Some research that suggests that species counts based on calling males
may be inaccurate measures of actual populations
http://www.rspb.org.uk/news/details.asp?id=tcm:9-168604
A girl for every boy?
Last modified: 29 August 2007
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 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.
Is this assumption a valid one? This study suggests not.
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.
Dr Paul Donald, 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.'
Why does this happen?
'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.
http://www.blackwell-synergy.com/doi/full/10.1111/j.1474-
919X.2007.00724.x
Adult sex ratios in wild bird populations
PAUL F. DONALD
Ibis (OnlineEarly Articles). doi:10.1111/j.1474-919X.2007.00724.x
Abstract
Offspring sex ratios in wild bird populations, and the extent to which
they vary from the equality expected by random genotypic sex
determination, have received much recent attention. Adult sex ratios
(ASRs) in wild birds, on the other hand, remain very poorly described,
and many of the questions about them posed by Ernst Mayr in 1939 remain
unanswered. This review assesses population-level sex ratio patterns in
wild bird populations, with an emphasis on the ASR. A quantitative
assessment of over 200 published estimates of ASR, covering species
from a wide range of taxa, regions and habitats, supported Mayr's
assertion that skewed ASRs are common in wild bird populations. On
average, males outnumbered females by around 33%, and 65% of published
estimates differed significantly from equality. In contrast,
population-level estimates of offspring sex ratio in birds did not
generally differ from equality, and mean ASR across a range of wild
mammal species was strongly female-skewed. ASR distortion in birds was
significantly more severe in populations of globally threatened species
than in non-threatened species, a previously undescribed pattern that
has profound implications for their monitoring and conservation. Higher
female mortality, rather than skewed offspring sex ratio, is the main
driver of male-skewed ASRs in birds, and the causes and implications of
this are reviewed. While estimates of ASR in wild bird populations may
be subject to a number of biases, which are discussed, there is
currently no quantitative evidence that an ASR of one male to one
female represents the norm in birds. A better understanding and
reporting of ASRs in wild bird populations could contribute greatly to
our understanding of population processes and could contribute much to
theoretical and applied research and conservation.
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