Report from a park user yesterday:
I saw two butterflies in Sleepers Field this morning – a brimstone and a peacock.
Both of these species hibernate in their adult form and emerge in the first sunny days of the year when the temperature reaches 13°C. In yesterday’s sunshine, walkers in the park unbuttoned their coats, and the first butterflies appeared.
Brimstones usually hibernate among tangles of bramble or ivy in sheltered, sunny places, while peacocks hibernate in dark crevices and holes in trees. Peacocks can also be found in garden sheds and outhouses, and they are one of the two species known to regularly hibernate inside houses.
Brimstone butterfly by Charles J Sharp (CC BY-SA 4.0) wikimedia.org
The underside of the wing of each of these butterflies gives them camouflage in their chosen hibernacula. A peacock’s underwing disguises it perfectly in a crevice in the bark of a tree, or the top corner of a spare bedroom wardrobe. The veins on the underwing of a brimstone, hibernating among evergreen plants, look like the veins of a leaf.
1 
2
Underwing of 1. peacock; 2. brimstone
The populations of both species is increasing:
Brimstone (Gonepteryx rhamni) – Distribution Trend Since 1970’s: +20%.
Peacock (Aglais io) – Distribution Trend Since 1970’s: +16%
Header picture: taken in the park by DKG
Southwick Country Park Nature Reserve 
It is interesting to read about the peacocks over-wintering in sheds and outhouses. This morning, as I went to open up our conservatory ventilators, a peacock was flying about inside trying to get out. It must have over-wintered with us. It didn’t offer to pay any rent, just flew off. This is a beautiful creature and I hope that it can survive outside.
This is the right time of year for the peacocks to wake up. They are nectar feeders and the nectar rich spring flowers will help the females build up strength enough to lay eggs in late April or in May on the new nettles. Leave some nettles in your garden for peacock caterpillars and raise the butterflies that will hibernate with you next winter.