They're Back - Chimney Swifts

Last summer we had a strange chimney swift incident at the cottage. In short, a pair of chimney swifts had built a nest in our chimney, as chimney swifts are known to do. Unable to perch, these birds spend their days flying through the air, eating pesky bugs as they go. They only rest when they cling to vertical surfaces, such as chimneys. Chimney swift nests are made of twigs glued with spit to the edge of the bricks. Last summer, a nest fell the entire length of our chimney to the very base of our fireplace, with only a thin wooden fireplace guard separating the babies from actually being in our cottage.
Fortunately, the dutiful parents tended to their babes the entire summer. Round the clock, they noisily swooped down the chimney, fed the birds and then exited again.
A few weeks ago, I smugly posted that the chimney swifts must have built a stronger nest this year. We could hear them, but only faintly since they were way up in the chimney.
I spoke too soon. A few mornings ago we heard a familiar sound and peeked into the fireplace to confirm our suspicions. Three eensy little chimney swift babies. Here's one little guy in part of the fallen nest.

I'm hopeful that mama and papa are as attentive this year as they were last. But, what do I do in the future? Should I put a guard on the top of my chimney so they can't build a nest again? I hesitate  because these birds eat so many mosquitoes and gnats and have few places left to nest. What would you do?
~Becky