15 minutes with the moon migration detector


This map shows the entire dome of the sky overhead flattened into a circle. The outer edge of the circle represents the horizon. Our imaginary moon migration detector is sort of like a fence the birds must cross to reach the other half of the sky. It should give us an approximate idea of how many birds pass through the entire sky during an evening of nocturnal migration. — created with Stellarium

After reading the news about all the broadwing hawks passing over Duluth’s Hawk Ridge Monday, I decided to look more closely at the moon last night through the telescope and actually count the birds I saw crossing its face (see 9/15 blog below). I observed for 15-20 minutes around 11:15 p.m. when the moon was well up in the southeastern sky. At 76x magnification, the little flapping silhouettes zipped by at the rate of one every five to 55 seconds. The average was about one bird every 15-20 seconds or three per minute. They were clearly migrating, since all the birds crossing the moon flew east to west and moved in a steady stream.

The four per minute rate, if extended later into the night, would come to about 3 x 60 minutes or 180 birds per hour. This isn’t a huge number until you realize that the moon covers only a tiny portion of the sky, an amount equal to 1/2 of one degree or a half a pinky finger at arm’s length. That’s when I realized that to get a clearer picture of how many birds were really flying over my house, I’d have to build an imaginary moon migration detector.

The detector would consist of an arc of moons reaching across the sky from the north to the south horizons. We all know that a circle contains 360 degrees. Our arc would be a half-circle and measure 180 degrees long. Since the moon’s a half a degree across, it would take 360 moons lined up side by side to stretch the distance. Still with me? Good. Now, no matter where in the east our migrating birds flew from, they’d have to pass in front of one of our imaginary moons as they winged westward.

Recall that about 180 birds per hour passed the moon last night. If we assume that same number crosses all 360 moons in our detector, the total number of birds flying through the entire sky would be something like 64,800 birds per hour. Holy heck! That’s a lot of birds.

OK, ready to take one more step out on a limb? The moon was visible in a dark sky from about 7:30 p.m. last night to 5:30 a.m. this morning — 10 hours. Just for fun now, let’s say that the rate of migration didn’t change at all during the night. That would mean that 64,800 x 10 hours or 648,000 birds flew over my house. Sounds crazy but who knows.


A "kettle" of hawks ride a thermal of air above Hawk Ridge in Duluth. Photo: Justin Hayworth / Duluth News Tribune

I realize these are all approximations and not solid science. I’m assuming the flight rate will be constant for the hour, and that the density of birds from horizon to overhead will average out. My estimates are also for one night only when the winds were favorable. Still it does give one an idea of how many of our feathered friends are fleeing the north for the south. It’s truly exciting to think of how alive the night air above must be this month with the flutter of their wings.


Moonlight shines through a picket fence last night (Sept.16). Photo: Bob King / Duluth News Tribune

3 thoughts on “15 minutes with the moon migration detector

  1. Cool! That is one heck of a lot of birds! All smart birds too..heading south! You are lucky to be so close to Hawk Ridge, I have read lots about it. Great Blog!:)

  2. Thanks Far! I think last night was an especially good night. Tonight they were still steady but fewer – of course the moon is shrinking now, too. This weekend is the big hawk weekend at the Ridge. It’s one of the best times to come to Duluth. Lots of people on hand to help identify birds, see hawks up close; excellent grilled bratwurst for sale, too!

  3. Pingback: Time to fall into that burning ring of fire | Astro Bob

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