Do a comparison with a high powered microscope, test after a uninterrupted night of silence, then repeat the process with birds chirping approx 1 hour before lights on then check stomata’s again
I play music 24/7 in my lung room loud enough to be heard inside the tents, it’s just music but I don’t think it’s spesifically the birds chirping more than just hitting the correct frequency here and there, most types of music vary in tones enough to give the correct vibrations of frequencies to the plant to know it’s wake up time
That’s be another good experiment, test with various frequencies vs sounds themselves