Welcome! What an interesting topic. I've never heard of a pet starling before, so this is pretty cool. I also agree with sourland. Transition the bird before releasing it.
I received a response from a wildlife rehab center I thought I should share as Starling information is hard to come by.
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Starlings are an amazing bird; smart, can learn to “talk”, will mimic many sounds and words and they are beautiful in their adult plumage. Having worked with many, many Starlings over the years, they can easily jump into your heart, we understand this.
I doubt what I have to say, is what you had hoped to hear, but we want to try and educate you on how things may appear through the eyes of a human raised Starling—it is quite different from the eyes of a wild Starling.
Understand first, that what we do here, is to try and get ALL wild animals back into the wild where they belong. This is where they are meant to be, to be with their own kind, flocking with other Starlings, finding a mate and raising young.
Even though it is not illegal (as it is with almost all of our birds in the U.S.) to take in and care for a Starling, we do not consider it the best for the animal unless the animal is permanently injured and has no other options.
The problem is, that your Starling has become a “Pet” and, due to its young age when you began caring for him/her, is almost certainly a “Human Imprint”--this is not something that can ever be undone.
Most human imprints completely lose their natural identity, they grow up being fed by what they perceived as their “mom” and or “dad” and so they become imprinted on them (humans) just as they would have been naturally “imprinted” on their parents and siblings had they been left with them.
We receive many young Starlings, each year that cannot go back into their nest to be raised with their families. As a licensed facility, we are equipped to handle them and raise them and in an indirect manner to “teach” them to become Starlings. We have foster parents (Adult Starlings) and “step-siblings” (young Starlings their own age) that we can raise them with, so they do not imprint on humans, but instead, on other Starlings, so when they go out into the wild, they are equipped to truly BE a Starling (flocking, mating, raising young and locating food).
It is too late for your Starling to be properly equipped to go into the wild and so you now have a Pet that will require your support and love as would any other pet, like a cat or dog.
One of the problems with an Imprint, it that they RARELY EVER get along with others of their own kind, even another Imprinted Starling. They tend to perceive “their home” (your house) as THEIR territory and will defend it from others who they perceive as invaders and not of their species.
Often, when Imprinted birds fight, they will fight to the death; they just do not perceive the world as they would had they been raised together.
We have learned this when people have brought birds to us and have not been honest about the length of time they have kept the bird they brought in and we later have discovered the hard way that the bird was imprinted.
YOU will likely become its perceived “mate”, much like most parrots that pick a human family member as their favorite (mate).
Therefore, even if your Starling would accept another in his territory, we would be taking a chance that we would be placing another bird (meant to be in the wild) in a permanent captive situation and then we have the problem of not just one, but multiple misplaced birds; we strongly advise against this.
If your Starling has a large cage and comes out to enjoy his time with you, he should be fine and will wait for you to come home each day.
Our biggest concern would be if he is free flying when the door is opened, that one day he may accidentally escape outside and may not return—outside he’d not be equipped to know and evade bird eating hawks, outdoor cats, dogs or to find food, everything will be strange and scary.
So, be very careful when people knock at your door, or enter/leave and when you come and go. He may just think he should go through the opening and will then find himself in a situation he is not prepared for—this can happen no matter how comfortable or happy he is inside, it happens to cats, dogs, and parrots all the time.
I know this is not what you had hoped to hear, but it’s best that you know his situation—he can be a happy Imprint with is daily human interaction, exercise and a proper diet.