Why are red-winged blackbirds the first birds to show up in spring?

They don’t check a calendar. Melting snow and longer daylight trigger an internal clock tuned to day length, kicking in hormones and fat stores. As short-distance migrants, they can handle cold snaps and start moving when days hit a certain length, even if it’s still freezing. So look for the red-shouldered flash in the marshes—they show up before the robins.