Monday, December 12, 2011

8,041 Broad-winged Hawks

I have greatly enjoyed many of the days spent hawk watching thus far in three seasons at Boothe Memorial Park in Stratford, though I do not think any surpassed the thrill of September 16, 2011, when 8,234 raptors were recorded including 8,041 Broad-winged Hawks. Contrary to the prelude to the day in my last writing on the historic Turkey Vulture flight, I felt extremely certain that we were about to be enthralled by a flight of thousands of Broad-winged Hawks in the week leading up to it. Even a full six days before the flight we were set to receive I was mentioning it to friends and fellow observers, saying in an email that a fast and strong cold front would be “hard to time…exactly a week away but it'll be fun, mega Broad-wing push”. Boothe’s previous Broad-winged Hawk daily record was only 1,884, this was only year three, and I am not a psychic – what made me so confident about that day?

Looks like a good day for hawks! (See the weather vane at the top of the tower in this northwest facing photo by Frank Mantlik)

I went back and examined some of the "mega" flights at various points in Connecticut's history in the year since the conclusion of the 2010 hawk watch season in an effort to determine when one would occur again. Thanks to Neil Currie and so many other vigilant hawk watchers over the last 40 years we have a very strong set of data that can guide you to general flight dates. The late teens of September are a superb time for Broad-winged Hawks to fly, and this has become generally basic knowledge among those invested in the hobby. Knowing when a given species has the itch to get on the move to the south is one very important factor, but what happens if the weather doesn’t cooperate? Some birds, especially the Broad-winged Hawk, would never want to move against a southerly wind or under particularly overcast and rainy skies.

Atmospheric success

I put on my weather hat and dove in to meteorological data, using the dates of some of the biggest flights of Broad-winged Hawks the region had ever seen and examining the weather conditions that made them possible. I looked for patterns in the atmosphere on a large scale – wind speed and direction, the placement of high pressure centers, any blocking weather in the days before, sky conditions, temperatures and precipitation, and more. Some of the major motivating factors fell in line with conventional hawk watch wisdom, namely that raptors often take to the skies after the passage of a fall cold front. However, this is where things get much more complicated as a simple frontal passage is not sufficient for a strong hawk movement, let alone a historic one.

The most important factor, in my eyes, is the recent placement of a strong high pressure center over the vicinity of the Great Lakes region during the daytime hours of a given flight day. Such a perfect placement is seen infrequently with a cold air mass filtering down from Canada in the wake of a low in September. For the most part, the cold front passage has to be swift enough and clean, without any other areas of low pressure remaining nearby, without the presence of residual precipitation via a subsequent shortwave or other means, and sizable enough to encompass at least the entire northeastern United States and southeastern Canada. This seems to be the best conveyor belt for Broad-winged Hawks, pushing them to the south across multiple states, and such a placement puts Connecticut squarely under northwest winds from the anti-cyclonic flow of the high. The timing of the movement of the high is important for the flight but also for the pressure gradient as the winds have to be light to moderate, as a howling wind from the northwest will move some birds, but not allow thousands to stream through.

There are actually many more ways a passing cold front could permit a small or moderate flight, though not a sizable one. Let’s say a cold front passes through with high pressure moving in from the south – this flow will be westerly and ultimately southwesterly, disallowing any substantial movement whatsoever. We could have a weak cold front passing through with showers followed by a lack of any significant air mass, or an upper trough causing completely overcast skies (limiting thermals) and some precipitation on its heels. Sometimes a cold front will pass through only to stall out just to the east or southeast of Connecticut, making it difficult to get raptors moving through our air in any great numbers. Sometimes you do not need a cold front at all, and one of the best periods of movement in 2011 proved that. The departure of a sluggish upper level low spinning over the region in early October was followed by a high that graced us with light northwest winds on October 5 and 6, pushing through 352 and 621 raptors, respectively, with October 6 allowing Boothe to break daily Sharp-shinned Hawk and Bald Eagle records, among others.

September 2011’s weather

The key to a massive Broad-winged Hawk flight is clearly the weather leading up to and on the day in question, and this held true in 2011. Early September was plagued by a nearly constant southerly or southwest flow in the wake of Tropical Storm Irene. The few fronts that passed through Connecticut stalled near or over the region, with one coming through in the first week of September stopping off our coast as Hurricane Katia came north through the Atlantic. Her sizable influence and cyclonic flow actually provided the best hawk watching day for Boothe Park prior to this Broad-winged Hawk flight, with 40 raptors seen under light northwest winds – yes, only forty. It is abundantly clear that the hawks were stopped from any significant movement whatsoever prior to the date of the major flight this September, and it is likely this contributed to the extremely large movement that followed.

