Wing moult on migrant Northern Wheatear

A distant Northern Wheatear in a field off Mary’s Lane, opposite the rear entrance track to the Paul Britten Hide at Summer Leys, attracted my attention yesterday. It turned out to be a rather smart male, showing features characteristic of the Greenland race leucorhoa (large, upright, long-legged and swarthy with rich orange throat/breast, extensively orange-washed underparts and noticeable brownish cast to upperparts).

 
While I was trying to digiscope it through the heat-haze, another wheatear appeared nearby – this time a female. On switching attention to it, I noticed it was asymmetrically marked. The uppermost and lowest tertials of its left wing had broad whitish-buff fringes, indicating unmoulted winter/1st winter feathers, while the middle one was the ‘normal’ plain colour of a 1st summer/adult. Its right wing appeared to show a single, plain adult middle tertial with the lowest and highest ones seemingly absent.

 
I have not noticed this on Northern Wheatear before and the effect was quite striking. I had always assumed that wing moult took place before migration, although this would indicate that this is not always so. I know that some species’ moults can be arrested during migration and, according to BWP, Northern Wheatear may or may not moult 1 or two tertials pre-breeding, but I would not have expected such asymmetry as was apparent with this individual.

Any comments welcomed.