I feel sorry for this pair of Reed Warblers, working hard to raise a chick that is not their own. Completely filling the nest cup, the grey-brown, ever-hungry youngster is an impostor, a nestling Cuckoo that has dumped the warbler’s own chicks or eggs overboard so that it may dominate the attentions of its foster parents. The chick is already larger than the two birds that deliver food into its demanding gape and it will grow much larger still. However, I have a certain amount of respect for this bloated, duplicitous lodger. After all, it is our only parasitic bird and it is fascinating to ponder on the evolution of such curious behaviour.
Globally, our Cuckoo is not alone in engaging in brood parasitism (to give this behaviour its technical name); it has 49 parasitic relatives and the behaviour also occurs within four other bird families. Even so, such parasitism is rare among higher animals and it is easy to see why our Cuckoo has been so well studied.
Adult Cuckoos are renowned for favouring hairy, sometimes toxic, caterpillars that are usually avoided by other bird species. Early in the season Cuckoos devour large numbers of Drinker moth caterpillars, collecting them from damp open habitats, but they also take other species, including the Magpie Moth, whose caterpillars show warning colouration and are distasteful to most would-be predators. Nestling Cuckoos are fed on whatever food each particular host species would normally provide for its own chicks and, for this reason, favoured host species are invariably insectivorous. These Reed Warbers will be feeding this particular young Cuckoo on aphids, moths and even the occasional butterfly. It is only upon reaching independence, at roughly five weeks of age, that the young Cuckoo will switch to feeding upon the kind of caterpillars favoured by the adults.
The act of parasitizing a nest is not simply an opportunistic one, since each female Cuckoo will specialise in a particular host. Only by doing so can she produce eggs that are close enough in colouration and pattern to the host’s own eggs to sneak one in undetected. Female Cuckoos will therefore work an area, locating nests of the host species and checking on their status. If a particular nest is unsuitable, perhaps because it is at too advanced a stage, then the female Cuckoo may predate the nest. This forces the unfortunate hosts to re-lay and thus creates a new opportunity for the Cuckoo.
Our Cuckoo has recently been flagged as being of conservation concern because of a long-term decline in its numbers. As such, it is good to see this particular Cuckoo nestling doing so well, even if one does feel sorry for its hosts.