Cliff Swallows sunbathing on a roof, Jackson Bottom Wetlands Preserve, Hillsboro, Oregon on 7 July 2010 by Greg Gillson.
We had a rather cool and wet spring. Thus, when it finally warmed up just after Independence Day [Editor's note to visitors: it always rains on all holidays in western Oregon.], I went out on the trails at Jackson Bottom one hot morning and found these Cliff Swallows sprawled out and panting on the roof of this wildlife viewing stand.
I got out my trusty reference guide, The Audubon Encyclopedia of North American Birds (1980) by John Terres.
It turns out all sorts of birds sunbathe, fluffing up their feathers and panting. Some take it to the extreme by flopping their wings forward wildly, gasping for air, while lying prostrate.
That seemed to be the case for the birds observed here, many of which were in a dusky juvenal plumage.
Why do they do it?
Terres' sources indicate that sunbathing benefits birds in several possible ways. Heat is absorbed (they were just cold and wanted to warm up); Vitamin D is produced; skin and feather parasites may move off the back and to a place where the birds can remove them. Some birds apparently sunbathe during the time of molt--perhaps it stops the skin from itching?