Hawks Like to House-Sit Baby Birds
Okay, so truthfully, hawks don't actually appear to be aware of the presence of young birds inside a house. I have never seen a hawk peer inside, or otherwise show interest in the house's content. I'm sure hawks just view birdhouses as the perfect perch from which to hunt birds and rodents who are out and about in the surrounding open areas.
Nonetheless, when a hawk lands on a house full of nestlings, the nestlings immediately quiet down and remain still. I don't know if the parents signal the nestlings of a threat, or if the youngsters just instinctively respond to the loud thud on the roof of the house - or both.
Looking at the hawks landing on these birdhouses, I imagine the thud is notable!
In the images below, we see a hawk (well, part of a hawk) perched on a house occupied by Momma Titmouse and her nestlings. Momma is clearly aware of the hawk's presence!
The last two pictures are of a hawk who spent considerable time hunting from the roof of this occupied Bluebird house. When the hawk FINALLY left, Daddy Bluebird was quick to check on his brood -- then head off to procure food; reassured all was well.
Overall, bird parents are on the watch 24/7. When the female is spending nights in the house (sitting on eggs or on young nestlings), it is clear the male is nearby -- prepared to warn her of any danger. I have observed nestlings huddling, still, in the corner of a birdhouse when a hawk is in the area -- warned by the parents to be quiet. Finally, I've watched the parents fly up and down in front of the house, loudly calling out a warning, when a snake or other predator threatens.
And yet, interestingly, when it comes down to the final straw, the parents will back away from a threat they can't control -- saving themselves. As a human parent, this is a bit hard to come to terms with -- but, in the wild kingdom, survival of the species depends on the survival of those who are able to reproduce.