They're both valid methods, but intramuscular is faster-acting and more painful to the bird. I always go SQ (subcutaneous) because I'm usually doing it on my own and it's easier to hold the bird on my lap and inject at the front of the wing (I'd do the thigh locations, since the vet recommended it; I learned to do injections with goats and we always did it at the shoulder)Very helpful info. Thank you for sharing.
I had inserted the needle with the procaine into the breast for 4 days and it worked. A year later she was sick again. I injected the needle same place and she violently reacted and jumped out of my arms, slammed herself to the floor repeatedly and died. It was horrible to watch. So I asked my vet where else to inject and maybe I injected into the wrong spot??? He said to put needle under the skin between the leg and body, that loose flap of skin. And I did this for another sick chicken and it worked, so it was not a muscular injection.