Monday, 23 November 2009 Annabel
McGilvray
ABC
The tiny fairy wren has a sophisticated
song, compared to larger birds (Source: iStockphoto)
Big birds may be the loudest, but their songs are unlikely to
have the complexity of their more petite feathered friends, according to new
research.
A bird's song is as much the outcome of the evolutionary battle for sexual
selection as its beautiful plumage.
And a survey of the diverse passerine family of birds suggests that in that
battle, complexity is traded off against volume.
In effect, species that have evolved to have more 'syllables' in their calls
have a quieter song, while than those that communicate with simple trills can
trumpet them out.
Surprisingly, the findings by Dr Gonçalo Cardoso, a Portuguese ornithologist
based at the University of
Melbourne, show that among passerine species - which include finches,
sparrows and ravens - the loudness of the call is even more closely linked to
its complexity than to the bird's size.
Cardoso's work, published in the Journal of Evolutionary Biology, is the first to
demonstrate such a link across species and points to an explanation for how the
diversity of birdsong evolved.
Avian extravagance
Birdsong has long been considered too complicated to act as a simple
identification tool.
But Cardoso says they are more like the acoustic equivalent to the external
ornamentation many animals have.
"When you see extravagant traits like that, it is very likely that they
evolved not because of their function for survival or breeding, but because of
sexual selection, the social games that animals play in seducing mates or
competing among themselves."
But there is still little understanding of why and how bird songs grew to be
so varied. Research is now being done around the world to determine the
evolutionary influences.
Cardoso chose to look at loudness because it is one factor that can be
measured across species.
To do so, he drew on the expertise of birdwatchers across the US, Canada and
Europe. They were asked to report on the complexity and volume of the passerine
calls they heard.
Reports were made on more than 140 species, and comparisons of the results
from birdwatchers in the same district were used to measure the accuracy of the
observations.
Attracting mates
The compilation and analysis of the results shows that while the birds'
physiology explains some variation in the volume of the calls made, the
composition of the songs provides a stronger link.
As Australian examples of the extremes, Cardoso compares the shrill loudness
of the common minor(?) bird to the delicate sophistication of the fairy wren.
Ultimately, he says that it is likely that birds pay attention to both factors
in courtship.
"When trying to attract mates, the attractive characteristics in songs that
females are attentive to are not necessarily just simple traits such as loudness
alone or complexity alone, they may evaluate all of that together."
Research is now underway to determine further explanations for birdsong
diversity, but Cardoso says that these findings provide a good beginning.
"It puts a little bit of order into birdsong diversity. Knowing this
trade-off gives a little bit of shape to the space of possible songs. We can now
understand a little more about the diversity of song."
http://www.abc.net.au/science/articles/2009/11/23/2746695.htm
