• bat;
  • torpor;
  • insect availability;
  • growth;
  • primary sex ratio

We counted the births of greater horseshoe bats born at three small breeding colonies in southwest Wales and south-west England at the extreme edge of the species distribution over nine summers (1984–1993). Overall birth timing was almost identical at the three sites (mean of mean birth dates = 13–14 July), but varied widely from year to year. Mean birth timing was synchronized at the three sites in a given year. Early births followed warm springs; a significant negative regression of mean birth date on mean April plus May temperature was evident. A rise of 2oC accelerated mean birth date by about 18 days. Population levels fell at all three sites following the very late mean birth date of 28 July recorded in 1986, after an extremely cold spring and summer. Recovery of populations in all sites followed a series of warm springs and early mean birth dates, but was hampered by birth sex ratios favouring males for several years. These findings confirm climate, through its effect upon birth timing, and possibly also on sex ratios, as a major factor controlling bat populations, and hence the distribution of bat species.