Thank you for all the information. Do sparrows do anything similar? I thought I saw a sparrow coming out of the box day before yesterday, but then figured I had to be mistaken because since then the bluebirds have steadily been in and out, and I've seen no sign of the sparrow. I measured the hole and it is 1 5/8 inches. I will look for the inserts to fix that-- is it all right to do while the bluebirds are nesting?

I can answer your question about what they look like. Young robins are a bit splochier than the adults. They are about the same size. Here is a picture I took last year of a young robin in a tree that the parents were still feeding.

welcome! As someone who is fairly new to birding as well (but absolutely loving it) my suggestion is to educate yourself. But do it in a fun way. If you're a smart phone user, download the Merlin app. It can help you id pretty much anything and you can hear calls and see multiple photos of your subject in question. I have found that once I know more about a particular species it pulls me in more to birding. Through this self education you can help target species you enjoy to frequent your yard. I'm personally in love with woodpeckers so my yard is littered with feeders to attract them. I now have different suet feeders so make Hairy, Red-bellied and Downy woodpeckers and fairly frequent visitors now. Best advice is to have fun!

It depends upon your point of view. The cowbirds grow faster, and will actively work to shove all of the other, smaller, nestlings out of the nest to their death. So, the smaller birds usually "leave" the nest first, with the remaining cowbird becoming the last to leave the nest. ETA: They will also hatch sooner and will push the other eggs out of the nest. Of course, it depends some on the configuration and species of the nesting birds. If the nest is too deep in a cavity for sibling removal, the much larger cowbird will demand most of the food, and the smaller birds often die from neglect and bullying.