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|Elegant Tern (Thalasseus elegans) Gulls, Terns and Skimmers (Laridae)|
|Aztekerterne Charrán Elegante|
2018-01-14 Estero Maipo, Región Valparaíso (V), Chile
Probably an adult judging from the photos of the bird in flight - both tail and coverts look completely white. My first thought about this bird was that it had to be a hybrid with Sandwich Tern [Cayenne/Cabot's] as the other parent because of the aberrant yellow bill with dark parts. This article from Mexico show birds with similar bills,
(VELARDE, E. & ROJO, P. 2012. Presumed Hybrid Elegant × Cabot’s Terns Thalasseus elegans × T. acuflavida on Isla Rasa, Gulf of California, Mexico. Marine Ornithology 40: 25–29.),
which is why I assumed so. However, further studies online and in literature have not made it possible to confirm that hybrid birds can look this way. As a matter of fact I'm not able to find any photos of hybrids after the pullus stadium. Scrolling through my own unpublished photos I managed to find a bird from Lo de Marcos, Nayarit, Mexico that also shows a bill with dark parts, though less than the present bird. Apart from the bill both these birds look very similar to the other Elegant Terns they associated with, which makes me conclude, for the time being, that this type of bill merely is an aberrant and unusual character.
Short update 2021-08-06: Now there are several hybrids claimed on ebird with photo, though not very similar to my bird. Looked around further, and will now ask if a Cayenne Sandwich Tern can be ruled out. A few photos on ebird do show dark on bill, somehow it looks like my bird has shorter legs, something that could correspond better with Cayenne than an Elegant...