Well it's a captains decison so your answer will have to come from Captains, but here's some tidbits based on experience.
Single birds on runways almost always get out of the way, having inherent ability too see and avoid via horizontal movement.
Its the suckers that are crossing the runway in the air (as singles) that get messed up with their timing and avoidance by lifting upwards just as the aircraft rotates.
Single birds are a small safety risk even if struck although they are expensive and can ruin your day.
However, flocking birds are an entirely different manner, if seen on the runway or alongside it. One should not assume ho-hum under these conditions even with small birds. There have been a couple of incidents where in excess of 30 starlings were ingested in multiple DC10 engines during takeoff roll resulting a a multiple engine powerloss but still enough power to fly.
On the other hand there have been incidents, ORLy comes to mind, where the business jet pilot was heard to say , no problems, watch this, and those were his last words.
I strongly avocate notifying the tower when multiple birds are seem lounging near the runway and refusing takeoff until a bird sweep is performed, however, a single bird is another judgemental matter