Bird Migration

Bird Migration Basics

Bird migration is a spectacular global phenomenon that has long captured the attention of human observers. But it wasn’t until the turn of the 20th century that ornithologists realized how much migration occurred at night.

Direct observation of nocturnally migrating birds is difficult. Seeing high-flying birds in the darkness is a fundamental challenge. Most information before the 20th century originated from limited illuminated observations, for example silhouettes of nocturnally migrating birds crossing the face of a full moon or sightings in anomalously bright skies above wildfires.

But the last century’s technological breakthroughs in radar, acoustic, electronic, and optical technologies have allowed us to see migrating birds and to explore the ecology of bird migration at finer scales. And these direct and indirect approaches of studying movements of birds at night have yielded a treasure trove of information.

We now know that in the U.S, at least 80% of migratory bird species migrate at night (check out Frank Döring’s cool image at the top of the page). An estimated 3-5 billion birds may be migrating, depending on the season. At times, more than one billion birds can be aloft in a single night.

Whereas many birds take off just after sunset, fly at night (all night when conditions are good), and land just before dawn, we have learned that some birds migrate in nonstop flights of 60–100 hours that span oceans and continents!

We know that flight altitudes vary by species and by local weather and topographical conditions, ranging from just tens of meters to several kilometers above the ground.

We know birds tend to fly with supportive tailwinds, which may, at times, occur at very high altitudes. And using technology, we know most migrating birds fly at altitudes where they are unobservable to the human eye.

Birds fly primarily northward in the spring and southward in autumn (where and when tailwinds are available play a significant role). But above some areas of the U.S., birds fly much more strongly eastward or westward. In all cases, seasonal timing, weather, and geography alter their flight directions and speeds. Additionally, birds may deviate around local and regional severe weather (e.g. strong thunderstorms) and may alter their flight directions during a night depending on the presence or absence of concentrating features of the landscape below them (e.g. coastlines).

Peak Migration Periods

In North America, most nocturnal bird migration in spring occurs from mid-March to mid-June, and in autumn from mid-August to mid-November. In the contiguous U.S. this peak migration period is typically from mid-April to mid-May in spring, and from mid-September through mid-October in fall. In the maps and table below you can look up the specific peak migration period for each county in the contiguous U.S.

The highest numbers of birds in flight generally occur in the first half of the night (usually 2–4 hours after local sunset). Peak migration traffic timing within a night can vary seasonally and geographically, with potentially dramatic departures from this typical pattern as a result of local and regional weather conditions and migrants’ origins and destinations relative to areas where they cannot land, among other factors. 

Knowing when the peak period occurs in a particular area can help inform birding and conservation action: for example, for making plans for visiting a particular area to maximize the chances of seeing large numbers of birds; or alerting people to nights that are most critically important to turn out lights to reduce collision risk and other hazards created by light pollution for migratory birds.

If you visualize the cumulative bird migration total passing a particular point as a line plotted on a graph, you will see that it starts very flat, and then rises most steeply during peak periods of nocturnal bird migration, finally leveling at the end of the migration season, at which point the highest point on the curve indicates the total numbers of birds that have passed over the region during the entire migration season.

We capture particularly intense times of migratory movement in what we call “peak periods”: the shortest seasonal windows during which 50% of total nocturnal bird migration traffic historically passes through each county. These periods are determined by examining radar measurements from 1995 to 2018. More detailed discussion and methods are available in this publication. The periods reported below have been calculated using the data of the Migration Dashboard.

Timing of spring peak migration

Place your cursor on a county on the map to display its spring peak migration period.

Timing of fall peak migration