Thanks for this, Eric. Fascinating stuff.
The info on adjustment of clutch size is particularly interesting. One
thing I've long wondered about is whether the instinct to chase the
young off the winter feeding territory also varies. If there's a
super-abundance of lemmings, one would think they could support a larger
number of owls than usual over the winter.
If you happen to talk to Therrien again any time soon, could you ask him
what is known, if anything, about the constancy of winter dispersal
and/or the numbers of young wintering over and how/if those things vary
with the lemming numbers?
On 12/9/2013 12:38 PM, Eric Hynes wrote:
> Hello Vermont Birders:
> All this Snowy Owl activity is terribly exciting. If you haven't had a
> chance to get out looking for one yet or if you haven't lucked into seeing
> one despite your efforts, don't worry, more are on the way.
> Bruce MacTavish reported to the Newfoundland listserv yesterday a
> conservative count of *over 200* in a single party outing along the eastern
> shore! Based on previous efforts and weather, he felt confident these were
> newly arriving birds. Obviously, we aren't on the East Coast and so are not
> as likely to get "those" Snowy Owls or concentrations of that magnitude,
> but clearly there are more Snowy Owls on the move.
> A friend and former colleague of mine at Hawk Mountain, Jean-Francois
> Therrien, conducted his Ph.D. research on Snowy Owls in NE Canada. He
> continues to spend time in northern Quebec in the summer doing research.
> There are several aspects to Snowy Owl behavior and plumage which seem to
> get debated frequently so I reached out to JF for some clarity. I thought
> some members of this birding community might be interested in his reply.
> In regards to plumage, most sources are in agreement that males tend to be
> whiter/lighter than females and immature birds tend to be darker than
> adults but these are just gross trends. There is a high degree of variation
> among individuals and some birds have been documented as getting darker
> with age. The darkest males can be darker than the lightest females.
> Thankfully, many birders are reporting to east coast listservs their
> sightings of Snowy Owls. Often the details include "immature male" or
> "adult female." Personally, I am cautious to label most Snowy Owls but I
> thought maybe I was missing something so I asked JF. Here is his reply:
> "Concerning age/sex classes: there is no specific criterion to tell them
> apart objectively (as of yet). Some folks have developed a way to tell sex
> among chicks at nest (see attachment), but in the field, especially in
> winter at low latitude, it is pretty much a guess in all cases. The only
> group that we can identify with confidence is adult males. For the rest,
> (except when you are facing a nest and you can tell female from male
> easily), it relies on feelings! People have long thought that snowies were
> getting whiter with age. This was mostly based on captive birds. We got
> lucky and photograph the same female wearing a satellite transmitter in 2
> winters (2008 and 2010). That female actually got significantly darker! We
> were amazed. A specialist of plumage coloration and molt in birds said he
> was unable to understand the actual pattern in snowies (since the number of
> wild bird being captured remains low). So, a lot more debates to come!"
> "(see attachment)" refers to a PDF JF sent me of a paper published in the
> Journal of Raptor Research in 2011. (I can forward the PDF if you contact
> me directly - not via the listserv). Researchers accurately predicted the
> sex of 140 nestlings 100% of the time by studying the remiges and retrices.
> Secondaries were best but outer primaries and central/deck tail feathers
> worked as well. The dark "spotting" on the flight feathers of the nestlings
> were comprised of pigmentation running the width of the feathers creating
> bars and circles of pigmentation on either side of the rachis not reaching
> the edges creating spots. Males had more spots than bars and females had
> more bars than spots. So if you feel compelled to label the birds you find,
> be sure to study/photograph the open wing and spread tail carefully. *Note
> - The paper does not talk about the effectiveness of using this protocol on
> anything older than a juvenile so you have to accurately age the bird
> first. It might work but has yet to be studied.
> The other area of discussion is the cause of the irruptions. Nobody debates
> the predator-prey connection. Lemmings make up the bulk of the diet on the
> breeding grounds and their populations are cyclical. The size of a Snowy
> Owl clutch is variable. Females assess how successful their mate is at
> bringing her lemmings at the beginning of the breeding season and she lays
> eggs accordingly. In years with lots of lemmings, she will lay lots of
> eggs. Debate comes in when the birds irrupt southward. So, has the lemming
> population crashed and starving birds are forced to migrate or do we see
> them down here when the Snowy Owls have big breeding seasons and there are
> lots of young birds dispersing? I posed this question to JF as well:
> "As for the irruption, you are quite right. Almost every year that we see a
> good reproduction in the eastern Arctic, we have some sort of winter
> irruption down in QC and New England. Last summer was fantastic in Northern
> Quebec. I was there at the tip of the province and we found several nests,
> all of which having large clutches and several lemmings piled up on the
> surrounding. We expected to see some irruption because all of those chicks
> are starting to wander around, and they are now reaching our latitudes."
> Die-offs eventually follow these spikes as nature corrects itself but I for
> one will be riding the high this winter.
> Enough out of me.
> Good birding,
> Eric Hynes