In August, birds officially don't do songs anymore, only subsongs. This is the biological term for the less prominent way birds sing in late summer (August) and early autumn and even winter. Subsinging is primarily for preserving food territories, not for attracting mates. It's also less frequent than the spring singing, which sometimes goes on throughout the day.
So yeah, birdsounds could be added and it would be realistic.
If you have any more questions about birdsongs, just post them: one of my friends had his promotional research at university about birdsongs.