[CT Birds] Gray Ghost - a skewed perception?
Spector, David (Biology)
spectord at ccsu.edu
Mon Dec 28 16:37:33 EST 2015
A 1984 article on the results of banding over 1000 Northern Harriers at several North American locations in the fall migration has some relevant data.
The order of average fall migration appears to be young male, young female, adult female, and, last, adult male, with much overlap in dates.
The authors of the study found only about 10% of their caught birds to be adults, with slightly more female adults (and slightly more male young birds). Thus, "gray ghost" adult males were only about 4% of the total. The authors caution that there may be sex and age biases in the capture techniques, so the data on captured and banded birds might not be fully representative of the population.
I am not aware of sex or age differences in non-breeding habitat or latitudinal range for this species, but it wouldn't surprise me if there were such differences, which could affect proportions of encounters with adult males in any given area.
The article is
Keith L. Bildstein, William S. Clark, David L. Evans, Marshall Field, Len Soucy, Ed Henckel. 1984. Sex and Age Differences in Fall Migration of Northern Harriers. Journal of Field Ornithology 55:143-150.
You can get a PDF of the article from SORA at https://sora.unm.edu/node/51088 .
By the way, does anyone know who first used "gray ghost" for male N. Harriers? My impression is that it is a relatively recently coined term.
From: CTBirds [ctbirds-bounces at lists.ctbirding.org] On Behalf Of Arthur Shippee via CTBirds [ctbirds at lists.ctbirding.org]
Sent: Monday, December 28, 2015 3:16 PM
To: Birds General reports
Subject: Re: [CT Birds] Gray Ghost - a skewed perception?
Do we know if migration patterns are similar for M and F Harriers? Might there be different routes, or days, or times during the day? Different migration patterns would likely produce different observational patterns.
Thanks for the discussion.
> On Dec 28, 2015, at 12:52 PM, julian hough via CTBirds <ctbirds at lists.ctbirding.org> wrote:
> I don't have access to Lighthouse data, so this is purely based from the hip, on what I see at Lighthouse. Adult harriers of both sexes, like accipiters, come through at the tail end of the season. During the mid-season (say mid-Sept to mid-October) the bulk (90%) of the birds are juveniles. I don't see many adult females going through at this time. When I analyzed the sexual make-up of a small sample of banded juveniles for some research I was doing on Northern Harriers, or sexed the birds passing lighthouse by eye-color (gray-olive on males, dark brown on females), I found that juvenile females were outnumbered by juvenile males. So, hypothetically, if a nest contained generally more young males than females, in two years' time, adult males should make up a larger percentage of the harrier population.
> But...I feel the suggestion that adult male "Grey Ghosts (awesome name by the way!!!) are less common, is down to a misconception. Again, all this is unscientific, since in fall, unsexed and unaged "brown" birds (young males, young females and adult females combined) obviously outnumber "gray" birds so it may be a skewed perception of the observer since they are seeing a much higher percentage of "brown" birds compared to a single age/sex class of a "gray" birds.
> I think the original comment may have assumed all the "brown" birds in fall are females. The key would be to compare the numbers of only adult females with "Gray Ghosts" to get a better feel of the numbers and this would require good views to differentiate adult females from juveniles in fall.
> Julian Julian Hough New Haven, CT 06519 www.naturescapeimages.wordpress.com
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