Published by the
Wilson Ornithological Society
Evaluating the Effectiveness of Select Visual Signals to Prevent
Daniel Klem Jr.
and Peter G. Saenger
The Wilson Journal of Ornithology 125(2):406–411, 2013
Evaluating the Effectiveness of Select Visual Signals to Prevent
Daniel Klem Jr.
and Peter G. Saenger
ABSTRACT.—Billions of birds are estimated to be
killed striking clear and reflective windows worldwide,
and conservation, ethical, and legal reasons justify
preventing this unintended human-associated avian
mortality. Field experiments reveal that to be effective,
UV signals used to prevent bird-window collisions must
minimally reflect 20–40%from 300–400 nm. Field
experiments reveal 3.175 mm parachute cord hung in
front of clear and reflective windows separated by
10.8 cm and 8.9 cm are effective bird-window collision
preventive methods. The results of the parachute cord
experiment and those of previous studies support the
importance of applying collision prevention methods to
the outside window surface reflecting the facing habitat
and sky. Comparison of field and tunnel testing
experimental protocols to evaluate bird-window colli-
Acopian Center for Ornithology, Department of Biol-
ogy, Muhlenberg College, Allentown, PA 18104, USA.
Corresponding author; e-mail: firstname.lastname@example.org
sion preventive methods suggest that tunnel testing is
useful for initial assessment but not as a definitive
measure of effectiveness. Received 6 January 2012.
Accepted 19 December 2012.
Key words: collision prevention, testing, ultraviolet
(UV) signals, visual information, windows.
Except for habitat destruction, the casualties
attributable to bird-window collisions total in the
billions worldwide and are estimated to be greater
than any other human-associated source of avian
mortality (Klem 1990, 2009a; Hager et al. 2008).
More accurate estimates of birds killed striking
windows and the implications of this and other
human-caused avian mortality are currently under
study for management of specific species and birds
in general (Arnold and Zink 2011, Bayne et al.
2012, Loss et al. 2012). Minimally, preventing
these unintended and unwanted deaths has ethical
and moral value, and a legal international treaty
obligation, implemented in the United States by
The Migratory Bird Treaty Act (MBTA) of 1918
and the Endangered Species Act of 1973, as
respectively amended (Bean 1983, Corcoran 1999).
Several review papers document extensive
published evidence in the ornithological literature
that birds behave as if clear and reflective
windows are invisible to them (Erickson et al.
2001; Klem 2006, 2009a, 2010; Drewitt and
Langston 2008). Visual patterns composed of
uniformly spaced elements covering an entire
pane are known to transform windows into
obstacles that birds see and avoid (Klem 1990,
2009b). To retain the utility of unobstructed
viewing, the seemingly most acceptable solution
to protect birds from windows is to incorporate
ultraviolet (UV) signals into visual patterns that
birds see and humans do not (Klem 2009b).
We evaluated the strength and wavelength of
UV signals necessary to prevent bird-window
collisions using field experiments, and we com-
pare these results to those evaluating the same
signals using a flight tunnel. We also reexamine
and evaluate the spacing of opaque elements
making up a pattern uniformly covering a window
to prevent collisions. Specifically, we tested: (1) a
commercial window manufactured for new and
remodeled construction named ORNILUX Mika-
do that offers a UV signal for the purpose of
preventing bird-window collisions, and (2) two
versions of a commercial window product named
Acopian BirdSavers that uses vertically hung
opaque parachute cords. We use our findings
and previously published results to comment on
the relative importance of using field experiments
as a final test in evaluating bird-window collision
prevention methods and the importance of apply-
ing preventive methods to the surface of a window
facing the outside environment.
Our field experiments were conducted on a
2-ha area of mowed pasture bordered by second
growth deciduous forest and shrubs in Hennings-
ville, Berks County, Pennsylvania, USA (40u279
530N, 075u409070W). The basic design of the
two field experiments was the same as reported
previously (Klem 1989, 1990, 2009b), consisting
of wood-framed picture windows, simulating
those in houses. All windows were placed in the
same habitat facing the same direction along the
edge of trees and open field (Klem 1989: fig. 1).
Each window measured 1.2 m wide 30.9 m high,
and was mounted 1.2 m above ground. Plastic
mesh trays were placed under each window to
catch casualties. Three window units were used in
the first experiment and four in the second; all
windows in both experiments were separated by
2.4 m. Simulating a feeding station at a commer-
cial building or residential home, a single
platform feeder measuring 30.5 cm on a side
and 1.2 m above ground mounted on crossed
wooden-legs were centered and placed 10 m in
front of each window. Feed consisted of a 1:1
mixture of black-oil sunflower and white proso
millet. All feeders were kept full throughout each
experiment. Controls and treatments were ran-
domly assigned daily to a new position with the
exception that no treatment or control was
permitted in the same position on consecutive
days. Windows were checked each day 30 min
after first light and checked and changed daily
30 min before last light. In an attempt to observe
active avoidance of treatments, all windows in the
second experiment were monitored during multi-
ple-hour continuous periods totaling 63.5 hrs over
16 days (22, 26 Feb, 18, 21, 22, 26, 27, 28, 29
Mar, and 4, 8, 9, 11, 15, 18, 22 Apr 2011). The
observer was positioned 20 m from and in the
middle of the window units in a camouflaged
blind behind the platform feeders. The flights of
individual birds moving from the tray feeders
toward the windows were recorded and assessed
as an active avoidance if a bird changed direction
and passed around or over a window.
