BIRTH TIMING AND POPULATION-CHANGES IN GREATER HORSESHOE BAT COLONIES(RHINOLOPHUS-FERRUMEQUINUM) ARE SYNCHRONIZED BY CLIMATIC TEMPERATURE

We counted the births of greater horseshoe bats born at three small br eeding colonies in southwest Wales and south-west England at the extre me edge of the species distribution over nine summers (1984-1993). Ove rall birth timing was almost identical at the three sites (mean of mea n 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. Ear ly births followed warm springs; a significant negative regression of mean birth date on mean April plus May temperature was evident. A rise of 2-degrees-C accelerated mean birth date by about 18 days. Populati on levels fell at all three sites following the very late mean birth d ate of 28 July recorded in 1986, after an extremely cold spring and su mmer. Recovery of populations in all sites followed a series of warm s prings and early mean birth dates, but was hampered by bith sex ratios favouring males for several years. These findings confirm climate, th rough its effect upon birth timing, and possibly also on sex ratios, a s a major factor controlling bat populations, and hence the distributi on of bat species.