John
Before the hybrid hypothesis catches too much fire. I am convinced that these birds are dark billed Elegants. Dark on the bill in Elegant Terns is common, the rarity in these cases is the extent. They can be nearly all black, like this one I saw in 2013 here in Half Moon Bay:
https://www.flickr.com/photos/alvarojaramillo/9442203562/in/photolist-XsSNif-wSAYus-wnvJR7-fCuq4m-fo8utM-fonLa9-fdmzyT-fdmziP-fdmzr8-dUozc2-d75zfy-d75yGs-d75BBs-cRqARh-agA1zx-9VJghg
The hybrids that have been identified on Isla Rasa, Baja California to my eye are again just Elegants with dark on the bill. There are color photos in the paper:
http://www.marineornithology.org/PDF/40_1/40_1_25-29.pdf
They spot various birds with dark on the bill, and assume they are all backcrosses from some original hybridization some time back. But they did not see any Sandwich (Cabot's) Tern while they performed their study. Again, I do not think they were considering that these were merely a variation in Elegant Tern. The bill shapes of these birds are classic Elegant, no sign of the shorter and straighter bill of Sandwich.
So in summary, I do not think the current bird is a hybrid. I think it is a dark billed Elegant Tern.
Alvaro
Alvaro Jaramillo
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