First of all, Itzjbean hit it right on the head, are you making sure you're not feeding formula cooler than 104 degrees F and no warmer than 110 degrees F? Any cooler will result in the formula spoiling in the crop and growing bacteria, fungi, etc., and hotter will cause crop-burn and infection...
***However, you are aware that their crops only need to FULLY-EMPTY once every 24-hours, right? And that is typically after their last nightly feeding and when you look at their crop in the morning first thing before their first feeding (depending on age, but at 8 weeks you're past the every 3-4 hour feeding, or should be)....It's perfectly fine if in-between feedings during the day if there is a very small amount of formula in the crop, that's completely normal. Again, their crops must fully-empty once every 24-hours, but not after every feeding, there can and usually will be a very small amount of formula left in the bottom of their crop between feedings during the day...And this is exactly the issue with an inexperienced person hand-feeding an unweaned baby bird, they don't know when something is normal, when something is a little off, when something is off but is not a big deal and can be treated at home, or when something is an emergency...How could you if you have not had years of experience? It's very much like a lay-person trying to treat medical conditions at home, with a total lack of medical education or training...It always ends badly...and as Itzjbean already wisely stated, properly Abundance-Weaning a Macaw is an extremely daunting task, not at all like properly Abundance-Weaning a Budgie, Cockatiel, Conure, or even an Amazon...
That being said, if your bird has a sour-smell coming from her mouth/beak, if she is acting lethargic, sleeping more than usual, vomiting/regurgitating anything at all, is constantly fluffed-up, shaking, or refusing formula completely, then you may have an issue...But if the only issue you're seeing is that there is a tiny amount of formula in the crop in-between feedings and the crop isn't completely empty before the next feeding, that's normal...If first thing in the morning the crop isn't completely empty, then there's an issue (assuming she is going at least 6-8 hours at night without a feeding at this point)...It takes a good 6-8 hours for their crops to completely empty, so obviously if you're feeding schedule at this point is, what, every 4-5 hours?, then the crop will not have time to fully empty...
***Now, if her crop is really full at the time of her next feeding, like very little to no formula is emptying, then you've got an issue...But that's not what you describe, so I'm assuming you're just experiencing the normal bit of formula that is in the bottom of the crop between the daytime feedings...And this is fine with a total lack of any other issues, signs, or symptoms...
****I'm not saying this to be nasty, rude, etc., but rather as an experienced breeder and hand-feeder trying to help you, so please take this as constructive-criticism:
It's extremely difficult to hand-feed a baby bird, let alone such a young Macaw, Cockatoo, etc. There are soooooo many things that can and do go wrong, and it doesn't sound to me like you have much experience hand-feeding/hand-raising baby birds, or much education/knowledge on the topic...That's not an insult, just an observation as you didn't know that this was normal...And also an observation of how dangerous what you are doing is with no experience...So my main question for you is going to be have you already found and contacted a Certified Avian Vet (not an "Exotics" Vet, that is not the same thing) and taken your baby in for a wellness-exam since you started hand-feeding him? This should be done the minute you bring-home such a young baby bird, as first of all, any breeder, pet shop, etc. that would sell you/adopt-out/give you such a young baby bird is totally irresponsible and only cares about making money and getting the babies out as soon as they can to save money and time. So you have no idea what may be wrong with the baby, what bacterial/fungal/viral/parasitic infections he may have, etc. And not only that, a Certified Avian Vet can teach you one-on-one, hand's-on, in-person the proper way to not only do the actual hand-feeding of the baby, but also will teach you the proper way to mix-up the formula, the different consistencies the formula should be depending on the baby's age, the proper and most efficient way to keep the formula always between 104-110 degrees F while you're feeding the baby, and most importantly what the things are that you should do when you encounter certain issues that often happen...You can't just willy-nilly give a baby bird ACV, Alka-Seltzer, etc. without knowing the correct quantities, the correct dilutions in the formula of the ACV (it will burn their crop's if it's too concentrated), etc. There are many, many, many little tips and "tricks" that experienced breeders/hand-raisers of birds just know because they learn them over time, from their mentor's that they were taught by, etc., and you cannot learn these things from reading things on the internet or from a book, or even from a forum of experienced breeders such as this one...There is absolutely no replacement for hand's-on, direct hand-feeding experience...and any Certified Avian Vet can and will be more than happy and should want to teach you for the sake of both the bird and yourself, as so many times in this situation the baby bird dies from any number of common issues...
