Melbourne, Nov 24 While big birds are very loud, they are not as creative and intricate as their petite feathered friends when it comes to singing, according to a 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 has indicated that in that battle, complexity is traded off against volume.

As a result, species that have evolved to have more 'syllables' in their calls have a quieter song, while those that communicate with simple trills can trumpet them out.

But, Dr Goncalo Cardoso, a Portuguese ornithologist based at the University of Melbourne, have found 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.

The study is the first to demonstrate such a link across species and points to an explanation for how the diversity of birdsong evolved.

"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," ABC Science quoted Cardoso as saying.

Cardoso focussed his study at loudness because it is one factor that can be measured across species.

For this, he drew on the expertise of birdwatchers across the US, Canada and Europe and asked them 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.

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 compared the shrill loudness of the common minor bird to the delicate sophistication of the fairy wren.

And he concluded bys saying 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," he said.

The study has been published in the Journal of Evolutionary Biology. (ANI)