The function of daylight flying in British bats

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Abstract
A national survey of the incidence of daylight flying of bats in mainland Britain was organized from September 1985 until March 1988. A total of 420 records of daylight flying were received by 1 May 1988. One hundred and forty‐four reports were from winter (October to March), 271 from summer (April to September) and five were undated. Peak activity occurred during April and in August/September.Activity in both winter and summer was greatest between 12:00 and 16:00h. The numbers of bats involved in each sighting varied between 1 and 200. In summer 87% and in winter 91% of observations were of single individuals. Numbers of daylight‐flying bats, relative to roost visitor reports sent to the Nature Conservancy Council, increased with increasing latitude during both summer and winter. This means that an individual is more likely to fly in daylight the further north in Britain it lives. The effect of day‐to‐day variation in climatic variables on emergence was investigated for records from 1987. In April 1987 emergence occurred on days which followed significantly cooler nights than nights preceding days without emergence. During the remainder of the summer of 1987, however, no climatic effects were significant. During winter 1987 emergence occurred on days which were significantly warmer and sunnier.These data suggest that during summer the primary function of emergence during daylight is to feed to make good energy deficits that have accrued because of inadequate intake during nocturnal foraging. During winter, bats time their daylight emergences to coincide with good feeding conditions, as has been shown previously for winter nocturnal emergence. It is possible daylight emergence occurs during winter primarily because the endogenous cycle during hibernal torpor cannot accurately synchronize arousal with periods of darkness.