Once we were around one week away, I noticed that long-range models indicated the likely possibility of a large and strong cold front stretching from southeast Canada to the Gulf coast pushing through late in the following week, clearing the entire eastern seaboard. A strong high pressure center would move through the northern Great Plains and possibly settle near the Great Lakes or Ohio Valley. This is essentially what occurred with wondrously fortuitous timing as the front cleared well off the Atlantic coast overnight on September 15, allowing for the air mass of the high to take over as it moved in to position near the Great Lakes near sunrise. The distance between pressure centers was enough to keep the winds at a moderate strength, around 8-12 MPH at our site, with occasional gusts lessening in frequency and intensity as the day went along. We were even able to muster up some light cirrus overhead to top off the near-ideal conditions.

Boothe Park’s position

A lot has been said about the position of Boothe Memorial Park in discussions about the high number of raptors occasionally passing over. It is situated approximately three miles off Long Island Sound from its closest point, actually in Milford. Somewhat like Turkey Vultures and some of the other buteo species, huge numbers of Broad-winged Hawks are reluctant to move much closer to the coast than our position, and indeed, many that come over are heading to the west or west southwest rather than south or even southwest. We can have streams of birds in a long train heading west while more are being added to it by incoming kettles from the north. Below is a shot of us on September 16, unfortunately the only photo taken that day during the nonstop action, counting the streams of Broad-wings going to the west southwest.

From left to right, Charlie Barnard, the clipboard on the chair, Scott Kruitbosch, Bill Banks - taken by Frank Mantlik

Once again, like the Turkey Vultures, the key at Boothe Park is to have a light to moderate wind from the north or northwest. Any easterly component pushes birds away from us due to geography, and when it comes to Broad-winged Hawks, they will not take to the sky in nearly the same force. As I discussed in the above section, the winds were mostly light to moderate, with occasionally stronger gusts that wore down as the day progressed. This ensured that the Broad-wings would be pushed from all inland areas down along the coast, but not so much that they were battered in the air, kept from attaining the appropriate altitude, or forced to fly to our south.

Nine exhilarating hours

The count began at 8:00 a.m., with Bill Banks joining me bright and early. We would remain there for the next nine hours, with Charlie Barnard and Frank Mantlik spending significant portions of the heavy afternoon flight there, and Penny Solum joining us for some time in the morning. As most hawk watchers know, it takes some time for raptors to get into the skies mostly because of the fact they are waiting for thermals, rising columns of warm air that help them almost effortlessly glide to the south. This is particularly true at Boothe Park for another reason as it is difficult to see much around ground level because of trees, buildings, and other impediments. The birds have to be relatively high in the sky to see them, and we typically only get early birds either directly over our heads or in gaps to the east and southeast near the Housatonic River.

We tallied a few quick Bald Eagles, Ospreys, Cooper’s Hawks, a bunch of Sharp-shinned Hawks, a Northern Harrier, and an American Kestrel. In the case of Broad-winged Hawks, it takes them an especially long time to wake up and get on the move, and we did not have any of them until 18 passed through between 10:00 and 11:00 a.m. By this point, I was beginning to get a little anxious about my prediction, and whether or not we would be able to pull off a large enough flight in approximately the next four or five hours. Catching even a couple thousand birds in such a time frame seemed like an insurmountable task at the rate we were going, but I kept repeating to myself (in my head and out loud, as others can likely attest to) that this was a perfect setup on a perfect date, and we were going to be paid off soon enough.

I suppose many of the Broad-winged Hawks we saw that day had some distance to cover after taking to the skies, moving to the south from other parts of Connecticut or coming in from nearby areas in the first few hours of the day before reaching us. Not long after 11 a.m. the major flight started, and before the hour was over, we had broken our hourly record of 876 birds (set on 09/20/10 by me, counting alone, from 3:00 to 4:00 p.m. that day) with a strong 892. Even if we maintained this rate we would have a tremendous afternoon, and we thought that if we could get a few more clouds than the 15% cover of cirrus we had that hour, this could be spectacular. Suffice it to say, we did, and it was more than we could have hoped for. We shattered the hourly record and the daily total record between 12:00 and 1:00 p.m. as we tallied 1,278 Broad-winged Hawks, pushing our total over 2,000.

I certainly did not anticipate doubling this over in the next hour, but from 1:00 to 2:00 p.m. we counted a mind-blowing 2,887 passing over our heads. For the most part this took the form of an exceptionally long and seemingly endless river of raptors nearly directly above our position, slightly to the north of straight up from the site. It spanned the sky from the trees and neighborhoods to the west all the way to the horizon over the Housatonic in the east. We craned our necks until we could not bear the physical pain any longer, the four of us trying to somehow keep track of and count every single bird in this enormous river. Part of the problem I recall having was that they flew in a wide track stretching beyond one or even two binocular views, especially tough for my “10s”.

