This is what Sibley says about the white patch on arctic loons. 

"extent of white on flanks of arctic loons varies depending on bird's posture: shows less white when active, more white when relaxed and floating high on water. other species may roll over to expose white belly, often when preening. injured or oiled birds may persistently lean to one side"
The Sibley Guide to Birds by David Allen Sibley

just wanted to put this out there

Elliot Potter
Ira VT

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