Q: A pair of bluebirds nested in one of our nest boxes this summer, and I'm wondering how summer heat affects the just-hatched young birds. Are they at risk?
A: That extremely hot weather in mid-July was a risk to all wildlife, but especially to young birds in nest boxes. The heat builds up in a box and doesn't dissipate as quickly as it might in a nest out in the open. Sorry to say it can get hot enough to kill a whole brood of chicks. I've read about some bluebird nest box monitors who rig up ice packs to keep their customers cool.
Loons underwater
Q: Watching the loons dive at our lake cabin, I'm wondering how much they eat and how long they stay underwater.
A: Loons are great divers and can stay underwater for 5 minutes or more. They prefer to hunt in shallower water, where they generally remain underwater for less than a minute. Once they catch a fish, crustacean or aquatic insect, they usually eat it underwater. Fish make up 80% or more of a loon's diet and they consume about 2 pounds of food a day.

Feather loss
Q: A barred owl sometimes lands in our backyard and today I found a feather, but no other traces of the owl. Might it have been caught by a coyote or fox? We've not heard any calls from it, and it has been very vocal lately, but birds don't call while molting, right?
A: The photo you sent shows a barred owl flight feather (according to "Bird Feathers," by S. David Scott and Casey McFarland), but if the owl had been caught by a predator, you'd almost surely find more than one feather. Instead, this single feather probably came from a molting owl, and they molt throughout the summer. The molt of their flight feathers is staggered so they always retain the ability to fly, and there's no reason why they wouldn't screech and call during the molting period.