Surfbirds News: BirdLife International Archives
Globally threatened birds pay for their sex
A new study published in the leading ornithological journal Ibis has uncovered that for the vast majority of bird species, there are more males than females. The discovery suggests that populations of many of the world’s threatened birds could therefore be overestimated, because scientists often base population estimates on counts of males.
Males are usually more brightly plumaged than females and the males of many species sing to attract mates and defend territories making them easier to hear and therefore count. Researchers then take this as an estimate of the number of breeding pairs, critically assuming an equal number of males and females in the population. But is this assumption a valid one? This study suggests not.
Stitchbird, one of the threatened species known to have a male-biased sex ratio.
New Zealand, Tiritiri Matangi Island 12th Nov 2005 ?? Ray Wilson
After carrying out a comprehensive review of hundreds of scientific papers, Dr Paul Donald of the RSPB (BirdLife in the UK) concluded that in the majority of bird species studied, there are more males than females.
“Most species have male-skewed sex ratios, but a wholly unexpected finding was that the rarer the species, the more highly skewed towards males the population sex ratio becomes,” says Dr Donald. “This means that many of the world’s rarest species may be much closer to extinction than we previously thought, because the number of females is lower than the number of males . It is much easier to save a population with an excess of females than one with an excess of males.” |
