Hello OBOL & Tweeters:

I received a number of private responses to my query about adult Snowy
Owls. Several people asked how the ages and sexes can be identified.
I have been identifying age-sex classes based in part on information
contained in the following publications:

Boxall, P. C., and M. R. Lein. 1982. Feeding ecology of Snowy Owls
(Nyctea scandiaca) wintering in southern Alberta. Arctic 35:282-290.

Josephson, B. 1980. Aging and sexing Snowy Owls. J. Field Ornithol.
51:149-160.

Kerlinger, P., and M. R. Lein. 1986. Differences in winter range among
age-sex classes of Snowy Owls Nyctea scandiaca in North America.
Ornis Scand. 17:1-7.

Portenko, L. A. 1972. Die Schnee-Eule. Neue Brehm-Bucherei, No. 454.
A. Ziemsen Verlag, Wittenberg.

Most people are familiar with the "classic" plumage differences: Adult
males vary from immaculate to very lightly marked, with markings
typically confined to the back, underparts, and wings. At the opposite
extreme, immature females are always very heavily marked all over. The
only parts of the plumage free of heavy marking are typically the face
and the back of the head. Barring is denser (i.e., more bars per feather)
and individual bars are broader than in the other age-sex classes. Some
individual immature females can be VERY dark, with more black than
white apparent on the plumage. On one such individual that I saw in
January at the West 90, the entire nape, upper back, and top of head
appeared to be black (thought the bird was quite far away). Immature
males and adult females are intermediate in density and breadth of
barring. Data in Josephson (1980) suggest that there is little overlap
between immature males and adult females in the width of ventral bars
(male bars narrower than females), but broad overlap in the density of
bars. Assessment of plumage differences can be somewhat subjective
in the field, but becomes easier with some comparative experience.

A nearly diagnostic feature for determining the age of owls is the
presence or absence of mottling on the remiges and wing coverts.
According to Josephson (1980:152), "Mottling, in the form of brown or
gray dappling of the remiges and coverts, was found in all first-year
birds examined in this study. These markings, present on the distal
portions of outer primaries, inner secondaries, greater primary coverts,
greater secondary coverts, median secondary coverts, and alular quills,
ranged from a few gray or brown flecks to extensive splotches and
sometimes covered much of the wings and back, obscuring the pattern
of barring. Dark, extensive mottling was found in some first-year males
and first-year females and was absent in all adults. The amount of
mottling seen in first-year birds was roughly correlated with overall
darkness of plumage." Determining whether this mottling is present is
usually straightforward with an extreme close-up view, but can be
subjective and tricky at a distance. I found examining museum
specimens to be particularly useful for sorting out this field mark.

All of these plumage differences are illustrated extensively in a series of
photographs in Josephson (1980), and he describes additional subtle
differences (as well as caveats). Unfortunately, photocopies of the
black-and-white plates do not do justice to the the rather subtle patterns
of mottling, so if you are really interested, you should definitely take a
look at the original version of the paper at the nearest research library.

Size is also a (surprisingly) useful criterion in the field. At both Boundary
Bay and Damon Point, the owls frequently roost communally, so it is easy
to make direct size comparisons among individuals. With a little
comparative experience, it is straightforward to determine the sex of
most individuals based on size alone. An extremely useful feature that I
have noticed, which does not seem to be mentioned in the literature, is
that females have proportionately more massive and broader heads than
males. I first noticed this while examining specimens at the Burke
Museum, then found that it held up invariably in the field. The best way
to get a feel for this is to look at birds in silhouette, without the
distractions of plumage. At Boundary Bay, several birds often sit on the
roof of that large home in front of the greenhouses, and it is relatively
easy to sex these birds from a great distance in dwindling light based on
head shape alone.

The principal ID problem is immature male versus adult female. Initially I
felt awkward about separating these two, since it struck me as odd that
there were NO adults around. However, I now feel quite confident. I
have identified all of the intermediate-plumage birds that I have seen as
first-year males based on a combination of: 1) presence of moderate to
extensive wing mottling; 2) width of ventral barring; 3) body size
(females obviously larger); and 4) size and breadth of head.

By the way, not only are the age-sex classes separable, but different
birds are individually identifiable (at least the immatures are -- I can't say
much about adults!). Each bird seems to have an absolutely unique
pattern of head spotting and bib depth and shape (i.e., how far down the
chest the white on the face extends, and the shape of the border where
the white meets the barring). I have individually photoidentified all of the
birds at Damon Point and plan to include a color plate with photos of each
bird in my eventual paper.

My interest in ages and sexes of owls derives from my wish to test the
"social dominance hypothesis," which predicts that behaviorally
subordinate (i.e., smaller or less experienced) individuals winter farther
south. Kerlinger and Lein (1986) examined museum specimens of
Snowy Owls taken in winter from different parts of the continent. In
general, they found that adult females wintered farthest north and
immature males wintered farthest south, consistent with a social
dominance hierarchy based on body size and experience. Among 65
specimens from Washington, Idaho, and northern Oregon, 36 were
immature males, 25 were immature females, 4 were adult females, and
none were adult males. The predicted distribution is also supported by
the populations at Damon Point (recent pop. = 12 males, 4 females) and
Boundary Bay (14 males, 11 females). I would be most interested in
hearing from anybody who is willing to take a stab at compiling a list of
the probable age-sex classes of all owls that have been in Oregon this
winter.

Thanks,

Bob Russell russell at afsc.noaa.gov
NOAA, Seattle