Peter Cowan - POST
By Peter Cowan, PhD,
Director of Conservation Science

Right now, thousands of birds are flocking to California, most of whom are escaping the freezing temperatures of their northern summer ranges. Some are coming to stay for the duration of the winter season but many will stay only for a brief, but much needed rest before continuing south, sometimes as far as Central and South America.

How these birds know where they are going on their long migrations is still a mystery. Studies indicate that some birds take their navigational cues from the sun, moon and stars. Others have found that they use geographic landmarks like rivers and mountains, simply following the path of least resistance. And some might be using each of these approaches in combination. But, nobody knows for sure and maybe we never will.

What we do know is that migratory birds consistently follow the same flight paths, converging every year on “flyways.” Think of them like nature’s interstate system, superhighways between seasonal habitats. California lies within the Pacific Flyway, an area that stretches from the Arctic to the coast of Mexico, and from the Rocky Mountains to the Pacific. North to south, it’s over 4,000 miles long and, in places, over 1,000 miles wide. Here’s roughly what it looks like:

Pacific Flyway - POST
The Pacific Flyway (roughly illustrated above) is one of four flyways in North America used by migratory birds for their annual migrations. The San Francisco Bay, and the diversity of habitat surrounding the Bay, create one of the most important stopovers for migratory birds on this flyway.

The San Francisco Bay is a critically important stopover for birds moving along the Pacific Flyway. As the largest freshwater estuary (tidal mouth of a large river) on the Pacific coasts of both North and South America, there is a cornucopia of habitat here that offers safe harbor for tired migrant birds. It’s an avian paradise, especially for those on the move.

Wilson Warbler
The San Francisco Bay is a critical stopover for migratory birds, like this Wilson Warbler which will cross nearly the entire North American continent on its seasonal migration. Photo: Judy Kramer

Freshwater from the southern Cascades, western Sierra Nevada and Coast Ranges converges here before colliding with the nutrient-rich waters of the Bay. It’s this abundance of both fresh and nutrient-rich salt water that feeds the wetlands of the Sacramento River Delta, the Bay’s vibrant marsh habitat and creates one of the most fertile habitats on earth. It’s no wonder why birds love it.

But it’s not just the Bay and its wetlands that makes this region so attractive. There’s a comfortable place here for almost every passing migratory bird thanks to the huge variety of climates and habitats in our area. The open marsh vegetation near the Bay (like Bair Island), oak savanna (like Arastradero), redwood forests (like Purisima Creek), chaparral (like Sierra Azul) and coastal prairie (like Wavecrest) offer a suite of habitat options for passing migrants. There is refuge here for almost every flight weary bird looking to rest and refuel.

The importance of Bay Area open spaces to the Pacific Flyway is clearest when considering the concentration of the region’s “Important Bird Areas (IBAs).” These areas were identified by the National Audubon Society who are working to monitor and protect the planet’s most important places for birds. The interactive map below illustrates our region’s IBAs and the overlap between these areas and the land POST has helped protect. Take a look and see how we’re working to protect and steward one of the planet’s most important refuges for migratory birds:

Get out and explore!

Join the Audubon Society’s Christmas Bird Count or download our Illustrated Guide to identify Bay Area Birds:

bay area bird guide - POST

Can’t see the form? Please click here for a simplified version.

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About Post

Peninsula Open Space Trust (POST) protects open space on the Peninsula and in the South Bay for the benefit of all. Since its founding in 1977, POST has been responsible for saving more than 76,000 acres as permanently protected land in San Mateo, Santa Clara and Santa Cruz counties.

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