Winter Raptor Research Updates From the Field – Season 2 Wrap Up

Kirsti Carr – Field Research Assistant

As fast as winter began, our winter raptor surveys have finished! We had a great few months looking for birds of prey in Addison County, with lots of snow cover and consistent cold temperatures. 

We spent 87 hours from December 10, 2021 through March 9, 2022 conducting road surveys on a 70-mile survey loop through Addison County. We surveyed Monday, Wednesday, and Friday as weather and scheduling allowed, though for seven of the twelve weeks we only surveyed twice. We recorded various weather conditions at the start and end of each survey. This was the second year VINS has spent significant effort surveying this area, and we hope to continue to grow our understanding of winter raptor movements and demographics in our state.

A map of raptor sightings during the survey period

Although we observed 931 raptors, many of these were likely the same individual seen on different days. The most counted bird, as expected, was the Red-tailed Hawk with 544 total observations. Our second most common raptor was the Rough-legged Hawk, with 263 sightings. These two were the only species seen on every survey. The third most numerous species observed was the Northern Harrier, with 59 sightings. We counted far more Northern Harriers in the first month of surveys than in the last two months. However, we began to see more harriers again in the first two weeks of March, mostly adult males. See our graph that shows the average snow cover per week, compared with the count of harriers. Some hypothesize that Northern Harriers will move away from wintering areas with snow cover. Although we didn’t find a significant statistical relationship between snow cover and harrier counts, perhaps with more years of data the trend might become more clear. In another graph, you can see the weekly averages of our five most numerous species. The trends in the number of Red-tails and Rough-leggeds seem to follow each other pretty closely. Could this be due to environmental conditions, survey limitations, or perhaps phenological cues important to both species? We’d love to hear your thoughts in the comments!

A Northern Harrier (
A graph showing the depth of snow cover and the sightings of Northern Harriers
A graph of the weekly averages of raptor sightings. (RTHA = Red-tailed Hawk, RLHA = Rough-legged Hawk, NOHA = Northern Harrier, BAEA = Bald Eagle)

We had a few single bird sightings this winter, which is always a treat! Perhaps most unusual was a young Golden Eagle seen in Bridport. We saw just one Turkey Vulture, a species rare in winter although coming back in droves now! We had a stunning but fleeting view of an adult Red-shouldered Hawk in Weybridge, and one Merlin just outside of Middlebury, way back in December. Although we inspected every Accipiter we could, we were disappointed to miss Northern Goshawk.

One of the best parts of surveying these roads so frequently was picking up on a number of dark morph Rough-legged Hawks in Addison Co.! We confirmed 71 individual sightings of dark-morph Rough-legged Hawks – birds whose body and underwing covert feathers were uniformly dark brown or black, with light silvery flight feathers and a dark trailing edge. Birds considered dark-morphs did not include “intermediate” or heavily-marked birds whose plumage still has a light background. This unexpectedly high count corresponds to 27% of all Rough-legged Hawk sightings, nearly one-third of observations! Due to our data collection methods, we lumped unknown-morph birds (i.e. birds too far away or back-lit to confidently see their plumage) and light-morph birds. Although it is not uncommon to see a dark-morph Rough-legged Hawk overwintering in the east, we are not aware of any formal census in Vermont.

A dark morph Rough-legged Hawks shows off its dark plumage

We will miss all these birds as they move onto their breeding grounds, but will patiently await their return! Until then, stay tuned for information on our upcoming projects and updates about our five GPS-transmitter Rough-leggeds from earlier this winter!

SpeciesTotal Observations
Northern Harrier59
Sharp-shinned Hawk2
Cooper’s Hawk6
Red-shouldered Hawk1
Red-tailed Hawk544
Rough-legged Hawk263
Unidentified Buteo4
American Kestrel9
Peregrine Falcon4
Bald Eagle31
Golden Eagle1
Black Vulture5
Turkey Vulture1
Grand Total931
Table showing the total number of observations during the survey period.

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