The phrase "chirping birds" refers to the sound that birds make when they sing or communicate with each other. It is a way to describe the pleasant and cheerful sound that birds create with their voices. Full definition
Waking to the sound of chirping birds at 5 am and my cat trying to rip out the screen in order to get to them (fortheloveoffoodblog.com)
Sipping frozen bevvies, sitting poolside under the warm sun, listening to the tunes of Bob Marley while crashing waves and chirping birds sound lightly in the background by day. (pumpsandpushups.com)
Finally it sprouts leaves and gains a nest with two chirping birds in it. (rogerebert.com)