You aren't going fast enough for it to be a problem for fibreglass and gelcoat unless the bird is quite large, like a large eagle or vulture.
You encounter birds all the time in a glider. You soar with them. You spot one thermalling, you join him.
A hawk or eagle will pretty much ignore you unless you pass within 10 ft. I encountered a large brown soaring bird, probably a juvenile bald eagle (they don't get white heads until they are several years old), last week at about 3000 ft in a thermal and passed under it several times, close enough to see it looking back and forth to keep track of me. Then he was gone...
To them a glider is just another big bird.
On tow, you have the tug in front making a racket that drives birds away, so that's not a problem unless the tug runs through a flock of gulls or something on departure.
Some eagles have been known to dive and attack gliders if you fly under them. Bad news for the bird if it takes on the leading edge. I've never heard of a composite glider being substantially damaged by a bird strike but I suppose it's happened and could be a problem if a large bird hit the horizontal tail or the wing while diving.
For me, bird encounters are always the highlight of any soaring flight.