Why do Insectivorous Bats in Britain Not Fly in Daylight More Frequently? (1991)

Previous studies suggest insectivorous bats occasionally emerge from their roosts during the day to make good shortfalls of energy intake. Other studies indicate that an inadequate nocturnal food supply may negatively affect bat reproduction. The question arises therefore why do temperate insectivorous bats not fly in daylight more frequently? Four hypotheses have been advanced to explain the infrequency of daylight flying. These are overheating, competition, predation and mobbing by non-competitor species. Analysis of behavioural observations in 420 reports of daylight flying, submitted in a national (UK) daylight-flying bat survey revealed that mobbing by non-competitors was very rare (4/420 reports) and its consequences trivial. Flying with aerial insectivorous birds was more frequent (19/420 reports) but antagonism was again uncommon. Predatory attacks, mostly by avian diurnal predators occurred in 13 cases, and in at least five of these instances the attacks were fatal. Large bats survived attacks significantly more than small bats. This may explain why large species of bats flew more frequently in daylight relative to their populations than small bats. In the laboratory pipistrelle (Pipistrellus pipistrellus Schreber) and brown long-eared bats (Plecotus auritus L.) avoided flying in the light. This effect occurred independently of body temperature and was not mediated by overheating. Furthermore, in the wild, bats did not avoid flying on days which were particularly hot or sunny, suggesting that in Britain overheating does not inhibit daylight flights. Heavier pipistrelle bats avoided the light more than lighter pipistrelle bats. This may reflect lower manoeuvrability and hence greater susceptibility of heavy bats (within a species) to predation, and may explain the reduced frequency of daylight-flying bats in Britain during late June and early July when they are heavily pregnant. Key-words: Activity cycles, aerial insectivorous birds, bats, competition, diurnal flying, feeding, hirundines, mobbing, predation, thermal stress Introduction Insectivorous bats of the temperate zone usually time their flights to coincide with darkness (Erkert, 1982). However they are also occasionally observed flying during the day (Speakman, 1990). A survey of daylight flying, throughout the British Isles (Speakman, 1990), has revealed the major function of daylight emergence by British bats is to feed. Estimates of the daily energy expenditure of insectivorous bats (Speakman & Racey, 1987; Racey & Speakman, 1985, 1987) in combination with estimates of their potential energy intake during their nocturnal flights (Racey & Speakman, 1985, 1990) suggest that bats may often be unable to sustain endothermy at normothermic levels (35-380C; Speakman, 1988) throughout their day roosting period. It has also been estabished that during torpor foetal growth is retarded (Racey, 1973) and this may lead to delayed parturition (Racey & Swift, 1981). Effects on lactation performance may also occur (Racey, 1982) but have yet to be demonstrated. To overcome this problem bats may resort to several tactics one of which is emerging to feed during daylight. This functional interpretation of the temporal activity patterns of bats presumes that there is an advantage to foraging at night. One potential advantage of nocturnal foraging is that net energy gains when feeding at night may exceed those during the day. The common temporal activity pattern would thus be consistent with the predicted pattern based on optimal foraging theory (Krebs & McCleery, 1984). This is however unlikely to be the case because flight energy costs are unlikely to vary between night and day, and bats exhibit a functional response to their prey which rapidly reaches an asymptote (Racey & Swift, 1985). Aerial insect abundance during the afternoon, when most diurnal flying bats are observed (Speakman, 1990), will generally always lie above the lower limit of the asymptote of prey intake (Williams, 1961; Johnson, 1969). The question arises therefore, why do bats not fly in daylight more frequently? There are four alternative hypotheses which have been advanced to explain this anomaly. These hypotheses concern fitness costs which may This content downloaded from 40.77.167.57 on Thu, 16 Jun 2016 06:10:08 UTC All use subject to http://about.jstor.org/terms 519 be associated with flying in daylight. Firstly, bats Day-flying bats may be unable to fly in the warmth of the afternoon and when they receive a high radiant heat load (from the sun) because in these conditions they are unable to dissipate the heat generated in flight (Thomas, 1981, 1987). Secondly, bats flying during the day may not achieve the potential energy gains suggested from the aerial insect availability because of direct competition with aerial insectivorous birds (Tugendhat, 1966; Moore, 1975). Thirdly, bats in daylight may be susceptible to avian diurnal predators (Moore, 1975) which have occasionally been observed attacking bats in daylight (Macy & Macy, 1969). Finally, other birds, which are neither predators nor direct feeding competitors may attack bats resulting in reduced feeding success or mortality (e.g. Miller, 1962). In this paper I further analyse data from the British national daylight-flying bat survey (Speakman, 1990) and combine this with laboratory observations of flying pipistrelle (Pipistrellus pipistrellus Schreber) and brown long-eared bats (Plecotus auritus L.) to assess which of these alternative hypotheses best explains the paucity of diurnal flying in insectivorous bats of the tem-