The usual antibiotics given for chickens are Baytril, Aureomycin or Tylosin (Tylan). Tylan or Aureomycin are the ones usually given for respiratory infections. If you put your ear to the hen's back you might hear a sort of rasping, rattle when they breathe if they have a resp. infection. Sometimes they do get colds, which of course are viral infections, and so antibiotics won't help with those -but then they can develop secondary bacterial infections, and this is what the antibiotics treat. So, sometimes you find that the antibiotics clear up the rattley breathing, but they go on sniffling for a while after, and then that clears up by itself. Depending on what it is and how bad they are, they might need injections of tylan, so you might need to see a vet. Are their eyes ok? Any swelling (and take a good look at both eyes of all hens) of the eyelids? Any bubbles or foaming in the corners? Any discharge? If there are any symptoms like this in the eyes, then you should really consult a vet for Tylan injections to start them off, as it could be Mycoplasma. If not, then Tylan soluble, which you dilute in their water, andmake sure they don't have access to any other water. The dose for that is .5g (i.e. 1/2 gram per litre- that's about 1/8 of a teaspoon or a little less) in each litre of water. Change the water every 24 hours with a fresh solution.
Investing in one or two good books on poultry health and diseases is well worth while, and can save a lot of worry.
Good luck with your hens.

PS. You won't be able to use the eggs during or for 7 days after treatment with antibiotics. But if you hardboil them and chop them up, you could feed some of them back to the hens - they would be your friends forever!
