What evidence is there of any connection between birds and dinosaurs at all?
When did this crossover occur?
We simply do not know, and there is zero proof of the connection between birds and dinosaurs.
Scientific theories need to be based on empiricism, not one fanciful wishful thinking like Kiplings, “how the rino got his horns”.
Previously the hip thing was just thought to be a shared trait or an independently derived trait - but the feathers clinched that they belonged to the “bird” clade based upon shared ancestry explaining a derived trait.
There is also protein sequence evidence. The closest sequence to the collagen discovered from a T-rex bone was that of a chicken.
It has long been known that dinosaurs were closer related to birds than to reptiles.
Science doesn't do “proof” it does evidence.
You can ignore the evidence if you want - but it is still there.
The evidence indicates that once there were lots and lots of dinosaurs but no birds. Then there were feathered flying dinosaurs. Then there were birds, including lots of flightless birds that were, without evidence of feathers, thought to be dinosaurs.
Science it keeps changing. Being a creationist means never having to change your opinion based upon evidence. Being a scientist means you must.