For many of us the distinctive songs
of our migrant birds are a clear sign of the start of spring, however our
recent study indicates that these songs may be masking bigger problems for
these species.
Using data collected by ringers we
explored the spatial variation in sex ratios of Willow Warblers at CE sites. Our analysis revealed that in 1994, the male-female
ratio was around 50:50 however, by 2012 males had started to outnumber females,
with the male-female ratio increasing to 60:40. Interestingly, it was also
clear that male-bias sites were most common in the south-east of England, where
populations have recently declined and Willow Warblers are at relatively low
abundances.
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| Willow Warbler. Photo by Edmund Fellowes. |
We wondered if it was possible
that male-biased sex ratios could be due to greater female mortality in the smaller
populations, where the greater costs of breeding for females may be exacerbated
by poorer resources. However, although male mortality rates were lower than
females, this difference was not greater in sites with strongly male-bias sex
ratios. Instead, it is likely that the increase in the male-female ratio is the
result of female choice, with individuals preferentially recruiting into larger
populations, leaving males unpaired in the small populations. This could mean
that conservation efforts will be most successful by focusing on sites capable of supporting large populations with
more equal sex ratios.
Our
work also has implications for how we monitor our bird species, as the higher
frequency of unpaired males, singing later into the breeding season can lead to
an over estimation of the breeding abundance in male-bias populations.
Further reading:
Morrison, C. A., Robinson, R. A., Clark, J. A. & Gill, J. A. (2016) Causes and consequences of spatial variation in sex ratios in a declining bird species. Journal of Animal Ecology. doi:10.1111/1365-2656.12556
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