Why so rusty?

 

 

Just a quick photo and a question.  As I was leaving work today, I noticed quite a few of the white pelicans roosting on the tidal flats in Bodega Harbor with rust-colored feathers on their breasts and sometimes their necks.  I hadn't noticed such dark coloration before, so I'm just curious.  Were their feathers stained by iron in the water at the breeding site?  Or is there something local (an abundance of diatoms)?  If you know more about this, I'd be interested in learning!