The parameter measured in the experiments
was the number of detectable bird strikes. A strike
was recorded when either dead or injured birds
were found beneath a window, or when fluid or a
blood smear, feather, or body smudge was found
on the glass. As in previous studies of similar
design (Klem 1989, 1990, 2009b; Klem et al.
2004), the data are likely to be incomplete and
conservative because some strikes may not have
left evidence of a collision. Additionally, preda-
tors and scavengers are known to remove some
injured or dead birds (Klem 1981, 2009b; Klem
et al. 2004; Hager et al. 2012).
Our field design can accommodate a maximum
of four window units; two experiments were
required to test the effectiveness of the preventive
methods studied. The first experiment was
conducted over 75 days from 3 October–18
December 2010, and compared clear (see-
through) and reflective (mirrored) glass controls
and an ORNILUX Mikado window offering UV
signals as a see-through pane simulating installa-
tion in a corridor between buildings, as a noise
barrier along roadways, or as glass walls around
zoo enclosures or building atria.
The second experiment was conducted over
68 days from 9 February–22 April 2011, and
tested the clear glass control, ORNILUX Mikado
pane covering a recessed non-reflective black
wooden board simulating a window that covered a
darkened room, and two vertically striped spacing
variations of preventive treatments known as
Acopian BirdSavers: (1) a clear glass pane
covered with 3.175 mm parachute cord spaced
10.8 cm from the center of one cord to the center
of the next, and (2) a reflective (mirror) glass pane
covered with 3.175 mm parachute cord spaced
8.9 cm from the center of one cord to the center of
CPFilms, Solutia, Inc. measured reflected UV
strength and wavelength for a sample of ORNI-
LUX Mikado and for the conventional clear and
reflective float glass controls using a Cary 5000
Spectrophotometer. ORNILUX Mikado reflected
7%UV from 300–380 nm, and 7–22%from 380–
400 nm. The clear and reflected glass controls
uniformly reflected 13%UV or less over 300–
400 nm. The parachute cord reflected 8%UV or
less over 300–400 nm; it was olive green and non-
reflective to the human eye.
Our experimental protocol was approved by our
Institutional Animal Care and Use Committee
(IACUC), and birds killed during the study were
salvaged under state and federal permits. Chi-
square goodness-of-fit was used to compare the
frequency of strikes among treatments in the two
experiments, and test results were considered
statistically significant when P,0.05 (Siegel
1956). We used SPSS (SPSS 2010) for all
statistical analyses of the experiments.
A total of 116 strikes were recorded in the first
experiment; 19 (16%) were fatal. The number of
strikes did not differ significantly across all
treatments with 32 (28%) at the clear glass control,
43 (37%) at the reflective glass control, and
41 (35%) at the ORNILUX Mikado (x
df 52, P50.41). The number of fatal strikes
differed significantly across all treatments with 2
(10%) at the clear glass control, 6 (32%) at the
reflective glass control, and 11 (58%) at the
ORNILUX Mikado (x
56.421, df 52, P5
0.040). Species numbers and treatment at which
fatalities occurred were: two Dark-eyed Juncos
(Junco hyemalis) at the clear glass control, two
Black-capped Chickadees (Poecile atricapillus),
three Northern Cardinals (Cardinalis cardinalis),
and one House Finch (Carpodacus mexicanus)at
the reflective glass control, and two Black-capped
Chickadees, one Ruby-crowned Kinglet (Regulus
calendula), one Hermit Thrush (Catharus gutta-
tus), one Gray Catbird (Dumetella carolinensis),
three Northern Cardinals, two Dark-eyed Juncos,
and one American Goldfinch (Spinus tristis) at the
One hundred and twelve strikes were recorded
in the second experiment; 22 (27%) were fatal.
