Window
collisions fatal
for birds
Window collisions second source of avian mortality, copyright Frania
Shelley-Grielen, all rights reserved

Another autumn arrives in New York with all the majesty and rich palette of changing
colors and leaf fall and with it another fall migration, bird watchers (and writers)
entranced by the seasonal avian journey find skies full of winged travelers for glorious
study.  Birds flying south for warmer climates let us know that at least someone gets to
get away for the winter, or so we think.  For the birds, navigating through this city
means that some make it and some will not make it out of here alive.

For all of wildlife, the number one human related threat is habitat loss or destruction.  
For birds, the second greatest danger is “window kills” or colliding with window panes.  
As biologist, Daniel Klem, Jr. notes: “birds behave as if clear and reflective panes are
invisible to them, and they kill or injure themselves attempting to reach habitat seen
through or reflected in windows.”  Actual figures of birds killed every year in the US as
a result of these collisions are unknown but Klem estimates them at 100 million to 1
billion with the modest assumption “that one bird is killed per building per year.”  
When it comes to dangers to birds, window-kills are not well publicized and are often
overlooked despite their staggering numbers.  Klem compares figures of other sources of
“human associated” bird deaths:  “120 million from hunting, 60 million from vehicle
road kills, 10,000 to 40,000 from wind turbine strikes” per year, concluding that “The
kills at clear and reflective glass and plastic are surely in the billions worldwide.”  
These sobering numbers highlight a need to raise awareness of a monumental
conservation crisis for our birds and to adequately address it.

How birds see, what, when and where they are seeing contribute to the danger.  A NYC
Audubon study done in 2009 collected data on 5,400 bird collisions in Manhattan over
a ten year period, focusing on factors associated with window collisions, the study found
that most victims (two thirds of the collisions resulted in fatalities) were migrating
birds from the warbler, thrush and sparrow families.  Bird strikes were also found to
occur more frequently along “portions of the exterior glass surface that reflect outside
vegetation.”   The study further found that collisions were more like to happen during
daylight hours as opposed to evening.  The Audubon study concluded that migrating
birds are more at risk of window kills while Klem notes that the more birds around glass
the more collisions, with our own misplaced bird feeders being just as likely, if not more
so, to place birds at peril.  Both statements are no doubt correct as long as glass is
present for birds to fly into.

Our human disconnect with failing to appreciate the severity of how dangerous
navigating the built environment is for birds may result from our failing to
adequately contemplate how very differently birds perceive the world.  Birds like
humans rely on vision as their primary sense and while bird vision is extraordinary
compared to human vision in some respects it is markedly different in how birds
process visual information-from the rate at which their brains perceive and process
information, to the spectrum of ultraviolet light they are aware of.  Structural
differences in anatomy add to differences in perception.  
Avian expert, Graham Martin
succinctly compares human vision to bird vision:  “The human visual world is “in
front” and humans move “into” it” and “The avian world is “around” and birds move
“through” it.” For a bird, with eyes on either side of the head, the field of vision is
mostly along the side or lateral with visual coverage above and behind the head.  
Frontal vision is employed for specialized tasks, beak control, chick feeding, nest
construction and targeting an object right before capture.  There is a blind area
directly in front of the bill and a limited binocular forward field, this forward field is
the bird’s peripheral vision.

Physical evidence of window-kills may not always be readily apparent.  Predators and
scavengers remove fallen birds and in heavily urbanized areas the task can simply
become more debris removal.  The NYC Audubon study made note that the collision
count was “underestimated” due to “removal bias” which includes building
maintenance staff and street sweepers in addition to animal forces.  The most fortunate
near casualties are those who can recover on their own or who are found by wildlife
rehabilitators who will care for injured animals.  In the metropolitan area the number
of licensed wildlife rehabilitators is limited even as the numbers of wounded birds are
overwhelming.  A recent look at two websites listing licensed wildlife rehabilitators,
nyswrc.org and wildliferehabinfo.org showed each listing four licensed rehabilitators
for New York City.  On one site, two of the rehabbers listed worked exclusively with
turtles, on the other site two of the rehabbers listed worked exclusively with squirrels
with two remaining working only with non-migratory birds.  Wild bird care, for all
birds, became available in 2012 at
The Wild Bird Fund, a dedicated center in
Manhattan.  Rita McMahon, a wild bird rehab expert, who began treating injured birds
in her home in 2001, founded the center.

For any wild animal a captive stay at any facility is a stressful stay in a foreign
environment.  Emergency care which may include medication and hand feeding is
mainly supportive (providing food, shelter and resting space). While rehabilitation
efforts are necessary for healing, McMahon concedes that “everything” in the rehab
process is stressful for birds with each bird having no doubt a family, friends and a
destination to get back to.  Mandatory solitary recuperation without affiliates is
stressful for birds who heavily depend on flock and social structure as mainstays for
survival.  Half of the birds that come to the center recover and once able to
demonstrate sustained flight are released.  A typical stay is 1 to 2 days.   Head trauma,
most likely induced by collisions, afflicted most of the migratory birds I saw when I
visited the center recently. Among the victims were an ovenbird, Northern Parula,
two different types of warbler (morning and black and white), a Red-eyed Vireo, and a
Scarlet Tanager.  Sparrows, gulls, doves and a swan were also being treated for broken
wings, poisonings and a host of other maladies.  On any given day the species can vary
with pigeons being treated at the center in the greatest number (it is New York).    The
Wild Bird Fund treated over 1,500 injured animals in 2012 and expects the number to
exceed 3,000 this year relying mainly on donations and volunteer efforts.

