Meadow browns overwinter as caterpillars that pupate in the spring and begin to hatch into adults just about now. Temperature and daylight hours are very important triggers in the life cycles of some species of butterfly, so many meadow brown caterpillars will have begun the process of pupation on the same warm day in spring. Now, on the first warm day after a week of rain, those adult butterflies all emerge together. Some years such hatches can be spectacular.
Meadow browns are still common and widespread, although, like most British butterflies, their numbers are in decline. Changing agricultural practices seem to have been the major culprit.
There is less haymaking now and more silage or haylage growing and farmers make several cuts per year, interrupting the life cycle of the meadow brown, the larvae of which feed on grass. The replacement of traditional meadows of native grasses by perennial ryegrass, which meadow browns cannot feed on, has triggered a decline of the species in areas of intensive arable agriculture.
It has been estimated that more than 95% of Britain’s traditional hay meadows have been lost since the 1950s. The park’s species-rich meadows, which are usually cut only once a year, are therefore important habitat for this butterfly.
Meadow brown caterpillar (first instar) by Giles San Martin (CC-BY-SA-2.0)