Feeding Habits of Noctule Bats

ELIMINUS MODESTUS

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Primarily an Australian barnacle, E. modestus was first recorded in British waters (Chichester Harbour) by Bishop in 1947, and it is thought that it was introduced sometime during the war on the hulls of ships, whose passage from Australia had been speeded up.

In 1947/ 48 it was recorded from different parts of the SufTolk coast and since then it has spread to many parts of our coast-line.

(Crisp 1958).

While small, E. modestus may easily be recognised by its shell being composed of only four compartments instead of six.

REFERENCES

Bishop, M. W. H. 1947. Establishment of an immigrant barnacle in British waters. Nature, Lond. Vol. 159 p. 501.

Crisp, D. J. 1958. The spread of Eliminius modestus (Darwin) in North-weat Europe. J. mar. biol. Assoc.U.K. (37) 483 - 520.

Darwin, C. 1854. A Monograph on the sub-class Cirripedia.

Sessile Barnacles. Ray Soc., Lond.

FEEDING

HABITS OF NOCTULE (.NYCTALUS NOCTULA)

by THE

BATS

EARL OF CRANBROOK

are common at Great Glemham and in summer are seen Aying in numbers on the park in front of Great Glemham House. In July and August of 1959 they had not been seen in their usual numbers on that area and Mr. Axell's obssrvations (page 271) led me to look at a similar but much larger dump on the edge of Framlingham Aerodrome about | mile away. This dump was also alive with crickets and at dusk many noctules appeared Aying low over the dump and were seen to take crickets in Aight on a number of occasions. My experience is that on the ground crickets are not easy to catch, running and jumping to avoid capture at the approach of a possible predator. On the wing in the air however they are poor Ayers, Ay in a straight line and continue to Ay in a straight line if chased, making no attempt to take evasive action. Their senses in Aight must be as acute on the ground and this apparent insensibility to the presence of danger while in the air is presumably due to the fact that they are physically NOCTULES

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NOCTULE BATS

incapable of changing direction. None of the crickets seen to be taken by bats made any attempt at evasive action and no bat was seen to try to catch a Aying cricket and fail.

On a number of occasions a bat was seen toflyabove a cricket within a few feet—so close indeed that the eddies caused by the wing beats of the bat made the cricket sway in itsflight—andyet make no attempt to capture it. The " swaying " seemed to be involuntary and not evasive action : it certainly had no effect upon the bat whichflewstraight on. The attacking approach of the bats was usually from the side or from behind the cricket, at a relatively low angle from the horizontal, but on three occasions a bat in horizontalflight20 to 30 feet above a cricket was seen to dive down almost vertically and seize the insect. This last of course is very typical of a noctule'sflightseen high up on any summer night, but so high that its prey cannot be seen. It would not, I think, have been possible to see from above a Aying cricket against the background of tins, rubbish, Vegetation etc., certainly at that intensity of light a human eye could only see them from below against the afterglow of the setting sun, and the bats must presumably have picked up the insects by echo location.

Echo location by bats has been not inappropriately likened to radar and a radar installation used to directfightersor gunfire on to the enemy aircraft must perform two functions. It must flrstfindthe enemy as he approaches, which is done by " scanning " the area within ränge of the apparatus with a revolving beam which picks up the aircraft just as a revolving searchlight would illuminate an aircraft as the beam lit upon it. Having found the enemy the apparatus must then keep track of him until the fighter or guns can be brought to bear. A bat's problem is exactly similar ; he mustfirstfindan insect Aying in the air above, below or in front of him—and then keep track of it until he can seize it.

D. H. Griffin (Listening in the Dark,) Yale University Press, 195 has shown that in the Vespertilionid bats, which include the noctule, the interval between the echo locating impulses and the duration of each pulse emitted by a cruising—and presumably " scanning "—bat lessens very considerably as it dives towards an insect in Aight or towards a pebble thrown into the air to simulate an insect in Aight. That is indeed what would be expected, the low rate of impulse emissionfindingan insect and the higher rate being necessary to obtain the greater accuracy needed to catch it in the mouth. Griffin has also shown that though the intensity or strength of the echo locating impulses emitted is greatest straight ahead, these are also emitted at a lesser intensity to the side. Since the echoes of the impulses are picked up by the bat's two ears which, just as two eyes will give stereoscopic vision, would give some indication of the direction from which the echoes

NOCTULE BATS

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came, it would be possible for a cruising bat to " scan " the hemisphere of Space in front of it by scattering echo locating impulses in every direction from straight ahead to 90° above, below, to right and to left. Having thus picked up its prey it could fly straight towards it using the impulses sent out at greater intensity straight ahead and of course increasing the frequency to obtain greater accuracy, An alternative method would be that used by a searchlight or radar whereby the whole target area is searched by a rotating narrow beam. The bat by moving its head could scan the air in front of it with the narrow beam of impulses of higher intensity emitted straight ahead and follow the insect until captured.

Noctules in flight will make sharp turns to right or left and dive down on an insect but do not make similar dives upwards. This may be due to aerodynamical difficulties but also to the fact that the bat's method of scanning does not allow it to pick up insects at a considerable upward angle to its line of flight. A captive noctule turns its head easily to right and left and down but does not turn it upwards for more than about 30°. If a cruising noctule scanned with an all-round scatter of impulses in all directions it would pick up an insect as far above as it obviously can below its line of flight. If it scanned by movements of its head it would not be able to pick up an insect high above its line of flight. The behaviour of the noctules seen catching crickets seems to indicate that the latter method is used. On several occasions they were seen to pass quite close to Aying crickets apparently without noticing them, when a very slight alteration of course would have been necessary to make contact. On other occasions they were seen to make alterations of course of 90°, dive down and seize insects. It is of course possible that in the first cases the bats were busy eating an insect and unable to take another or in hot pursuit of some other and, by me, unobserved insect. These unobserved insects, if there were any were certainly not crickets : they were not Aying in crowds but as individuals and in no cases was there another cricket near one of those seen to be un-noticed by a bat. The happening moreover was not infrequent and it seems more likely that they were " looking the other way ".

It has been suggested that the nose leaves of the Horseshoe bats act as horns, efficiently concentrating the echo-locating impulses emitted by these bats in a forward directed beam, with which they scan by movement of the head. If, as would appear to be the case with noctules, the Vespertilionid bats also scan with the beam of high intensity impulses sent out straight ahead, the scatter to the side noted by Griffin must be of little additional use and merely due to a less efficient beaming apparatus.