EVEN UNHATCHED YOUNG CAN LEARN
Whenever there is a case of rearing the offspring mothers do have some extraordinary qualities compared to fathers, whether it is in humans or other animals. Among birds there are many species in which females are able to communicate with their chicks even when they are not hatched.
Mallard (Anas platyrhynchos) females, belonging to the group of dabbling ducks, emit a species-specific maternal call, during incubation or brooding, which unhatched ducklings listen, understand and respond. The instinct of the mother tells her, when to communicate. It starts making call when in-embryo duck’s head projects into the airspace of the egg, about 17 to 19 days after laying. This helps the ducklings to recognize the call, which they will hear in the near future after coming out of the shell.
Member of the family Anatidae mallards are placed under the group of dabbling ducks because they feed mainly at the surface rather than by diving. Another interesting aspect of these birds is that can be fooled into laying, much beyond their normal clutch-size by removing their eggs as they are laid. In this way, mallards were fooled to lay, up to 146 eggs!
These birds are killed in large numbers. In 1984 alone, it was among the most commonly shot bird in the USA, when 3,954,100 mallards were killed whereas the number in the following year was 3,234,800.
Besides shooting, they also die in large number due to lead poisoning. Usually they are poisoned through anglers’ discarded or lost split-shot and shooters’ discarded shotgun pellets. These birds along with some other species of dabbling ducks are especially attracted to these pellets because they feed largely on hard, round seeds of aquatic plants, which resemble pellets. Destruction caused due to lead poisoning can well be judged by the fact that on one occasion in Illinois (USA) 110,000 mallards died because of lead poisoning.
Other bird which communicates with its unhatched chicks is Superb fairy-wren (Malurus cyaneus). The female sings to its unhatched eggs to teach the embryo inside a ‘password’ — a single unique note — which the nestlings must later incorporate into their begging calls if they want to get fed.
This allows the parents to differentiate between their own offspring and those of the two cuckoo species that frequently invade their nests. The females also teach their mates the password.
Fairy-wrens are known to make a distinction between their own nestlings and those of the cuckoos’ on the basis of their foreign begging calls, says Sonia Kleindorfer, an animal behaviourist at Flinders University in Adelaide, who led the work. But it wasn’t known that wren nestlings learned the passwords before hatching (Current Biology – November 2012).
Discovery by accident
It was an accidental discovery for the researchers. While they were making recording inside the birds’ domed nests in search of anti-predator calls, they noticed that female wrens were singing to their unhatched eggs.
When Kleindorfer and her team analysed recordings made over the full nesting cycle, they found that the all nestlings in a given nest had the same unique begging call. This call contained a signature element present in the call the mothers had made while incubating the eggs, and in the call she used to solicit food from the father. When the researchers broadcast a foreign nestling call at the nests, both the parents refused to feed the chicks.
To test if the begging call was learned or genetic, Kleindorfer swapped around eggs across 22 nests. When these eggs hatched, nestlings used the call taught by their foster mother, not their biological mother.
Although cuckoo eggs get incubated alongside the wren’s eggs, it seems that cuckoo embryos don’t have enough time to learn the password well. The lessons begin about 10 days after the eggs are laid, giving wren embryos around 5 days to pick up the call before hatching, but cuckoo embryos, which hatch earlier and then push out any other eggs, only get about 2 days. This means that victimized parents can escape having to feed an enormous baby bird that isn’t their own, and can leave to start a new nest.
Wren’s aren’t perfect at spotting cuckoos, though. They can always identify one species, but catch the other only around 40% of the time. Kleindorfer says there is evidence that, in the latter species, the cuckoo nestlings attempt to guess the password by trying out different calls.
Kleindorfer agrees. “There are many different scenarios where mother-to-egg communication would be useful,” she says, “for example to identify relatives or non-relatives.” It could also offer females an extra chance to favour certain cultural traits in the next generation. “It is a new perspective on the battle of the sexes,” says Kleindorfer.
Canary mothers (genus Serinus) give a very important message to their developing chicks about the life they will be going to face after they hatch. In response, nestlings adjust the development of their begging behaviour.
If the message is that they will be reared by generous parents then they beg more vigorously for food after hatching. But if they are destined to be raised by meaner parents they end up being much less demanding.
By attending to messages in the egg, nestlings gain weight more rapidly because they match their demands to the parents’ supply of food, and can avoid either begging too little or wasting effort on unrewarded begging.
The Cambridge team made the discovery using fostering experiments, exchanging eggs between canaries’ nests so that the chicks grew up in an environment that they were not expecting.
“This work changes our understanding of the pre-natal environment in birds,” says Dr Rebecca Kilner of the Department of Zoology, University of Cambridge, who led the research.
“We’ve known for about twenty years that maternal substances in the egg can influence how chicks develop, but the common assumption is that they are a means by which mothers manipulate their offspring in a way that suits the mother more than the chick.
“What we’ve shown is the reverse: these substances are actually there to suit the chick. If we muck up the message in the egg experimentally, it is the chick that is penalised directly rather than the mother.” The work was funded by the Natural Environment Research Council, and was published in Science on March 12, 2010.
In the domestic chickens also mothers and their offspring begin communicating before the chicks the chicks are born, with the mother clucking softly to her babies, and the chicks peeping back from inside their shells to let her know that they will soon begin hatching.