The Rio Negro is one of the breeding grounds of the black skimmer (Rynchops niger, talha-mar). On Fazenda Barranco Alto this year there was an exceptionally big breeding colony of over one hundred birds on a single sandbank.
Note the much larger lower mandible.
Their nests are well protected from caymans because these can´t reach down into the little depression in the sand.
Skimmers have a light graceful flight being one of the favorite birds of the many photographers ...
They feed usually in small flocks, flying low over the water surface with the lower mandible skimming the water for small fish and insects caught by touch. 

A skimmer skimming !
Skimming next to a pied lapwing.