Bird migration is something I watch for. Migrating songbirds should start showing up soon. There is a broad rule of thumb that the first hummingbirds show up on tax day, although I think they are unaware of any official postponements. Some resident and early migrating raptors are courting or passing through. Near my house there has been traditional Red-tailed hawk nest for at least the last four years and I just noticed a pair circling in tandem with some likely courtship displays. There is a Golden Eagle nesting area in the foothills that I pass on my bike rides and I've been watching for them but haven't see them yet.
When I was a kid, robins used to be a sign of spring, but there are a few here year round now. Maybe it was always that way and I never noticed.