Population changes of Greater horseshoe bats studied near Bristol over the past twenty-six years

Capture censuses of Greater horseshoe bats involving about 35 hibernacula were consistently carried out over twenty-six years in two separate areas. Hibernacula were visited three times per winter. A control area (reduced disturbance) was visited once annually. Censuses showed similar trends in all three areas, and counts of bats at a breeding site showed similar trends to those hibernacula, hut much greater stability. All numbers fell sharply to about 50%, of the original level between the winters or 1962/3 and 1966/7, then more-or-less stabilized until the mid 1970s; rose in the late 1970s and early 1980s, and finally declined precipitiously in all areas in 1986 to about 30% of the 1962/3 figure. Numbers of juveniles born annually showed no relationship to the numbers reaching hibernacula during the subsequent winter, but was negatively related to the mean birth time in July. The final forearm length of juveniles is negatively related to birth time. A population control mechanism based on birth timing and subsequent growth is postulated and supported by reproductive studies.