Glass, also known as Ice, is light blue in colour. Among the three kinds of blocks, this one can be easily broken by all kind of birds. Levels with these blocks are often provided with Blue Birds, as they are very good at breaking these blocks. It is suggested to multiply. the bird in front on the glass block to maintain the greatest strength of the bird. Most birds slow down when they hit glass, but Yellow Bird slows down after only breaking one bit of glass.
Blue Birds are very helpful on destroying glass, especially when "shotgunning" or better, when split.
Strangely, Yellow Birds are weak against glass, despite glass having a much weaker strength than wood & stone.