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Early bird gets the better song

Mothers transfer the gift of music to the first eggs of their brood.

Which one of these eggs will produce the best singer? Credit: KAKIMAGE / Alamy

Birds that come from the earlier eggs in a brood are more likely to be better singers, scientists have found.

In most bird species, song is used by males to demonstrate their fitness to potential mates, and many studies have shown that the healthiest males tend to sing the longest, loudest and most complex songs.

Masayo Soma — who researches biolinguistics at the Riken Brain Science Institute, in Wako, Japan — and her colleagues wanted to find out if the order in which birds hatch affects their song. "I expected to detect age hierarchy in song, because older siblings are stressed less and obtain more resources growing up," says Soma.

To test the idea, her team cross-fostered Bengalese finches (Lonchura striata domestica) so that the age hierarchies formed in fostered broods were independent of the order in which the eggs were laid. Nine pairs of finches raised a total of 16 clutches of four chicks. Nine more adult males were also introduced to breeding cages at time of fledging so the young heard more than one bird's song.

When the fostered finches had matured, the researchers recorded their songs. As they report in Behavioural Ecology and Sociobiology1, their initial hypothesis was wrong: hatching order and nest hierarchy had no noticeable impact on the songs.

But the order in which eggs were laid was important in determining how complex a particular hatchling's song would be when it matured. Finches hatching from earlier eggs in a brood consistently sang songs that were more complex than those of siblings coming from eggs laid later.

Egg surprise

"Evolutionary ecologists are increasingly realizing that females of some species can differentially allocate key resources in their eggs," says Roxana Torres, a biologist at the National Autonomous University of Mexico, in Mexico City. This can have profound long-term effects on the abilities of their offspring.

But scientists are still unsure about what these resources are. "Androgens are involved in the mechanisms of sexual development, and androgen concentrations in egg yolk are likely to be influenced by egg order," Soma says. Other researchers have already examined whether experimentally enhanced yolk-androgen levels improve individual bird conditions and the results have been mixed, she says, with some species showing substantial health advantages from increased androgens and some showing adverse effects, with immune function being compromised.

László Garamszegi, a biologist at the University of Antwerp in Belgium, suggests that maternal testosterone could be helping to build the parts of the bird's brain responsible for song production. However, he concedes that other molecules, such as carotenoids and the vitamins A and E, may also be stimulating the maturing brain in the embryo.

Soma says that whatever the chemical cause, it is unusual for altricial birds - those that have chicks who cannot fend for themselves, such as these finches - to give their first offspring additional advantages over their siblings.

She now hopes to conduct experiments that add androgens and other compounds into the eggs of finches to see what effects it might have on song quality.

References

  1. Soma, M., Hiraiwa-Hasegawa, M. & Okanoya, K. Behav. Ecol. Sociobiol. doi:10.1007/s00265-008-0670-9 (2008).

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Kaplan, M. Early bird gets the better song. Nature (2008). https://doi.org/10.1038/news.2008.1188

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  • DOI: https://doi.org/10.1038/news.2008.1188

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