BioEd Online

Birds that live in bunches work each other up into a reproductive frenzy with their songs, according to research that confirms an old hypothesis.

As far back as the 1930s, ornithologists proposed that large, sociable colonies of birds would tend to have earlier, bigger and more closely synchronized clutches of eggs.

Known as the Darling hypothesis, after F. Fraser Darling who first suggested the idea, it has finally been supported by experiments in the laboratory, and the research appears online in the Proceedings of the Royal Society¹.

Postitive feedback

To test Darling's hypothesis, the researchers set up two indoor colonies of the zebra finch (Taeniopygia guttata), a smart little Australian bird often seen in pet shops.

The first group of birds was played recorded sounds of its own colony, but the second group heard a playback that blended its own colony sounds with noises from extra finches.

Females in the second group had more eggs, laying them earlier and more synchronously than controls, confirming the theory.

Peter Boag, a biologist at Queen's University in Ontario, Canada, who worked on the study, explains the advantages of this mating pattern. It is probably beneficial for a bird to have its chicks at the same time as the couple on the next nest, he says. With more chicks around, the risk to each individual chick from predators is reduced.

It is also advantageous for a female to start laying early in the season, because this gives her more time to invest in her brood and makes it likely that she will fledge more chicks successfully.

However, laying too early will isolate chicks and put them at risk, so how do females decide when to lay?

Safety in numbers

Finches are known to use environmental cues like rainfall and length of the days, but the experiment by Boag and his colleagues shows that they also respond to bird calls associated with reproduction. After all, if everyone else sounds as if they are laying, it's probably safe to lay your eggs too.

"A single bird on its own might be right or might be not right," says Boag, "but if you have fifty birds reacting to those cues, and if the majority decides it's time to breed and they get excited, then they sing a lot."

He suggests that the volume of social sounds acts as a kind of information feedback loop. "It leads almost to a crescendo and feeds upon itself and suddenly, boom: everybody mates," he says.

The exact mechanism for the effect is still being worked out, says Boag, but other studies have shown that hearing social sounds can cause changes in hormone levels in birds.