The butterfly is multi-brooded in warmer parts of it's range, but in Britain it normally produces only a single generation, emerging from July to mid August - the progeny of adults that migrated northward from the continent in May or June. In particularly warm summers there is often a partial 2nd brood that emerges in October. These late emerging adults and any resulting eggs or caterpillars normally perish in early winter, but at a tiny handful of sites on the south coast larvae occasionally overwinter successfully, producing a few adults in February or March.

The eggs are laid singly or occasionally in two's or three's, on the upper surface of leaflets of red clover Trifolium pratense, lucerne Medicago sativa, horseshoe vetch Hippocrepis comosa, bird's foot trefoil Lotus corniculatus, sainfoin Onobrychis viciifolia, kidney vetch Anthyllis vulneraria, or black medick Medicago lupulina. They are pale yellow at first, quickly turning to pink and finally to orange. They hatch after about 7 - 10 days.

Clouded Yellow caterpillars are rough textured, dark green in colour, with a pale yellow lateral stripe and orange dashes between the spiracles. They feed diurnally on the leaves of the foodplants.

Caterpillars hatching in May and June feed up and become fully grown within about 3-4 weeks, but those of the 2nd brood feed very slowly and usually perish with the arrival of the first frosts in early November. In warmer regions of Europe they continue to feed during the winter months, and do not enter a state of diapause.

The pupae are yellowish green, marked on the wing cases and abdomen with tiny black dots. They are extremely difficult to find in the wild, attached by the cremaster and a fine silk girdle to stems of the foodplants or other nearby plants. The adults emerge after about 18 days.

Both sexes fly very rapidly and purposefully, migrating in search of breeding sites, stopping for a day or two to refuel at nectar-rich hillsides. Once they discover a suitable area with an abundance of the larval foodplants they tend to remain for several days.

On warm summer days they spend long periods flying restlessly from flower to flower, nectaring at small scabious, field scabious, devil's bit scabious, knapweeds, cat's ear, stemless thistles, mouse-ear hawkweed, marjoram, clovers, trefoils and vetches.

When feeding they remain extremely alert, and rarely settle for more than 3 or 4 seconds at a time. A minor and temporary drop in temperature however is enough to cause them to enter a resting phase, when they settle in rabbit scrapes or other depressions, often remaining stationary for several minutes at a time.

On sunny mornings males patrol back and forth over the breeding sites, exhibiting a regular cycle of nectaring, resting and searching for potential mates. Females are nearly always mated prior to their maiden flight - I have often found copulated pairs in which the females still had limp wings, and had clearly only just emerged. The butterflies remain joined for about 2 hours, during which time they usually remain stationary, hidden among grasses. If disturbed however they readily take flight, with the male carrying the female. Normally they resettle nearby on the ground, or on bushes, but I have known them to fly up to land high in the tree tops.

Gravid ( already mated ) females are often intercepted by patrolling males. In October 2009 at Beachy Head in Sussex I watched a male pounce on a freshly emerged resting female. He danced around her, then landed behind her and tried to walk into the gap between her almost-closed wings. She then opened wide and raised her abdomen, and he tried to pull it down with his forelegs. Within about 2 seconds he realised he was onto a loser and flew off. The female then flapped her wings a couple of times ( difficult because her wings were still limp ) and then sat in the grass with them half open for another couple of seconds. Then she closed her wings and remained stationary to complete the wing drying process. I was very interested in the fact that she rejected the male, even though she had clearly only just emerged. The implication is that she must have already mated with another male, immediately after emergence. Although it was a cold day, wing expansion and drying should still only take about an hour, so in that time she must have mated, parted with her mate, and then had the encounter with the second male :

In late afternoon both sexes seek roosting sites. Sometimes they spend the night beneath bramble leaves or other foliage, but normally they tuck themselves away among grasses. If a large number of Clouded Yellows are present at a site, they often roost in pairs, or even in threes.