I think many birds will gladly eat them while they're smallish. Once they get large, it's probably only the more dedicated insectivores that will go for them. Like cardinals. Here's video of a cardinal eating a similar sized caterpillar that could be a hornworm. https://www.youtube.com/watch?v=ktWIEnqKK34 (Warning: the video is a little gross, as the bird really mangles the caterpillar). I haven't personally seen cardinals in the act of eating a hornworm, but something must be doing so because I only find about 2 a year, but I see the moths in my yard. And cardinals do like to nest in my neighbor's hawthorn bush and use my garden as an insect larder.

Nope, the who in a great horned is a distinctive who with a deep tonal quality. This was definitely more of a child's slide whistle quality, more of a toot. I'm thinking it was a northern saw-whet as it most closely matches that tonal quality. Here is a link that might be useful: Listen to

Yep, colours work out. Today I had the male show up too. Both birds are walking very diligently through my flower bed in search of seeds. They make quite a different sound when startled.

There are half a dozen subspecies of American robin. The ones from Newfoundland are sometimes called "black-backed robins", and in addition to being much darker than the more common subspecies are somewhat larger. Birds from the rain forests of the pacific Northwest are also very dark. Probably what you saw was an adult male bird from one of those subspecies.