What is "Bird Strike Speed"?
Aircraft structures are required to withstand the structural damage caused by birdstrike. According to 14 CFR 25.571 - Damage - tolerance and fatigue evaluation of structure:

(e)Damage-tolerance (discrete source) evaluation. The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of -
(1) Impact with a 4-pound bird when the velocity of the airplane relative to the bird along the airplane's flight path is equal to Vc at sea level or 0.85Vc at 8,000 feet, whichever is more critical;

In case of aircraft with high cruise speed, the speed may be limited below 8000 ft to comply with this regulation. As Bird Strike Requirements for Transport Category Airplanes notes,

... to meet current bird strike criteria, some manufacturers specify relatively low $V_{MO}$ and $V_{C}$ airspeeds up to 8000 feet, that increase above that altitude.

The 'bird strike speed' is likely this.