Integrated air temperature between January 1 and March 31 was recorded from 1983 to 2023 near a nursery roost of Pipistrellus pygmaeus in West Wales, U.K. Integrated air temperature was defined as accumulated heat above a threshold temperature above 5°C when Chironomid prey flew. Between 2003–2023 integrated air temperatures were in the same range (71–264 degree days) as those from 1983 to 2002 but the incidence of warm years increased. There were eight years from 2003–2023 when records were > 203 degree days compared with four years from 1983–2002. The effect on P. pygmaeus was investigated and weekly exit records from April 1 to September 27 during 1983–2002 showed six years when the first peak exit count in May–June (N_JN) occurred earlier than anticipated (weeks 6–8, April 1 = week 1) so N_JN counts were not within the U.K. national monitoring period (weeks 9–12). Subsequently, the probability of early N_JN counts in weeks 6–8, based on the changes in the integrated air temperature January to March, was calculated. A binary logistic model predicted that for 95% of the time, the likelihood of an earlier peak exit count in six to eight weeks exceeded 1.0 (OR = 1.06, 95% CI = 1.02–1.10) for every one degree day of increase in the integrated air temperature. The probability of an early peak exit count in weeks 6 to 8 stabilized to 0.80 to 0.95, when the integrated air temperature increased linearly from 210 to 264 degree days. The annual integrated air temperatures from January to March at 66.7% of 24 nursery roost sites in the U.K. from 2019–2023 ranged from 150–259 degree days, so prediction of early peak exit events could optimize nursery roost counts by addition of counts in weeks 6–8 at selected maternity roosts when the annual integrated air temperature ranged from 210–264 degree days and enable assessment of the extent and pace of climate change on P. pygmaeus.