Which feathers make up the black patch on a Northern Pintail?

On a swimming male Northern Pintail in basic (bright) plumage there is often a black patch visible on the side, just below the long scapulars.
On the pictures of flying birds in Sibley and Nat Geo the only dark feathers shown on the wings and back are the secondaries. The patch seems to far in the front though for being secondaries. So what feathers are those?

I spent a few happy hours looking at photographs on the internet. On some flying or mounted birds a black patch was visible between the wing and the elongated scapulars. I suspected that those were outer scapulars. Links to some of the photos I found follow further down.

The Handbook of the Birds of Europe, the Middle East and North Africa confirms this:
"Outer scapulars black, streaked cream or buff-grey on inner web and along shaft; elongated lower scapulars black, broadly edged cream. Lower scapulars partly vermiculated grey."

Incidentally, Sibley uses a Northern Pintail to illustrate duck feather groups in his "Birding Basics" on page 88. He points out the black patch as the secondary coverts there, which is wrong!

Comparison to the white patch on wigeons

It is very interesting to compare the black patch on the Northern Pintail to the white patch on the wigeons, which looks to be in almost the same place on a resting bird.

The white patch on wigeons consists of the greater coverts. (Had Sibley illustrated a wigeon he would have been right.)
This means that they are only molted once a year when the wings are molted in the summer while the bird is in eclipse plumage. The white patch is thus present year round, it is also visible on eclipse plumage males.
It is on the other hand not present in males in their first cycle yet. Wigeons in their first basic (=juvenal) plumage have mottled gray greater coverts. The wing feathers are retained while the body feathers are molted in the formative molt which results in the formative (= first bright or first winter) plumage of the body. In the early summer the birds go through another body molt, the first alternate (eclipse) plumage, while the first basic wing feathers are still retained. While having this alternate body plumage the birds start their second basic molt by molting their wings during the summer, now for the first time acquiring white greater coverts. Soon after the wing molt the basic molt proceeds with the body feathers and the birds acquire their bright aspect.

The black patch on Northern Pintail consists of the outer scapulars, which are body feathers.
This means that they are molted twice a year. In basic (bright) plumage their visible outer webs are all black. In alternate (eclipse) plumage they are not, they do not have the black patch in alternate (eclipse) plumage.
Northern Pintails should also have the black patch in their formative (= first bright or first winter) plumage, since it consists of body feathers and body feathers are molted in the preformative molt. I have not been able to explicitly confirm this though. Exceptions are always possible. The possible exceptions in this case would be that the formative outer scapulars do not have completely black outer webs or that the scapulars are not molted in the birds' preformative molt, that first basic (= juvenal) scapulars are retained until a later molt.

(I have followed Peter Pyle's plumage terminology proposed in his paper "Molts and Plumages of Ducks", Waterbirds 28(2), 2005)

Photographs of Northern Pintails showing the outer scapulars

Photos of swimming or standing birds showing both the speculum and black outer scapulars are here, here, here, here, here and here.

Though most often visible, the black scapular patch can be concealed under body feathers just like the rest of the wing. Some examples of birds showing their speculum but not their black scapular patch are here, here and here.

Flying birds showing the black outer scapular patch are here, here, here, here and here.