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Birds chirp before the occurance of an earth quake as they have keener senses. so they can sense the smaller waves that travel a bit faster than the larger waves from the source of the earth quake. so, these birds act weird or chirp continiously either cause they are frightned or are trying to warn us about the earthquake. 

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Birds chirp before the occurrence of an Earth quake as they are have very sensitive body which enables them to recognise the phenomina of earth quake and perhaps they chirps to tell others the the earth quake is about to occur.
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