Notes on a Foraging Group of Serotine Bats

NOTES ON A FORAGING GROUP SEROTINE BATS ( E P T E S I C U S SEROTINUS, THE

EARL

OF

OF SCHREBER)

CRANBROOK

A small number of serotine bats has been known to frequent the main road A.12 between Wickham Market and Mariesford for a number of years, " s i g h t " identifkation in the early 1950's having been confirmed by the capture of a couple of serotines in a mist net in June, 1960. Miss M. Lynn Allen who has watched them for the last four years gives a rough estimate of the number normally feeding there as fifteen to twenty : it is not possible to be more precise since a few noctules (Nyctalus noctula) feed and fly with the serotines and it is difficult to distinguish between them with certainty. There seemed to be about that number of bats Aying on a warm night during the period under review. In late June and early July both species fly very low taking summer chafers (Amphimallus solstitialis) as these fly from the privet hedges on either side of the road.

On 15th June, 1964, chafers were found to be Aying in large numbers and between then and 6th July, when the chafers had more or less ceased to Ay, this stretch of road was visited and mist nets set to catch the bats.

Technique U s e d The mist nets were set on two poles 14' long. If help was available a net 45' X 9 ' was used and sometimes two : if single handed a smaller net 22' X 4 ' 6". These were either set parallel to the hedge and kept set or, if enough help was available, held more or less horizontally by two people and swept up as a bat flew over the net. Using this second method the main body of feeding bats could be followed as it moved along the road.

When taken from the mist net the bats were put into a coarse fisherman's keep-net until the evening's work was over. They were then taken out, examined and released, a note being made of sex, weight, length of forearm and breeding condition. The results are set out in the Table below.

BATS

15 16 19 20 24 26 27 28 30

June June June June June June June June June

1 July 4 July 6 July

Ad.

Male

E. serotinus Juv.

CAPTURED

Female Ad. Juv.

4

N.

noctula Notes

Warm, many bats Aying Warm, many bats Aying / Cold and wet, no bats

\ 3 2

— 2

;

Aying

Warm, many bats Aying Warm, many bats Aying Warm, many bats Aying Cold, no serotines Seen Cool, noctules but few serotines Cool, noctules but few serotines Cool, noctules but few serotines Cool, few serotines, very few chafers Aying

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The bats were not ringed or marked and some may well have been recaptured on successive nights. The young female caught on 16th June was kept in captivity until mid July, so the minimum number of individual bats examined was eight, i.e., one juvenile male, four adult females (captured together on 16th June) and three juvenile females (one captured and removed from the population 16th June, two captured together 27th June) : the maximum number thirteen (the above eight plus the adult females captured 26th and 27th June). The weights and measurements noted below are based on those of every bat captured, nine adults, four juveniles. The proportion of total captures noctules : serotines was two : thirteen and judging from " sight " identifications of the bats in flight this is not far from the actual proportions between the two species.

Behaviour of Bats It is difficult to distinguish with complete certainty between serotines and noctules in flight, particularly when they are Aying at heights above thirty to forty feet or so, which noctules do normally and serotines not infrequently. When both species are Aying together at that elevation and in pursuit of the same prey both fly in the same sort of way : though the shorter wider wings of a serotines can sometimes be distinguished from the longer narrower ones of a noctule, it is easy to make a mistake. At lower elevations, and especially when taking Amphimallus, both species fly much more slowly and often within a yard or two of the observer when the shape of the wings is easily distinguished and in the early part of the evening when it is still light both can often be recognized by the colour of their fĂźr.

The evening temperature varied considerably : no thermometer readings were taken, the terms " cool " or " cold " in the table above are expressions of the effect on the observer. It will be seen that Over this short period noctules were Aying on cool nights when only a few serotines appeared.

On those nights when Amphimallus was Aying a few beetles started to Ay at or shortly before the time of sunset as given in Whitaker's Almanack and at about the same time a few large bats appeared Aying high : these seemed to be noctules but may have been serotines. A little later what were undoubtedly serotines would be seen " patrolling " up and down the road at a height of Afteen to twenty feet, diving on to chafers at intervals. Then as more chafers Aew from the hedges the bats would come down to within a few feet of the ground, Aying up and down and across the road and hedges in pursuit of the beetles. The traffic, particularly on Saturday and Sunday nights, was fairly heavy ; the bats chased their prey at " car " height across two lines of traffic without lights or with side lights or headlights as the night got

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darker, without so far as could be seen any collisions. Often it looked as if a collision was inevitable : a bat would fly at a height of two or three feet from the ground from one side of the road to the other into the path of an oncoming car and it seemed impossible that any creature could keep its attention fixed on the prey in front and also see or pick up by echolocation approaching cars in the two Iines of traffic : suddenly it would turn back or up into the air and escape destruction.

I am told by the roadman responsible for that length of road that he has never picked up a dead bat.

The bats usually kept relatively close together as they fed, spread along a hundred or a couple of hundred yards of road, the whole group moving as a group along the road as they hunted.

