Posted by: jkennedy366 | 25 October 2013

Light and dark broad-winged hawks in later October, whence and why

Most of my commentary recently on this blog has been about the extraordinary numbers of dark broad-winged hawks around starting on Saturday October 19th and repeated on the 20th and 22nd with one rain delay in between.

Several years ago we were really excited when 3 dark birds were seen on one day with 2 in sight at one time with (if I remember right) 5 or 6 for the season and a really great year.

Sunday, I had 30 for the day with up to 6 in sight in the binoculars at one time. And lots more on Tuesday with 130 since the flight started.

Dark broad-wing over the tower

Dark broad-wing over the tower

Just as important is the unprecedented numbers of plain old broad-winged hawks for the latter half of October. If you add all the birds seen on a date like October 22 for the last 11 years, this year had as many birds in one day as the total for those 11 years. Saturdays count doubled the 11 year total.

What is strange is that there are no other hawk counts around the country that account for these birds. Where have they been and why are they so late. Martin Reid had about 30,000 birds on that Saturday which were different that the Smith Point birds. Looking at the great lakes concentration points that have large numbers of broad-wings go by showed that there was a total of a dozen birds for all the sites for the entire month of October. The Appalachians were also empty with only Duluth have about 1,000 birds for the entire month. There has been no mention on the listserves etc that I look at saying anything about late broad-winged hawks.

So where did they come from and where have they been. The great number of dark birds provides a clue as the dark birds generally nest in the very northwest part of the breeding range way up in Alberta. Many of them go south far west of the UTC (with a few crossing San Francisco Bay) and are seen at many mountain sites in the west. They also tend to appear at Smith Point and other points later in the season. Will they be missing at the western sites this year?

Looking back at the weather for the last several weeks there were two events that could have led to both the late and dark broad-wings at Smith Point. Two major winter blizzards moved through Wyoming and the Dakotas and then curved northeast into Canada.

The first started in Wyoming with up to several feet of snow in places before killing large numbers of cattle and wildlife in South Dakota and easing as it curved north into Canada before reaching eastern Dakota. A second storm 2 weeks later again brought a foot of snow and took the same path. At some point we will probably hear about the loss of birdlife in the area as the first storm occurred before many birds had migrated out of the area.

Areas north of the storm were not really affected. Hawks moving south toward the storm(s) would have encountered strong west (counterclockwise) winds tending to push them east before they reached the blizzard area. As the storm moved east and then north, hawks drifting east would have been stopped as the storm swung north. After the storm, there was a wide area with deep snow, no thermals and contrary winds limiting the ability to go south across the snow belt. The second storm provided the same effect to a lesser degree.

This scenario would provide for the numbers of both color phases here on the UTC. The birds were drifted east but not as far as Lake Superior. And they were held up and stopped by deep snow and no thermals just like the delay and confusion that occurs here when they do not want to cross Galveston Bay.  But the snow area was a couple of hundred miles wide instead of 10 miles.

It is a truly extraordinary event and one that has been thoroughly documented at the Smith Point tower. Now all we need is some data from further north to actually provide clues or evidence of the cause. As mentioned, Corpus Christi had a pulse of broad-wings too and 13,000 went by Veracruz yesterday.

 

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