Too many males?

Cat Morrison is the lead author on a new paper that, using CES data, found biased sex ratios in small migrant populations, as Cat explains:

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. 

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

Turtles migrating north

For many, the song of the Turtle Dove is synonymous with warm summer days in our countryside. Thetford Forest, Norfolk has traditionally been a great place to hear them purring; however over recent years it has become increasingly more difficult to find them. The 2015 Breeding Bird Survey (BBS) report, published recently, puts the decline into perspective "Turtle Dove down by 93% in the UK from 1995 to 2014!". The full BBS report highlights other declines, but also some big increases.

The decline in Turtle Dove is also noted in the ringing totals, with an average of c. 40 birds ringed in the whole country since 2010 (and only 12 in 2015) compared to an average of almost 140 in the 1980s. With so few birds ringed, the re-encounter rate of a Turtle Dove is very low, but however slim, there is always a chance to provide useful data.

Colour of location: Ringed in Britain & Ireland, Found Here; Ringed Here, Found in Britain & Ireland

The majority of the recoveries of Turtle Dove have been in Portugal, Spain and France with just four sub-Saharan records. Soon the map above will need to be updated to show the 2016 records of a BTO-ringed Turtle Dove being found on the Faeroes! This second year Turtle Dove (below) was ringed on 23 May 2016 on Fair Isle only to be seen alive and the ring read on 30 June 2016 at Vidarlundin Park, Torshavn.

Turtle Dove on Fair Isle about to make another epic journey North. Photo taken by Lee Gregory.

Not only is this an exciting report for Fair Isle (the only other previous recoveries of this species were of a bird caught on 5th May 1982, which was shot in Spain on 01 September 1982, and another ringed on 6th June 1974, which was shot in France on 4th September 1977) but it is also a great report for the whole of Britain & Ireland. It is great to hear that this bird wasn't shot or found dead either.

Thanks to David Parnaby and the rest of the Fair Isle Bird observatory team for highlighting this.