How old is the bird?
If the bird has only been with your mom a few months, you probably still need to build trust----even if the bird likes you...It can take a long time and you want to get good enough at reading body language that you prevent biting by avoiding triggers (without allowing your bird to manipulate a situation). You have to think it through ahead of time and make sure you are one step ahead so that you don't put yourself in the situation of A. giving your bird extra biting practice and B. trying to figure out how to react when bitten (both should be avoided because a misguided estimation as to the function of the biting could be quite bad).
That having been said--
Hormones are a huge deal with birds and they are often tied to screaming, plucking, aggression etc...
1. NO shadowy spaces should be allowed (no tents, huts, hammocks, drawers, boxes, under furniture, blankets etc).
2. NO petting anywhere other than the head or neck.
3. NO access to bedding material in the cage (shavings, piles of paper etc).
4. Minimum of 10-12 hours uninterrupted sleep on a schedule (sleep regulates immune health and hormones).
**even if this bird is too young to be hormonal, you do not want to encourage practices that will be unsustainable later (e.g., petting under wings, huts etc).
A bird can often like someone and not want to be touched by them...This is common and it means that you need to keep working on your relationship without overstepping boundaries. Building trust doesn't start with physical contact-- it starts with you just hanging out for hours in the room, doing your routine, talking about what you are doing (but not pushing that in-your-face interaction). Then you might start putting treats in the bowls (without forcing the bird to take them from your hand). If they seem relaxed enough to take a treat from you, great, but don't force it. Once that is done, it is sort of a waiting game, but don't push physical interaction on your bird--let them seek it from you.
I personally would advise against putting yourself in a situation where you have to pull your finger away because lots of birds DO test---and that makes you a bit of a weak link (I don't mean in a dominance sort of way, as birds don't really have "dominance" dynamics...it is something else). I am not saying that biting is a power thing because it isn't--but if they see that they can manipulate you to get a reaction, often times they will (biting is communication---and yes- it can be used to manipulate, but it can also mean "hey, man, DON'T TOUCH ME RIGHT NOW!", or, "why weren't you hear?" or, "why can't we mate?" or, "take me to the kitchen dummy!"
Baby birds are always nice but older birds (like teenage people) are more skeptical and less trusting (even if nice when babies).
My cockatoo tests everybody after she is comfortable with them and they key is to not be scared...and not trying to handle the bird may be your solution to that right now. If you are bitten, only react if you are sure you know why the bite happened. In this case, it does sound like it has a purpose---possibly fear, possibly territorial, possibly testing, but DO NOT muscle through (e.g, keep pushing the issue) and push your will onto the bird. This will destroy trust. If you are bitten and it isn't an accident, try not to jerk or yell (or call for the preferred person) but you want to AVOID being bitten at this point in time...It's still VERY early in your relationship to be pushing it.
