FYI
Rob Blye
East Coventry Township
Chester County, Pennsylvania
----- Forwarded Message -----
From: "Scott Weidensaul" <scottweidens...@verizon.net>
To: pabi...@list.audubon.org
Sent: Sunday, January 26, 2014 8:16:04 AM
Subject: Re: [PABIRDS] Snowy Owls: Age, Sex and Plumage
Thanks to Barry and Dave for bringing up this timely subject, and linking to
Art McMorris's DVOC 2011 presentation. There has actually be a lot of movement
on this question recently, including three important publications in the past
two years that refines our understanding of snowy owl plumages. The take-home
message is that birders tend to simplify (often greatly) the degree to which
snowy owls can be aged and sexed by plumage, but that in some cases it is
possible.
In 2011, Seidensticker et al. published a paper ("Sexing young snowy owls,"
Journal of Raptor Research, 45[4]:281-289), based on work with nestlings in
Alaska. They had 100 percent success predicting sex by assessing the barring or
spotting pattern on the middle secondaries, especially S4. In a nutshell, males
had more spots (markings that did not touch the feather shaft) than bars, and
females the reverse -- but you have to have a clear view of the spread wing and
tail to tell, and this was tested only with juvenile birds.
In 2012, Solheim published a paper from Norway ("Wing feather molt in snowy
owls Bubo scandiacus," Ornis Norvegica, 35:48-67), which examined molt in 53
museum skins. In the author's assessment, it is possible to age a snowy owl in
the hand or in a sharp photo of the spread wing up to at least fourth year,
based on contrasts between new and old feathers. He notes that the 24-hour,
harsh summer sun to which snowies are exposed causes extreme fading even in
juvenile birds.
Also in 2012, Eugene Potapov and Richard Sale published an excellent monograph,
"The Snowy Owl" (T & AD Poyser) that synthesizes a lot of what's known about
snowy owl plumages. They discuss work by Hawk Mountain's J.F. Therrien and his
colleagues on the breeding ground with marked birds, showing that snowies
sometimes get lighter with age, sometimes darker, and some may do both at
different stages of the same bird's life. As Norman Smith in Massachusetts has
been pointing out for years about snowy owl plumages, it's complicated. For
example, some online ageing guides note that mottling on the tertials is a
juvenile characteristic. It is, but as banders have discovered, some of those
mottled tertials are retained for several years, making them largely useless
for ageing.
All this should give pause to those birders who, with breezy assurance, are
assigning age and sex to the owls they see this winter based mostly on body
markings. But with a good, clear photo of the spread wing and tail, it's
possible in some cases to make an age/sex assignment. David Sibley and I hope
to create an online ageing/sexing guide to snowy owls this winter, if we can
find a spare moment. And we continue to encourage photographers who have good,
clear wing and tail shots to upload them on the SNOWstorm website
(www.projectsnowstorm.org) so we can quantify age/sex classes in this winter's
irruption.
As to speculation about different plumage patterns for different regional
populations, remember that snowy owls aren't like other birds. The breeding
grounds work that J.F., Denver Holt and others have been doing strongly
suggests there *are* no regional populations -- that snowy owls are almost
entirely nomadic, moving back and forth across the Arctic like water sloshing
in a basin. Some of J.F.'s tagged birds moved a thousand miles between breeding
seasons, for instance. One thing we're doing this winter with SNOWstorm is
collecting DNA for genetics testing, but I'll be surprising if it shows much
that suggests regional variation.
As always, you can find more information and regular updates on our work at
www.projectsnowstorm.org.
Scott Weidensaul
Schuylkill Haven, PA
On Jan 26, 2014, at 3:50 AM, Dave DeReamus wrote:
> Barry's post about the plumage variation of the Snowy Owl rekindled something
> I've always wondered about: Has anyone ever found a correlation between the
> birds found at the southern edge of their breeding range trending darker
> (more heavily marked) and the birds at the northern edge (closer to the
> Arctic Ocean) trending whiter (less mottled)?
>
> I have absolutely nothing scientific to support this thought, but using some
> of my very limited common sense and realizing how important camouflage is to
> many birds, I've often wondered if the birds at the southern edge of the
> Snowy's breeding range might generally be darker in order to blend in better
> with their surroundings during the months with little or no snow cover. I'm
> assuming (possibly incorrectly) that there might be less snow cover for a
> little longer period at the southern edge, which would make it tougher for a
> whiter bird to be inconspicuous. Since Snowys don't have a drastic
> color-changing plumage like that of ptarmigans, could this be a possibility?
> This could all mean nothing at all, but does anyone know if this has ever
> been assessed or not?
>
> Good birding,
> Dave DeReamus
> Palmer Township, PA
> becard -at- rcn.com
> Blog: http://becard.blogspot.com/
> PicasaWeb Photo Albums: http://picasaweb.google.com/becard57
> Eastern PA Birding: http://users.rcn.com/becard/home.html
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