I see huge numbers of these butterflies in those parts of the countryside where its main food plant, Lotus corniculatus (Bird's-foot Trefoil) grows. If that is in shortish grassland then you can easily spot the pupae attached to grass stems. The male is blue but the female varies in colour in the British Isles being mostly brownish blue in the South and quite blue in Scotland.

Newborough Warren, Anglesey 29th May 2008

Added on 25th June 2008, updated November 6th 2008, updated 9th June 2010