I was fixing breakfast for the cats when there was suddenly a ruckus outside, a flock of robins attacking some predator near the nest (from the angle of the kitchen window, I cannot see the nest or the predator). This was more than just the nesting pair, this was at least eight robins all swooping and squawking. Soon, a crow flies off with what I am assuming is one of the baby robins in his mouth, and an army of adult robins go off after him.
Back at the homestead, I can hear a robin still sounding an alarm call either from the nest or the roof right above it, and there are several robin sentries posted on fences, sheds and wires around the yard. After a few minutes, the crow tries coming back, but before he even gets within a block of my house, four or five robins are on his tail and they chase him back down the hill.
Sentry robins continue to patrol the yard, and a couple of time a robin flies in to check on the nest, but things seem to be calming down, so I leave the kitchen for a minute to go get the newspaper. When I come back, there are robins and grackles sounding the alarm in the big spruce in my yard, and eventually they flush out a crow, and this time it's the grackles that chase him down the block. (Maybe the grackles have a nest in the spruce? Maybe the grackles just like a fight.)
A few minutes later, two crows fly in and perch on the wire across the alley, but neither of them must be the nest snatcher, because nobody pays them any mind and a few minutes later they fly off on their own.
Half an hour after this all started, the robins are pleasantly singing again, and I see one fly into the nest with a large earthworm. But I'm afraid the robin family is facing an uphill battle now; about an hour after the first incident I was upstairs and heard distressed robins, so I went to look out the window, and there's the crow again, being mobbed by half a dozen robins. Me approaching the window must've spooked him, because he took off again and the robins chased after him. A hour after that I went outside, and there is a robin hunkered down on the nest, and the nest itself looks undamaged. The only time I got a clear view of the babies, I saw two little heads with gaping maws, although there could have been more. There's one less now.