Tom - interesting observation...I suggest exposure to brown coloration from tannin in tree bark. At my Adirondack bird banding station at Jenny Lake near Corinth, NY, where I band year round, I have captured some adult Hairies during the breeding season with extensive brown staining on their flight plumage (primaries and secondaries) as well as elsewhere on their bodies.
Then as their young fledged leaving the nest, and the adults began their annual flight feather molt, the new feathers appeared black and white with no tannin staining. It's a nesting phenomenon, or in this case a roosting phenomenon, not likely pigmentation.
Bob YunickSchenectady. NY
From: Tom Berriman <[log in to unmask]>
To: VTBIRD <[log in to unmask]>
Sent: Wed, Feb 27, 2019 4:25 pm
Subject: [VTBIRD] Hairy Woodpecker color
Interesting coloring to a female Hairy Woodpecker at Damon's Crossing this
afternoon. Don't know if it was caused by rust colored tree it was digging
into or is pigmentation. Noticed it under tail, chest, cheeks and base of