Thursday, October 8, 2009

Pepperspecks and the Visibility of Hawks

It was a mezzo mezzo Broad-winged Hawk migration at Wachusett Mountain in Princeton, Massachusetts, this fall. The numbers were less than half our long-term average and about three quarters of the average for the past ten years.

Compounding the disappointment, many of the broadwings were pepperspecks, so high in the sky that some people thought we were counting floaters in our eyes. How high were these pepperspecks? In Flight Strategies of Migrating Hawks (1989) Paul Kerlinger says that at Cape May, NJ, broadwings were difficult to see with the naked eye when 625 meters, or roughly 2000 feet, directly overhead against a cloudless sky. Broadwings were generally seen with the naked eye below 550 m (1800 ft) but somewhat difficult to see beyond that. When using 7X binoculars, single broadwings directly overhead were difficult to detect at 1,100 m (3600 ft). Kerlinger notes that in Texas flocks of hundreds of broadwings observed on radar could be missed by observers with binoculars when the hawks were flying less than 1600 ft above ground level about a mile from the observers.

Today I was hawk watching at Lighthouse Point in New Haven, Connecticut, one of my favorite sites for looking at accipiters and falcons. It is a great site to see and compare Sharp-shinned Hawks with Cooper’s Hawks, and today I was trying to photograph them, with limited success. I was in a parking lot between two small woodlots, somewhat sheltered from a 10-13 mph northwest wind gusting to 22 mph. Early on, the birds were low, often just above treetop levels, but they gradually worked their way up in late morning to the limits of unaided vision.

Occasionally, they would stack up, soaring and hanging into the wind. One time, I had 8 Sharp-shinned Hawks stacked up in a single column from the treetops to the limits of unaided vision. It looked like the skies over Kennedy airport early on a Monday morning, but these air travelers weren’t looking to land. They were trying to decide if they wanted to fly 5-7 miles over the bay in that wind, or circumnavigate the bay. Kerlinger says that sharpies become difficult to see against cloudless skies between 400-500 meters (1300-1600 ft.) overhead and disappeared above 700 m. (2300 ft).

Later in the day when I had given up all hope of photography because of the birds’ altitude, one of the excellent observers at Lighthouse would find a sharpshin or kestrel flying at the limits of naked vision. When one bird was found, we would often find another half dozen birds in the immediate area, once our eyes could focus on something. On one occasion I found a Peregrine rowing across the sky almost directly overhead, beyond unaided vision and approaching the limits of binocular vision. While directing other observers to the Peregrine, I discovered two more following in an almost direct line at the same altitude, which suggests these broadwing-sized birds were probably around 3600 ft. high.

A number of factors that I have not addressed affect the visibility of hawks. Several books and articles published in the '80s addressed the issue to an extent, but I’ve not seen much done “recently.” Is anyone aware of “visibility studies” done at a hawk watch near them?


  1. Paul, I had pepperspecks, too, at Waggoner's Gap. Still, it was a nice number of pepperspecks. Or perhaps I was just glad to be on a mountain top and out of the office!!

    Carolyn H.

  2. I've done some size/distance measurements through 8X bino and 20X scope for broadwings and it amazing to see "wings appear out of thin air to join kettles in progress at no more than 3000' away!That suggests the light from these birds just doesn't reach our eyes at really not that far away.Against a bright cloud,we see the shadow or light displacement.
    The bird blocks the light from the cloud and stands out.Notice how birds against the clouds look all dark?Even they're color doesn't transmit a whole lot of light against a "blue" sky. We become almost color blind to brown/orange/faint red,from stareing at that blue...Thom

  3. Years ago Hawk Mountain ran an experiment about how far away you could see broadwings with 10X bnioculars. Someone created a plywood life-size Broadwing silhouette cutout, painted black. A person walked out along the ridge of the mountain holding the cutout while someone else looked at it. This resulted in deciding that if the birds were more than .75 mile high, you wouldn't be able to see or identify them.

  4. Observers at Tadoussac in Quebec have been looking at this issue. They gave a presentation on their findings to date at the NAOC conference in Veracruz in 2006. Sorry I don't have a reference handy, but perhaps someone reading this does.
    Further, I once did an informal evaluation of distance/ID at Eagle Valley, WI. At that point it is one mile across the Mississippi River to trees in Iowa. Through my 9X bins I was able to identify an Osprey flying toward those trees from about midway across. By the time the bird reached the other side I decided that if I had picked it up only then, I may still have been able to ID it as an Osp -- only because of its distinctive shape. Given the size of that bird, I figured that a bird half the size MIGHT be seen (depending, as Thom said, upon the lighting/cloud background, etc.) and MAYBE accurately IDd at 0.5 mile (2000' +/-).