Question

why birds chirps before the occurrance of an earthquake

Answer 1

birds have the power of sensing low frecquency waves and pressure changes so they try to escape from the danger to warn its fellow beings birds generally do chirpp as other beings like  humans  dont have such power . 
 its just like why do you shout when you see a house on fire

Answer 2

The senses of birds are more sensitive than humans. The occurrence of earthquake is felt by humans only when the ground shakes. But animals and birds can detect the the earthquake much before the ground shakes. Birds can detect some waves which travel faster but not detected by human. Infrasonic waves are one example of that. Other changes include the change in magnetic field which are also detected by animals but not by humans.
So birds can detect the changes in the environment prior to the occurrence of an earthquake. So they become restless and chirp to warn other birds.