I love these little guys! They are black-bellied whistling ducks. As you may have guessed from their name, they "whistle" instead of quack. They sit in trees and prefer not to swim too long in the pond. They first made their appearance at Run*A*Round about 7 years ago. (They were so odd, I had to search in the "rare" section of my bird reference book to find out what they were. I even reported them to the Texas Ornithological Society since back then they had not yet registered sightings in our county.)
That first year, 14 of them showed up one day in April. Ever since then, we've gotten returns of about 10 to 12 per year. We had a pair nest on our pond one year, but after losing all of their ducklings to snapping turtles (and maybe a few to our dogs!) we've not seen babies here since. But they happily fly in every day, several times a day, and squawk and squeal and raise holy heck to try to scare away other pairs while they enjoy munching under the bird feeder tree. Then in fall, they gather into a small flock and fly off once again for lands south of the border.
P.S. This photo was taken through my office window - even through the screen. But since we recently got new windows installed, the screens are clear and crud-free. :)