Singing for their supper
The superb fairywren is a bird that uses special methods to keep free-loaders out of her nest.
S.Ananthanarayanan
The Old Testament has an account of the device that the army of Gilead, a mountainous area on the Jordan River, used to identify the raiders from the tribe of Ephraim. The raiders were fleeing after a military defeat and were trying to pass off as Gileadites.
The Gileadites asked suspects to say the word, ‘Shibboleth’. The Ephraimite habits of speech made Ephraimites pronounce the word as ‘Sibboleth’, which gave them away.
Diane Colombelli-Négrel, Mark E. Hauber, Jeremy Robertson, Frank J. Sulloway, Herbert Hoi, Matteo Griggio and Sonia Kleindorfer from Flinders University, Adelaide, City University of New York, University of California at Berkley and the University of Veterinary
Medicine, Vienna, have written in the journal, Current Biology, of a similar trick that a species of bird employs to identify parasitic, look-alike but alien fledglings that have found their way into the nest.
The superb fairywren is a perching songbird native to Australia and is frequently a victim of parasitic egg-laying in her nest. This is an instance of brood parasitism, where birds, insects and some fish manipulate a host, of the same or another species,
to raise the young of the parasite as her own. The parasite usually manages to do this by brood mimicry, or laying eggs that resemble the eggs of the host and can hence pass off as one of the host’s own.
A well known brood parasite is the common cuckoo. While individual females parasatise specific species, the whole cuckoo species parasatises a variety of hosts. This may be because individual birds learn which species to parasatise according to where
they were hatched and hence pass on the trait to their female offspring. The males, however, do not distinguish between females and there is no birth-place-based genetic separation within the species.
The incentive for brood parasitism is relief from the travails of incubating and rearing fledglings and the freedom to do other things, including having more offspring for other birds to bring up. There is hence evolutionary adaptation to lay eggs that
pass off as those of other species. The host species, however, bears a high cost, in terms of its own survival. The cuckoo fledgling, in fact, is larger than its foster siblings, and pushes them out the nest, to get all the foster parents’ attention. The
host species hence adapts away from being parasatised and there is an ‘arms race’. The parasites may even attack and destroy nests if the hosts act to reject the alien eggs. Where hosts could peck and damage alien eggs, the parasites have evolved to have eggs
with a tougher shell, so that it is the host’s own eggs that may be damaged. And so the race goes on.
The paper in Current Biology says the superb fairywren has developed a unique defense. Many host species, including the fairywren, cannot identify alien eggs, but can make out the parasitic nestlings. When this happens, they abandon the nest and start
afresh. The fairywren is also able identify alien fledglings, but when she does not, the parasite fledgling pushes out the rightful occupants of the nest and develops an imitation of their begging vocalization, to obtain more food from the unwitting foster
parents. And hence, the paper says, the arms race in evolution of the host-parasite species becomes an acoustical one, of recognition using sounds. One species, the Horsfield’s bronze cuckoo is able to imitate the begging calls of its host, but another species,
the shining bronze cuckoo is not, and is less frequently a brood parasite of superb fairywrens, the paper says.
The authors of the paper then say the superb Fairywren has developed a complex adaptation to make out alien chicks, a method based on an acoustical identifier learnt by the fledglings even before the eggs have hatched. Her own eggs hatch some 15 days after
they are laid. When the embryos are largely formed, about 5 days before hatching, the mother produces a short, high pitched trill, some 1.8 seconds long, about every 4 minutes, or about 16 times every hour. And the vocalization stopped as soon as the
eggs had hatched.
And then, when the eggs had hatched, it was seen that the chicks used a portion of the mother’s call, the ‘signature’ element, as their begging call. This element was different for different mother birds and significantly similar to the calls of the birds’
own chicks. The mother also repeated the call to the male, the father, so that the father could also make out the specific begging calls of the fledglings. Both the parent birds could hence make out the begging calls of their own chicks and hence tell when
the chicks were alien.
A fair question to raise is why alien fledglings also do not learn the signature trill while they are incubating in the hosts’ nest. The reason is that the parasitic cuckoo eggs that are laid in the host’s nest hatch within about 12 days of being laid.
The cuckoo embryos hence received only3 days of exposure to the mother’s calls, against the 5 days of the wren embryos. Alien chicks, when they hatch, usually push out the other eggs in the next. This, however, does not take them far, as the alien chicks
were have not learnt the begging calls, their ‘Sibboleth’ is made out. And the parent wrens do not feed them.
Control tests, where the eggs laid in one nest were swapped with the eggs of another nest showed that the chicks learnt the begging calls of the foster mother. This shows that the calls were not innate but were learnt. It was also found that playing incorrect
begging calls from a loudspeaker stopped the parents from feeding their nestlings.
” There is now no doubt that some seemingly innate traits are the result of experience during the embryonic stages,” the paper says. The paper notes that our current understanding of how connections between cells in the brain develop and change had its
beginning in the bird model. Further work, with rats and monkeys, showed how changes in the brain led to movement of muscles, and then to use thought to work robotic arms. The work reported in this paper, of learning by superb fairywrens before they are born,
shows that the bird model could be suitable for more study of prenatal learning and how it is organized in the structure and interconnections of the brain cells.
[the writer can be contacted at response@simplescience.in]