Help with Tern ID - SE Brazil, Sao Paulo (1 Viewer)

[marciomotta]

Hello!

I would like some help with this identification of terns. This individual was found in a flock composed of approximately 300 Thalasseus sandvicensis eurygnathus, 20 Thalasseus maximus and 25 Sterna trudeaui. Due to the size of the tarsus, we initially identified it as Sterna paradiseae.
The sighting location was the central region of the coast of São Paulo, Southwest region (Coordinates: -24.016979769592247, -46.400040679715666) .

Thanks for the help!

 

[THE_FERN]

Plausible.

Not seeing obviously dark primaries. I guess the standard confusion tern would be South American. That has longer legs and (usually) a brighter red bill than arctic. I find red-billed arctics to have a darker blood-red bill. The colour here looks quite bright but may be within arctic range

 

[njlarsen]

A Sterna paradiseae with this much white on the crown should have blackish legs and bill it seems? While South American Tern (hirundinacea) should have red more or less year round?

Could leg length be problematic because it is fluffing up the feathers on the belly, so hiding the legs from view?
Niels