Mystery bird: western gull, Larus occidentalis

This leucistic individual is difficult to ID, but after staring at this bird through a magnifying glass for 3 hours and comparing it with a hundred or so photographs, I think I’ve got it!

Western gull, Larus occidentalis, photographed at Golden Gate Park, San Francisco, CA, USA.

Question:  This beautiful North American mystery bird is rather special when compared to its brothers and sisters. Can you name the species and tell me what is so special about this individual?

Response: The photographer says this is a leucistic western gull, Larus occidentalis. Because the individual lacks most (all?) pigments, I am relying exclusively on morphological features to ID this bird:

The bill: a western gull has a large, heavy bill with a pronounced hook. The gonys (on the lower mandible where the red spot is) is enlarged and curves up sharply to the bill tip. When I look at a western gull’s bill, it looks larger at the gonys than at the base to my eye.

The forehead: a western gull has a flatter forehead than a glaucous-winged gull.

I admit this is a very difficult identification from just one photograph, especially when there’s not another gull in the image to compare to, but I’ve been staring at photographs for the past 3 hours (using a magnifying glass!) and this bird does look like a good western gull from this one photograph. If anyone manages to get some DNA from this bird and does a little sequencing of this bird’s mitochondrial genes, I’d be very interested to write about what they’ve learned if they wish to share their findings with me. (hint, hint!)

You are invited to review all of the daily mystery birds by going to their dedicated graphic index page.

via Mystery bird: western gull, Larus occidentalis | Science | guardian.co.uk.