Short Communications 
The Wilson Journal of Ornithology 123(2):373-377, 2011 
Use of Ultraviolet Light as an Aid in Age Classification of Owls 
C. Scott Weidensaul , 1 - 5 Bruce A. Colvin , 2 David F. Brinker , 3 and J. Steven Huy 4 
ABSTRACT.—Use of ultraviolet (UV) light, which 
causes porphyrin pigments in feathers of some birds to 
fiuoresee, provides a simple, effective means of 
distinguishing multiple generations of flight feathers 
in owls. This permits easier and more accurate 
classification of age of adult owls. This lighting 
technique has been used extensively with Bam Owls 
(Tyro alba) and Northern Saw-whet Owls (Aegolius 
acadicus) and works well on a variety of owl species at 
night in the field, and should have wide applicability 
among owl researchers. The relative ages of the feathers 
can be easily distinguished by the intensity of 
fluorescence they exhibit when the ventral surfaces of 
primaries and secondaries are exposed to UV (black) 
light. This allows rapid and accurate assessment of molt 
and, in turn, the assignment of an age classification for 
the owl. Received 10 August 2009. Accepted 9 January 
2011. 
Feather molt among owls is complex but occurs 
in a relatively predictable sequence for an 
individual species, varying from complete or 
near-complete annual flight feather replacement 
in some species to a much lengthier process that 
may require 3 to 6 years (e.g.. Great Homed Owl. 
Bulw virginianus). The replacement sequence of 
primaries and secondaries is relatively predict¬ 
able. and it is believed possible to accurately 
assign age of individuals of some species to the 
third, and possibly fourth, year (Pyle 1997). 
However, distinguishing subtle differences be¬ 
tween third- or fourth-generation feathers by 
looking for contrasts in wear and color can be 
difficult, especially at night under incandescent 
light. 
Porphyrins are a large group of pigments 
characterized by nitrogen-containing pyrole rings 
including chlorophyll and. in animal blood, heme 
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Corresponding author, e-mail: 
scottweidensaul@verizon.net 
(McGraw 2006). Porphyrins are used by many 
birds to pigment eggshells in the oviduct, but 13 
Orders of birds also use porphyrins as a plumage 
pigment, most notably owls, goatsuckers, bus¬ 
tards, and turacos (Gill 1995. McGraw 2006). 
Porphyrins are easily destroyed by exposure to 
sunlight, and are most abundant in new feathers; 
many of the so-called natural porphyrins also 
fluoresce brightly when exposed to ultraviolet 
(UV) light (Gill 1995). Most natural porphyrins 
contain iron, but several are based on copper, 
including turacoverdin, which produces intense 
green coloration in some turacos, two galliforms, 
and the jacanas (Dyck 1992); and turacin, 
responsible for magenta coloration in turacos 
(Gill 1995). Porphyrins were first isolated from 
bird feathers in the early 20th century, but their 
role in feather structure and function, and their 
synthesis with regards to plumage formation, 
remain largely unexplored (McGraw 2006). 
In this paper, we describe a technique using UV 
fluorescence of porphyrins to more easily classify 
age of owls by examining flight feathers and molt 
patterns. 
HISTORY 
In 1982, Colvin was studying the interactions of 
Bam Owls (Tyto alba) and farm rodents by lacing 
non-toxic rodent baits with tetracycline, which 
would make rodent bones and teeth (collected 
from Bam Owl pellets) fluoresce under black UV 
light. Colvin was also trying to find easier ways to 
quickly distinguish molt limits among adult Bam 
Owl flight feathers, especially when working at 
night under weak incandescent light (e.g., a 6-volt 
flashlight). He subsequently tried bodi “white” 
and long-wave “black” hand-held fluorescent 
lights, discovering they both made molt patterns 
easier to see. because newly molted feathers have 
higher concentrations of porphyrins and fluoresce 
much more brightly, contrasting with weaker 
fluorescence in older feathers. 
Colvin did not publish his findings and. for 
many years, the technique was used only by a 
limited number of researchers who had been 
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