ivermectin is not effective against tapeworm and not the drug of choice against gapeworm either fyi. Plus 'one or two drops' per bird is unlikely to be the correct dose and in fact at 1% you are very likely under dosing.
under dosing off lable drugs is a great way of promoting drug resistance. Ivermectin is useful against ectoparasites but should only be used with your vets say so. and I've no idea if ivermectin is carried on into eggs so if anyone has collies or other dog breeds with potential ivermectin sensitivity they should avoid feeding eggs from hens treated with the drug.
Do not be tempted to use ivermectin on waterfowl.
if any animal has a high worm burden and then you worm it-it can cause problems because when the worms die off, they release toxins into the animal. best to worm regularly from the offset using a licensed wormer-just because you don't see worms, doesn't mean your birds don't have them!