- Living on the Wind: Across the Hemisphere with Migratory Birds
After three nights and days of rain, storms, and winds from the south, last night was finally clear and calm. The full moon was so bright that no light was required as I walked up and down my driveway and around our yard. While I couldn't see any birds, a few short flight calls overhead told me that a great river of birds was flowing in the sky above me.
Bald eagles and other bird of prey migrate during the day, when they can take advantage of thermal soaring to help lift and carry their large bodies long distances. But many songbirds migrate in large flocks or swarms at night. Avian predators are less active, skies are often less turbulent, the cooler night air is slightly denser and helps birds dump heat and like sailors, birds can use the stars for navigation across an otherwise relatively featureless landscape.
How many birds were migrating last night? Two words: Green Doughnuts.
The map above shows reflectivity, defined as the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. In short, when radar hits stuff, it gets bounced back. The more radar that gets bounced back relative to the reference, the more intense or denser the stuff it is hitting: precipitation, flocks of birds, migrating bats, insects rising into the sky, and so on.
If you look at the map above, you can see a line of storms moving to the east. Some of these storms are dropping quite a bit of precipitation, as shown by the yellow and red colors in Michigan's Upper Peninsula. But you can also see blue and green doughnuts or blooms blowing up around sunset (which occurred at 6:47pm CT in eastern Minnesota and Iowa last night). The unique pattern, lower reflectivity, behavior (an ebb and flow that begins at sunset and begins to peter out at about 1:30am), weather, and time of year all tell us that these are migratory birds.
So why was I excited about green doughnuts? Check out the reflectivity key below...
The colors represent the strength of returned energy to the radar expressed in values of decibels (dBZ). As we stated earlier, the amount of radar reflected back to the ground unit is proportional to the number and diameter of stuff - drops/flakes/birds/insects - per unit volume. The US Weather service defines rainfall rates and types here, while the Cornell Lab of Ornithology defines bird 'rates' (but not types) as follows:
- Minimal migration: < 5 dBZ — fewer than 59 birds per cubic kilometer
- Light migration: 5-10 dBZ — approximating 59-71 birds per cubic kilometer
- Moderate migration: 10-20 dBZ — approximating 71-227 birds per cubic kilometer
- Heavy migration: 20-30 dBZ — approximating 227-1788 birds per cubic kilometer
- Extreme: >30 dBZ — more than 1788 birds per cubic kilometer (actually occurs at some times in very rare circumstances)
Blue doughnuts are pretty typical, at least during the summer months. But green doughnuts are something else! Last night's radar shows heavy migration that approached extreme migration in some places. As I walked and listened, I thought it was probably a pretty special night. I can't believe how special it really was! I'll be out looking and listening for birds again tonight.
How do normally diurnal birds become temporarily nocturnal? Read more about that here (warning - this is a dense read): https://www.nature.com/articles/srep34207
The Cornell Lab of Ornithology publishes bird forecasts and migration reports on their website at http://birdcast.info/. Check it out and keep those feeders filled for hungry seed, nut, and suet eaters that are resting and refueling in your yard!
BirdNote did an excellent story about nocturnal migration. You can listen to that here: https://www.birdnote.org/show/nocturnal-migration-songbirds.
The Old Bird studies nocturnal flight call activity. While he isn't the only one, almost everyone involved in this activity cited him as the original. Check out his website at: http://oldbird.org/.
Thanks to the University of Wisconsin, we have a national radar map here: http://www.raptorresource.org/maps/UW_Composite_Radar/ and one showing migration for the great lakes/upper midwest here: http://www.raptorresource.org/maps/UW_Composite_Radar/. These maps archive data over several months. We encourage you to check them out and compare nights - summer versus winter, stormy nights versus calm nights, and good flight nights.
Good luck birds, and we'll see you next spring!