Common Cuckoo (Cuculus canorus)

The drama begins in April, when the Cuckoos arrive from their African winter quarters and the males begin making their loud advertising calls. Few sounds are so resonant of the season. The less demonstrative females, whose call is a loud but infrequent bubbling trill, cannot fail to make contact with such noisy operators, but once met the sexes have no need of much co-operation in a season, so relationships between them are brief, and both sexes are probably promiscuous. The males hold “song-territories” for the purposes of attracting females, while the latter hold their own exclusive “egg-territories”, containing a supply of the nests of their host species.

A female instinctively seeks out the nests of the same species that reared her, a trait that is passed genetically along the female (but probably not the male) line. A club of females specialising in the same host is known as a “gens”, plural “gentes”. The gentes present differ from place to place. In Britain, for example, the four main hosts and their gentes are Reed Warbler, Meadow Pipit, Dunnock and Robin, whereas in Scandinavia they are Redstart and Brambling, and in some parts of Central Europe Great Reed Warbler and Pied Wagtail are important. All small insectivorous birds are potential foster-parents, but a certain species’ degree of vulnerability to becoming a regular host depends on several factors. Reed Warblers, for example, have rather variable eggs, so they are relatively poor at noticing the new incumbent. If a species has eggs of consistent colour the Cuckoo will often mimic them, but this does not necessarily mean that the host will accept them – it all depends on how “wise” a certain species has become to parasitism. Cuckoos and hosts are set in a sort of permanent “arms-race”, with the hosts working towards the rejection of all Cuckoo eggs and the parasites aiming for full acceptance.

A Cuckoo with plans for its hosts is a bird-watcher. It spends much time hidden away on a high perch in a tree, observing the stage that certain pairs have reached, and working out where the nests are. It is important that it remains concealed because it neither wishes for its cover to be blown, nor does it wish to evoke an aggressive mobbing response from the local community, which can at times be dangerous. On some occasions, however, to be mobbed is useful. If a host, such as a Meadow Pipit, has widely spaced, well hidden nests, the only way to find them might be to make a full frontal approach, and gauge where the nest is by monitoring the degree of agitation of the hosts – a technique equivalent to the game we play as children, leading someone to a hidden item by saying they are “warmer” or “colder”.

Ideally the female needs to act when the clutch is incomplete; that way, an egg can be added and a host’s egg removed before the host starts incubating. The timing is careful, too. Most small birds lay their eggs at dawn, but the Cuckoo strikes in the afternoon, at a time when, with incubation not yet begun, both adults may be away from the nest regularly. The female steals in and lays its egg within a few seconds, departing with a host egg in its mouth in exchange; the latter egg will be eaten. In the course of a season, the female may lay in as many as 20 nests.

So long as it is not rejected the young Cuckoo develops faster than that of its hosts, having been given a head start by beginning its growth in the female Cuckoo’s oviduct. Once it hatches it has a long rest and then, as is known throughout the world, it ejects all its nest-bound competition there and then. It hauls eggs or nestlings to the rim of the nest in the small of its back and, with a final effort like a pregnant woman’s last push in childbirth, it despatches them over the side, to certain death. Occasionally two Cuckoos hatch in the same nest, and a real battle is joined.

The young Cuckoo goes on to drain every drop of effort from its overworked foster-parents, begging constantly for more with the bottomless pit of its gape opened wide. It grows fast on their labours and, since its beseeching calls are such a powerful stimulus, it will often enlist the efforts of other small birds roundabout which, against their better judgment, find themselves queuing up to take part in food deliveries, too. Finally, gorged on ill-gotten gain like a benefit fraud, it takes leave of these services and goes out to prepare for its lone migration southward. All Cuckoos, adults and young, are gone by the end of September.