A long and detailed piece with lots of images and graphs. I hope it's available to non-subscribers to the Chronicle.
As a lover of alliteration, from the piece "letting seabird birders see seabirds off the seashore."
About a billion birds migrate through California as they head south for the winter. For many, the Bay Area is a perfect rest stop.
By Christian Leonard and Jenny Kwon | Dec. 25, 2024 4:00 a.m.
They come in the millions.
From as far as Alaska, massive flocks of birds leave their breeding grounds as fall weather portends the arrival of frigid temperatures and the loss of their best food sources. The journey south is a long one — thousands of miles for many species. Some will travel all the way to South America, where the warmer weather means food is plentiful.
But there are havens along the way, rich habitats still lush with fish and insects to fuel their trek along the Pacific Coast. And for many species of birds, few havens in California are more important than the San Francisco Bay Area.
Each fall, upward of a billion birds fly from Alaska and Canada to warmer climates near the equator, blanketing skies and shores as they travel along the coast. The Bay Area has something for all of them. Sandpipers comb Marin’s shores for tasty mussels, while some cardinals like western tanagers hop from branch to branch in East Bay forests. Many species spend the entire season here, taking advantage of the region’s mild winter.
“You have one of the most diverse … environments in California, indeed, (of) anywhere along the West Coast of the United States,” said William Sydeman, a marine ecologist and president of the Petaluma-based Farallon Institute.
The key is water. The San Francisco Bay, combined with the Sacramento-San Joaquin Delta, is one of the largest estuaries in the country. The mix of fresh water and saltwater nurtures a host of bugs, fish and plants, making it a vibrant feeding ground for birds. The estuary supports so many species that it’s considered one of the most important sites for shorebirds in the Americas.
Bay birds also benefit from something oceanographers call the “continental shelf,” the part of the coastline that’s under relatively shallow water. In Northern California, shelf waters are cooler and nutrient-rich, making them another important habitat for hungry birds.
That’s because unlike the East Coast, where the shelf can extend more than 100 miles into the ocean, the West Coast’s shelf is only about 20 miles wide. In the Bay Area, the shelf can drop off after only a few miles, said Scott Shaffer, a professor of ecology at San Jose State University. That means many ocean-bound species like albatrosses get much closer to the Northern California coastline than they would in other regions — letting seabird birders see seabirds off the seashore.
All of those interconnected factors — the rich food sources, the diverse set of habitats, the geographic shape of the region itself — make for a lively fall migration. More than a million birds flew through San Francisco from late summer through fall this year, and 4.5 million traveled through Santa Clara County, 2024 data from research organization BirdCast shows.
But even in late winter, the birds keep coming. Several species of ducks migrate in December, many of them stopping or even spending the winter in Bay Area ponds and lakes.
Here’s why the Bay Area is so important for birds — and where to find them
Greg