On Willow Warblers

The facebook group, 'Kent Birding', recently found itself discussing Willow Warblers.

This cold spring has so far been most like 2013, in that it has been possible to see their spring migration around the estuary; downed birds along the seawall, some days reaching double figures. In the better conditions of 2014 and 2015 records were pretty much singletons only, when and if you were lucky enough to bump into one. And for me they are local breeders no more.

During the Facebook discussion I guided forum users to the excellent BTO BirdTrends pages and to regional results but for this supplemental note I decided to look elsewhere.

One of the old-style correlations for habitat preference, not used so often nowadays but heavily relied upon at the time of  'Birds of the Western Palearctic' and Simms' excellent New Naturalist 'British Warblers', was temperature. Species populations were often judged to breed between relative minimum and maximum isotherm which for the maximum in Chiffchaff was 26c in July, for Willow Warbler, 22c.

One needs only to play on the excellent Met Office website to pull up a map of the UK showing the mean average maximum July daily temperature during 1981-2010, the period for much of the local decline; the whole of the North Kent Marshes is mapped at "greater than 22c". Local warming appears to be just another of the many nails in the coffin for Willow Warbler here.

(For birders in other parts of Kent, bearing in mind the debate on strength of remaining local strongholds, that map is well worth studying. The south face of the North Downs, under a cooling influence from the prevailing south-westerly winds, still came in below the 22c threshold. Perhaps part of any explanation why some in those parts still feel able to comment Willow Warbler remains relatively simple to find there? Another reason to always think beyond the 'political' county overviews.)

It is also easy to stick with those older texts to answer an imprtant question as to what the differencereally  is between a Willow Warbler and a  Chiffchaff- not in appearance, but in habitat preference. They were, after all, 'sister species' (I say 'were' due to the taxonomic fluctuations in species/races- they certainly remain within a recently evolved sister clade) so not really any shock they are hard to separate in the field for some. But there must have been something that triggered the specialisation, something perhaps rooted in habitat/behaviour.

Simplistically, Chiffchaff is more purer woodland, Willow Warbler more a scrub/wood (with good understorey growth) mix.

Scrub has been a long-neglected habitat, with many of the regrowth transitional zones between, say, woodland stands and grasslands, lost in recent decades. Thinking is slowly coming around to appreciating scrub more, but all too often 'scrub bashing' is a required 'norm', and birch automatically cleared (guess which species of tree provides most food for Willow Warblers during the breeding season?).

Are the larger amounts of shade in ideal Chiffs territories why they can put up with higher temperatures? Questions, questions, questions.

In another interesting twist to the local tale, one of the last Willow Warblers holding territory alongside the estuary, a bird at the Lower Halstow Brickfields in 2013 and 2014, was a 'song switcher'. This is a bird that switches between phrases of it's own song and phrases of another species in the same burst (in this case singing Willow then Chiff).

'Song switchers' should not be confused with 'mixed singers', where individual notes from two species' songs are 'mashed up' and come out as an essentially unrecognisable song, not resembling either species. There is some debate to the causes for both conditions- switching being thought down more to copying, mixed song perhaps down to hybridisation. For anyone wanting to know a little more on the phenomenon, this article is worth a visit. Perhaps, when young, this Brickfields bird simply didn't hear enough Willow, and ended up picking up a Chiffy accent? Whatever the reason, it was perhaps a little fitting that such an interesting individual helped note what might yet be the end, locally, for breeding Willow Warbler.

One of this spring's passage birds, ringed April 12th.
Seen to be still carrying plenty of fat reserves for the
remainder of it's migration. 

So, I could say this year's Willow Warbler spring passage has been relatively good so far. Good in the 'selfish' sense there have been a few more to see, but poor in that most of these sightings will have been birds not really wanting to be stopping off here. I'll certainly still try to find a local territorial bird or two over the next couple of months before the return passage in late July/ early August and youngsters start practicing their songs. However they sound.