The 250 below 10k restiction in the USA was originally brought in as a ATC measure in the early days of jet transports and the vast speed difference between them and the older piston powered airliners.
Initially proposed, in 1957, was a 160kt limit in the terminal area then 250 kt below 14,500' both of which the airlines argued would be too comercially prohibitive so 250kt below 10k was the eventual result.
Nothing to do with birds at the time. I've got a reference if anyone is interested, it's about two pages worth.
Regards,
BH.