The shift in Earth's tilt toward the sun triggers a "chain reaction" of natural events:

This definition is fluid. It depends on latitude, altitude, and soil temperature. A true biological starts when the average daily temperature reaches a specific threshold (often 50°F or 10°C) for several consecutive days—a phenomenon known as "green-up."

Spring is defined differently depending on whether you are looking at the sky, the calendar, or the natural world

Many animal species begin their breeding cycles to ensure young are born when food is most plentiful. The Symbolism of Spring