High speed sync flash unit for Canon 5D Mk4.

Could I please have some advice for a suitable flash unit with high speed sync capability to use on a Canon 5D Mk4 mounted with a 100-400 Mk2. I am often trying to photograph small birds in shady/dull conditions and would like to use fill flash to give a bit of a lift. My current flash (Nissin) will only sync at max 1/250 shutter speed. I like to try and use a SS of around 1/800 to 1/1000 on small birds. Any advice on suitable Canon or third party units would be most welcome.