In late April and early May, female Red Admirals can be observed flying around nettle patches in woodland glades. They periodically alight on nettle leaves Urtica dioica, upon which they lay a single egg. Usually a dozen or so eggs will be laid on any sizeable nettle patch. Egg-laying is interspersed with periods of basking and nectaring - typically at blackthorn, crab apple, and wild cherry.

2nd brood adults, which may be of either UK or European origin, are less particular about oviposition sites. They will lay their eggs in any sheltered sunny lowland habitat where the foodplant grows e.g. in July 2008 in my Hampshire garden I watched 2 Red Admirals laying eggs on the same small clump of unkempt nettles. Each laid about a dozen eggs between about 11.00am and 1.00pm. The eggs were all laid singly on the upperside of terminal nettle leaves. The egg-laying bouts, which each lasted about 2-3 minutes were interspersed with periods of basking, and periodic flights to nectar at a Buddleia bush a few metres away. Interestingly a Comma was ovipositing simultaneously on the same nettle patch, and neither species seemed disturbed by the presence of the other.

The larva lives and feeds within a tent of folded nettle leaves, spun together with silk. Every few days, as the leaf-tent gets devoured, the larva moves house, and spins a new tent nearby. If these tents are opened, the spiky greenish-brown larva can be seen within, normally curled in a J shape, head-downwards.

On sunny days the butterflies often bask on tree-trunks, always assuming a head-downwards or sideways facing position. In warm but overcast weather they like to bask on the foliage of bramble, hazel and other bushes.

In early spring Red Admirals nectar at sallow catkins and the blossom of blackthorn and hawthorn. Favoured summer nectar sources include dogwood, hemp agrimony, bramble, marjoram, devil's bit scabious, ragwort, burdock, spear thistle and ivy blossom. They also visit dung, or imbibe mineral salts from damp ground, and attend sap runs on oak trunks.

In gardens they will nectar at buddleia, ice-plant and michaelmas daisies. In orchards they will feed at fallen apples or pears.

Red Admirals are inquisitive, and will often investigate humans entering their territory. On several occasions I have been dive-bombed by individuals that have swooped to make contact with the top of my head. Males are highly territorial, chasing after Commas and Peacocks as well as other Red Admirals. When nectaring however they will accept the presence of other species, and I have often seen mixed groups of Red Admirals, Silver-washed Fritillaries, Commas, and Large Whites gathered on a single flower head of hemp agrimony or buddleia.

At night, on cold overcast days, and during the winter months, Red Admirals roost head-downwards on the trunks or lower branches of oaks, larches and other trees, where the bark-like underside of the wings provides them with excellent camouflage.