Chronic oiling of marine birds in California by natural petroleum seeps, shipwrecks, and other sources.

Highlights: [•] Seabirds in California are affected by chronic petroleum oiling. [•] Natural petroleum seeps are the primary source of chronic oiling. [•] The majority of birds affected are diving birds. [•] A seasonal trend in oiling may be related to wave energy from large storms. [ABSTRACT FROM AUTHOR]