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Plumages and Molts
Juveniles undergo a partial
first prebasic molt into first year plumage by late September. After
September, young of the year and older birds are similar in
appearance. First year birds retain the juvenal primaries, secondaries,
tertials and tail for a year or more. The first annual molt of flight
feathers starts at about 13 months of age (Bull and Duncan 1993). Not
all primary and secondary feathers and often lesser secondary coverts
are molted annually, so after about 1.5 years of age, they usually
show a mix of old and new flight feathers. A few juvenal primaries and
secondaries may be retained for 3-4 years. Year-old and older Great
Gray Owls normally molt all tail feathers annually (Bull and Duncan
1993), but Pyle (1997) on page 88 stated that they often keep juvenal
tail feathers for two years, so such birds would have substantially
abraded tails. More study is needed. Most second year and probably
older birds from the 2004-2005 irruption showed an abnormal plumage
because they retained many old feathers, which normally would have
been molted. The retention of considerable old feathering indicated an
inhibited or partially skipped molt and probably resulted from food
stress during the molt period before these owls irrupted southward.
Nero and Copeland (1997) reported a similar inhibited molt during the
1995-1996 irruption. We postulate that heavily worn birds with much
retained old plumage may be typical of many irruption years.
Adults
We arbitrarily define adults
as birds in their second fall and older. The folded wingtips are dark
brown without pale tips and contrast with the paler gray tertials
above and paler tail below, shown in Figures 1 and 3. Some adults have
one or two new grayer secondaries among contrasting older brownish
secondaries on the folded wing panel (Figure 3). Other
adults have the entire visible secondary panel brownish, indicating no
recent molting. In first year birds, the secondary panel is more
uniformly gray and less contrasting. The overall plumage of adults
(not all) in 2004-2005 showed a greater mix of older browner feathers
(probably due to an inhibited stress molt) contrasting with newer gray
feathering. Even normally molting adults often retain brownish lesser
wing
coverts, which is a good indicator of age.
First Year
A first year is a bird
during its first fall, winter and spring. First year birds have
distinctly fresh pale-tipped primaries on the folded wingtips, shown
in Figures 1 and 2. The tail feathers of first year birds usually have
tiny sharp points where the juvenal down (mesoptile) adhered to the
tips of tail feathers. These tips remain into January or February.
Figure 1 shows a comparison of first year and adult tails. Also see
the photo of a first year bird on page 243 in Duncan (1996).
Caution: Do not confuse the pointed tail tips of first year birds
with the exposed quills on the tails of heavily worn older birds (Figure 5). First year birds lack the mix of older browner feathers,
which is typical of many but not all adults. We saw no first year
Great Gray Owls in 2004-2005, indicating a widespread failed breeding
season in 2004, as reported by Pittaway (2005). However, we did see
second year birds whose retained juvenal folded primaries suggested
first year birds, as in Figure 4. These birds did not molt normally.
They differed from first year birds in having retained two-year-old
frayed tails and variable amounts of contrasting old brown feathering.
Second Year
Sometimes this age class can
be recognized in the field. Second year birds that have molted
normally usually show a mixture of pale primary tips and darker adult
tips. Some third year birds are similar, but most should have more
adult primaries. Most second year birds in 2004-2005 had retained old
brown feathering similar to that described by Nero and Copeland
(1997). They stated, “The inhibited molt shown in these 2-year old
birds was presumably the result of a nutrient shortage (lack of
sufficient food) in winter 1994/1995, and/or spring and summer 1995.
As a result, these birds had retained much plumage that was more than
1 year old, hence largely worn and faded, thus giving them a strongly
brown color overall. Often such birds could be identified at a
distance, whether perched or in flight.” Figure 4 shows a second year
bird with first year folded wingtips and a mix of old and new feathers
elsewhere. This molt contrast indicates that it is not a first year
bird. Figure 5 shows heavily abraded tail feathers, suggesting a
second year or possibly a third year bird with a retained juvenal
tail. See Caution under the heading First Year above.
Subspecies
There is one subspecies
(race) in North America, which is the first named or nominate
subspecies, S. n. nebulosa. The Great Gray Owl was described in
1772 from a specimen from the Severn River in northern Ontario (AOU
1998). There is very little geographical variation in size and plumage
colour across North America, probably because its nomadic behaviour
allows out-crossing and gene flow among populations. The Great Gray
Owl is one of the few bird species described first from the New World
that also occurs in the Old World. A second subspecies, S. n.
lapponica, occurs across northern Eurasia. The three Royal Ontario
Museum specimens of lapponica are slightly paler overall with
more distinct long streaking below. A third subspecies, S. n.
elisabethae, described from Mongolia, is not recognized by most
authorities (Michel Gosselin, pers. comm.).
