Mystery Grasshopper Warbler remains so.

The unusual pale olive-grey Grasshopper Warbler caught on 25/04/14 (previous post can be found here) remains just that an unusual pale olive-grey Grasshopper Warbler. Much trawling of the internet and searching through various reference works and publications hasn't really thrown much light on the matter. I now know a lot more about Grasshopper Warblers but not much more about this particular bird. It still seems to be a potential toss-up between eastern sub-species and plumage aberration or rare colour morph. Whatever it is I haven't come across a comparable photograph including those of the eastern sub-species, straminea. However, some illustrations of straminea show spring birds can be greyish but seemingly not quite as pale as this particular bird.

Below are a couple of images of more typical olive-brown Grasshopper Warblers that were caught in the following days.

To try and progress matters I emailed Martin Garner to see if he had come across anything similar or to see if he could suggest any lines of enquiry. He agreed it was an interesting bird and requested some images for the Birding Frontiers website (link here) which focuses on identification issues and has a much better reach than my blog. Hopefully having some images published on that site may spark some suggestions but we still have to accept that we may never really know what it is in the absence of DNA testing. Whatever the outcome I still plan to submit it informally to the British Birds Rarities Committee (BBRC) for their consideration and for reference. If a bird like this can turn up once then it can turn up again.

One of the things I came across when researching via the net was a paper (link here) reviewing claims of Eastern Grasshopper Warblers in Britain (Harvey & Small 2007). This paper reported that the 4th primary was slightly emarginated in just over 50% of a sample of 20 of the eastern subspecies straminea and that this feather wasn't emarginated in any of the 12 nominate birds (naevia) examined. However one of the nominate birds I caught on the same day as the pale bird showed a slight emargination on the 4th primary. 

I had only photographed this bird's wing because it had clearly gone through some kind of eccentric moult and had replaced a number of primaries in both wings. These new feathers also stood out as they appeared longer than they should be relative to the old feathers either side. I only noticed the slight emargination of the 4th primary when comparing photographs of various Grasshopper Warblers subsequently. Although the emargination on the 4th is only slight it is clearly there, especially when the feather is compared with the 5th, and would be recorded as such for the wing formula of this bird. The presence of a slight emargination of the 4th primary may not be the potential indicator of a bird worth further examination as that paper suggests and perhaps more importantly shows how much we still have to learn about Grasshopper Warblers.

What a Gropper.

I have to admit to getting caught out yesterday and it was not by the weather. No, it was by an olive grey Grasshopper Warbler that made some very unusual noises to boot. This bird was striking, very very striking and it was clearly a Grasshopper Warbler of sorts but it was not on Fair Isle, North Ronaldsay or the East coast of England, no it was in Wigan.

pale olive-grey looking Grasshopper Warbler ringed 25/04/13

Now if it doesn't seem that pale to you lets have a look at a more typical warm brown Gropper.

Grasshopper Warbler ringed 20/04/14.

The only reference I had with me was Svensson, the ringers bible, and having checked all the specific features I was satisfied that it was a Grasshopper Warbler but given its pale appearance there was clearly a chance it was one of the eastern subspecies. The main contenders are straminea from Eastern Russia, Western Siberia and Central Asia and obscurior from the North Caucasus.

Now where I was caught out is that I now know that it would have been useful to take more measurements than I actually did and especially of the tail. A bit of a cock-up with hindsight but then there is so much overlap between the subspecies they may not have helped anyway. The wing length of 64mm means it wasn't particularly small but it could still be OK for straminea, just. At the time I was more concerned about confirming that it was actually a Grasshopper Warbler as it looked so different. However, I did take a good series of photographs as I thought it may be identifiable as to the subspecies on its appearance alone.

Further enquiries are currently being made to see if it can be attributed to one of the eastern subspecies although we may only be able to say that it showed some characters of an 'Eastern type' Grasshopper Warbler. Unfortunately it didn't drop a feather so taking the DNA route isn't an option. A remarkable bird whichever way you look at it and one that seems likely to spark quite a bit of debate over its appearance and origin although it may well remain a mystery bird. I will keep you posted when I get some feedback from my enquiries.