Study was conducted by researchers at University of Wisconsin-Milwaukee . They measured the male and female plumage colour of between 20 and 20,000 bird species . The researchers found some species show differences between the sexes while others have evolved so both sexes have the same plumage colours . The findings challenge Charles Darwin 's theory of sexual selection in birds .
Study was conducted by researchers at University of Wisconsin-Milwaukee . They measured the male and female plumage colour of 977 bird species . The researchers found some species show differences between the sexes while others have evolved so both sexes have the same plumage colours . The findings challenge Charles Darwin 's theory of sexual selection in birds .