Insects thrive in summer, providing a bug banquet for insectivorous birds. To defend territory and mates, these insects produce sounds, often creating loud choruses which define summer for many people.
In noisy situations, people are able to focus their auditory attention on sounds of interest, like listening to your dinner companions in a loud restaurant. This "cocktail party effect" may be why birderwatchers can tune out background noise, including loud insects or city sounds, while listening for birdsong and calls.
Birds seem to adapt to noisy environments and competition from other birds by singing at different pitches, taking turns, or getting louder. Whether they indulge in an occasional cocktail is yet to be studied.