Mobian birds in my universe work a little different, than the standard Sonic birds.
List goes as follows;
Beaks
Bones
Wings
Tails
Moulting
Beaks
Birds usually have either a very smooth or rough surface, depending on species. The beak is the strongest and most hardest part around their muzzle, the cheeks are the softest spot, closest to skin. The entire muscle has an almost- plastic/like texture and feel to it, as it's real portruding bone. They have no skin on these muzzles, unlike most mobians. Beak shapes and lengths vary on genes, heritage and species. Birds don't usually kiss with mouths, they usually nuzzle heads, as most common alternative, or show a different kind of affectionate-gesture, aimed at the head, when possible. Whenever birds have fluff around their muzzle that's feathers, it's considered a beard, though some bird-mobians got normal-hair beards, if their roots stem from a furred animal or are hybrids.
Bones
Birds's bones, just like their real-life inspired counterparts, have very brittle and delicate bones. Real-life birds have "hollowed out bones", why? They're required for flight. The air passes through those bones, to help add balance and buoyancy to their flight and be lighter, to lift off smoothly. A majority of the population in hospitals, are of bird-species or a hybrid, due to injuries. A majority also, are dead, which is why there's usually warnings in school, extra made for bird students as warnings. Special safety gear aimed at them, is something most industries on Mobius profit off of them. Be very gentle with birds, you might just unintentionally break a bone of theirs, due to their sensitive and delicate nature.
Wings
"Every bird species that has wings, is destined to get a pair eventually" , as the saying goes for many generations now.
Birds get born with the bone structure, for both arms and wings, unless their wings WILL be the arms OR the wings are disfunctional, causing you to only be able to use your arms.
If you become especially unlucky, you might end up with disabilities, caused by genom failure to adapt to a more animalistic feature; You can become paralyzed (ANY paralysis possible)[i.e. Marco's father, Mark], or have a lifelong deformation such as wings and arms growing together and making them extraordinarily long or have overgrowing feathers, that cover and muddle their arms, making them so heavy, you cannot lift them anymore.
While some treatable, others are not, for obvious reasons and specific circumstances.
Most birds on average, have their wings sprout at the age of 65. Some already get them successfully at 30, some get them later, possibly at 80. It all purely depends on genes and heritage. The younger the wing-sprouting happens, the smaller or shorter the wings could be. The older, the longer and larger they could be. If it happens too soon, it'll cause lifelong disabilities or need surgery, in order to be fixed. Prying or forcing them open, also breaks their ability of flight. They will have sprouted-wings, but underdeveloped muscles'll cause them trouble and not the desired outcome. Rarily, sometimes, those rules are ignored, if you're lucky to hit a gold-streak of luck in the gene-lottery.
What does the above exactly mean? A bird gets their skeletal structure already done and attached, as mentioned above. As they grow up, those bones grow with them. If they do any sort of substance/smoking habits or eat too unhealthily, they stunt the wings's growth, from their potential actual size. The wings are stuck to their backs, making their torsos from the backside very secure and fluffy, unlike the chest, where they're more extremely vulnerable. Once they reach their time to sprout, their backs start to slowly have their wings stick out. First signs are to be seen, when feathers fall off or come loose from the bunch. Comparable with a tooth fairy, most birds are excited to witness, when their wings get to grow out into the final stage of sprouting.
Tails
Bird tails are linked to psychological mobian-associated studies. Often times, their most emotionally-revealing form of expression is their tails. Due to being long and large most of the time, it's hard for them to hide their true emotions at all. If their tail is spread, it can often mean an emotion of fear, danger, upsetness or sadness. Sometimes, with a few, even anger. They come in all variations but clear is; Is that they're a great tool for honesty. No facade or forceful binding, will stop them from revealing actual emotion, unless they clip them off voluntarily or get broken off. Criminal birds, tend to clip them off, so interrogaters and detectives in police-stations questioning them, won't know when they're lying or not. Some countries require you to keep your tailfeathers, if you don't, you'll be easily seens as suspect and will be monitored, by most governments for that decision. If it's a legally-recorded process, with papers to prove, you will be freed from any surveillance and suspicion, they would have of you, needing to state why you clipped them, in said documents as one of the things you're obligated to answer.
Moulting
Moulting naturally happens with most species-types. In a sense, it's like a teenager-outburst. Almost unpredictable and slow-process. When a bird has to moult, usually twice or thrice a year (rarely quadruple times), they lose all their feathers and their new feathery dress grows out. Most birds, tend to stay home or choose to avoid public, when it happens. It's such a common phenomena of excuse, that you can legally state it as a vacation, if needed. Moulting can be mentally stressing to some, due to feelings of shame, guilt or embarassment. Those who are especially gullible or religious, see moultings as a sign for their religion or a prophecy, birds should avoid fortune-tellers around this period, as most-often, they get scammed in moulting season. Their dress changes commonly around spring and autuumn. To some, it's winter and summer. When a bird has lost all their feathers, some tend to look bald, due to their feather-strands being said hair or like they had their hair cut. Teenagers moult more often, than adults, with unpredictable cycles, due to hormone-skipping, bring part of the reason, why their moulting occurs faster.
Hair
Birds can have normal hair like anyone else, rarily. A majority, use their feathers as hair. Either roughing them up, so they seperate to thin- stringlike anatomy, making a very fluffy and voluptuous appearance, or choose to keep it in a clump, group or even tangle them together, so they stay together. It's up to the bird, to decide, what they do with it. Hybrid/unpure birds, tend to have normal fur/hair or added combination of feathers too.
Feathers
Due to mobians being natural- human-height, you can imagine how giant their feathers are. When a bird moults, it gets rid of sick or old ones. When they pluck/clip or have a large, healthy feather falling out, they are considered a good of high interest, they sell well and usually are considered a high-quality souvenir. A hobby of some people collecting these, has even formed, as a small, high-class mobian community. Middle- and low classes tend to stay out of such hobby, finding it rather creepy and weird. Among pop-culture, it's considered a "rich-people hobby for the hobbyless". Some involuntarily get trafficked off, getting all their feathers torn or clipped off, around criminals. You can be especially unlucky, if you're an exotic bird. Due to birds being known-by-fact to be vulnerable, they often get taken advantage of.
A bird living in such modern times, is rough for them. They are often protected by special laws made just for them globally, due to them being treated as rather endangered, for all their disadvantages they got, just existing, purely by itself.