One of the pairs of reed warblers on my local nature reserve is playing host to an unwanted visitor, in the shape of a young cuckoo. Perhaps this should be unsurprising as cuckoos have been seen and heard locally since the end of April. The cuckoo is an unusual bird; not only does it lay its eggs in the nests of other species, but it also tackles the hairy caterpillars which are unpalatable (or even toxic) to other creatures. The first of our cuckoos to arrive are male; the females, on average, arriving a week or so later. Each bird (both male and female) will set up its own territory, the size of which appears to be dependent upon food availability, the number of vantage points and, importantly, the abundance of host species. A female cuckoo, for example, may have a territory that is as small as 30ha if the density of host species is sufficiently great. This circumstance may come about if she happens to specialise in parasitizing reed warblers, which often nest semi-colonially, and so occur at high density.
Although young cuckoos have been recorded in the nests of more than 40 different species of bird, it is the reed warbler and meadow pipit that are most commonly targeted in Britain (together making up more than 85% of the cuckoo nest record cards held by the British Trust for Ornithology). Each female specialises in a particular host species and will seek out unguarded nests into which she may deposit her eggs. Over the course of the short breeding season (adult cuckoos usually depart towards the end of June) she may have deposited 25 eggs. Such specialisation is the key to her being able to pass off one of her eggs as that of the host species.
The cuckoo chick instinctively ejects any young or unhatched eggs belonging to its adopted parents. In this way it will receive all of the food that the parents bring in to the nest. Such large quantities of food are essential if the chick is to grow rapidly, often gaining 10 times its hatching weight during the first week alone. The rapid develop means that it soon outgrows the nest and will leave in under 18 days, though it does remain dependent upon the hosts for a few more weeks. Although the presence of a cuckoo can mean that as many as 20% of the suitable nests in the area are parasitized, this appears to be the exception rather than the rule. What is clear, however, is that declines in favoured host species could be behind the current decline in cuckoo numbers. The presence of this cuckoo chick, then, should be welcome.