King County's night sky turned into a bird highway this week, with radar detecting an estimated 751,700 birds crossing the county overnight on September 10, according to BirdCast estimates. The number marks a dramatic jump from the roughly 420,000 birds counted crossing the county just a week earlier, and a staggering leap from the 1,700 birds detected on September 1.
The overnight totals were part of a broader statewide surge, with more than 3.6 million birds migrating across Washington that same night, as reported by Axios. Birds crossing the state traveled southeast at an average speed of 25 mph, per BirdCast estimates cited in that report, while the flock over King County reached a peak of 279,000 birds simultaneously aloft around 12:50 a.m. Flight altitudes averaged 3,700 feet above the ground that night.
Cornell Lab of Ornithology's BirdCast migration tracker shows fall migration is ramping up, with huge numbers of birds moving south under cover of darkness even though the calendar still says summer. Migration generally begins 30 to 45 minutes after sunset and reaches its greatest number aloft two to three hours later, the tracker notes.
How Radar Turns Night Skies Into Data
BirdCast relies on weather radar to detect and estimate the birds migrating through the night sky, the same radar network used to track rain and storms. The system draws on 143 NEXRAD weather surveillance radar stations operated by the National Weather Service across the contiguous United States, combining radar reflectivity and velocity data with machine-learning models to measure nocturnal bird volume, altitude, and direction, according to research published in the journal Oxford Academic. Those models update migration intensity estimates every six hours.
Species typically moving through the region at night include willow flycatchers, black-throated gray warblers, western sandpipers, and western wood-pewees. The nighttime timing isn't random: most migratory songbirds fly under cover of darkness to take advantage of cooler temperatures and calmer atmospheric conditions, relying on celestial cues like stars and the Earth's magnetic field to navigate, per the National Audubon Society.
Washington's Place on a 10,000-Mile Flyway
King County sits along the Pacific Americas Flyway, a corridor spanning up to 10,000 miles from Arctic breeding grounds in Alaska down to South America and the Pacific Islands, according to the Pacific Birds Habitat Joint Venture. The flyway serves as a critical seasonal route for millions of migrating songbirds, shorebirds, and waterfowl. Among the flyway's most extreme travelers is the bar-tailed godwit, a shorebird that completes a non-stop migration flight of roughly 6,700 miles in nine days from Alaska to New Zealand without landing to eat or rest, according to The Nature Conservancy.
Nationally, BirdCast's models recorded more than 4.7 billion total bird crossings across the continental United States during peak migration periods in 2024, offering scale to single-night totals like the one over King County. These nightly surges occur against a backdrop of long-term decline: North America has lost nearly 3 billion breeding birds, a 29% overall population drop, between 1970 and 2019, according to a landmark study led by the Cornell Lab of Ornithology and published in the journal Science. Roughly 90% of that net loss occurred across 12 common bird families, including sparrows, warblers, and finches.
Why Lights Out Matters for Migrating Birds
Artificial urban lighting disorients night-migrating songbirds and can pull them toward built environments, where window strikes contribute to building-collision deaths that exceed 1 billion birds nationwide annually once delayed mortality is factored in, according to the National Audubon Society, citing a study published in PLOS ONE that found 60% of collision-injured birds brought to wildlife rehabilitation centers ultimately die. Earlier estimates had placed annual building-collision fatalities between 365 million and 988 million birds.
Data from a study led by the Smithsonian Conservation Biology Institute and U.S. Fish and Wildlife Service, reported by Mongabay, found that low-rise commercial buildings cause 56% of U.S. building-collision bird deaths and residential homes cause 44%, while high-rise skyscrapers account for less than 1% of total collision mortality. In practice, that means low-rise structures and homes combined account for roughly 100 times more collision deaths than downtown skyscrapers.
Audubon Washington's Lights Out, Washington! initiative urges residents, building managers, and property owners to turn off or dim non-essential indoor and outdoor lighting between 11 p.m. and 6 a.m. during peak spring and fall migration windows. Locally, Birds Connect Seattle, formerly Seattle Audubon, coordinates the Seattle Bird Collision Monitoring Project and has partnered to expand dBird.org, a crowd-sourced database where residents can report dead and injured birds to help identify urban strike hotspots.
