Yesterday's golden-plover event--and a question for Bryan Guarente

[Ted Floyd]

Hey, everybody.

American Golden-Plovers were reported from eleven (11) sites in Colorado yesterday, Sunday, Oct. 14. To put that in perspective, there were two (2) previous reports for Colorado in 2018: one (1) in Washington County, Sept. 4-8, and one (1) in Kiowa County, Sept. 18.

The previous analysis is based on eBird data-mining.

When one ponders such matters, one's thoughts turn instantly to Bryan Guarente. Bryan, what caused this? The snow, obviously. But why this particular snowfall? And why this particular species?

Ted Floyd

Lafayette, Boulder County

P.s. Other than an American Golden-Plover, goodies yesterday in the general vicinity of Waneka Lake, Boulder County, included an Eastern Bluebird, hundreds of southbound Sandhill Cranes, two Hermit Thrushes, FOS Gray-headed and Pink-sided juncos, FOS Townsend's Solitaire, a Long-billed Dowitcher, Wilson's and Orange-crowned warblers, a getting-latish flock of 15 Lesser Goldfinches, and a Wood Duck.

P.p.s. This Monday morning, Oct. 15, a quick stop at the Legion Park overlook revealed the Valmont Reservoir complex to be very birdy, harboring a Sanderling, a Semipalmated Plover, a couple dozen Mountain Bluebirds, and distant gulls, geese, and grebes galore. It would be very much worth the effort, I suspect, to walk in from Red Deer Drive and watch from the Open Space tract beyond the end of the road.

[Bryan Guarente]

Cobirders, 

when Ted beckons... you get a really long email...

So the question is:

1. Why did this situation bring more birds to the Front Range?

TL;DR (Too long; didn't read) -- Super-short snarky answer just for Ted: it was the wind!  The weather had a lot to do with it and which end of the cold front Colorado ended up on helped dictate that flow of migrants.  Based on percentage of the total flow area behind the cold front compared to the overall flow, it looked like a 30-40% chance that birds would end up in the Front Range due to funneling or convergence.

Full version:

• Why did this weather situation bring more birds to the Front Range?
  

Let's look through the computer models because it is sexier, and makes it easier for everyone to understand because I can give you data everywhere on the globe.  One could also do this with satellite imagery, but it is harder to get you to see what I want to see, so I will work with the easier option.  

• https://earth.nullschool.net/#2018/10/14/0000Z/wind/surface/level/orthographic=-105,40,1706/loc=-105.000,40.000
  

That animation of a single time gives you the idea of what is going on that made Colorado a hotspot for any migrants yesterday.  Any bird trying to make its way to the southeast from Canada may have started out with good intentions, but depending on which side of the flow it started from or ended up in over time, it had a strong chance of ending up heading toward the Front Range.  The cold front itself is the "blue" area with no wind that curves from Lake Nipigon down through Iowa, Nebraska, then curving into Colorado.  All of the airflow behind that cold front (to the north and west) is what we want to focus on.  The flow had multiple possible end points at that time: near Lake Nipigon, along the cold front just south of Lake Superior, along the cold front in Iowa, or into the Colorado Front Range.  

The highest likelihood location for the birds to end up was actually along the Front Range.  The percentage of the total area of that flow behind the cold front that was showing a distinct convergence into the Front Range was about 30-40% (guesstimated).  So any birds within that 30-40 percentage of the total area had a strong likelihood of ending up in Colorado's Front Range.  That means that birds ranging from Alberta through Montana, North Dakota, Minnesota, and western Iowa and then everywhere southwest of that behind the cold front, had a strong chance of ending up in the Colorado Front Range.  The door was wide open so to speak.  The flow was broad initially, then came crashing in on itself converging into a small area (Colorado Front Range).  So think of this as your funnel for bird convergence.  On the broad end, you put in any birds you'd like, then on the other end, you get a stronger concentration of birds because the winds they like to follow are forcing them together more over time.  Other places are getting lower concentrations of migrants due to the divergence of the birds from their area into our area.  

