Butterfly recording in Lancashire, north-west England, goes back to early 19th century collectors and the development of natural history societies. With the advent of computers, formal records (which include details of date, number, observer, grid reference etc.) began to be stored and the Lancashire & Cheshire Fauna Society now holds more than 26,000 records of butterflies up until 2017.
The Lancashire branch of Butterfly Conservation produce annuals reports summarising data for vice counties 59, essentially North Merseyside and Lancashire north to the Ribble but which now includes Greater Manchester north of the Mersey, and 60, which stretches from the Ribble to the northern county boundary, small areas of which are now in Cumbria. For the purpose of this book, however, the area considered is limited to present-day North Merseyside (Liverpool, Sefton, Knowsley and St Helens) plus 'political' Lancashire, the latter including the various unitary boroughs and those areas within Vice County 64 for which records were provided by the Yorkshire branch of Butterfly Conservation.
For the majority of butterfly species there are too few regularly-monitored transect walks in the county to give a clear idea about changes in population size. The species accounts therefore largely deal with changes in distribution rather than populations and are usually expressed in terms of the number of tetrads occupied. In order to get a handle on changes over time, distributions are compared prior to and after 2005, a date chosen because approximately half of all records of all species come from either side of it. Thirty-four species have been recorded annually in recent years.