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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 2, June 2011 
associated with him or his colleagues in Ohio or 
New Jersey, working primarily with Bam Owls. 
In the mid-1990s, Brinker learned of the method 
from Scott Butterworth. then with the West 
Virginia Division of Natural Resources, and 
taught it to Huy, who began Using black UV light 
in October 2000 on Northern Saw-whet Owls 
(Aegolins acadicus), netted for banding during 
autumn migration in Maryland. Brinker and Huv 
alerted Weidensaul, and all three used the 
technique extensively over the next several years 
to assign age to adult owls. 
This method has since been adopted widely by 
participants in Project Owlnet, a collaborative 
network of more than 100 owl migration banding 
sites, which annually band 8,000 to 15,000 Nonhem 
Saw-whet Owls (http://www.projectowInet.org/). 
It has proven especially helpful in distinguishing 
after-seeond-yeur/afler-third-year (ASY/ATY) 
adults, which arc marked by the presence of three 
generations of feathers, a frequently subtle 
distinction that can be difficult to make in the 
field, at night, under artificial light. The recapture 
of marked, known-age owls in subsequent years 
has demonstrated that intensity of UV fluores¬ 
cence in the flight feathers corresponds to the 
relative ages of the feathers themselves, and is 
consistent with accepted, age-linked molt se¬ 
quences described in Pyle (1997). The importance 
of tbs technique lies in the ability it gives even 
inexperienced workers to quickly and easily 
distinguish molt limits in owls, and thus facilitate 
accurate age classification. 
OBSERVATIONS 
Use of UV light to read molt limits has proven 
successful in a variety of North American owl 
species. We primarily refer to Nonhem Saw-whet 
Owls, but given the assumed universality of 
porphyrins in owl plumages, this technique should 
be applicable to most, if not all, tytonids and 
strigids. 
Colvin originally experimented with a variety 
of “white' - fluorescent and long-wave “black” 
fluorescent lights, but most banders now use 
commercially available long-wave black UV light 
bulbs. Good results have been obtained with a 13- 
watt compact fluorescent backlight (eg Feit 
Electric BPESLI5T/BLB, available from on-line 
distributors) with a screw-in base for use in lamps 
taking household incandescent bulbs. There are a 
variety oi handheld battery-powered lights (e g 
Arachnid A49 LED flashlight), powered by AA 
batteries, that are useful for field applications 
where 120v AC is not available. 
The ventral surfaces of newly molted flight 
feathers fluoresce an intense magenta color with 
the UV light positioned - 15 cm away, brightest 
in the proximal third of the feathers, and fainter or 
absent from the distal third (Fig. 1). Undenting 
coverts fluoresce similarly. Porphyrins arc gener¬ 
ally reddish or brownish pigments, bur the 
fluorescence is often brightest in areas that appear 
in natural light to he white or lightly tinge j with 
pink. 
Most individuals exhibit little fluorescence on 
dorsal wing surfaces, although it is unclear 
whether this is the result of rapid degradation of 
porphyrins in sunlight, or of limited deposition in 
those areas. Prior to widespread use of UV on the 
ventral wing surface, molt limits were evaluated 
using incandescent light to assess the differences 
in feather wear and sunlight-related fading on the 
dorsal surface of the feathers. These differences 
are often subtle and difficult to detect, making 
accurate assignment of age to owls more prone to 
error. 
There is usually little fluorescence on the 
rectrices, except where the bases of the feathers 
are covered by coverts, even though Northern 
Saw-whet Owls undergo a complete and nearly 
simultaneous replacement of the tail during the 
prcbasic molt (Collins 1961, Rasmussen et al. 
2(X)8). Tarsal and adult ventral down feathers 
glow with an especially bright, ruby-red color. A 
hatch-year (HY) Northern Saw-whet Owl. molting 
from its juvenal “chocolate” plumage in July in 
coastal Washington, had a mix of fluorescence on 
its underparts: the brown or fawn juvenal feathers 
exhibited no color under black UV light, while 
newly molted feathers glowed brightly (Jamie 
Acker. Dawn Garcia, and Stan Rullman: pet*- 
comm.). Tlie remiges showed a bright, even 
fluorescence, while the rectrices had no fluores¬ 
cence at all. Northern Saw-whet Owl ventral 
contour feathers show almost no fluorescence in 
all ages and plumages. 
HY Northern Saw-whet Owls captured in fall 
migration show an even fluorescence across llie 
underwing surfaces, being most intense in the 
inner primaries and outer secondaries, and lading 
significantly across the innermost secondary, 
which show' little or no fluorescence (Fig- I A 1 
Second-year (SY) owls captured in fall, which 
have replaced outermost primaries and innermost 
secondaries (most often primaries 6-10 and 
