For some it’s robins.
Most depart south by the end of August and begin to return north in February and March (exact dates vary with latitude and climate).
For others, it’s crocuses.
Crocuses typically appear shortly after the snow melts, sometime around April 1st-21st. Blooms only last for two weeks, so the window to see them is short.
I didn’t snap a picture but for me, another “first sign of spring” (literally) is the sign that led Sasha and I to our first garage sale today! 😉