09/29/2025
Read this!
Recently, many TV meteorologists in eastern North America have been publishing social media posts about bird migration. It’s a logical connection, because migrating birds show up on weather radar, and some nights have provided spectacular radar images of vast numbers of birds on the move.
Unfortunately, every time a meteorologist posts one of these images, commenters jump in to say these couldn’t possibly be birds. I don’t expect to convince the doubters, but for those who are genuinely curious, here is some information.
“Birds don’t fly at night!” Surprisingly, many birds do fly at night during their migrations northward in spring and south in fall. Songbirds like thrushes, warblers, sparrows, buntings and others, which are active strictly in the daytime at other seasons, will fly all night when traveling between their summer and winter homes. Some birds do migrate by day—hawks and swallows, for example—but the majority of migrant species travel at night.
“Radar couldn’t detect something as small as a bird!” Think about it—weather radar can detect things as small as raindrops, and can sense them with enough precision to show whether the rainfall is light, moderate, or heavy. When thousands or even millions of birds are flying high overhead, weather radar certainly detects them.
“Why would birds travel in big circular flocks?” They don’t. On a big migration night, they are spread out like a wide, thin, scattered blanket of birds, all moving roughly the same direction. The radar reflections look like circles because the radar beam sweeps in a circle and reflects the birds that it intersects at certain heights.
“Are all these birds leaving because we’re going to have an extreme winter?” No, it’s normal for birds to migrate south at this season. Hundreds of millions of small, insect-eating birds leave Canada and the northern states in fall to migrate to warmer climates, with many going to the Caribbean, Central America, or South America. Others just migrate a shorter distance within North America. Most migrate before cold weather arrives, and there’s no connection to the severity of the coming winter.
“If all these birds are migrating at night, why don’t we see them in the daytime?” Nocturnal migrants usually land around dawn and look for some safe habitat where they can spend the day, scattering into woods and fields and marshes. For a thought experiment, let’s say ten million birds land in the state of Pennsylvania one morning. Why don’t we notice them? Pennsylvania has about 44,000 square miles of land area, so those ten million birds come out to about 228 per square mile, or about one bird for every three acres. If you walk into a three-acre woodlot, will you notice that one bird?
Bird migration is an amazing phenomenon, with staggering numbers of birds traveling incredible distances and navigating with astonishing precision. For much more information on the topic (although with an emphasis on spring, not fall), may I recommend my book “A Season on the Wind: Inside the World of Spring Migration” (Mariner Books, 2019).
