Brimstones are quite common especially in spring, the almost white females contasting with the darker yellow males.
Fleam Dyke (from Balsham to Fulbourn) is a good spot to see Brimstones. On a hot still day (like 19 April 2011) up to 20 or so can be seen along a short stretch on the Fulbourn side of the A11.
Except when hibernating they may be seen on the wing at any time of year with a gap in mid July before the fresh new hatchings appear.
They have just one brood per year, the adults hatching in August, and hibernating as adults before taking to flight again in the warmer days of spring. They breed in early summer, laying their eggs on Buckthorn or Alder Buckthorn.