Isotherm when first wave of robins seen. First earthworm appears. First robin heard singing. First female robin seen. First males seen in battle, fighting over territories.
Nest building begins.
Both sexes gather nest materials, but usually only the female builds. She may begin nest building within a day or two after arriving. Incubation of eggs underway. Watch for adult flying with worm, rathern than eating it. Both parents feed the babies.