It has been my experience when flying in areas with high bird concentration i.e. coastal areas with large groups of birds using the updrafts of inland winds from up to 1000m (3000´) rock cliffs that flying at speeds at or under 60 kts will give them enough time to notice you and get out of the way. If you have to fly in bird areas you usually are doing a job which requires you to fly at slower speeds anyway f.eks. filmwork, exploration, seal counting or such. Birds in these ares are of course vigilant and awake since they are hanggliding and probably hunting for food.
If flying at these slower speed I find that its usually better to let the bird get out of the way self, if he sees you, avoid though flying under birds, depending in race theyr first instinct is often to dive when startled.
However birds can be anywhere at any altitude. One friend of mine that used to fly a Super Puma actually had a birdstrike from above, it came through the roof window just in front of the throttle quadrant, the carcass stopped in the throttles and the insides continued all over the radios, instruments and pilots, they sure where happy to have the helmets on and clear visor down.
I find the birds of prey to be the worst, Hawks and such tend to take a break and fall asleep while flying. I have only had 3 birdhits in 3600hrs and two of those were with hawks at 1500 and 3000 feet. Needless to say I´m glad that the windshield in the Bell 222U that I fly now can withstand a 2 lbs birdstrike at 150 kts.
Walter