London, May 13: Why do the most brightly plumaged male birds attract more females in the mating game? It is a question that has spawned countless studies. Now a new Swedish study claims that it may actually boil down to something as basic as sight.

Scientists have hitherto assumed that birds see what we see. But apparently they do not - human sight might not be as good as that of birds. "The results mean that many studies on sexual selection may need to be re-evaluated," said Anders Odeen of Uppsala University, who led the study.

The significance of birds' plumage, both in terms of richness of colour and particular signals, has been shown to be a major factor in birds' choice of partner.

In order to assess the colours of birds, everything from binoculars to RGB image analyses are used. However, most studies are based on the hypothesis that human colour vision can be used to assess what birds see.

"It's a bit like a colour blind person describing the colours of clothes - it's often quite accurate but sometimes it can go badly wrong."

This problem has been discussed in the research arena, but so far no study has been able to show its extent. The Uppsala researchers used a mathematical model to investigate how bird and human retina work.

Using the model combined with information on differences in the colour-sensitive cones of the eye, they have been able to figure out how colour contrasts are perceived. Greater colour contrast can be translated as more "brightly coloured".

The differences were partly due to the fact that human vision cannot perceive ultraviolet light, while avian vision can. There are several differences between human and avian perception of colour, which show that certain shades that can be seen clearly by birds are not perceived at all by humans.

Through evolution, our colour vision has developed from a more primitive version. This means that we have gone from having two types of colour sensitive cones in our eyes to having three. Birds have four.

"Most other animal species only have two, which means that their colour vision is rudimentary. It is human colour vision that differs from the norm, so in reality it's ridiculous to use our colour vision to assess the colours seen by other animals."

The results are not only significant for basic research on sexual selection. They also illustrate the risks of making certain decisions on the basis of human vision, for example, in designing and legislating on lighting systems for domestic fowl.