Cuckoo (Cuculus canorus)
C. canorus is a familiar herald of spring and a species that is thinly distributed over a wide breeding range throughout most of Britain, Europe and the Palearctic. It is a fairly common but rapidly declining summer visitor to Sussex, where the males commence singing soon after their arrival; typically in early April. Recent satellite tracking technology has shown that the males usually leave the UK to their wintering area south of the Sahara only a few weeks after first arriving here in the spring.
C. canorus has a well-known breeding strategy as a brood parasite, laying its eggs in the nests of other birds; typically those of the Dunnock (Prunella modularis), Meadow Pipit (Anthus pratensis), Pied Wagtail (Motacilla alba) and Reed Warbler (Acrocephalus scirpaceus) in the UK. The female, who will specialise in one particular host species, will only ever lay one type of egg and will always target her egg-laying appropriately. The genes for egg pattern and colour are thought to be carried on the female chromosome, so they are passed down from mother to daughter regardless of who the female mates with. This means that a Cuckoo targeting a Reed Warbler always produces eggs with a Reed Warbler pattern and she knows which nests to target thanks to the process of imprinting. As a young chick, she will have learned to recognise the song and appearance of her foster mother; and as a returning adult she will seek out the nests of females that match this mental image and lay her eggs accordingly.
The above images of a female were recently taken on North Uist in the Outer Hebrides.
Yates, B. (2014). Common Cuckoo. The Birds of Sussex. Thetford: British Trust for Ornithology (BTO) Books on behalf of the Sussex Ornithological Society, pp. 360-361.
Whitcomb, P. (1996). Cuckoo. Birds of Sussex. Sussex Ornithological Society, pp. 358-359.