Noctule Bats feeding on Cockchafers

NOCTULE BATS ( N Y C T A L U S NOCTULA) COCKCHAFERS H.

G.

BARRETT and

THE

E A R L OF

FEEDING O N

CRANBROOK

bats usually hunt their prey at tree top height, with a rapid, powerful flight, though they have been recorded as Aying low to take cockchafers. Cockchafers only fly over a short period and the combination of the flight of cockchafers, noctule bats and observer occurs too seldom.

NOCTULE

In 1960 one of us (Cranbrook) found noctules and serotines (Eptesicus serotinus) hunting summer chafers (Amphimallus solstitialis) as they flew from the roadside privet hedges a mile north of Wickham Market on A.12, and watched them on three successive evenings. The hedge was about 4 feet high and it was possible to stand right against it to see what was taking place at very close quarters, though the insects, which were crawling about the hedge in large numbers, were spread for about a hundred yards up and down it. For most of the time the bats flew up and down, parallel to and a few yards from the hedge in which the chafers were crawling, seizing them as they flew from the twigs into the open. At fairly frequent intervals a chafer would make a loud buzzing noise from inside the hedge, as if it was caught up in some obstruction. If this noise lasted for more than five seconds or so it almost invariably brought a bat so close into the hedge that it seemed to be picking up its prey from amongst the twigs : on one occasion a bat dropped onto the hedge with outspread wings but took flight again almost immediately. On no occasion was a bat actually Seen to pick a chafer from the hedge but they were obviously attracted by the " loud buzzing " ones.

For ten days in 1963 we were able to watch noctules catching June-bugs (M. melanontha) Aying in and about some oak trees Standing in the park and garden of Great Glemham House. One of us (Cranbrook) has watched noctules Aying on the park at Great Glemham House for many years. Usually on any flne, warm night in the summer they can be seen hawking their prey in the normal way at tree top height, when they are easily seen against the sky. An exact count is obviously impossible : the bats are not concentrated but diffused as individuals over a wide area. Looking from the place where they were seen hunting cockchafers in 1963, on a normal night something of the nature of six to a dozen noctules can be seen hunting over an area of about ten acres.

When hunting cockchafers they were much more concentrated, presumably because their prey too was concentrated. Over a

348 Transactiom

of the Suffolk Naturalists',

Vol. 12, Part 5

small area of about a couplc of acres a score or more of noctules hunted each night, showing remarkable control and manoeuvreability at low speeds. In general the bats circled round amongst the trees picking up the cockchafers as they flew out but frequently a bat would be seen to fly right into open spaces amongst the branches. At times the bats would slow down to pick up a cockchafer as it flew from the twigs, sometimes almost hovering with rapidly beating wings, sometimes stopping abruptly, the wings rigid and outstretched with the body held almost vertical. The bats took the cockchafers at all heights, not infrequently Aying below the lower branches to pick up the insects as they feil from above.

Some of the cockchafers were in the Virginia Creeper which Covers the house. On two occasions, early in the evening when the insects were still easily distinguishable against the sky, a noctule was seen to fly right in amongst the leaves of the creeper but no cockchafer was seen : on two others, following a similar approach to the creeper, a cockchafer was seen to fly out and be caught by the bat. These episodes took place too far away for it to be possible to hear if the cockchafers were buzzing amongst the leaves but recalled the bats Aying into the privet hedge after buzzing cockchafers.

DĂźring the years 1960-61-62 we caught in mist nets a large number of noctule bats which were feeding on house crickets as these Aew from cricket infested rubbish tips. DĂźring these Operations we could often bring a high Aying noctule down by lobbing a stone up into the path of the Aying bat to fall into the net. T h e success of this method depended on the accuracy of the thrower. If the stone reached the summit of its trajectory a few yards in front of and below a bat, the animal, presumably mistaking the stone for a cricket or other insect, would dive down onto it and would not infrequently be caught in the net.

Using this technique we have been able to catch noctules Aying in their normal way at tree top height over the park, though occasionally a bat would neglect altogether a stone thrown with such accuracy that our experience with cricket hunting noctules led us to expect an immediate response.

T h e behaviour of the June-bug hunting noctules was very different. Quite consistently these neglected stones, however accurately thrown and though we caught several of them it was only those who Aew low in pursuit of low Aying June-bugs.

T o the human ear crickets are silent Aiers, cockchafers and other large beetles noisy ones. Noctules in pursuit of silently Aying crickets can be deceived by a silently Aying insect Substitute

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in the shape of a stone, which is neglected by those in pursuit of the noisily Aying June-bugs. T h a t bats use echo-location to catch their prey is well known and a bat searching for insects by this method could obviously be deceived by echoes reflected from a Aying stone. It would not be deceived if it were chasing its prey by sight or by listening for sounds given off by the insect itself while in flight. There is strong evidence that noctules are attracted by the loud buzzing noise made by cockchafers when not in flight and it seems possible that they pick up and pursue these noisily Aying insects by listening for the sounds given out by the insects themselves. If so engaged amidst an abundance of noisily Aying prey they would not be diverted by a silently Aying insectsubstitute.

Our experience with high-Aying noctules on the park, some neglecting stones, some Coming down to them, can be explained by this hypothesis. Those in pursuit of silently Aying insects and using echo location would be deceived, those listening for, e.g., large dung beetles would not.