The numbers of birds and the variety of species cared for at the Wild Bird Fund reflect
the everyday jeopardies faced by wild animals in our built environment.  New York
City is located in a major flyway or highway for migratory birds (add to that the
native species and the numbers increase to more than 370 species passing through or
making their home in the area).  When asked in an email exchange, about the ratio of
migratory birds compared to native birds typically treated, McMahon wrote: “It's the
migratory birds who suffer most from window strikes.  We have 12 to 20 at a time in
recovery.  During fall migration, many to most of them are first-year birds who have
not traveled through NYC before. For example, we have 8 yellow-bellied sapsuckers
right now, 6 of them are first-year birds.” In response to human-associated dangers to
native wild birds McMahon writes: “Though some resident birds strike windows, more
fall victim to deadly run-ins with vehicles: cars, trucks and bicycles. The greatest
number of collisions with vehicles occur with birds that live on the street: gulls and
pigeons. Avian diseases take their toll in great measure as well.  After that it is
predation by pets: dogs and cats. In the urban environment great numbers of birds fall
prey to pollution and getting entangled in trash. Many pigeons and waterbirds suffer
from lead poisoning and a surprising number of birds are entangled by human hair,
from songbird babies in the nest to pigeons on the street. Natural (non-pet) predation
accounts for less than 10%.”

Keeping birds out of emergency care is a far more daunting proposition.  We cannot
know what birds are looking at while they are flying but employing their side vision to
see other birds or predators is probable as is looking for food or habitat.  And even if
birds were able to look ahead for obstacles would they be able to slow down to avoid
them?  The answer is most likely that they cannot slow down due to flight mechanics.  
Many birds cannot see ahead of them with peripheral frontal vision lacking the high
resolution their side vision offers.  Human oriented solutions to bird collisions often
assume that birds are looking ahead and can effectively respond to obstacles in their
flight paths.  How birds navigate flight tells a different story and requires rethinking
from a bird point of view.  Avoiding collisions for birds requires a warning system such
as our own “slow down hazard ahead” road signs but needs to be avian friendly and
meaningful to birds in order to be effective.

Graham Martin suggests a system that would warn the birds well in advance of the
obstacle in order to “prime attention” with things that are much larger than we might
believe necessary in order to register visually for birds and not be fixed in space rather
be mobile and or able to divert or distract from their flight path.  Foraging patches,
models of other birds or alerting sounds placed well enough away from the building to
allow reaction time for the birds are recommended.  Such warning systems require the
creativity of an informed avian themed perspective along with installation and
acceptance in the human landscape.  McMahon’s observation and data on the
proportion of younger birds being susceptible to collisions is a rich area for further
investigation and study.  Are older birds learning to avoid windows?  And if older birds
have learned to somehow circumvent windows how did they put the puzzle into place
to avoid them and what can we do to enhance the process and better understand how to
help?

For an immediate remedy, Klem advocates protective measures which include
screening windows and placing bird feeders as close to windows as possible to lessen
impact.  Protective decals are popular but are only effective as long as they are in
sufficient number.  Another popular treatment is ultraviolet window marking; while a
study released this month by Swedish researchers found that this may only be of use
for those birds which are ultra violet light sensitive such as gulls, parrots and certain
perching birds but not for most other birds which are violet light sensitive.  And for
right now in NYC there are those volunteers working at the Wild Bird Fund with
whichever of the latest lucky victims which might get to get home after all.  Maybe.

References

Klem, Jr.,D. (2010). Avian Mortality at Windows: The Second Largest Human Source of Bird Mortality on
Earth.
Proceedings of the Fourth International Partners in Flight Conference: Tundra to Tropics. 244-251

Gleb, G. & Delacretaz, N. (2009). Windows and Vegetation: Primary Factors in Manhattan Bird
Collisions.
Northeastern Naturalist 16(3):455-470

Martin, G.R. (2011). Understanding bird collisions with man made objects: a sensory ecology approach.
IBIS (153) 239-254

Hastad, O. and Odeen, A. (2014), A vision physiological estimation of ultraviolet window marking
visibility to birds.
PeerJ 2:e621; DOI 10.7717/peerj.621
(Copyright Frania Shelley-Grielen) An injured ovenbird is treated at
The Wild Bird Fund in New York City
"For all of wildlife, the number one
human related threat is habitat loss
or destruction.  For birds, the second
greatest danger is “window kills” or
colliding with window panes.  As
biologist, Daniel Klem, Jr. notes:
“birds behave as if clear and
reflective panes are invisible to them,
and they kill or injure themselves
attempting to reach habitat seen
through or reflected in windows.”
(Copyright Frania Shelley-Grielen) This recovering swan was brought
to the center suffering from botulism poisoning
Request a presentation
(copyright Frania Shelley-Grielen) Head injuries suffered during
collisions with windows are the most common
(Copyright Frania Shelley-Grielen) Injured wild birds offered care are
the "lucky" ones.
"Human oriented solutions to bird
collisions often assume that birds are
looking ahead and can effectively
respond to obstacles in their flight
paths.  How birds navigate flight tells
a different story and requires
rethinking from a bird point of view.  
Avoiding collisions for birds requires
a warning system such as our own
“slow down hazard ahead” road
signs but needs to be avian friendly
and meaningful to birds in order to be
effective.”
(copyright Frania Shelley-Grielen) At The Wild Bird Fund in New York
City, the majority of injured birds are pigeons.  Note the protective netting
set off of windows to soften possible collisions
info@animalbehaviorist.us
212-722-2509 / 646-228-7813


Entire website copyright Frania Shelley-Grielen