Science Corner - Pacific Flyway

Kristie Ehrhardt • December 5, 2024

Using data that was collected over a span of many years from hunters returning the leg bands from harvested waterfowl, biologists in the mid-1930’s were able to determine that migrating birds followed four different geographical routes, or flyways, during spring and fall migrations throughout North America. Today, these long-established flyways still accurately represent the major north-south migration passageways used by billions of migrating birds. In 1947 the flyway concept was formally adopted to help manage resources for migratory birds within the United States. Named for their geographic regions, the four flyways are the Pacific Flyway, the Central Flyway, the Mississippi Flyway and the Atlantic Flyway.


Because each of the flyways has unique biological and ecological qualities, state and federal wildlife agencies have adopted the flyway classification to help monitor and protect resources within the United States for migrating birds. California is a vital part of the Pacific Flyway which is approximately 4,000 miles long and 1,000 miles wide. It is utilized by waterbirds, shorebirds and songbirds and extends from the Arctic tundra in Alaska south to Patagonia in South America. In the United States, the Pacific Flyway includes the western states of Alaska, California, Idaho, Nevada, Oregon, Utah, Washington and the portions of Colorado, Montana, New Mexico and Wyoming that occur west of the Rocky Mountains. The Pacific Flyway alone is estimated to host over one billion (!) birds annually.


Each flyway is jointly managed by biologists from state and federal agencies (California Department of Fish and Wildlife and the United States Department of Fish and Wildlife) as well as other land and wildlife management agencies, university faculty and staff and interested non-profit organizations such as Ducks Unlimited and the Audubon Society. Management plans are developed with input from all United States agencies involved as well as Canada and Mexico. Under the authorities of bilateral treaties, the federal governments of the United States, Canada and Mexico are responsible for the conservation and protections of the migratory birds in North America. The Migratory Bird Treaty Act (MBTA) affords protection to listed migratory species and allows individual states to enforce their own conservation rules and regulations, including game regulations, as long as they do not conflict with the federal regulations. Under the MBTA, state protections are allowed to be more stringent than the federal protections but they are not allowed to be more lenient. The MBTA was enacted to help protect all migratory bird species that are native to the US and its territories but does not cover those species that were introduced such as European Starlings or House Sparrows.


California’s wetlands and rice fields are a critically important refueling, resting and nesting spot for birds traveling along the Pacific Flyway. Sadly, California has lost 95% of its historic wetlands yet the Central Valley remains the most important wintering area for waterfowl in the Pacific Flyway. Wetlands and rice fields here support about 60% of the total population of migratory birds by providing open water, food and shelter. Some species such as Snow Geese and Greater White-fronted Geese migrate south to winter in California and other species such as Swainson’s Hawks migrate north from Argentina to summer and breed here in California. Exactly how these birds accomplish such a hulking pilgrimage is not completely understood. Some biologists believe that birds use navigational cues from the sun, moon and stars while others believe that they follow geographic landmarks such as mountain ranges and bodies of water and still others believe that birds use the Earth’s magnetic field to help navigate. Recent studies indicate instinct and learned behavior also may play a role in birds ability to migrate across multiple countries. What we do know is these birds take the same routes year after year and species’ arrival can be predicted fairly accurately.



There are many significant stops for the birds along the Pacific Flyway. Some of them are refueling and resting stopovers while others, like the Central Valley’s rice fields, are the termination of their seasonal journey. One of the closest hotspots to view these miraculous birds that made a miraculous journey is the Vic Fazio Wildlife Area in the Yolo Bypass. If you’d like to go visit check out https://birdinghotspots.org/hotspot/L443535 and don’t forget your camera and binoculars!


-Kristie Ehrhardt (kehrhardt@tuleyome.org)

Tuleyome Land Conservation Program Manager

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