These are male Anhingas overseeing their nesting sites at the UTSWMC rookery in Dallas, Texas. These birds are sporting their breeding plumage as indicated by the mohawk-like crests on the backs of their heads.

Wikipedia has this to say about Anhingas:

Anhinga species are found all over the world in warm shallow waters. The American Anhinga has been subdivided into two subspecies, A. a. anhinga and A. a. leucogaster, based on their location. A. a. anhinga can be found mainly east of the Andes in South America and also the islands of Trinidad and Tobago. A. a. leucogaster can be found in the southern United States, Mexico, Cuba, and Grenada.

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage

Anhinga - Males in Breeding Plumage