Poor photo, worse birder...
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At rest the best way to distinquish them from Black-tailed this the Frosted look om the back - Black-tailed has a plain back
Amadan. Your photo is certainly good enough for identification
purposes, and you can't be that bad a birder as your ID was correct.
for the comments - the frosted back should be a good identification aid.
That ID by the pattern on the back is a pretty good guide through the winter, especially for adult birds. Be careful with younger Black-tailed Godwits, and those with an element of summer plumage coming through, as then they can be less plain on the back. Like all ID features, it is best used in combination with other features, like the longer legs and the longer, straighter bill of Black-tailed.
My Flickr photos...
Lat/Lng: 54.6312, -1.1386
OS grid ref: NZ557265