A study in Dorset, England, of a very large nursing colony of the common bat Pipistrellus pipistrellus provided some much needed basic taxonomic data for this species in the British Isles. Despite the fact that 389 of 429 bats caught were juveniles the measurements were demonstrated to be comparable to adults. The sex ratio of juveniles were shown to be 1:1. No adult males were caught. Amongst the 40 adult females, eight were nulliparous (never having had young) and the remainder had all given birth and were lactating or just post lactating. It appeared that at least some Pipistrellus do not become pregnant in their first year. It was estimated that this colony consumed a minimum of 100 kg of flying insects during July and August.
The individuals tend to be 0·5 g heavier one year after weaning and a further 0·5 g heavier the following year.
Adult females leave the nursery roost immediately after weaning and it was suggested that this may be to prevent competing for food with the juveniles.
Behaviour in roost, upon emergence and in captivity is described.