So sorry to hear about your babies. There are some things you can do to help prevent against ants--assuming your nestbox is mounted on a metal pole. Many here actually put axle grease up and down the pole, underneath the baffle (your box IS baffled, I hope). The baffle will prevent any birds from making contact with the grease. Also, make certain that the hole at the top of the metal pole is sealed either with an end cap or duct tape (I learned the hard way that ants can climb up the inside of the pole to get to a nestbox). I've also heard of some using a q-tip dipped in an ant insecticide called Tero and stapled to the bottom of the nestbox, though I've never tried this myself. I would definitely avoid spraying any insecticides during nesting season, since adult birds can be affected by eating contaminated insects and, if they feed those insects to their young, their babies might be affected as well.
Kathy

It is possible that the babies died first and attracted the ants to them. Do you have HOSPs in the area that may have killed them?

Ants will destroy eggs and babies. As Kathy mentioned, AXLE GREASE works and that is what I use. I take it from the ground to as far up under the BAFFLE as I can reach. Kathy's tip on duct tape at the top of an open pole is a great idea and I've been doing that (too) since she mentioned it last year. It never occurred to me how easily the ants could go from the ground to the box using the inside of the pole to get there. THANK YOU KATHY, great tip! All three of my nest box poles are open at the top and all three are duct tapped.

I am so sorry this happened to your babies. I don't really enjoy monitoring because my male is a champion dive bomber (*I* am his favorite target) but one reason I just put on my big hat and go for it is because so many things can go wrong inside a box and w/o monitoring, you won't be able to prevent it until it's too late.

Sorry to hear, my bluebirds third attempt ended up also being killed by ants. Took down wooden post and put up 3/4 electrical conduit pipe, and put the cap on top. I've also came out some days to find spider webs on outside of box, those type you don't see and walk through. Been monitoring that situation everyday, never actually seen any spiders though, maybe they got ate by the bluebirds. Anyone else had any problems with spiders.
Keith

Shymouse,

I use vasoline petroleum jelly on my metal post. It's less messy than axle grease if you get it on you. I put a real heavy thick coat on with a rag from the ground to the baffle. That seams to work well.

Tim

Tim, Vaseline will work but one application of axle grease (I use throw away gloves) will last an entire nesting season. I used Vaseline a few years ago but several rains will wash it off.

Ants will NOT destroy eggs, and they will NOT eat healthy babies. I have watched many nests with a video camera and am sure of it. I have seen a robin nest and a brown thrasher nest crawling with ants (because the nests were in a bush) - and the babies were unharmed. One night there were so many I wasn't able to even look at it- I went to the nest, scared the mother away and spread sevin dust all over it. Then I placed a deck lamp there so the mother could return (it was dark outside). While I was there, I didn't even see one ant ON the babies - they were just crawling around doing their own business. But they did irritate the mother bird - she was pecking them, or maybe she was just having a midnight snack :)

Aleksandras, you are wrong. FIRE ANTS most certainly WILL destroy eggs and kill babies. If for no other reason than that the adults will abandon a nest with fire ant infestation. I've never had it happen in my nest boxes but have seen it, first hand, in two neighbors boxes. They don't EAT the healthy babies, they sting/bite them and the babies eventually die due to the bites and being abandoned.

I can't comment on other types of ants but FIRE ANTS are deadly to eggs and nestlings when they infest a nest box.

Hi, one of my nestboxes had little black ants, and they didn't harm the babies, nor did they seem to bother any of the birds. I think it is important to specify what kind of ants we are talking about in each particular situation. Obviously, the fire ants are deadly, but the black ants don't seem to be harmful to healthy birds. So both answers are right in different situations.

I have fire ants all over my yard. I have never seen them in the nests. They would go to dead/nearly dead animals, but not healthy ones. Unless it's in their way or near the ant nest and poses danger to colony.

well I dont know about your fire ants, but mine killed a dwarf rabbit that was not sick, so I assume they will kill a baby bird

I have to agree with you, BlueBars. No way would I put insecticides/chemicals inside a nestbox, regardless of who says it is safe. There are simpler, less harmful methods that accomplish the same thing--that's way too extreme AND the bottom line is that who really knows just how much damage is done.
Kathy

There are many birds other than bluebirds and other species are usually getting highly infested with mites in their nests. I have been using it on nests for years. Purple martin landlords are using it as well. I don't want a debate. If someone wants to let these parasites eat these helpless babies - fine with me. But it will not happen in my yard. Especially with such cool birds like the brown thrasher. Their nest were full of mites. I had planned to use sevin dust on their nest anyway because of this.
I recommend you read the article bellow from purplemartin.org
If anybody would like to discuss this subject, they can always visit that site and discuss it in the forum there.

Here is a link that might be useful: Purplemartin.org - Sevin

Debbie, I guess your rabbit was being caged outside and the rabit food attracted ants?

https://www.earthworkshealth.com/how-to-use-diatomaceous-earth-for-pests/diatomaceous-earth-for-ants