Male bias may mean rare birds are even more endangered.
New research, published in the leading ornithological journal Ibis, has revealed 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 birds facing extinction could be overestimated because scientists often base population estimates on the number 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 detect. Researchers then take this as an estimate of the number of breeding pairs, assuming an equal number of males and females in the population. However this study suggests that this an erroneous assumption.
After carrying out a comprehensive review of hundreds of scientific papers, Dr Paul Donald of the RSPB concluded that in the majority of bird species studied, there are more males than females. He said: ‘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 becomes.
‘Therefore, 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.’
‘It’s not that females are producing more sons than daughters, because at hatching the sex ratio is generally equal. The only possible explanation is that females do not live as long as males’, Dr Donald adds. ‘As generations grow older, they become increasingly dominated by males as more females die off.’
One possible explanation for this higher female mortality is that females may experience higher physiological stress. In many bird species, females are the dispersing sex, while the males stay closer to home. In migratory birds, the females often fly the furthest, despite being smaller.
Why do more threatened species have such strongly skewed sex ratios?
‘One possible explanation is that many threatened species are endangered because of introduced predators, which have been shown to kill females when they are incubating eggs in the nest.’
Backing up Dr Donald’s argument, some studies have found that populations of threatened species in New Zealand have reverted to a more balanced sex ratio after predators were removed.
Because most bird population estimates are likely to be overestimates, it is crucial that researchers consider the sex ratio skew when devising strategies for saving globally-threatened species.
The species showing this skewed sex ratio include: the wandering albatross; the Raso lark, from the Cape Verde islands; and the Mauritius parakeet. Several examples from New Zealand include: the kaka, the kea and the kakapo (three species of parrot); the Stitchbird; and the weka, a large moorhen-like bird.
