we can't actually hear most of the details in birdsong.... what sounds like the same song repeated to human ears/brains can actually contain extremely fast variations within it that are used for communication.... we can't hear fast enough to get it all! yo.....
oh! it's actually more complex than that. so for complicated reasons, being able to hear fast trades off directly with being able to hear precisely (like perfect pitch). temporal resolution vs pitch sensitivity. bird hearing optimizes for one or the other differently at different times of year!
the example given is the carolina chickadee. in the fall they form big flocks and need to be able to communicate efficiently with all those other birds, so their hearing is very fast. in the spring, the flock breaks up as individuals pair off to mate-- and the breeding season song is much simpler than the other year-round chickadee call! just four notes, and what makes it sexy to the girlies is how consistently the male can hit the exact perfect tone
but other birds have it inverted! the white-breasted nuthatch has a very fast courtship song with very fine structure, so their hearing becomes faster during the breeding season, and less sensitive to pitch
also-- sometimes it's different for males and females :0
in house sparrows, the females have seasonal hearing, but the males don't. so they hear the same in the fall, but in the spring the females get better at pitch and worse at speed. this is also interesting because speed of hearing improves your ability to locate an object in space based on sound, so when the hearing gets slower their ability to map sounds also gets blurrier.
and I wonder if this could have anything to do with rates of getting caught by predators in dull-colored females vs dull-colored males vs brightly-colored males, cause I remember those numbers being kinda counterintuitive
