You think 2020 was tough.

Just be glad you’re not an indigo bunting.

The 2020 fall migration proved lethally difficult for many of the migratory species that funnel through Arizona, New Mexico and Texas on their way back to the tropics for the winter.

A national network of bird watchers reported thousands of dead, migratory birds this year, starting with reports of mass die-offs in August at White Sands, N.M. and continuing through the fall migration — especially in Arizona and New Mexico.

No one knows how to account for the mass deaths.

It might have reflected the impact of the Southwest’s epic drought, which has likely reduced insect populations.

It might have reflected warm temperatures that triggered the start of migration before birds had accumulated enough fat reserves for the journey.

It might have partially reflected a series of storms and a regional cold snap in early September.

Or maybe the effects of climate change are just adding up, sending ripples through the food chain we only notice when we come across a pile of dead birds.

The discovery came on top of comprehensive research suggesting that in the past half century the number of birds in the U.S. and Canada has declined by about 29% — a decline of about 3 billion.

That study compiled the results of annual bird counts of some 500 species, according to the publication in the scientific journal Science. The decline affected both the numbers of common species like robins and sparrows as well as less common and increasingly endangered migrants.

That study also didn’t offer a clear explanation for the decline, which likely involves a host of factors including pesticide use, climate change, feral cats, invasive species and loss of habitat both in northern spring and summer breeding grounds and winter foraging grounds in the tropics.

Bird migration represents a high stakes gamble that enables birds to take advantage of the bounty of spring and summer in the north, but returning to the warm consistency of the tropics in the winter. Historically the gamble has paid off, although between half and 85% of bird mortality occurs during the migration.

However, reaping that bounty requires epic feats of endurance, navigation and timing from creatures that may weigh a couple of ounces. Billions of birds make the unerring journey, often flying at night at altitudes up to 23,000 feet — at least for high fliers like bar-headed geese. Most cover between 15 and 600 miles a day at speeds up to 55 miles an hour. Some, like the Arctic tern, cover 50,000 miles a year. The bar-tailed godwit can cover 7,000 miles without a rest. Scientists aren’t certain how birds can achieve such epic feats of navigation, but it likely involves an ability to sense the earth’s magnetic field, a prodigious memory and perhaps the sense of smell.

The research on North American bird declines relied on the twice-annual, national bird counts involving thousands of bird lovers and scientists. The study focused on 526 species that comprised 76% of all bird species in North America. The study also included weather radar measures of the once vast flocks of migrating birds. The radar tracking at 143 stations nationwide documented a 14% decline in migrating birds just between 2007 and 2018.

Among the worst-hit groups were warblers, but even seemingly indestructible populations of starlings have seen a massive decline. The biggest single category of decline came among species reliant on grasslands.

The die-offs this year in the Southwest seemed focused on migrating songbirds, with many year-round birds not found dead in large numbers. The die-offs were accompanied by many reports of unusual behavior, like birds that normally rest in trees instead foraging on the ground.

Some researchers suggested that the weather played the leading role, including both the impact of the drought on the insect food base and the impact of abrupt cold snaps during the migration. The smoke from wildfires in California, Washington, Oregon, Colorado, Arizona and elsewhere could have also played a role by reducing the birds’ lung capacity and endurance.

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