A yellow thought in a purple shade …
Of the three species of Clouded Yellow recorded in the British Isles, the Clouded Yellow Colias croceus, Berger’s Clouded Yellow C. alfacariensis, and the Pale Clouded Yellow C. hyale, C. croceus is by far the most frequently encountered and the easiest to identify. The latter two species are very similar in appearance and occur only as very rare immigrants.
In flight, croceus shows a rich orange coloration that is distinctive among British butterflies. When settled, the lemon-yellow underside provides a useful diagnostic feature, separating it from both alfacariensis and hyale, which exhibit noticeably paler undersides. This distinction generally remains reliable even in the helice form of the female croceus, a well-known polymorphic variant in which the usual orange ground colour is replaced by a soft creamy white. By contrast, distinguishing alfacariensis from hyale in the field is considerably more difficult; without regular experience of either species, confident identification of the adult butterfly is unlikely.
As a strong-flying and migratory species, C. croceus has a wide and variable distribution, and may occur throughout much of the British Isles. It is found in a range of open habitats, including coastal cliffs, calcareous grassland, and agricultural land supporting its larval host plants (primarily Fabaceae). Many immigrants remain near coastal areas, where they feed, mate, and oviposit, while others disperse inland, reaching as far as Scotland and Ireland in favourable years. Annual abundance varies considerably, but during major immigration events, such as that of 1947, very large numbers have been recorded, with estimates reaching 36,000 individuals.
References:
https://www.ukbutterflies.co.uk/species.php?species=croceus [Accessed, 20th March 2026].
https://www.ukbutterflies.co.uk/species.php?species=hyale [Accessed, 20th March 2026].
https://www.ukbutterflies.co.uk/species.php?species=alfacariensis [Accessed, 20th March 2026].