The period in which adult butterflies are present in nature is known as the flightperiod, and differs for each species. Some species fly only in spring, others in summer or late summer. The flight-period may differ from year to year, due to varying circumstances. The Dutch Butterfly Foundation has recently done a research on the flight-period of butterflies, based on data from collections, literature and field surveys. Results with 26 trial butterfly species showed, that most species had an earlier flight-period using collection data than with field data. This is due to collectors’ preference for fresh, undamaged specimens. Male butterflies were found to appear earlier than females. For some species, significant regional differences in flight-period were found. Seasonal weather conditions had a very distinct effect on the flight-period of most species. A warm spring results in an earlier flight-period than a cool spring-time. The effect of summer temperatures was less clear.