The Flight of Noctule and Pipestrelle Bats Compared

THE FLIGHT OF NOCTULE A N D PIPISTRELLE BATS COMPARED by

H.

G.

BARRETT and

THE

E A R L OF

CRANBROOK

FOR the past three years we have been capturing noctule bats (Nyctalus noctula) marking, releasing and in a large number of cases recapturing them. (Trans. S.N.S. X I : 3 p. 271, 1959.)

Having captured the bats we put them into a coarse fisherman's keep-net the rings of which are 10" in diameter, the whole net 18" deep. In this, and even in a larger net 15" in diameter which we used occasionally, the noctules made no attempt to fly though an occasional individual would climb about inside the net using its hind feet and thumbs but with the fingers separated and wing membrances slightly spread.

(When crawling about a bat " walks " on wrist and hind feet, with fingers and wing membrance folded against the forearm.) After climbing about inside the net for a short time they would crawl to the apex from which it was suspended and hang there packed tightly together like a bunch of dark fruit.

Having examined the bats, measured, weighed and marked them we released them at the place where they were caught. On a number of occasions we put individual noctules on the ground, the concrete run-way of a disused aerodrome. T h e bats would crawl about on wrists and hind feet for a time, finally lying stationary on the concrete with out-stretched wings. Suddenly they would spring into the air, Aying away immediately. It was certainly a downward thrust of the wings which jerked them into the air but we were unable to see by the light of a torch or car headlights if this was accompanied by a kick off with the hind feet.

Our usual practice was to hold the keep-net mouth upwards when the bats would climb up the inside and over the rim to hang by their hind feet from the ring at the mouth. They would then open and shut their wings two or three times finally Aying away, dropping towards the ground as they took Aight. Sometimes a bat would leave the keep-net altogether, climb up the arm, body and even neck and head of the person holding the net until it had climbed to the highest point attainable, when it would Stretch its wings and Ay away. A noctule, in fact, seems to climb as high as it can, when climbing is possible, before taking Aight.

If the bats had been in the net for more t h a n a c o u p l e o f h o u r s , as happened when we had eight or a dozen or so to deal with, they were very loath to Ay. T h e same would sometimes happen on a cold night after a shorter period and in both cases we had to throw them high into the air to get them to Ay. When this is done, even on a warm night and with an active bat, a noctule does not

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get " under control " immediately it opens its wings. It swoops downwards with beating wings for a yard or two and then upwards or horizontally away.

Even on warm nights and after a short time in the net we found them loath to take wing when the net was held low, with the top ring from which they hung 2 ' 6" to 3' from the ground. If it was raised to 4 ' to 4 ' 6" they quickly flew away. When they took flight from the lower level the invariable drop towards the ground before they were in fĂźll control brought them very close to it and and on a few occasions a bat actually landed on the ground before it could get into fĂźll flight. It seems probable that it was proximity to the ground which made them loath to take flight when the net was held low.

The only other bats we caught were pipistrelles (P. pipistrellus).

These we usually released as soon as they were caught, merely opening the hand in which they were held. They took flight at once from the open palm, Aying upwards almost like a helicopter.

Hanging from a finger, as the noctules hung from the top ring of the keep-net, a pipistrelle seems to fly straight off with no marked drop toward the ground. If thrown into the air a pipistrelle seems to be in complete control immediately it opens its wings : once these Start to beat the bat remains at the same level, with no fall toward the ground. Occasionally we put a pipistrelle or two in the keep-net to take home and examine. Whether or not there were noctules in the net as well, the pipistrelles were very active, Aying from side to side across the narrow expanse, scarcely wider than a pipistrelle's wingspread of 7 to 8 inches.

The difference between the take off Aight of the two species is therefore very marked. A pipistrelle with a weight of about 4 to 5 grammes, relatively broad wings and a wingspread of 7" or 8" has much more control at low speeds than a noctule with a weight of about 30 grammes, narrow wings and a wingspread of 14" or 15 ". T h e reverse of course is obvious when the two species are seen Aying together, the rapid dashing flight of a noctule is in marked contrast with the normal slow twisting one of a pipistrelle.