Many song birds have extremely complex mating rituals that are very taxing, especially for the males involved. Males of certain bird species are typically brighter and more colourful than the females, who tend to have a very drab appearance, often in a neutral plumage. In most bird species, only the males sing because they sing only when they are courting a female.
However, in some bird species, the mating rituals may be more complex that originally thought. The colour, size, and voice quality of the mate is very important, and the colour of the plumage is a particularly interesting key in certain bird species like the exquisite Gouldian finches (Erythrura gouldiae).
Gouldian finch species display an unusual trait called polymorphism. Polymorphic birds means that there are several members of the same species that have different colour variations and patterns of feather. For example, in Gouldian finches, it is not uncommon for some birds to have a red coloured head and for other birds to have a black coloured head. What’s more intresingis that both the males and the females of the species bear extremely bright, colourful plumages.
Studies have shown that males of the species prefer courting females that have similar colours to them. However, a recent study shows that these birds can only make the distinction between mates by using their right eye. If the right eye is somehow blocked, the birds do not seem to make the colour distinctions that they usually do in the wild. For example, black Gouldian finch males prefer females who also have black head, while mostly ignoring females with red or yellow heads.
Scientists are starting to understand why the right eye seems to be so important in determining the mate preferences of birds. For one thing, the right eye is directly linked to the left hemisphere of the brain. Unlike mammals, which have a corpus callosum that connects the two sides of the brain, birds do not have this structure, leading to a very specilized set of activities that are locked away to only one side of the brain.
To test this, Jennifer Templeton together with her colleagues in Knox College, Illinois, the United States and Macquarie University in Sydney, Australia, collected 16 homozygous Gouldian finches. The finches were all male and still juveniles, so they have not been able to experience sexual contact with a mate. The scientists then applied cones of cotton over the eyes of the birds using false eyelash cosmetic glue.
The birds were then exposed to three potential mates, a black headed male, a red headed female, and black headed female. When the birds were allowed to see with their right eye, they would almost always show a preference for the black headed female, but once their eyes are covered, they make no real distinctions over their preferred mate. One bird even sang to the male when its right eye was covered.