How did the sound of firing affect the birds on the battle front?

From the text:

When the sound of firing rang through the hills, the squalling birds gave a start“and took flight, their hearing hurt, as if a hand had struck some discordant note. The end of September gently embraced the beginning of October; it seemed as if heat and cold were being seamlessly reconciled to each other; and over clear blue skies, like bits of carded cotton or white budgerows, light feathery clouds scudded by.