Careful study of the Mallard and American Black Duck has allowed me to reliably distinguish them from one another when I get good, clear views of plumage and bill color.
The birds are slim and dark with black heads, necks, and breasts. Their wings are a light gray and sometimes you can catch a glimpse of white undertail coverts. Black Terns!
At Fair Meadows Sanctuary, one fact has been made abundantly clear: there is keen competition for nesting sites, both in artificial structures and in tree cavities.