Birds are fine to feed if he'll take them. My pituophis diet is roughly 30% chicken and while I often offer one to my regius, I have yet to have any show interest. Luckily the pits practically inhale the things and nothing is ever wasted.
I don't weigh my feeders, just eyeball everything, but I guess it'd take about 2-3 chicks to equal 1 small rat in weight. I'd wait until he is in the 500-700g range to offer a hatchling chicken. If you want to try fowl sooner, and spend more money, you can get quail he'd be able to handle now.
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