A blog mainly about birds and birding, to supplement my website I shall add new posts on an ad hoc basis as and when I have something I think is worth sharing, whether that’s an interesting bird, something I’ve learned, perhaps about identification, or something that’s aroused my curiosity. Often there will be questions, some of which you might be able to answer... please use the comments!

Thursday, 26 March 2015

Brown-bristled Marsh Tits?

A recent post on BirdForum drew attention to a new feature for separating Marsh Tits and Willow Tits.  The claim is that Marsh Tits always have black nasal bristles whereas Willow Tits have brown nasal bristles once they mature (young birds may have black nasal bristles like Marsh Tits).  See posts #10 & #11 on this thread.

On Saturday I had the opportunity to watch and photograph several Marsh Tits coming down to seed at Lynford Arboretum.  Lynford has certainly hosted Willow Tits in the past and is not far from where Willow Tits continue to persist, but whether any recent claims of Willow Tit there have been reliable or not, I am not sure.  While I was watching I didn't see anything that made me suspect Willow Tit.  When I used to see both species regularly I found the bull-necked appearance of Willow Tit to be pretty reliable and quite different from the more delicate and elegant Marsh Tit (at least more reliable than the recent literature suggests it should be - but I do agree with these papers in so far as it's a subjective feature so easy to misapply, especially if identifying from photos).  Another feature that's dissed in recent literature is the wing-panel.  Well rightly so as there's huge overlap, but then again I wonder if birds at the extremes might be identifiable on that feature alone?  Anyway, I digress - my point is that all the birds I saw looked like Marsh Tits.  Many of them called and I definitely only heard Marsh Tits.  I suppose it's not impossible that a Willow Tit sneaked in without calling while I was looking through the camera lens rather than paying attention to what the bird looked like, but I doubt it.

These are some of my pics:

Marsh Tits, Lynford Arboretum (Norfolk, UK), 21st March

Some of these birds seem to have black nasal bristles, like they're supposed to.  But a couple of them seem to have at least a slightly brownish tone to the nasal bristles.  It's very subtle, but I don't think it's just my imagination - can you see the brown in these?

Marsh Tits, Lynford Arboretum (Norfolk, UK), 21st March

The first of these two is a tight crop of the second photo above.  The pale spot on the mandible isn't obvious in this photo but it is in the other photos of the same bird (including the top photo of the six above).  In any case it looks like a pretty standard Marsh Tit and doesn't have anything to make me think it might be a Willow Tit.  The second is a tight crop of the fifth photo in the set above (same bird in the fourth photo too).  That bird does look a bit more Willowy in the photos, structurally, but I didn't pick that up in the field.  It also shows a clear and strong pale mark at the base of the upper mandible which, while not held out to be 100% reliable, is surely a pretty clear pointer to this being a Marsh Tit.

Now I don't think the brown tones on my birds here are as strong and clear as they are in Ashley's photos of brown-bristled Willow Tits on BirdForum.  But to my eyes there's a clear contrast between the slightly brownish bristles and the blacker forehead and crown, especially on the lower photo.

What does this mean?  Four possibilities so far as I can see:
  1. Brown bristles on a Marsh/Willow Tit is not a reliable means of identifying it as Willow Tits, contrary to Ashley's claim on BirdForum.  I know Ashley is experienced with both species but I don't know how extensive his data is on which he has based this claim.
  2. It IS a good feature but instead of saying black for Marsh Tit we should say black or blackish brown - as distinct from the more clearly brown of Willow Tit.
  3. It is a good feature but can be misleading in photos.  Mine aren't of the finest quality and some colour aberration in the photo may have contributed to a more brownish appearance, which is more evident in the bristles which were a subtly different shade of black.  Not convinced, but I throw it out as someone might want to make the case for it.
  4. My birds are Willow Tits.  I'm sure they're not, but I'm capable of being wrong.
  5. My birds are hybrids.  Being a hybrid nut I like this idea, but I don't seriously think it's likely.  Worth thinking about though, especially if the evidence for Ashley's claim is robust and black really does mean black, as opposed to blackish brown. 
There may be other possibilities - let me know if you think so.

If possibilities 2 or 3 are the explanation then this has implications for how we should apply Ashley's new feature, and clearly considerable care is required (and much more so if possibility 1 is the answer).  I am personally content to rule out possibility 4.  I think we need more understanding before embarking too strongly on the hybrid theory, though I would point out that (a) hybridisation between these two species has been reported, and is very likely to be overlooked when it does occur so can be expected to be massively under-reported; (b) hybridisation between other Poecile species (e.g. in North America) occurs quite frequently and (c) hybridisation is most likely in places where one species is either spreading into an area already occupied by the other (not relevant here) but perhaps also where one species is withdrawing from an area still occupied by the other (possibly very relevant here).

What do you think?