We are now in the midst of the big bird migration time of the year and weather radar can help documents the huge flux of birds overhead.
But even more fascinating, we can skillfully predict bird migration using numerical weather prediction.
One of my favorite sites to check out bird migration is BirdCast, run by the Cornell Laboratory of Ornithology. I really appreciate this group and as an undergrad at Cornell I frequently visited their Sapsucker Woods Wildlife Sanctuary.
One of the wonderful things they have on their website is real-time bird migration maps based on the clever use of weather radar (see below). Turns out that weather radar is a very effective tool for tracking birds, particularly when there is not much precipitation (when the National Weather Radars are in clear air mode).
The map below provides an example: a real-time snapshot of bird migration at 12:50 AM PDT early this (Saturday) morning. The colors shows the intensity of bird migration and the orange arrows provide the direction of migration (which can be determined from the Doppler weather radar signal!). Not surprisingly, most birds are moving northward and the Pacific Northwest is an active migration route. The largest migration is from Texas to Wisconsin. By the way, the units of this map is THOUSANDS of bird per km line per hour. That is a lot of birds!
To confirm the bird invasion, here is the composite radar image the night before (1:38 AM on May 1st). Wow. There was very little precipitation that night, so virtually all of this is birds. Keep in mind that there are major gaps in weather coverage (such as east of the Cascade crest). You note lack of echos offshore…our feathered friends prefer to stay over land!
Our bird friends prefer to fly at night, and the Cornell migration graphic at 9:40 PM Eastern Time yesterday shows this, with the red line indicating the location of sunset at that time. Very few birds while the sun is up (west of the line), lots of birds to the east of the sunset line.
But this site has even more! It include forecasts of bird migration activity. Using decades of radar information to provide migration ground truth, they correlated bird migration activity with forecat weather parameters (from NWS prediction models), day of year, and much more, using a machine learning algorithm. This approach is based on the work of Van Doren and Horton (Science Magazine, 2018) . To illustrate, here is the migration prediction for tonight (Saturday-Sunday), which includes the amount of forecast precipitation as well.
Less birds tonight..and that has to do with the change in the weather. This bird forecast research noted above (Van Doren and Horton) found the bird migration correlates best with temperature (more migration with warmer temperatures), with precipitation also discouraging our feathered friends. The strong front moving through today will result in both cooler temperatures and showers. Thus, our migrating bird visitors will take a well deserved rest this evening.
Finally, I should mention that there is all kinds of fascinating information in that Van Doren an Horton papers, such as the annual variation in bird migration over the U.S. based on the weather radar data (see below). Peak migration is in early-May, with a huge ramp-up in April…so we are very near the peak now. Thus, watching the radar now is of particular interest for all bird lovers.
So during the next mild night this month, look up and imagine the thousands of birds that are moving northward above your head. Kind of reassuring during these difficult times.