The Willow Warbler is a species causing concern, especially in southern Britain, and the maps from Bird Atlas 2007-11, organised in partnership with BirdWatch Ireland and SOC, have been eagerly awaited.
They provide us with our first full picture of distributional changes for twenty years. There is a strong message for Willow Warblers: Scotland good – England bad.
Take a look at the map left, generated over the four summers of Bird Atlas 2007-11 and there does not seem to be a problem for Willow Warblers.
The species is shown as present – if not proven to be breeding – in almost every ten-kilometre square in Britain & Ireland. The distribution appears exactly as it did in the two previous atlases of 1968-72 and 1988-91.
However, Atlas volunteers were not just collecting breeding evidence in the breeding seasons of 2008 through to 2011; they were also undertaking timed visits to tetrads (2km by 2km squares) and counting all of the birds seen and heard, in a standard one or two-hour period.
The second map uses the number of birds encountered during timed visits to show the relative abundance of Willow Warbler over the whole range.
Now we see a different story, of a species that is at home in the north and west but not in the south. If you have a copy of the 1988-91 atlas on your bookshelf turn to page 350 – just twenty years ago, there were as many areas of high density Willow Warbler habitat in England as there were in Scotland
One of the new developments for the latest atlas is to compare the relative abundance of species at the 20km by 20km level, which copes with biases caused by chequerboard sampling in Ireland. It does this by looking at tetrad occupancy, and taking account of differing levels of coverage. The third map shows how things have changed over the twenty-year period. Grey through to black shades illustrate increasing losses, whilst pale through to dark salmon shows gains.
The Atlas changes over the period between 1988-91 and 2008-11 represent multiple visits to every 10km square in the country, for both time periods. It’s not possible to maintain this level of coverage on an annual basis so BTO also runs the Breeding Bird Survey (BBS) with our partners, JNCC and RSPB. By visiting just over 3000 1km by 1km squares twice each year, BBS volunteers give us an annual FTSE-type check of the winners and losers in the bird world.
For the Willow Warbler, travelling to and from sub-Saharan Africa each year, things still look fine in Scotland, with a slight increase in numbers since 1995. Compare this to the story for the East and Southeast England, where BBS shows us that two-thirds of Willow Warblers have been lost in the same period.
The Willow Warbler has been the focus of a PhD by Catriona Morrison, supervised by Jennifer Gill at the University of East Anglia, and the BTO’s Jacquie Clark and Rob Robinson. Using national survey data from Britain and Ireland (BBS: BTO/RSPB/JNCC Breeding Bird Survey and CBS: BWI/NPWS/Heritage Council Countryside Breeding Survey) from 1994 to 2006 she modelled the variation in Willow Warbler population trends over time and space.
Across Britain and Ireland, these annual population trends follow a gradient from sharp declines in the south and east of England to shallow declines and/or slight increases in parts of north and west England, across Scotland and Ireland, as is reflected, over a longer period, in the Atlas map.
The rates of population change also vary over time; declines in the south of England are shallower now than at the start of the time series, whereas populations further north in Britain have undergone periods of increase and decline (Morrison et al. 2010). The regional-scale gradient in breeding season population trends of Willow Warblers in Britain & Ireland suggests that there are regional-scale drivers, such as changing climatic conditions, across different parts of Britain & Ireland.
This paper does not rule out an influence of the conditions experienced in the non-breeding season, which could arise if Scottish Willow Warblers are wintering in different areas to English ones, or variation that might be caused by the timing of passage of birds from different parts of Britain & Ireland. If either of these is shown to occur, then regional-scale environmental changes in Africa or on passage sites could also be influencing these patterns within Britain & Ireland.
We know, from Nest Record Scheme data, that Willow Warbler nesting attempts are not as successful as they used to be. Potentially this may be due to habitat changes caused by a drying out of the countryside and by increasing numbers of deer, which eat the grass in which Willow Warblers build their nests, and eggs too if they come across them. Cat Morrison and Dave Leech have been delving into nest record and CES ringing data, to investigate the reasons for the north-south split for Willow Warblers, and we expect to see further results shortly.
It’s a tough life being a Willow Warbler. For youngsters which successfully fledge from a nest, the chance of surviving long enough to return to breed twelve months later is just about one quarter, which implies that three-quarters won’t make it. Even for adult birds, which have been through the process at least once before, the annual survival rate is just 31%. It’s amazing, therefore that the longevity record was set by a bird that was ringed as a youngster in 1999 and still known to be alive in 2010, when caught in Scotland. Even for a Scottish bird, that’s very good going!