What's interesting about these juvenile birds is their huge variation in progression of their moult into 1st winter plumage. When the birds hatch they immediately start growing their juvenile plumage which generally consists of bright reddish/orange upper part and upper wing feathers that are fringed with white. During their southward migration they begin moulting into their first winter plumage. Ultimately, this moult is "complete" - meaning that all feathers are replaced (note that most sandpipers have an "incomplete" moult into first winter plumage, WRSAs are an exception). Although all feathers are replaced eventually, only the body feathers, scapulars, and possible wing coverts are replaced during migration - the flight feathers (primaries, secondaries) are replaced on the wintering grounds.
The replacement of mantle feathers, and scapulars is visible in the field and although it doesn't serve much help in identifying them, it can provide an appreciation of bird moult and is just plain interesting!
This juvenile White-rumped Sandpiper has replaced about 10 mantle feathers, and 2 scapular feathers on both sides. The arrows point to the new feathers which are noticeably duller than the old juvenile feathers. These new feathers are part of the 1st winter plumage. So technically this bird is transitioning from juvenile to 1st winter plumage.
These are the feathers to look for when studying/observing the moult of White-rumped Sandpipers at their stop over sites.
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The following 11 photos are of White-rumped Sandpipers taken at various locations and times this Autumn. I've arranged them, as best I could, from full juvenile plumage to first winter plumage.
1: this bird has replaced no scapular feathers, but has replaced a limited # of mantle feathers.
2: one scapular feather and a number of mantle feathers replaced
3: similar to above
4: two scapular feathers and about 10 mantle feathers replaced
5: at least 4 scapulars and what seems like a majority of mantle feathers replaced
6: ~7 scapulars and 80%+ mantle feathers replaced
7: ~eight scapular feathers and 80%+ mantle feathers replaced
8: similar to above
9: similar to above - a whole row of scapulars appears to be replaced here
10: similar to above, but the arrow points to one of the scapular feathers that borders the mantle feathers. So far in the sequence of photos, this is the first bird to have replaced a feather along that tract/row.
11: the most advanced plumage I have photographed this season - many scapulars replaced including those that border the mantle feathers; no wing coverts have been replaced - they continue to have the white-fringing associated with the juvenile plumage of most shorebirds.
Once these birds reach their full first winter plumage they'll have replaced all their feathers.
Come the spring, they will begin another moult which generally involves replacing only body feathers, the scapulars, and some wing coverts.
