Monstrous cuckoo in the nest
‘Is there anything more extraordinary in the natural world?’ asks Nick Davies, Professor of Behavioural Ecology at Cambridge.
Standing on a ditch bank in the Fens, he parts the reeds with his long hazel stick and recognises the neat, woven cup of a warbler’s nest.
‘Sprawled on top, its wings dangling over the rim on either side, sits an enormous cuckoo chick,’ he says.
Before long, its reed-warbler ‘foster mother’ arrives with a bright blue damselfly in her bill. Minuscule in size compared with the squatter in her nest, she responds to its frenzied calling, bending deep into its mouth, her head nearly engulfed, to deliver her offering.
‘Another twitch of the reeds, and she slips away in search of another meal,’ says Professor Davies.
This ‘monstrous outrage on maternal affection, against the dictates of nature’, as the Reverend Gilbert White of Selborne judged it, could scarcely be more extraordinary. The reed warbler is after all a most resourceful and intelligent creature – as it has to be in making its annual 5,000-mile journey from its African winter home, navigating by the stars and memorising the landmarks that guide it to its territorial breeding grounds in the Fens.
How almost inconceivable, then, to human understanding at least, that she should be deceived into so grotesque a parody of motherhood. Almost blithely, she tolerates the destruction of her own offspring, devoting all her energy instead to nurturing this alien intruder.
This is a complex, fascinating (if ultimately unfathomable) story, painstakingly unravelled by Professor Davies and others over the past 30 years.
Much was already known, thanks mainly to the efforts of Edgar Chance. A wealthy businessman and assiduous egg-collector, he filmed, for the first time, the cuckoo in flagrante delicto, as it were.
The silent, 12-minute, black-andwhite documentary, with captioned commentary, opens with Mr Chance in tweed cap and plus fours clambering into a specially constructed hide.
Then we see a cuckoo who, the caption tells us, had the previous year laid 25 eggs, all in different meadow-pipit nests. She swoops down, steals one of the pipit’s eggs, lays one of her own in its place, and departs – all in a matter of seconds.
Twelve days later, we see the recently hatched cuckoo chick, naked and still blind, manipulating one of the pipit’s eggs into a hollow in her back and pushing it over the nest rim. He then turns his attention to the two already hatched pipit chicks and disposes of them in a similar manner. His foster mother returns and settles down to keep him warm, oblivious of her own young writhing just an inch or so away. In the final sequences, he grows ever larger, dwarfing his hard-working foster parents who must stand on his back to feed him.
‘The cuckoo’s secret,’ notes Professor Davies ‘ranks as one of the greatest feats of ornithology.’ He has elaborated on his findings in a protracted series of experiments, superbly described in his book Cuckoo: Cheating by Nature.
The best-known of its many deceptive stratagems is in laying an egg coloured similarly to that of the host who will rear her young.
This presupposes the seemingly implausible necessity for there to be several different races of cuckoo – warbler-cuckoos with green speckled
eggs, pipit-cuckoos (brown), wagtailcuckoos (greyish white) and so on.
Investigating this further, Professor Davies placed green, brown and white model eggs in reed-warbler nests. Sure enough, the reed warbler ejected any discordant egg, retaining only the green one.
The stealth and speed with which the cuckoo lays her egg would suggest the reed warblers’ awareness of her extended presence might alert them to the danger of being victimised. Professor Davies confirmed this by placing a taxidermist’s mount of a cuckoo in close proximity to a nest in which he had placed a mimic green egg. This too was rejected.
It is one thing for the reed warbler to be deceived by an egg similarly coloured to its own. It’s quite another for it to behave as if there is nothing amiss in nurturing the cuckoo chick for several weeks. Here the deception is not visual but vocal. The foster parents respond to the chick’s characteristic loud and demanding calls, but not to those of a blackbird temporarily substituted in their nest.
This brief account can only hint at the panoply of subtle ploys with which the cuckoo and her chick perpetrate their crime. One might reasonably wonder at how they synergistically arrived at so monstrous a confidence trick ‘against the dictates of nature’.
‘The cuckoo’s secret ranks as one of the greatest feats of ornithology’