I think the bluer bird is younger, not knowing that summer ends, and being even more surprised that this involves immense journeys

I think the blue bird doesn't migrate, but idk. Just a guess.

Pretty sure it's just supposed to be one of those how is it already this late in the year type things

I think the joke is kind of like the talking muffin joke. One bird sees a leaf changing color and apparently has a pattern recognizing brain enough to identify that summer is ending and remembering from last year that that means they'll have to fly south.

The other bird is a bird and it's a miracle he recognized that the leaf changed color from day to day. He probably doesn't even remember what migration is, it's all instinct.