Forecast and Analysis

New feature: bird numbers for forecasts and live maps

Adriaan Dokter and Benjamin Van Doren The Cornell Lab Apr 13, 2021

The BirdCast team has added a new feature to our forecasts and live maps: numeric estimates of the predicted and observed numbers of migratory birds in flight.

As of today, you will notice a small white box with a number in the lower left corner of the maps. For the live maps, this box indicates the number of birds in flight at the indicated time, in millions of birds. For the forecasts, the number indicates the approximate number of birds predicted to be in flight above the US at approximately three hours after sunset. As a rule of thumb, the predicted number approaches the peak number of birds observed on the live maps within a night. Because the moment of sunset varies from location to location, the forecast number and the observed peak number for the live maps are difficult to compare exactly.

Throughout the season you will notice huge changes in the numbers of birds aloft, both throughout the night and throughout the season, with numbers varying from a few to several hundred million birds. Our models currently tend to underpredict nights with most intense migration – such mass movements are relatively uncommon (see this new paper for some details!) and therefore more challenging to predict. But the team is working diligently to refine our forecasts to be as accurate as possible.

To convert radar observations (in so-called “reflectivity”) to bird numbers, we assume that all birds in flight are the same average size. For more information on quantification, please visit our the FAQ section.

Forecast: 77 million birds are predicted to be in flight on the night of April 13 at approximately three hours after sunset.

Live map: on the night of April 12 2021, the number of birds in flight above the US changes from about 25 to 100 million birds. A peak number of just above 100 million birds is reached at 2:00am Eastern Time.