This was only one snapshot of the winds at the surface though.  For a period of about 12 hours, this was still the case around this.  Earlier it was less convergent into the Front Range, but picked up, then maximized around the time I showed you earlier, then tapered off a little.  Importantly though, the time I linked you to was right around sunset when the snow started to pick up all along the Front Range.  This was a bonus for birders, hindrance for the birds.  Both the sunset and the snowfall made this more important for the birds to get to the ground, and then they likely stayed the night to try their luck at adding some munchies in the morning.  

This is the time for American Golden-Plover migration.  It also happens that the location this storm started from had a good chance of grabbing some of those migrant AGPLs trying to make their way through the Central Plains like they normally do.  However, as luck would have it, they ended up on the wrong side of the flow behind that cold front.  They got stuck on the Colorado Front Range side, and then we got lucky to see them here.  The number of AGPLs that migrate through this corridor in a short period of time is HUGE.  That also gives us a higher chance of getting them here in CO.  I remember from my days in Illinois that this time of year would produce fields upon fields of AGPLs numbering in the thousands easily.  They would take off in huge flocks and migrate quite broadly through the area during the day.  You could easily go a day with seeing 20-40 flocks numbering 500-1000 birds a piece.  It is kind of surprising that there weren't more AGPL found along the Front Range when you think of it that way.  

Yes, you may say as a counterargument to my arguments about the wind that birds have wings, and they don't have to follow the winds.  True.  They don't have to follow the winds.  If you ended up on the wrong side of that flow though (the west side closer to Montana or Alberta), the chances of you covering enough ground to not end up in Colorado was pretty slim without a LOT of extra effort to cross the flow.  Ask your pilot friends which way they spend more fuel with a tail wind or with a cross wind and you will get some idea of why they ended up here instead of Iowa like they were "supposed" to.  

Hope that helps.  This was my quick response.  If you want to hear more, just ask and I will see what I can do to respond.  If you get to this email soon after I sent it, you can see the same type of wind pattern play out in the satellite imagery here: 

https://www.ssec.wisc.edu/data/geo/#/animation?satellite=goes-west&end_datetime=latest&n_images=all&coverage=conus&channel=03&image_quality=gif&anim_method=javascript

This is real-time data though, so you won't be able to watch that loop for too much longer as it purges the old stuff.  

Hope that helps, Ted.  And I hope others gleaned some knowledge from this as well.  It was a fun situation to analyze and even more fun to bird.

Bryan Guarente

Meteorologist/Instructional Designer

UCAR/The COMET Program

Boulder, CO

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[Sandra Laursen]

Bryan,

thanks for this analysis.  One lesson I have absorbed from you in the past is that it's not the surface-level winds that are important, but the wind up a bit higher where the birds are migrating.  Why do you focus on surface winds this time?

thanks

- Sandra Laursen

[Bryan Guarente]

Sandra and others,

In this case, it was because of the snowfall that made me focus in at a lower level.  The snowfall drives the birds lower so they have more visibility to something on the ground (at least that is what is suspected).  In the earlier times of this event, the 850 millibars (units of pressure abbreviated mb; approximately 1.5km up from sea level) was similar to the surface winds, so not too big of a difference.  I appreciate you calling me out on this.  It would have been better for me to say that I was looking at the surface information because of the snowfall associated with the system coming through.  This may actually lead to other confusion for the birds parts and drive them closer to the ground making it more likely for them to descend earlier rather than later. 

I did evaluate where the precipitation was falling prior to the period I posted about and had been watching it throughout the event (for meteorological not ornithological reasons).  The precipitation was falling behind this cold front ever since it was in Alberta, so it was a good bet that the birds would be flying lower to the ground.  Ask anyone who monitors nocturnal migration, and you will find that more migrants are heard when the cloud cover is lower.  This is true when there is precipitation as well, but there may be a bias in observers for comfort reasons.  

Hope that helps, and please feel free to keep asking more questions.  There are more intricacies to this, but we only scratch the surface on COBirds.  

Thanks for calling me out.  I appreciate the fact that this may have taken extra gumption to do in a public forum, and I deserved it.

Bryan Guarente

Meteorologist/Instructional Designer

UCAR/The COMET Program

Boulder, CO

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