Why do birds suddenly stop singing at certain times of day?

They don't really stop so much as shift what they're doing - birds sing most intensely in early morning and late afternoon because that's when they're hunting for food or defending territory, and singing burns energy they'd rather spend eating. Around midday when it's hottest, many species go quiet and find shade instead of belting out songs. You'll also notice they hush up at dusk right before roosting, though some do have an evening chorus. It's less about a sudden switch and more about birds being pretty practical about when to invest in singing versus surviving.