September 14, 2010

The Birds Are Back

I have mentioned before that birds can be viewed on weather radar, and we have a great example tonight.

It seems like the big migrations tend to occur at night (less chance of being caught by predators?). Here is the radar image that shows the amount scattered by targets in the atmosphere (we call this reflectivity) at 5 PM. Not much there, just ground clutter and some weak returns from a few birds, planes, density discontinuities in the atmosphere.

But here is at 8 PM....the sky is filled with echos..the birds!

And which way are the birds going? We can tell, because we have Doppler Radars, which reveal the velocity towards or away from the radar. Cold colors (e.g., green) indicate a velocity towards the radar, warm colors (e.g, yellow) motion away. Clearly, the birds are generally heading to the south or southeast, just what one would expect.

This is is good time for the birds to get going---it is about to get wet here again, as a frontal system makes landfall later tomorrow.


  1. The birds migrate at night for one major reason, and a number of smaller reasons. The main reason is that at night the winds tend to be more calm, so the birds can fly with less resistance from the wind - especially important in the fall when our winds tend to come from the southwest.

    Fewer predators could be a factor, and another is that scientists have found that birds navigate at night by using the stars as a guide - which is especially important for newly born birds who would be flying south for the first time, so they wouldn't recognize landforms as easy as the reliable stars.

  2. I'm sorry, but as a bird watcher first hand and a weather watcher second hand, I disagree with your viewpoint that these are birds. I suggest you talk to the Seattle Audubon Society and they will tell you that this is the wrong time for birds to be flying, and that there is no way that there is that many birds in the sky that it would show up on radar like a plague. This is radar noise.. basically whatever is maker the air so dense, the radar is picking it up and displaying it incorrectly.

  3. are not correct. In fact, I have talked to bird experts on this and in fact there is a large literature on the subject.

  4. Rob,
    You are not correct about the winds..aloft, where the birds fly, the winds are NOT more calm at night. The use of sky for navigation is a possibility...but they do the nightly flights even when there is a high overcast, which would make the stars impossible to see...cliff

  5. Birds are probably a lot smarter than we think they are, and just haven't told us why they fly at night. Has anyone even bothered to ask them?

  6. Rob is somewhat night, the winds may be stronger (depending on the location), but due to the lack of solar heating, the air is less turbulent, making for a more efficient, predictable flight.

    It's also cooler...these little guys are doing some serious exertion, and it's always nicer to do a workout when it's cool.

    It's definitely time now for bird migration...shorebirds are nearly done, songbirds will be going for a while, and waterfowl in Oct-Nov.

    (I work in radar ornithology on migration studies)

  7. Biking home after dark last night (8:30pm) I could hear mass birds flying over around the Duwamish – it sounded very strange. I think there were seagulls among them but I’m not sure.

  8. It's difficult not to be skeptical when viewing that noisy radar image, but it's true. Just today Wired provided some disturbing evidence. I take it that in the referenced literature there are methods to calculate flock size/density using reflectivities?

    Your post was right on queue, Cliff, as yesterday just after the fog lifted I observed a disoriented Townsend's Solitaire fly into and around the top of the Columbia Tower. Definitely a migrant, and hopefully it was able to evade the local Peregrine Falcon contingent!

    I wonder if the Audubon Society or Cornell might be interested in BirdWatch? You could combine radar imagery with audio data to provide species-specific bird migration nowcasts...

  9. Cliff, you will have to take it up with the Smithsonian! In trying to discover when birds in the NW typically depart for the south, I did discover this from the Smithsonian Nat'l Zoological park:

    At what time do birds migrate?

    Most long-distance migratory songbirds and shorebirds, and some waterfowl, migrate at night when conditions are more favorable (cooler temperatures and calmer air) and predators are few.

    Whereas the nocturnal migrants (that is, the ones that migrate at night) travel through the air by flapping their wings, birds such as hawks and vultures fly by soaring and gliding on rising currents of air.

    These soaring birds must migrate by day, since the rising currents of air which enable them to soar form only during the day as the sun’s rays heat the earth. Swallows, swifts, and nighthawks are also diurnal migrants (that is, they migrate by day) because they feed on flying insects that are active only by day.


    I do wonder if the birds are flying earlier than usual this year because of the cooler weather.

  10. I was saying last night before I saw this post that I could hear the shorebirds flying overhead. Listen for real high pitched "Peep"s.

    The shore birds fly at night because they don't land for most of the migration south. The shorebirds I hear last night may have been airborne since they left N. AK. It varies from species to species but in general they fatten up on the breeding grounds and make these insane flights 1000s of miles south with out rest.

  11. It seems like birds would want to take advantage of the updrafts from solar heating. Even on cloudy days sometimes, there seems to be some mixing. Since arrow dynamics has been seen with a v formation for the x plane, air mixing would be the second piece of the puzzle for the z plane. This is just a thought....

  12. DJ, by "the wrong time for birds to be flying," did you mean wrong time by calendar year/season or wrong time of the day?

  13. The calmer atmosphere at night is in fact a generally accepted main reason for birds migrating after sunset/before sunrise.

    Maybe it is windy at night, but air is not as choppy due to thermal mixing? Plus I am not sure at which altitude they are flying - probably not very high - and at that particular altitude air could be calmer.

  14. There seems to be a lot of interest in the nighttime bird migration issue.

    The winds several thousand feet up don't vary that much between day and night, so that can't be it.

    This time of the year convection is weak, so differences in mixing sounds unlikely.

    Nighttime flying might be cooler for sun.

    At night they would have a chance to see the stars for navigation, although during the day they would have the sun, of course.

    And there is the issue of being seen by predators...that would be better at night I would assume.

  15. There are more fun facts on the Smithsonian page Celia is referencing:

  16. Birds fly at night to avoid the rush hour traffic! Dah!

    Sorry, I find some of these comments amusing.

    I sometimes thought it was the radar? Different mode at night, something to do with visable light?

    Its mostly a question of how birds migrate and some of how radars actually work.

  17. As a pilot who spends most of his time between 60- 140agl (14,000 feet) I can attest to updrafts this time of year especially over broad areas of surface area (dry lake beds,large open fields,) or on west facing slopes of foothills or ridgetops. Yes, summer days can give you a little more bumpiness but it is still around. Last year I spotted a migration of geese at flight level 3000 ft following a ridge-line parallel to the sacramento valley. It was a beautiful site. V formation.

  18. Although it seems any misinformation has been challenged - I'll chime in anyway.

    These definitely are birds and they are probably mostly songbirds, with a few shorebirds and a couple Green Heron's tossed in (not ducks or geese or birds of prey). Navigating with the stars and avoiding predators are always what I've heard as reason for migrating at night. From a logical perspective, these birds have to refuel occasionally, they can't find food at night. Thus they sleep and eat during the day (probably sleeping much less than normal) and migrate at night. Additionally, as Rob said, land masses don't help so much if you've been blown off course!

    If you've ever stood out in a quiet place this time of year, you can hear birds all night long (so literally thousand over one spot). Even in the middle of Seattle I'll hear Swainson's Thrushes, White-crowned Sparrows, and various species of Warblers. I've even watched the moon with my spotting scope (which is bad for you eyes, take note) and caught, the silhouette of a small, presumably, Green Heron.

    Migration is a wonder and we're lucky to have technologies that can help us imagine it!


  19. Oddly enough I was going through and cleaning out my stash of old podcasts right after reading this and ran across the following NPR Talk of the Nation podcast. They discuss this very thing with a number of bird migration experts. Worth a listen.

    It is from 10/9/2009:


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