[CT Birds] snowy owls (long)

Chris Elphick elphick at sbcglobal.net
Sat Dec 7 00:35:18 EST 2013

Given all the recent talk, I spent a
little time looking through the research literature on snowy owls.  It turns out there is relatively little
information in the scientific literature on many of the topics under discussion
– and what is out there is mostly from a couple of decades ago – but I put
together a quick summary of what I found.

1. Whether owls can be aged/sexed by
plumage? (bottom-line: don’t assume too much)
From the Birds of North America account (Parmelee
1992):  “Adult males generally whitest
overall, sometimes lacking barring altogether. Adult female and first-year male
difficult to distinguish, and there is some plumage overlap. First-year females
are the most heavily barred, usually having barred upper breasts and crowns."  BUT, and here's the key part: "Ageing/sexing generally not safely accomplished in the field.”
See also: http://www.dvoc.org/OrnithStudy/Presentations/Presentations2012/Snowy%20Owl%20plumages.pdf
2. Which
birds are most likely to be in the south? (bottom-line: young males, but could
be anything)
Kerlinger and Lein (1986. Differences in Winter Range among Age-Sex Classes of
Snowy Owls Nyctea scandiaca in North America. Ornis Scandinavica): “Although
there was nearly total overlap in the distributions of the four age-sex
classes, the proportions of each class varied dramatically and consistently
along a north-south axis. On average, immature males wintered farthest south,
while adult females wintered farthest north.”
3. Are southern owls likely to be in bad
condition? (bottom-line: maybe not as bad as you think)
From Kerlinger and Lein (1988. Causes of mortality, fat
condition, and weights of wintering snowy owls, Journal of Field Ornithology): “Necropsies of salvaged specimens and
information from museum skin labels suggest that starvation is not as common
among wintering Snowy Owls (Nyctea scandiaca) as previously suggested. Moderate
to heavy fat deposits were found on 54 (45%) of 121 specimens. Traumatic
injuries, including collisions with automobiles and wires, were the major cause
of mortality of birds wintering in Alberta.” (Note the geographic focus of this
study - hard to say whether it applies to unusually large invasions or to the East.)
4. Are snowy
owls more likely to hunt by day or night? (bottom-line: no one really seems to know)
Surprisingly, I
couldn’t find any data on this question in the BNA account, and I couldn’t find
anything more recent.  What information
there is seems to be entirely anecdotal.  
5. What causes
irruptions? (bottom-line: food abundance plays some role, but it’s probably not simple)
Again,  there appears to be less information than you might think.  From the BNA again (albeit >20 years ago): “Migratory movements relate in ways not fully
understood to the abundance of prey species”.  Detailed studies of irruptions in other species (northern finches etc.),
however, suggest that the largest irruptions occur when there is a combination
of a really good food crop leading to a good breeding season followed by a poor
food crop, see: http://cedarcreek.umn.edu/biblio/fulltext/t1855.pdf.
6. Coolest tid-bit I stumbled across:  
one field of approximately 500 m2, 31 to 35 Snowy Owls roosted by
day” (from Holt and Zetterberg 2008. The 2005 to 2006 snowy owl irruption
migration to western Montana http://www.bioone.org/doi/pdf/10.1898/NWN07-19.1).  Makes that 13-owl day at Plum Island, MA, last
week sound like a bit of a snore …
Chris Elphick
Storrs, CT
elphick at sbcglobal.net

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