The Japanese researchers became curious about the begging behavior of the Horsfield's hawk-cuckoo after seeing a photograph of a feeding chick displaying its unusual wing patches.
To investigate further, the biologists took to subalpine forests in the foothills of Mount Fuji, where the cuckoos lay their eggs in the nests of red-flanked bush robins.
Upon hatching, the single cuckoo chick evicts its bush robin nestmates to gain the undivided attention of the unwitting foster parents. But, the researchers write, "eviction carries a cost, because lone parasitic nestlings attract a reduced provisioning rate."
For these cuckoo chicks, boosting the volume of their begging calls to compensate wouldn't appear a sensible option, because bush robins nest on the ground, making nestlings extremely vulnerable to predation.
The researchers say that nestlings in at least half of the nests they studied were eaten, mainly by martens and weasels. Not surprisingly, hawk-cuckoo chicks keep noise levels to a minimum.
Having observed the chicks quivering their yellow wing patches during feeding, Tanaka and Ueda tested whether this was, indeed, an alternative way to solicit extra food.
When the two biologists covered the wing patches of some chicks with black paint, the length between feeding intervals increased markedly. This suggested that the patches act as begging signals.
According to Tanaka, the amount of food provided to the painted chicks also decreased by up to 30 percent.
The researchers suggest these wing patches are used to mimic the key visual begging cue for bush robin parents: a nest filled with bright, gaping beaks. The biologists say they even observed adult bush robins trying to stuff these imitation beaks with food.
Even though the wing patches aren't shaped like open beaks to human eyes, they closely match them in color. Tanaka says that in the dark nest, parents probably can't tell the difference.
"Parents have a mechanism for assessing the need of [their] whole brood, including the number of gapes on display and the rate of calling," Kilner, the Cambridge University cuckoo expert, said.
She adds that her own studies of reed warblers and great tits in Britain suggest that these vocal and visual begging cues are roughly of equal importance to parents.
A cuckoo chick must exploit these begging signals, because its food requirements closely match those of its host's entire brood. This is graphically illustrated by the sight of a huge, grub-guzzling cuckoo chick dwarfing its "parents" as they struggle to keep up with its demands.
"The European cuckoo decides to prioritize the vocal cue to exploit the parents, so uses a vocal trick," Kilner added, "whereas this Asian cuckoo has a visual trickthe fake patches simulate the presence of many mouths."
And what a neat trick it is.
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