****The other thing that you may or may not know anything about is the proper weaning-process, and with a Macaw or Cockatoo specifically, if you do not properly Abundance-Wean them, you're going to have not only physical health issues, but severe, severe behavioral and psychological issues. This happens with even experienced breeders (usually the irresponsible ones who want to rush the weaning-process, and any who practice "Force-Weaning")...In fact, we just had a very experienced member have to surrender her bird to a sanctuary due to his severe neurological issues resulting from him not being properly Abundance-Weaned by his breeder that she got him from...So you really need to start reading everything you can about Abundance-Weaning a baby Macaw, specifically the proper age that you need to START the process, which is much earlier than the time he'll be fully-weaned, obviously, and even before he fully-fledges. You must start the Abundance-Weaning process, in a properly set-up Weaning-Cage or "Starter Cage", very early with a Macaw. This is the only way they will properly learn, at their own paces, how to play with toys, how to eat food out of different containers, how to perch on many different surfaces, how to climb, how to drink water from different containers, basically how to properly interact with their surroundings, no matter what they are, and how to properly eat many different types of solid foods and drink water from any type of container/dish/bowl. And many baby birds have no idea how to climb, how to perch on anything but a plastic, uniform perch or wooden dowel, how to do anything but sit/stand and pick at food...
I don't know the situation in-which you got your baby, it's not your fault if a breeder or pet shop sold you this unweaned baby that is far too young to be in it's new home, but it angers me sooooo much, especially being a breeder who did it the right way, and knowing all of the other breeders here in our community that do it the right way, and seeing that member here have to literally surrender their poor bird, which they loved dearly and tried everything they could to help him overcome the damage done by his breeder...I don't want any of this to happen to you and your bird, and I especially don't want to see you lose your baby because you're not experienced at hand-feeding/hand-raising...
I just realized I didn't ask you about what ambient temperature you're keeping him in, Itzjbean probably did, but if not, do you have your baby in a Brooder (either a proper Brooder or a homemade one in which you can control the temperature)? I don't know what a Macaw's feathering is like at 7-8 weeks old, but if he still does not have all of his down yet, then he must be kept in a Brooder at about 95 degrees F, and if he has all of his down feathers grown-in but not his outer feathers (he definitely doesn't have all of his outer feathers yet), then he needs to be kept in a Brooder with a constant ambient temperature around 85 degrees...This is not optional at all, as this will cause him to not be able to properly digest his formula just as quickly as feeding him formula that isn't between 104-110 degrees will, and will absolutely result in slow-crop, crop-stasis, and a potentially-fatal fungal and/or bacterial infection in his crop and throughout his entire GI Tract...
If you need any help at all setting-up a homemade Brooder, getting his formula to stay between 104-110 degrees F, setting-up a proper Weaning-Cage, etc., please just post any and all questions, PM me, Itzjbean, or SilverSage, all are very experienced bird breeders and hand-feeders, or just PM one of the Mods and they will forward your questions to the appropriate people...Anything we can help you and your little guy with we will, as again, this isn't your fault, just please be sure to not buy/take-in any more unweaned birds, it's just not worth the extreme risk...
Also, if you need help finding an experienced CAV we can definitely help you with that, as there are some very good ones in the LA area, one in-particular that I'm aware of who owns his own Hyacinth Macaws and is just wonderful and soooo knowledgeable....
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