I photographed the tiny ducklings and  mom above at Estero Llano Grande State Park .  They are Black-bellied Whistling Ducks, a species only that are found in a few southern states in the U.S. but in Mexico, Central America and all the way well into South America.  They both perch and nest in trees and not uncommonly perch on overhanging wires. Their name fits their frequent calls that include a whistling sound.
There are more than 20 ducklings with this pair of Black-belllied Whistling Ducks, a species of 'tree ducks' . The AllAboutBirds.org website notes, "Females often lay eggs in the nests of other whistling-ducks�a behavior known as egg-dumping."  So it seems likely that some of these babies came from other ducks but that these parents don't seem to care.  SeEtta