Before northward migration, shorebirds moult into breeding plumage, and many will retain some of this until they return after breeding. While giving a value to a breeding plumage score is somewhat subjective, the following scale is fairly robust:
1 = full non-breeding plumage
2 = mostly non-breeding with a trace of breeding plumage
3 = ¼ breeding plumage
4 = ½ breeding plumage
5 = ¾ breeding plumage
6 = mostly breeding plumage with just a trace of non-breeding remaining
7 = full breeding plumage
This scale works well for birds that colour up completely, such as Red Knots and male Bar-tailed Godwits. Female godwits have a much duller breeding plumage, and the colourfulness of the plumage may not relate that well to how much moult it has done.
The sex of many godwits can be determined visually in the field by three characters:
Body size. Females are larger in all structural measurements than males, having on average a 28% longer bill, 11% longer tarsus, and 6% longer wing than males. They also weigh about 20% more. It is often possible to tell males from females in mixed flocks.
Relative bill length. Females have longer bills relative to their head length than do males. The ratio of bill length to head length averages 2.6 in females (ie. the bill is 2.6 times the head length) but only 2.2 in males.
Breeding plumage. When birds are well moulted, many males can be told by their rich red plumage. Females never moult such colourful plumage, and are typically dull, often with barring rather than red feathering on the underparts. If you see a very red godwit, it is almost certainly a male. Some females migrate north with almost no discernable breeding plumage; a complication is that young males also seem to put on little breeding plumage so could be mistaken for a female if structure and size are not taken into account.
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