The buffy streak on female type Green-winged Teal

Female type Green-winged Teal show a buffy or whitish streak along the sides of the tail on the undertail coverts.
Examples are here, here, here and here.

The Handbook of the Birds of Europe, the Middle East and North Africa says about the underparts of the female:
"Feathers of flanks sepia with broad pale buff edges and streaks. Breast, belly and vent white with variable subterminal pale grey mottling, under tail-coverts white, spotted or streaked dusky-brown."

The National Geographic Guide says:
"..largely white undertail coverts that contrast with mottled flanks."

The buffy streak is in the place where the bright male has the buffy triangle, but restricted to the upper portion of it as this picture nicely illustrates.
It seems that the amount of white or buffy varies, with the streak along the side of the tail being the minimum amount.
I only found one photograph in which ist is not prominent, but still present.
Here is a picture of a bird with the pale patch being of about the same size and shape as on a bright male and here is one with the whole undertail coverts white.
An example of the other end of the spectrum is here, with most of the undertail coverts, everything but the buffy lateral streak, being mottled and not contrasting with the flanks.
Most birds seem to have largely pale or white undertail coverts with variable amounts of spots and streaks on them, but not on the outermost ones which form the pale streak. Some more photos that nicely illustrate that are here, here, here, here, here and here.

The male

On the basic (bright) male the undertail coverts are black and buffy, with the black forming a border along their front and coming up along the central undertail coverts to a point. Two buffy triangles, one at each side, connect above this point under the middle of the tail. The National Geographic Guide illustrates a male seen from behind. It shows too much black coming up to the point on the central undertail coverts, the buff part above it widens trangularly on bith sides, it is not just a streak as it appears in the illustration. The pictures here, here and here show this.