CHANGES IN THE FLIGHT PERIOD OF THE HEDGE BROWN BUTTERFLY PYRONIA-TITHONUS DURING RANGE EXPANSION

(1) The range of the hedge brown butterfly Pyronia tithonus (L.) has extended northwards in Britain, certainly over the last three decades and probably over a longer period. (2) During the latter part of this period of expansion (1976-89) regular counts have been made throughout the flight period of this butterfly at sites in England and Wales. Data are presented from thirty-five sites. (3) The length of the flight period, averaged over all years, was significantly shorter close to the edge of the range. (4) The flight period has become significantly longer during the years of monitoring and the mean flight date has become earlier. (5) Extension of the flight period and change in mean flight date were not significantly related to distance from the range edge. (6) It is suggested that the extension of the flight period and expansion of range are likely to be related. It is not known whether the relationship is causal or whether both parameters are influenced by further, unknown, factors.