As the feathers drop off, the bare skin swells as it begins to hold more water and its blood vessels expand, making the patch almost as hot as the bird’s inner body temperature. When an incubating female sits on her eggs, her skin muscles will widen the patch, and she can put the skin directly on her eggs.
Patches also may develop in some male birds — such as brown thrashers, barn swallows and downy woodpeckers — that help their mates incubate eggs.
IN THE SKY: From David Dundee, Tellus Science Museum astronomer: The moon will be new on Monday; on Tuesday evening, look for a thin crescent moon in the west. Mars is low in the southwest at dark and sets in the west a few hours later. Jupiter and Saturn rise in the east just after midnight.
Charles Seabrook can be reached at email@example.com.