Taken this spring at Pheasant Branch Conservancy.
All of the Prothonotary Warblers I've observed so far this spring have had what appears to be a reddish-orange stain above the bill, sometimes extending over the head. I've recently observed the same feather feature in prothonotary photographs taken by other birders as well.
I first noticed this "field mark" a few years ago on a bird at Pheasant Branch Conservancy (see photograph below), but it was faint enough that I thought it was merely a plumage characteristic, perhaps something adult birds acquired over the course of subsequent molts. However, photographs I've taken this year show that it isn't just coloration, but appears to be something sticky that’s matting their feathers.
Just a hint of the stain is visible on this 2011 bird.
While conducting researching on the Internet, I discovered a message thread on the Illinois Birders’ Forum about a "weird prothonotary warbler" regarding this plumage mark. An experienced bird bander who has handled many Prothonotary Warblers had this to say about the mark:
"Having handled a few hundred male Prothonotaries over the past 8 years in southern Illinois, the brown feathers at the base of the bill like the Busey bird are fairly common. As they commonly stick their head in small nooks and crannies amongst the foliage at the water edge, they will routinely 'foul' their head feathers. This can last for weeks, but will wear off over time."What do you think it is, though? Dirt? Tree sap? Something else?
All images © 2013 Mike McDowell