Morphs
There are no morphs of the
Great Gray Owl, but some birds are grayer or browner than others due
to individual variation, age, wear, and amount of retained older faded
feathers. Fresh first year birds with all new feathers often appear
more uniform gray in the field. It is important to keep in mind that
the same bird will appear different depending on light, time of day,
and background. Observers seeing the same individual in the morning
and then in the afternoon often think they have seen two different
birds. A Great Gray Owl’s true colours and patterns are often altered
in photographs.
Albinism
Albinism is more frequent in
Great Gray Owls than in other owls (Alaja and Mikkola 1997). A male
bird with white plumage that mated with a normal bird in Idaho
produced normal young (Whitfield and Kelley 1995). It was considered
an incomplete albino because its eyes and toes were not pink. Many
partial albinos with some white feathers and a few leucistic birds
with diluted or muted feather coloration have been recorded.
Melanism
Great Gray Owls exhibit
varying degrees of melanism (Pittaway 2005). Figure 6 shows a
partially melanistic individual from the irruption in 1995-1996, with
more information in the caption. A slightly melanistic individual
along Halls Road in Whitby, Durham Region, during January and February
2005 was intermediate between Figure 6 and normally-coloured birds. An
almost solid black individual was photographed in Minnesota in
November and December 2004 (see photo in Alt 2005).
Male or Female
Females are larger than
males, but there is considerable overlap (Bull and Duncan 1993).
Judging size in the field is deceptive and unreliable. If several
birds are seen together, one might “guess” that an extremely small
individual is a male or an extremely large one is a female, but not
with certainty. It is impossible to tell males from females by
appearance in the field on winter range (Pittaway 2005).
Ethics and Photos
Please allow Great Gray Owls
the space they need to hunt and rest. To age them, a scope is
essential. If you are taking photos of perched birds to determine age,
the best photos are of the sides and the back, showing the wingtips,
secondaries, wing coverts and tail.
Conclusion
The above information on
aging Great Gray Owls in the field is preliminary. Some will be
difficult if not impossible to age in the field. We recommend using
our information in combination
with the paper by Peck and
Murphy (2005) in this issue.
Acknowledgements
Mark Peck and Glenn Murphy
allowed us to examine and photograph specimens in the Royal Ontario
Museum. We thank Michel Gosselin of the Canadian Museum of Nature for
advice. We thank Albert Kuhnigk for use of his photograph of a
partially melanistic Great Gray Owl. Ron Tozer made many helpful
suggestions.
Literature Cited
Alaja, P.
and H. Mikkola.
1997.
Albinism in the Great Gray Owl (Strix nebulosa) and other owls.
Pp. 33–37 in Biology and Conservation of Owls of the Northern
Hemisphere. Second international symposium, February 5-9, 1997,
Winnipeg, Manitoba (J.R. Duncan, D.H. Johnson, T.H. Nicholls,
editors). General Technical Report NC-190. USDA Forest Service, North
Central Research Station, St. Paul, Minnesota.
Alt, M.
2005. Melanistic Great Gray Owl in St. Louis County. Loon 77: 118.
[AOU] American Ornithologists’ Union.
1998. Check-list of North American Birds. 7th Edition. American
Ornithologists’ Union, Washington, DC.
Bull, E.L.
and J.R. Duncan.
1993. Great Gray Owl (Strix nebulosa). In The Birds of
North America, No. 41 (A. Poole and F. Gill, editors). Academy of
Natural Sciences, Philadelphia, Pennsylvania, and American
Ornithologists’ Union, Washington, D.C.
Duncan, J.R.
1996. Techniques to sex and age Great Gray Owls: A bird in the hand
worth two in the bush? Birders Journal 5: 240–246.
Godfrey, W.
E.
1986. The Birds of Canada. National Museums of Canada, Ottawa.
Nero, R.W.
and H.W.R. Copeland.
1997. Sex and
age composition of Great Gray Owls (Strix nebulosa), Winter
1995/1996. Pp. 587–590 in Biology and Conservation of Owls of
the Northern Hemisphere. Second international symposium, February 5-9,
1997, Winnipeg, Manitoba (J.R. Duncan, D.H. Johnson, T.H. Nicholls,
editors). General Technical Report NC-190. USDA Forest Service, North
Central Research Station, St. Paul, Minnesota.
Peck, M. and
G. Murphy.
2005. The Ontario Great Gray Owl invasion of 2004-2005: Mortality,
sex, molt and age. Ontario Birds 23: 122–137.
Pittaway, R.
2005. Great Gray Owl observations 2004-2005. OFO News 23(2): 8–9.
Pyle, P.
1997. Identification Guide to North American Birds. Part 1. Slate
Creek Press, Bolinas, California.
Whitfield,
M.B., M.E. Maj, and J. Kelley.
1995. Incomplete albino Great Gray Owl in Idaho. Blue Jay 53: 197–199. |
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