It turns out that when an owl lays an egg and then vanishes at 8pm on a night when it’s 33 degrees, leaving the egg uncovered for the entire night, it doesn’t mean she has abandoned the nest after laying only a single egg. You can mourn and grieve and kick yourself for putting a strange new camera in the nest box and ruining everything, but in the morning you'll realize that all that fretting and despair was for naught.
It turns out that owls don’t lay all their eggs at once. They lay one every 2 or 3 days, and don’t start incubating them until the entire clutch is laid. And the eggs are just fine not being kept warm in the meantime. At least that's our theory at this point. There isn’t actually a lot of information available on the nesting behavior of screech owls, probably because all of their activity happens in the dark, making them hard to observe, so we haven’t found any official literature to confirm this theory. We did, however, find multiple sources citing this pattern in birds other than owls. Those sources say that birds do this so that the eggs don’t hatch all at once, suddenly giving the parents four or six or ten hungry mouths to feed. Ultimately they will have all those chicks to feed at the same time, but staggering the hatching increases their survival rate.
This appears to be what’s happening here in our nest. On the night in question, when it looked like DH had abandoned the nest, and all was lost, so soon, after a mere three days, she came back at sunrise, preened a bit, finished off the last of something dead she had drug in with her, tucked her single egg underneath her, fluffed and ruffed, and dozed off to sleep. And then she did it again. Twice. She stayed out all night for the next two nights, with no apparent concern for the solitary, cold egg she’d left behind.
(That's a sparrow carcass in the lower right corner.)
And then, three days after the first egg, she laid a second one. A third egg came two days after that, a fourth in another three days, and THEN she started sitting on them for most of the night. With incubation apparently underway, we thought that must mean that the clutch was complete at four eggs. But on the night of March 29, ten days after the first egg was laid, a fifth egg appeared.
Now she is definitely in incubation mode, staying on the eggs almost the entire night while B delivers cockroaches, crickets, moths and sparrows to her.
She does leave for up to an hour at a time, presumably to go find something to eat. This video shows one of those instances, when B arrived at the nest with a dead bird for her, only to find her absent:
Word on the internet is that incubation lasts 28 days, so if all goes well for the next few weeks, we can expect the first egg to hatch on April 25ish.