Preventing bird-window collisions.

Avian mortality resulting from collisions with clear and reflective sheet glass and plastic is estimated to be in the billions worldwide (Klem 1990, 2006). Collisions are predicted and expected wherever birds and windows coexist (Klem 1989, 1990, 2006). Birds behave as if windows are invisible, and it is important to prevent this unintended killing, estimated to represent the largest human-associated source of avian mortality except habitat destruction (Klem 2006, 2009a, b). The diversity of species and the invisible threat suggest that birds in general are vulnerable to windows, but documented casualties of species of special concern indicates that avian mortality from window collisions is contributing to population declines of specific species and birds in general (Klem 2009a, b).

I evaluated several methods to prevent bird strikes at windows using previously effective outdoor flight cage and field experiments (Klem 1989, 1990). Most preventive treatments examined the use of ultraviolet (UV) signals to alert birds to windows, and the availability of materials affected the composition of what was tested in each experiment. The ability of birds to avoid clear plastic and the ability of one-way films, fritted glass, and feathers to prevent collisions were also evaluated. Specifically, I tested: (1) clear plastic with a UV-absorbing component, (2) single and uniform covering of multiple UV-reflecting maple leaves, (3) a string of colored contour feathers, (4) a one-way external film having an unobstructed view from inside and an obstructed view of dot pattern from outside, (5) a ceramic frit glass with a uniform covering of translucent dots, (6) a variety of UV-absorbing stripe patterns created by plastic strips, and different UV-absorbing and UV-reflecting complete covering, striped, and grid patterns created by external films.

METHODS

Flight cage and field experiments were conducted on a 0.2-ha open mowed grass suburban backyard surrounded and isolated from neighbors by mature shrubs and evergreens in Upper Macungie Township, Lehigh County, Pennsylvania (40[degrees] 34' 35" N, 75[degrees] 34' 57" W). Four field experiments were conducted on a 2-ha open rural area of mowed pasture bordered by second growth deciduous forest and shrubs in Henningsville, Berks County, Pennsylvania (40[degrees] 27' 53" N, 75[degrees] 40' 07" W).

Flight Cage Experiments.--These tests were conducted from 13 March to 30 April 2004. The basic design was reported previously by Klem (1990) and consisted of a trapezoidal flight cage 1.2 m high, 3.6 m in length, and 0.3 m wide at the narrow end and 2.6 m wide at the broad end. Five Dark-eyed Juncos (Junco hyemalis), one White-throated Sparrow (Zonotrichia albicollis), and one House Sparrow (Passer domesticus) were captured in March for use as subjects, housed in small cages, and tested from mid-March and throughout April. Except for the House Sparrow which was an adult female, age and gender of all other subjects were unknown; previous studies of collision casualties document equal vulnerability for all age and gender classes (Klem 1989).

Individuals were released from a holding box at the narrow end and forced to discriminate between left and right flight paths as they attempted to escape to wooded evergreen habitat visible outside the broad end of the cage. One half of the cage at the broad end was left unobstructed in all experiments. The other half was obstructed by clear plastic or objects tested to prevent bird strikes. During testing of a subject, the obstructed and unobstructed sides were changed for half the trials to ensure no bias flight path preference for one side or the other. Actual clear plastic was tested with two Dark-eyed Junco subjects to learn if they were capable of discriminating between clear plastic and unobstructed airspace. Previous studies revealed that Dark-eyed Junco subjects were not capable of discriminating between clear glass and unobstructed airspace (Klem 1990). Objects tested were hung on the obstructed side with clear monofilament line to appear as if taped, stuck, or applied as a coating to clear glass or plastic to prevent accidental collision injuries to subjects in subsequent experiments. No Institutional Animal Care and Use Committee existed during this study, but guidelines for the care of wild birds in research were followed (Gaunt and Oring 1999). All subjects were released unharmed at the end of the experimental period.

Eight flight cage experiments were conducted. Each experiment tested one to five subjects, and each subject flew a minimum of 10 trials per experiment with additional trials (up to 24) to clarify results (Table 1). A trial consisted of recording a subject passing through the unobstructed side of the cage or the side containing the object tested. If the subject chose the obstructed side it was scored as a window strike; if the subject flew through the unobstructed side it was scored as avoidance. Two to three objects were evaluated on any test day. …