One had to sweep their binoculars back and forth simply to count all the birds in even one small section of the river all while they moved to the west. Bill, Charlie, and Frank were fantastic, calling out the numbers to me as I added them up in my head and occasionally scratched down totals on the data sheet. We all tried to count these overwhelming numbers as best we could while keeping on the conservative side if we ever lost track of a few, being careful not to double count any. Our independent counts of sometimes hundreds of birds were often extremely close to one another. A few times we had to stop and stare in awe, being overwhelmed by the sheer outrageousness and splendor of the sight, unable to think correctly or process it fully. I thought we would be able to cruise past 3,000 in the hour, but they started to slow down from their peak movement before we got to 3 o’clock.

That is not to say the movement was anything but astonishing in the next hour as we totaled more 2,468 Broad-winged Hawks. Our bodies would finally get a respite from constantly staring upwards as the flight rapidly slowed down before 4:00 p.m., and we concluded at 5:00 after a final hour of 175 more. Here are the unreal totals for the raptor migrants tallied during our nine hours of observation:

Osprey - 30
Bald Eagle - 11
Northern Harrier - 5
Sharp-shinned Hawk - 122
Cooper's Hawk - 12
Broad-winged Hawk - 8,041
American Kestrel - 12
Un. Buteo - 1
TOTAL - 8,234

Broad-winged Hawk flight history

I had some idea of the magnitude of our total in that it was of limited historic nature, though that it was nowhere near the top days seen in Connecticut, as some flights have been over 10,000 birds, with Quaker Ridge in Greenwich having a couple of days over 30,000. I surely thought the other coastal sites that day, Quaker or Lighthouse Point in New Haven, would have numbers similar to ours, probably greater at the former and lesser at the latter. However, the conditions I described apparently did play out perfectly for us, as we beat the more-inland Greenwich total of 6,342 raptors, 6,176 being Broad-winged Hawks. Lighthouse Point, being right on the water, had 3,713 raptors with 3,116 Broads, having had more of the other species, also to be expected.

Neil Currie, one of the founders of the NorthEast Hawk Watch and raptor migration expert, has been a force in creating and promoting hawk watching for 40 years. It is because of him and others that we have all of the information on migration that we do now, from prime locations to observe raptors, to the dates of flights, and decades of data. Neil was kind enough to compile and send me information on the largest flights of Broad-winged Hawks ever in Connecticut so that I could get a sense of the significance of this flight and compare it further to when others like it occurred. All of the data discussed below does not yet include the 2011 season.

There have been 456 days in which over 1,000 hawks were observed at a given location in Connecticut, with 2,000+ being seen 192 times, 3,000+ recorded 99 times, 4,000+ tallied 67 times, and a sensational total of 5,000+ on 47 occasions. All of the dates of the sightings of 5,000+ raptors fall between September 12 and 25, with the majority being between the September 14 and 19.  Unsurprisingly, Quaker Ridge leads the pack with 14 of these days, followed by a tie for second of four each for Lighthouse Point and Whippoorwill Hill, out of 20 different sites. In consideration to our total of 8,234 at Boothe Park on September 16, there had been 19 previous counts of 8,000+ hawks in Connecticut history, this being the 20th in the last 40 years.

On the surface, this leads one to think such an event happens every other year, but this is not the case. Some of the mammoth flights have been recorded at multiple locations on the same day at these extreme levels, such as a movement on September 19, 1993 that featured 25,307 raptors at Booth Hill in West Hartland, 23,371 at Woodchuck Lane in Harwinton, and 22,475 at Quaker Ridge in Greenwich. Some have occurred back to back, like a flight of 30,786 raptors at Quaker Ridge followed two days later by a movement of 10,022 more at Greenwich Point. Interestingly, 24 of the flights over 5,000 took place in the 1980s, and 19 took place in the 1990s, with only three being recorded in the first decade of the 21st century.

As far as I can tell, the 8,234 raptors we recorded at Boothe Park on September 16, 2011 was the highest one-day total seen in Connecticut since 11,107 were counted at Booth Hill in West Hartland on September 13, 1998, when I would have been just shy of 13 years-old and quite clueless as to the very existence of such phenomena possibly occurring down the road from me at Boothe Park. That is an extraordinarily remarkable time period, and only makes the observations and memories of that day even more special. Let us hope that more days like this are coming in the future, and the population of Broad-winged Hawks to our north did not peak in the past. I cannot wait to see what September 2012 has in store for us and if we will be able to predict it so accurately once again.

You can see more information, from directions to all of this data, on our HawkCount page here:

Scott Kruitbosch
Conservation Technician

Photos © Frank Mantlik

1 comment:

  1. I just got Bedford Audubon's newsletter in the mail, and they report that on September 17 (the day after your big flight) the Chestnut Ridge hawk watch in Mount Kisco recorded 9,655 broad-wings. Mount Kisco isn't that far from Stratford. Hard to believe so many broad-wings could be passing Stratford one day and then even more passing Mount Kisco the next. I actually wouldn't be surprised if their date is a typo.