8 month old flapping hands

Mine is 7 months and moves her whole arms up and down as if she’s a bird.

Hand flapping in an 8-month-old is often a normal developmental behavior called "stimming," used to express excitement, self-soothe, or cope with intense emotions. It can be a typical part of development, and it's not automatically a sign of autism, but it is a behavior to watch in conjunction with other potential signs. If you are concerned, it is best to consult a doctor to rule out any potential underlying issues and discuss next steps.  

Common reasons for hand flapping

Excitement: Babies often flap their hands to show happiness or intense feelings like being tired, overstimulated, or frustrated.  Self-soothing: Hand flapping provides sensory input that can help a baby regulate their system, calm down when over-excited, or cope with overwhelming feelings.  Communication: Before they can use words, infants may use hand flapping to communicate how they're feeling. 
…

… Motor development: At this age, babies are exploring hand movements, which can be a normal part of development. 
When to be concerned
While hand flapping is often normal, it could be a cause for concern if it is accompanied by other developmental delays or differences. These may include: 

Avoidance of eye contact
Lack of other gestures, such as waving or pointing
Delayed crawling or other motor skills
Difficulty with social interaction and limited response to their name
Self-injurious behaviors, such as flapping so hard it causes physical injury
Difficulty focusing or an inability to replace the behavior with other forms of play

My first born started hand flapping around 8 months. Something my second is not doing. I didn't realise until looking back at videos of him how obvious it was.

It’s age appropriate so I wouldn’t spend time over thinking it. But if you’re really concerned contact your health visitor.

I’m in the same boat with my little girl having 2 half sisters that are autistic - both on completely different sides of the spectrum albeit. My little girl crickets her feet and bashes them on hard surfaces and has done since 3 months old. If it’s any reassurance, I know how you feel - the constant worrying if this is an early sign of it or if it’s just “normal” development… but I am with you - when you are aware of autism and it being in your family already, it’s hard for your mind not to take you there. Sending you love because it’s a complete mind f*ck 💕

No formal assessment yet but it has been flagged down and had a sogs. He would be high functioning but still flaps a lot especially when overwhelmed or excited. He was delayed in his crawling and walking and when he did start to crawl he kept one leg straight. He walked on tip toes a lot but that's stopped now. He does get hyper fixated on things and can do his timetables and he is only 4. The only sign I had that started as early as 8 months was flapping and then he started endlessly spinning wheels like on cars ect.

If you do online research you will see that hand flapping in isolation is not an early indicator. It mentions other milestones like eye contact, interaction with toys, and other age appropriate milestones if not met could be a cause for concern. But even then it’s not definitive. I understand why it can cause worry.