250 knots
Was for collision avoidance although in the good old days it didn't exist and once saw 365 knots flying up the Thames estuary on the extended ils for 27 right at heathrow.
Most I've done is 300 knots below 3,000ft coming out of Tripoli.
Regularly asked heathrow departures if they had speed control on departure and accelerated to VNE under the Biggin pattern which restricted our climb clearance.
As for birds they fly above 25,000ft.