One of the coolest things that happens in California during each year's spring migration is the return of the Swainson's Hawks to the Central Valley. This bird comes in great numbers to the United States from as far away as South America. They mostly come to the central part of the United States, but a unique population persistently comes every year to California to breed. This animated graphic completed by the Cornell Lab of Ornithology illustrates this marvelously. Just look at the Central Valley light up just before the rest of the country:

The video above is built from sightings to eBird, and current maps from the site provide us with a great picture of what's happening this year. Below is what eBird was showing at the end of February:

And here's another image from the end of March. Note the arrivals in the Central Valley:

And here's another map created just today from the first 12 days of April, showing all kinds of birds throughout the region. According to the experts, arrivals should peak right about the middle of April:

(photo courtesy the USFWS)