McGILL BIRD OBSERVATORY
Many owls can be aged by the pattern of moult among their primaries and secondaries. In the case of Northern Saw-whet Owls, patterns for hatch year (HY) and second year (SY) birds are widely accepted; some believe that third year (TY) and even fourth year (4Y) birds show predictable and recognizable patterns of moult, while others believe that those distinctions are unreliable and prefer to classify older birds as after second year (ASY) or occasionally after third year (ATY).
A recent poster by Nova Mackentley, Eugene Jacobs, and David Evans (presented at the Raptor Research Foundation meeting in Duluth MN, October 5-9, 2011) reported that based on recoveries of known-age owls, 95% of individuals showing a "typical" TY pattern in fall were in fact TY birds, but only 59% of TY birds showed this kind of pattern. Similarly, 95% of individuals showing a 4Y pattern were actually 4Y birds, but only 63% of 4Y birds showed the expected pattern. This suggests that it is likely safe to age Northern Saw-whet Owls as TY and 4Y if moult matches the expected patterns (see below), but that a number of individuals deviate from the norm and need to be assessed as ASY (irregular mix of two generations of feathers, or three generations), or ATY (four distinct generations of feathers).
The photos below show wings of dead owls, collected over time (with permission) by Prince Edward Point Bird Observatory (PEPtBO). In each case, the photo on the right shows the dorsal view of the wing under normal light, while the photo on the left is a ventral view of the same wing under ultraviolet light. Porphyrins in the newer feathers fluoresce pink, and these fade over time, often allowing for an easier distinction among different generations of feathers than under regular light.
All age determinations were made by PEPtBO master bander David Okines, who has banded several hundred Northern Saw-whet Owls each fall over the past few years; written commentary is by MBO master bander Marcel Gahbauer, based on extensive discussion of the specimens with David. Numbers given in the descriptions are intended to be representative of usual variation in the species, but exceptions no doubt occur in all cases, especially for older birds. Feedback on these photos and comments are welcome at email@example.com.
year (HY): The simplest pattern, in which all primaries and
secondaries are a uniform shade of brown above, and all show a similar
amount of pink below under UV light.
year (SY): Again a relatively simple pattern, with the outermost
3-7 primaries and innermost 5-9 secondaries replaced (darker brown
above, pink below) contrasting visibly with the unreplaced inner primaries
and outer secondaries (paler brown above, whitish below).
year (TY): An increasingly fragmented pattern, now showing three
distinct generations of feathers. Only one juvenile flight feather
has been retained, s1 (the first secondary), which appears pale brown
above, and white below. On either side of it are four new feathers,
bright pink below and dark brown and fresh above. Other feathers
(p5-p8, s5, s8-s13) are intermediate in age, somewhat faded under both
views, but not as pale as s1.
year (4Y): A complex pattern, in which four generations of
feathers can be identified. Characteristic of the 4Y pattern is a
very old retained juvenile s1 (first secondary). It is distinctly
paler than other feathers both above and below. The next oldest
feathers are primaries 4-5, and secondary 5; these would have grown in the
owl's second year. Feathers grown in the owl's third year, now two
years old, still show a fair amount of pink below and dark above; these
are primaries 1-3 and 8-10, as well as secondaries 2-4 and 8-13. The
newest feathers are fresh and dark above and bold pink below:
primaries 6-7 and secondaries 6-7.
second year (ASY): This designation is applied to owls with a
moult pattern that shows three generations of feathers but does not
conform to the typical pattern of a third year bird. In this
example, s1 has been recently replaced along with p1-p2, but p3-p4 were
replaced last year and s2-s5 the year before that; all others were
replaced this year.
third year (ATY): This designation is applied to owls with a
moult pattern that shows four generations of feathers but does not conform
to the typical pattern of a fourth year bird.
All photos by Marcel Gahbauer, of wings collected with permission from dead birds by Prince Edward Point Bird Observatory in Ontario.
© 2002- The Migration Research Foundation Inc.