The total number of strikes differed significantly
across all windows, with 69 (62%) at the clear
glass control, 31 (28%) at ORNILUX over dark
interior, 7 (6%) at parachute cords spaced 10.8 cm
apart covering clear pane, and 5 (4%) at parachute
cords spaced 8.9 cm apart covering reflective
595.00, df 53, P,0.001). The
number of fatal strikes differed significantly
across all treatments with 1 (5%) fatality of
Northern Cardinal at the clear pane covered by
parachute cords spaced 10.8 cm apart, and all 21
(95%) other fatalities at the clear glass control
that included: one Mourning Dove (Zenaida
macroura), one Black-capped Chickadee, three
Northern Cardinals, one Purple Finch (Carpoda-
cus purpureus), one White-throated Sparrow
(Zonotrichia albicollis), and 14 Dark-eyed Juncos
558.364, df 53, P,0.001).
THE WILSON JOURNAL OF ORNITHOLOGY NVol. 125, No. 2, June 2013
Flight paths of 44 individual birds flying from
bird feeders toward the experimental windows
were recorded to determine avoidance perfor-
mance. Of six individuals flying toward the clear
glass control, four (67%) moved to avoid and two
(33%) hit the window. Of 16 individuals flying
toward the ONILUX over dark interior, 12 (75%)
moved to avoid and four (25%) hit the window.
Of 12 individuals flying toward the parachute
cords spaced 10.8 cm apart covering the clear
pane, 11 (92%) moved to avoid and one (8%) hit
the window. Of 10 individuals flying toward the
parachute cords spaced 8.9 cm apart covering the
reflective pane, all 10 (100%) moved to avoid the
Several studies document evidence that birds
perceive UV wavelengths from approximately
300–400 nm (Burkhardt 1982, Bennett and
Cuthill 1994, Vitala et al. 1995, Bennett et al.
1996, Hunt et al. 1998), and although Martin
(2011) questioned static UV signals as a
collision deterrent, external films consisting of
contrasting UV-absorbing and UV-reflecting
patterns applied to sheet glass have been
effective in deterring bird-window collisions
(Klem 2009b). The use of UV signals to deter
bird-window collisions is arguably the most
practical solution because they preserve the
properties that humans expect and enjoy from
sheet glass while seemingly transforming clear
and reflective panes into obstacles that birds see
and avoid. External films can be used to retrofit
existing panes to render them bird-safe, but
uniquely manufactured sheet glass with UV
coating (glazing) patterns to be used in new and
remodeled construction will be required for a
long-term solution to protect birds from the
harmful effects of window strikes worldwide.
ORNILUX Mikado is a uniquely prepared and
commercially available sheet glass for new and
remodeled construction that claims to use UV
signals to prevent bird strikes. Our first field
experiment revealed that an ORNILUX Mikado
pane installed in see-through settings was more
lethal to birds than conventional clear or
reflective panes. We suggest these results can
be explained by the quality of the UV signal
offered by ORNILUX Mikado to birds. The UV
signal from ORNILUX Mikado reflected a
maximum 7–22%UV from 300–400 nm, reach-
ing above 20%reflection only at 397 nm. By
contrast, previous tests using the same field
experimental design found external films with a
UV-reflecting component of 20–40%over 300–
400 nm to effectively deter bird-window colli-
sions (Klem 2009b). We suggest that the
inability of ORNILUX Mikado installed in a
see-through setting to deter bird strikes is
explained by the lower level of reflected UV
that is available for bird perception, and to offer
an effective collision deterrence the UV-reflect-
ing component of the signal minimally must be
20–40%and be adjacent to contrasting areas of
UV-absorption to further highlight the UV
In our second experiment, ORNILUX Mika-
do covering a darkened interior exhibited a
55%reduced number of strikes compared to the
clear glass control, and this compares to the
58%and 66%deterrence reported for ORNI-
LUX Mikado in a see-through setting using
tunnel testing experiments (American Bird
Conservancy 2011). Tunnel testing experi-
ments consist of releasing birds at one end of
an enclosure where they are attracted to fly to a
brighter lighted opposite end at which they
choose one of two flight paths though an
unobstructed side serving as a control, and the
alternative side containing the collision pre-
vention treatment being tested (Rossler and
Zuna-Kratky 2004, American Bird Conservan-
cy 2011). Why the ORNILUX Mikado window
was more effective in deterring bird strikes
when covering a darkened interior is not
known, and may or may not be associated with
the UV signal offered by the pane.
The frequency of bird strikes at clear and
reflective panes covered by the two spacing
versions of vertically hung parachute cord provide
additional evidence, further validating previous
studies that vertical stripes separated by 10 cm or
less are effective bird-window collision preventive
methods (Klem 1990, 2009b; Rossler and Zuna-
Kratky 2004; American Bird Conservancy 2011).
The observations documenting flight paths of
individual birds toward the window treatments in
the second field experiment further supports the
level of deterrence recorded for ORNILUX
Mikado covering a darkened interior and the two
vertically hanging parachute cord patterns.
