) would allow statistically more valid generalisations to be made than can be gleaned from a single site alone. Potentially interesting questions to answer might address some or all of the following topics:
- Inter-specific variability – are some bat species more active in winter than others?
- Inter-annual variability – is higher activity recorded during milder winters?
- Intra-annual variability – in which winter months are bats least and most active and are there any regional differences?
- Activity triggers – how do environmental (mainly weather) factors influence the onset of winter activity?
- Phenology of winter bat activity – is winter activity affected by climate amelioration?
- Benchmarks – how can winter activity be measured (and methods standardised) and what can these measures tell us about site quality?
Lyndon Roberts, Alar Ecology – E. lyndon.roberts:alar-ecology.co.uk
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