ELI5:Sound propagation in high humidity air.

My ornithologist cousin told me the reason birds make more sound in the morning and at dusk is that it tends to be when air humidity is the highest. The reason is that sound travel better in these conditions - higher concentrations of water vapor. In the bird world it is all about being heard apparently.

So I was thinking about sound propagation in water, which we know to be much faster. But water vapor is an entirely different thing. It is lighter than dry air because of its lower density. But what are the mechanisms at play, that makes higher frequency sounds (eg. bird sounds) travel better in a mix of dry air and water vapor? I don´t get it at all.