Apple itself is fine but I had an experience quite a few years ago when breeding canaries.
I had about a dozen young birds that were about two months old and in their own large cage and along with their seed, I had given them an apple as a treat that I just quartered by cutting.
They really seemed to enjoy it but returning a few hours later, found half of them dead.
I checked with my vet and found out that apple seeds contains a chemical that when digested it's converted into a very toxic cyanide compound.
The chemical is in the apple seed and just a slice or piece normally would not be a problem but in my case some of the seeds were there in the core of the whole apple. Just a warning on apple seeds. Apple flesh is fine. I'd just keep an eye on the droppings without any new foods. Sometimes very different foods can cause dropping changes.
Were these regurged seeds?