Birds of a feather really do flock together, study finds

A tiny bird which has evolved into five different colour variations on a small tropical island prefers to mate with others of the same colour plumage, according to new research.

Dr Yann Bourgeois, of the University of Portsmouth, found that a DNA study of the grey white-eye, which lives on Reunion Island in the Indian Ocean, showed that the 4in (10cm) long songbird is likely to breed with similar mates.

He said both natural and sexual selection had led to colour differences in the varieties of the bird which lived in five distinct territories only a few miles apart.

Reunion Island, which is just 31 miles (50km) long and features volcanoes, rainforests and beaches, is divided into five different territories by rivers and ancient lava flow streams as well as changes in altitude.

Dr Bourgeois said: “It surprised us that birds living just 10km (6.2 miles) apart already have differences in their DNA.

“Birds are usually seen as good dispersers, but these birds stay close to where they are born. Based on the results of this study, it’s possible they may be reproducing mostly with birds of the same colour.”

For the study published in Molecular Ecology, Dr Bourgeois and colleagues sequenced the DNA of birds in each of the five colour groups and found that natural selection to adapt to the local environment influenced changes of colours of birds living in the island’s mountains while sexual selection drove colour variations in the lowlands.

He added that hybrid birds which were the offspring from neighbouring territories might not breed successfully.

Dr Bourgeois said: “Sexual selection is common across many species – birds like the grey white-eye are not alone in the animal kingdom in preferring to mate with those who look the same as them.

“It’s also known that big changes in altitude put a demand on all species – animals at higher altitude need more haemoglobin in their blood and tend to be bigger than their counterparts living closer to sea level, for example.

“It appears that we observe that most of the divergence between colour forms is found on the sex chromosome, and that other chromosomes seem to be involved mostly when comparing forms of high and low elevation.”

Dr Bourgeois added that he suspected that the birdsong of the different varieties of the island’s grey white-eye might have evolved a regional accent, such as has been found of birds in New Zealand, but research was needed to prove this.