This is an adult female Anhinga suffering from the heat of a mid-June Texas summer. She is panting hard in an attempt to cool off.
Wikipedia has this to say about Anhingas:
The Anhinga is placed in the darter family, Anhingidae, and is closely related to Indian (Anhinga melanogaster), African (A. rufa), and Australian (A. novaehollandiae) Darters. Like other darters, the Anhinga hunts by spearing fishes and other small prey using its sharp, slender beak.