Thursday, May 15, 2014

In search of Wood Warblers

Having spent a couple of mornings in a fruitless search for Yellow Wagtails in my local birding areas I decided to change tack and go and look for birds I knew I had a pretty good chance of locating. Mixed woodland of predominantly oak and birch with a fairly open understory and sparse ground flora is a favoured habitat of Wood Warblers as well as Pied and Spotted Flycatchers, Tree Pipits and Redstarts. The neighbouring county of Derbyshire has some of this very habitat about an hour's drive from my house. Padley Gorge was my destination this morning as I have managed to see all of these species there every year for the past dozen or so. However, Wood Warbler is becoming more and more difficult to locate, last year I managed to find just the one so I thought it might prove to be difficult this time round. I was not wrong! Two hours in the gorge and no Wood Warblers. Likewise at another suitable location further north and nearer to Sheffield. A site that Sheffield birders swear by for Wood Warblers.
Spotted Flycatcher was waiting at the edge of the gorge as I arrived...good start. Pied Flycatchers were all over the shop...there were more Pied Flycatchers than you could shake a stick at, as a mate of mine would say. But absolutely no Wood Warblers.
Willow Warblers singing, Tree Pipits singing and calling from the trees along the edges of the wood and some belting male Whinchats out on the more open moorland.
Quite a lot of these:
Male Pied Flycatcher Ficedula hypoleuca
 Just one of these:
Spotted Flycatcher Muscicapa striata
 A few of these:
Tree Pipit Anthus trivialis
But none of these: (So no picture)
So what could be happening to Wood Warblers? Why are they becoming more and more difficult to find?
Wood Warblers winter south of the Sahara and breed across much of Europe making them long-distance migrants. There have been declines in numbers across nearly all of its breeding range. According to the 'Bird Atlas 2007 - 2011' BTO 'Between 1968-72 and 1988-91 there were marked losses...Subsequently there has been a 37% range contraction since the 1988-91 Breeding Atlas. There has been a 65% reduction in numbers recorded in the UK during 1995-2010 (BBS report 2011)'
The RSPB state that: The UK Wood Warbler population has declined by 69% since the mid 1990s and has been lost from 50% of its former UK range. However, the causes of this decline are unknown, and, like other migrant birds, may be driven by factors at their breeding grounds, wintering sites, on migration routes or a combination of these.'
To look for reasons for this decline the RSPB have been running a project since at least 2009 in which they have concentrated on looking at possible drivers of Wood Warbler decline occurring at UK breeding grounds. Key questions have been: Has nest predation increased; have nest failure rates increased; have food sources declined or changed, particularly caterpillars. Has climate change affected the phenology of main prey species so that the timing of peak prey activity and nesting activity no longer match. Or have changes in woodland structure contributed to the decline. It can't be habitat loss...the habitat is still there, I was in it today, it's the birds that are not there.
Results in so far suggest that there is no evidence to suggest changes in rates of nest predation have driven declines. Predation is heavy, particularly by Jays, but this was the case before the decline. 'Lower abundance of caterpillars and a mismatch in timing had no impact on Wood warbler productivity, mainly due to their ability to successfully switch diet to flying insects and spiders.' (Mallord et al)
The RSPB report continues: Habitat changes 'have not been sufficient to drive observed declines of Wood Warblers and the three other species for which this habitat is important: Pied Flycatcher, Common Redstart and Tree Pipit. The report which can be found here
concludes: Overall, our work so far has found very little evidence to suggest that factors operating in the breeding grounds have driven the Wood Warbler's decline in the UK.
The causes of the decline then must be on the wintering grounds. The Atlas ends its section on the Wood Warbler by stating that Ockendon et al. (2012) included Wood Warbler among a range of species wintering in the humid zone of tropical West Africa that are in decline, and suggested that these trends could be driven by changes in climate or land use in the wintering area. Let's hope we can find out what is causing the decline before it's to late and we lose all of our Wood Warblers along with other long-distance migrants from tropical West Africa.
Tomorrow the Yellow Wagtail!

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