On one evening they were first seen hunting round a small tree (A on the map) ; a mist net set there caught one noctule though serotines could easily be recognized in the sunlight. They then moved slowly southwards in a discrete group, hunting the while : for a time most of them circled another tree (B) and then around a large thorn bush (C) after which they moved slowly, hunting together, down to the five crossways (D). Thereafter in the gathering dusk they were more widely spread up and down the road but drifted, feeding, up and down the length of the road as a group.

When working single handed the net was set at C, partly because it was more or less in the centre of the bat's usual ränge and partly because there were some heaps of sand by the road side into which the poles supporting the net could be stuck. Standing at that point the behaviour described above could easily be followed.

Once they started to feed there seriously there was a fairly heavy concentration of bats : this would gradually thin out as they moved away down the road until no bats could be seen, though chafers were still Aying. After a period of about fifteen to twenty minutes a few large bats would reappear, sometimes turning back a hundred yards or so down the road, sometimes Aying past and then back to rejoin the main body. Then more bats appeared and came closer until the original concentration was repeated, when they would thin out again until once more there were no bats to be seen. This might be repeated a third or even a fourth time.

Miss Lynn Allen teils me that she has frequently watched this group of bats and that they nearly always keep together in this way while they feed. Noctules and serotines seemed to move together. On their first appearance in more or less fĂźll daylight and Aying low the two could be distinguished with certainty : on subsequent appearances sight identification in the failing light was less accurate and had there only been one or two bats I would hesitate to say that both species reappeared but with fifteen or twenty Aying and many opportunities of identification it is possible

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to be reasonably sure. Certainly on one night at least noctules and serotines moved together. On their first appearance both species were easily recognized Aying very low in füll daylight, after which all the bats disappeared. On their subsequent reappearance a noctule was captured and a serotine caught in the net with outstretched wings for long enough for it to be identified with certainty, though it dropped out and escaped.

The bats could easily be brought down by throwing a pebble into their path in contrast to the behaviour of cockchafer-hunting noctules noted by Barrett and Cranbrook {Trans. Suffolk Nat. Soc. 12 : 347. 1964).

The chafers were Aying in large numbers in mid June and the bats Aew low in pursuit of them. These conditions continued until the end of the Arst week in July : on 6th July very few chafers were Aying and the few bats present were Aying out of reach of the net for most of the time.

Juveniles Juveniles were relatively easily recognized by weight and lack of wear on the teeth. The average weight of adults caught was 30-7 ± 2-5 gms. of juveniles 21-8 ± 1-7 gms. There was no signiAcant difference in forearm length, that of adults being 52-7 ± 1-4 mm. of juveniles 51-8 ± 1-2 mm. These young bats therefore were fully grown though light in weight, in which respect young serotines are like young noctules (Cranbrook and Barrett : Proc. zool. Soc. Lon. in the press) and probably do not go on foraging Aights until they are fully grown. In all the teeth were needle sharp and unworn, in marked contrast with those of the adults.

There are very few records of the date on which juvenile serotines have been caught Aying. Blackmore (quoted in Handbook of British Mammals, H. N. Southern, 1964) says that young serotines born in captivity 16th June were able to Ay about twenty days after and Cranbook records the capture of a free Aying juvenile at Bentley on 25th September [Trans. Suffolk Nat. Soc. 11 : 453. 1960). This capture of free Aying juveniles in late June seems to be the earliest record and is a month or six weeks earlier than the capture of the Arst juvenile noctules recorded by Cranbrook and Barrett (loc. cit.).

The capture of three females to one male out of a total of four taken gives no statistically signiAcant Information about the sex ratio amongst juveniles : it may well have been the normal and expected 50%-50%.

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Adults

In all the adults the teeth showed much sign of wear, the canines in particular looking almost like truncated cones in some of the individuals examined. I have looked at the teeth of a large number of adult noctules and very few of those had teeth showing as much signs of wear as did those of this group of serotines.

All the adults captured were lactating females. This preponderance of females at certain times has been recorded with other species of bat and in other countries : in SufFolk with noctules at Great Glemham by Cranbrook and Barrett (loc. cit.) and with pipistrelles at Westleton by Lovett (Trans. Suffolk Nat. Soc. 12 : 39. 1961). To account for it by saying that each observer was dealing with a " nursery colony " of female bats is only stating in different words the observed fact that in each case a preponderance of females was caught, though it may well be true. The alternative suggestion that female bats are more easily caught than males is certainly not true of noctules: Cranbrook and Barrett caught a preponderance of female noctules in June and July, males and females in equal numbers in August and September and a preponderance of males in October.

Summary

DĂźring late June serotine bats were found to be moving in a more or less discrete group up and down a two mile Stretch of road feeding on summer chafers and a small number was captured in mist nets. When in pursuit of their prey the bats showed a remarkable facility in avoiding the relatively dense motor traffic along the road : no casualties were observed.

All the adult bats captured were lactating females.

Newly Aying young bats of both sexes were also captured.

These were identifiable as juveniles by weight and tooth wear.

A few noctulesflewwith the serotines in pursuit of the same prey. The serotine seemed to be more affected by the weather than were the noctules.