The discrepancy in the test results that occurred
using our field experiment compared to tunnel
testing experiments conducted by the American
Bird Conservancy (2011) suggests caution in
relying on tunnel testing as a final assessment of
effectiveness of any bird-window collision pre-
vention method. When comparing the protocol
of field and tunnel testing experiments, tunnel
experiments are markedly not as accurate in
simulating hazardous clear and reflective win-
dows installed in human structures. Components
of tunnel experiments that limit a more accurate
simulation of installed windows include: (1) the
stress of captured individuals of several different
species released to fly within restricted space to
alternative decision areas, (2) controlling illumi-
nation to simulate clear and reflective panes, (3)
netting placed in front of test panes that subjects
can potentially see and thereby influence their
choice, and (4) not being able to control for
variable weather conditions during test periods.
By contrast, our field experimental design far
more accurately simulates installed windows in
commercial and residential buildings, and each
preventive treatment and control are monitored
under the same conditions. Moreover, field
experiments can control for bias of installation
location by randomly moving treatments and
control daily over the experimental period. The
ability to conduct experiments by randomly
moving treatments and controls at existing
structures is extremely difficult and most often
Tunnel experiments are most useful in evalu-
ating several potential preventive options to deter
birds from striking see-through windows. Most
results from tunnel tests are similar to field tests
(Rossler and Zuna-Kratky 2004, American Bird
Conservancy 2011), but we suggest that the
limitations of tunnel experiments to accurately
simulate windows in actual buildings precludes
this protocol from serving as a definitive tool in
evaluating bird-window collision prevention
methods. We further suggest that the field
experiment protocol described here provides
accurate results to serve as a definitive assessment
of evaluating bird-window collision prevention
methods, because the effectiveness of treatments
are compared in an environment like that of
windows installed in actual buildings. Moreover,
given the limitations of tunnel testing to simulate
windows installed in human structures, we
recommend field experiments with the protocol
we report here be used to verify tunnel testing
results to qualify for Pilot Credit 55 Bird Collision
Deterrence used by the United States Green
Building Council (2011) in their Leadership in
Energy and Environmental Design (LEED) used
to promote the construction of environmentally
The effectiveness of vertically hung parachute
cords to deter bird collisions is attributable to the
critical spacing between cords, and also as
important is the placement of the cords over the
surface facing the outside. Architects and other
building professionals number the surface of
windows from the outside inward such that
surface number one is that surface facing the
outside environment. Unlike the parachute cords,
glass manufactures that produced the ORNILUX
Mikado using UV signals and other patterns such
as ceramic frit that uses dots, stripes or other
more creative shapes that are also visible to the
human eye are applied to interior surfaces of
multi-pane windows as a means of protecting the
collision deterring pattern from the weathering
effects of the environment. The physical proper-
ties of light absorption, refraction, reflection, and
transmission influence how humans, and almost
certainly birds, perceive patterns applied to
interior window surfaces (Knight 2013). When-
ever interior lighting is equal to or of greater
intensity to that of the outside environment,
patterns applied to surfaces other than surface
one will be visible to birds and humans looking
at the window from the outside. Clear windows
on either side of a corridor (link way) or where
glass walls meet in the corners permit patterns on
interior surfaces of windows to be visible when
viewed from the outside. But when windows
cover darkened interiors such that interior light is
of less intensity than outside, surface one reflects
the facing habitat and sky masking any patterns
on interior window surfaces and rendering them
ineffective in preventing bird strikes. Thus, like
the parachute cords, methods used to prevent
bird-window collisions at windows reflecting the
facing habitat and sky that are applied to surface
one will be most effective. Additional support of
the importance of placing bird-window collision
prevention methods on surface one are results of
previously reported studies in which all external
films applied to surface one, no matter what their
visual appearance, reduced the risk of bird-
window strikes by 59%or more (Klem 2009b).
We thank CPFilms, Solutia, Inc. and T. Port and B.
Lawless-Coale specifically for measuring the UV signal
from ORNILUX Mikado and the parachute cord, and
THE WILSON JOURNAL OF ORNITHOLOGY NVol. 125, No. 2, June 2013
discovering and reporting the error in UV signal measure-
ment from the external films previously published (Klem
2009b, UV-reflection strength reported as 80%should read
20–40%). We thank C. Mathers and M. K. Erdman for
collecting the individual flight path behavior, and J. D. Flood
and M. Jacob for technical help interpreting the properties of
sheet glass. We are especially grateful to Arnold Glas and
Jeff Acopian for financial support and supplying samples of
ORNILUX Mikado and Acopian BirdSavers for our
experiments, respectively. We thank the anonymous review-
ers and the Editors for extensive suggested modifications that
markedly improved the manuscript. D. Klem, Jr. holds a
patent (US 8,114,503 B2) on the critical spacing between
elements forming a pattern to transform windows into
barriers that birds will avoid. The authors and the Acopian
Center for Ornithology at Muhlenberg College have no
conflict of interest and have not and do not expect to receive
any financial benefit from products described in this study.
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