Behaviour of a Bat Flying Over Water

BEHAVIOUR OF A BAT FLYING OVER WATER L O R D MEDWAY

AT the lake at Great Glemham House, from 18.10-18.20 hrs. on 9th April, 1975—the first warm day after a period of unseasonably cold, snowy weather—I watched, in bright evening sunlight, a single bat Aying low Over the water and behaving in a fashion I have never before observed. Its flight was rather slow and fluttering, and its circling, convoluted course covered the open central and western part of the lake. In excellent light, I was able to watch through 7 x 42 binoculars and could clearly see its action as it repeatedly dipped into the water both hind feet, together with the tip of the tail and the hind margin of the interfemoral membrane. A slight splash was produced at each dip and, in a few instances providing a lateral view, I received a clear impression of a forward scooping action. I did not observe any associated movements of the head.

On investigating the water surface, I found two species of midge (Chironomidae) emerging: a large midge, which predominated (identified by Mr. P. Langton as Chironomus thummi Kieffer), and a small (identified by Mr. P. S. Cranston as Eukiefferiella coerulescens Kieffer). Many emerging midges (as always) were stuck on the water surface and drifting downwind in the westerly breeze.

The bat itself was of moderate size (i.e., larger than a pipistrelle, P. pipistrellus, and smaller than a noctule, Nyctalus noctula, both of which are common and familiar at Great Glemham). It was markedly bicoloured, warm light brown on the upperparts (perhaps seeming a little ruddier than normal in the evening light), and much paler below, a dull white tinged grey-brown. When seen as the bat approached, i.e. in head-on aspect, the two colours were sharply demarcated. Although of course the identification cannot be certain, size and colour conform with Daubenton's bat, Myotis daubentonii. Of this bat, van den Brink (1967, A field guide to the mammals of Britain and Europe, p. 52) wrote: 'Usually flies very low and fairly fast, with a vibrating wing-beat, in wide circles over the water, often touching the surface'. This description conforms with my observations which, however, go further in identifying the parts of the body touching the water.

Daubenton's bat belongs to the group of large-footed Myotis (subgenus Leuconoe), many of which are characteristically associated with water. In guano samples from a cave in France known to be a roost of Daubenton's bat, Brosset and Delamare Deboutteville (1966, Mammalia 30, pp. 247-251) identified the remains of non-volant aquatic arthropods and of small fish; these were presumed to be among the natural prey of the bats, although

BEHAVIOUR OF A BAT FLYING OVER WATER

65

the method of capture was not ascertained. The subgenus Leuconoe includes the Asian species M. macrotarsus which Griffin (1958, Listening in the dark, p. 220) has suggested may feed, like the true fishing bats Noctilio spp., by hooking prey from the water with the use of its elongated hind feet. The bat seen at Great Glemham, however, was touching the water surface not only with its feet, but also with the tail-tip and margin of the interfemoral membrane. The use of the flight membrane to capture and hold insect prey is known among several species of bats, including the little brown bat, Myotis lucifugus (which van den Brink, p. 52, has suggested may be the North American representative of Daubenton's bat): 'Insects . . . are usually captured with a wing tip, immediately transferred into a scoop formed by the down and forwardly curved tail and interfemoral membrane, and then grasped with the teeth' (Barbour & Davis, 1969, Bats of America, pp. 44, 47). Although the conclusion remains tentative, I suggest that the bat seen at Glemham was obtaining (or, since no certain feeding action was seen, in many cases attempting to obtain) floating midges by scooping them directly from the water surface into the interfemoral membrane.

I have not since repeated the rare chance of watching a bat in such good light. The lOth April was cold; I watched from 18.00-18.30 hrs., but no bat came to the lake. There were no midges emerging. On my next evening at home, 13th April, the weather was warm but the sky overcast. No bats came before sunset; at dusk (18.50-19.10) there were about five bats over the lake, but none behaving in this fashion.

Acknowledgements I am grateful to Mr. P. S. Cranston for providing identifications of midges, and to Dr. R. E. Stebbings for his comments on these observations.

Lord Medway, Ph. D., Great Glemham House, Saxmundham.