If you thought birdsong sounded different during lockdown, it turns out you were probably right. The uniquely quiet circumstances of the covid-19 restrictions in San Francisco saw birds respond by lowering their pitch, singing sexier songs and making their songs clearer.

We know some birds react to human noise, particularly the low-frequency city sounds of car engines and air conditioning units, by singing more loudly and shifting to a higher frequency.

“What we didn’t know was if you take sound out, what exactly happens – does it drop by the same …