Thanks to you both. My initial reaction was an owl but when I saw the picture on the camera I thought the eyes looked smallish for an owl, besides its head seeming so small relative to the hole. I didn't factor in the different types of owls. I've lived here ten years and it's the first owl I've seen in that time. It's not at all uncommon to hear them but you never see one. I think it's neat and it's got me looking up there each afternoon hoping for another peek so I could maybe take another picture. It was snowing pretty decent that day and between that and the distance it threw the focus off. I even have the camera sitting near the back door.:-)

they like the ducts because they are warm. if it gets in the duct as your title says then you find the hole it made to enter. if the insulation is missing around the supply box..it may be comming for the water. if the crawlspace is cold, the supply box is metal and warm air is moving in the supply box..it will condensate. may either be a thirsty critter or a cold one. find hole(s) seal with critter resistant material. sheet metal, flashing..depends on where the hole is located. knowing what type of duct work you have will help to advise you on how to seal. can you take a picture of the access hole you've sealed? and take the supply grill off and take a pic inside the supply box so we know what you have. its hard to see from here. if someone installed the duct to this area..there is some type of access. get a piece of plastic, lay on it and scoot around to see what is going on under the house. it could be a racoon, armadillo, possum, skunk..lots of critters out there. luckily it probably isn't a zombie. best of luck

If you go out there with a ruler and measure the animal droppings, their length should help you figure out what animal left them behind. I'd imagine rats because they look too big to be from mice, but I'm just guessing. With all the trouble I've had with voles in the garden, I've never seen droppings in the garden and I've never had mice or rats in the garden....only voles. Yes, rats will damage gardens, as will virtually any mammal with a mouth (mice, rats, voles, squirrels, skunks, chipmunks, gophers, etc.). Some folks who have rat trouble have a hard time harvesting anything at all unless they enclose their entire garden in a cage made of quarter-inch hardware cloth, so it is important to get rid of them. The last thing you want to do is to let them establish a population in/around your garden. If it is rats, sometimes a good dog (like a rat terrier) will control them, but only if the rat terrier is in the garden at the same time the rodent is (which might be at night). You can trap them or you can hire a pest control company to do it. I wouldn't put any transplants out into the garden (other than what whatever you already have out there, if anything) until the issue is solved because you'll just be feeding them. We have tons of squirrels on our property, and they come and steal food from the bird feeders every day, and they are blatant....doing it while I stand a few feet away watching. Yet, they've never once shown up in my garden and I've never found damage I think was caused by a squirrel. They have occasionally feasted on peaches or plums, but the fruit trees aren't fenced in, so I expect a certain amount of fruit to be lost to animals and birds. I've always assumed that the squirrels are finding enough stuff to eat outside my garden so they don't have to venture into it. Our woodland is full of native fruits and nuts so I think the squirrels have enough there to keep them happy most of the time, along with food pilfered from the bird feeders or the chicken coop. If you don't have native areas around where squirrels can feed, then they might come to your garden. They are notorious for taking one bite out of every tomato instead of just eating a tomato.....so, if you have squirrels, you do want to discourage them too. If you're not seeing squirrels, though, then they might not be there. I see them being active during the day, whereas rodents like rats are more often active at night. At the below link, there is info that might help you figure out what kind of poop you've found. Rodent Poop ID

Thank you for your help Patty, Ann, & wirosarioan. I was afraid of that news, I know I already have a bad case of "rose slugs" in my garden, those little suckers ravaged my poor roses last year, but I didn't think that they would be the ones causing this damage. I'll have to keep an eye out for this caterpillar you're describing, Ann. I have yet to find anything that looks like that. I have some new tomato plants I've just planted, I'm sure they'll go after those too. Does anyone have any non pesticide tips on controlling these things? Or is the pick & squish the most affective method?