Gyrfalcon on Trilby Road s of Larimer Landfill on 2/18/2020 (Larimer)

[DAVID A LEATHERMAN‏]

Peter Burke and I saw the gyrfalcon this morning on the big poles w of Taft on Trilby about 10am.  After maybe 20 minutes, the bird flew off in pursuit of something toward the sw corner of the landfill and appeared to land on the ground but we lost sight of it.  About an hour later another big falcon landed on a pole east of the one the gyr had been on.  Eventually the second bird moved one pole to the west and perched on the same pole the gyr had chosen earlier (I am guessing the 5th big pole w of Taft).  The second bird proved to be a prairie falcon.  Interestingly, we actually saw the prairie fly n to pursue a common raven that was heading s out of the landfill with a big mouthful of something distinctly yellow.  What could the prize have been?  The falcon tried to shake down the raven but we did not see anything drop.  This episode was very reminiscent of what jaegers do with gulls. 

Gauging the size of birds is tough on those big poles.  Obviously, if the choice is between gyr and prairie and one sees the underwing pattern, an ID is rather simple.  Back in the comfort of 70-degree computer chairs, Peter and I went to school on our photos and would now suggest the following:

Both gyr and prairie are big gray falcons.

 

Both have dark malars, with a typical prairie falcon having a darker and narrower malar than this gyr.

Both have bright white throats.

Both have pale areas on the side of the face behind the malar.  On the prairie, this area matches the throat in whiteness.  On the gyr it is bigger and grayer than the throat, not nearly as contrasting.

The prairie has a small white patch immediately behind the eye, the gyr does not.

 

The supercilium of the prairie shows some bright white, the gyr's is pale but not bright white.

Cere of prairie seems brighter yellow than the pale yellow-green of gyr.

As anyone who has seen this gyr knows, if it is on a pole, it may not, probably won't, stay put for long.  As soon as getting on a big gray falcon, check the head.  If it has bright white anywhere other than the throat, it is likely a prairie.

We are trying to be helpful here and in no way quibble with anyone who has wrestled with an ID of the falcons in this situation.  The guides do not say enough of help with this difficult ID challenge.  One would think, myself included, there would be little confusion between birds 6 inches different in length and with one being twice the weight of the other.  Not true at the distances involved, being puffed up in cold weather, having full crops/bellies and being under the optical illusion those monster poles seem to generate.  See my poor photos attached that, nevertheless, mirror the views one gets in terms of distance and lighting.  Left bird is gyr, right is prairie.

                                                  

And thank you to Andy Bankert and Robert Beauchamp for first finding the gyrfalcon last winter and this winter, respectively.

Dave Leatherman

Fort Collins

[Anne Price‏]

Dave, thank you for your excellent comparison of markings, a fine discussion and the two helpful photographs.  If I may further add to your description, as a falconer and someone who has worked with males and females of both species:

 

I've been told by several experienced raptor watchers who've seen the Ft. Collins gyrfalcon in person that it is a male.  If the prairie falcon appears to be about the same size when looking through binoculars, or even with the naked eye, then it is likely a female.  The healthy flying weight of a female prairie falcon is in the range of 27-30 oz.  The equivalent for a male gyrfalcon ranges  from about 32-37 oz. Two falcons so close in weight would almost certainly appear to be the same size when viewed perched on a pole, etc. My point being, this can definitely be a tough call if lighting and other conditions aren't ideal…and both falcons are very close in size.

 

Good birding to all,

~Anne Price

Littleton, JeffCo

 

[David Tønnessen‏]

A nice analysis by Dave Leatherman. I'd also point out that part of what gives Prairie Falcons a dark-armpitted look in flight, the contrastingly dark flanks, is also quite apparent while perched. This is demonstrated well by Dave's images; compare the even streaking/barring through the Gyrfalcon's underparts to the Prairie Falcon's denser-streaked flanks.

David Tonnessen

[Bryan Tarbox‏]

Nice to see some discussion of this. When I saw the Gyrfalcon a week or so ago, it caught and began feeding on what I presumed was a meadowlark. While feeding, it was buzzed by another very similarly sized falcon. The other falcon disappeared almost immediately, so I was unsure of what it was, but assumed it was a Prairie Falcon (though of course couldn't help but wonder that it seemed very similar in size and general pattern/coloration). Good to know that there is definitely a similar size and plumaged Prairie Falcon in the area, chasing other birds for meadowlarks, as well.