Breeding plumes in North American Passerines?

I have been taking a lot of photos of birds in eastern NA this spring. I have noticed that there are several species that show two fine filoplumes extending from the lower nape. These are very difficult to see and are much more obvious if they are seen against the background. Here are a few examples of what I have been seeing.

If you know of any studies that indicate the presence, function or other information about these structures I would very much appreciate an email from you.

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Thank you very much,

Wil Hershberger

Ovenbird male Wood thrush male

Orchard oriole male Field sparrow male

Northern parula male