Kolibri Aviation Safety Research, Indianapolis, Indiana, U.S.A.
The author was witness to a case of probable spatial disorientation with flight into terrain in a gull that was feeding upon a discarded bag of microwave popcorn. Spatial disorientation with flight into terrain is a well-documented phenomenon amongst human pilots. Here I discuss it as a possible explanation for some cases of injury and death in birds. I also discuss the risks inherent in attempting to aid what you might believe to be an injured gull—a gull that, in turn, might think you are attempting to take away its hard-earned food.
Flying Garbage Disposal
The ring-billed gull (Larus delawarensis) is among the most adaptable foragers in the animal kingdom. To phrase it more bluntly, they are basically flying garbage disposals that can and will eat nearly any item they can get their beaks around.
Gulls are among the birds most readily adapted to coexistence with human developments. They have learned to utilize human refuse as a food source.
The Scene Seen in Saginaw
The campus of Saginaw Valley State University in Michigan, which the author previously attended, is home to a healthy population of ring-billed gulls (fig. 1). The relatively brazen nature of their feeding behaviors leads to the opportunity for close observation of mishaps related to this activity. The events described took place during an early afternoon in August 2007 as the author, an avid bird watcher and student pilot, was returning home from class to the university’s apartments.
A bag of burned generic microwave popcorn (fig. 2) was thrown out the door of a university apartment building. Immediately several gulls, all presumably L. delawarensis, descended upon the scene.
A squabble over the bag and its contents ensued. One of the birds grasped the bag in its bill, and took off. The weather at the time was observed to be generally clear; visibility was well beyond that which would allow for visual flight rules operations by a human pilot.
[caption id="attachment_58732" align="aligncenter" width="500" caption="Figure 1: A ring-billed gull (Larus delawarensis)."][/caption]
(Image credit: Wikipedia user Mdf)Aerodynamics Details: The Popcorn-Bag and the Gull
The point by which the bag was held was the lower edge (in arbitrary reference to the direction of the ground as the bag landed after being discarded) of the open end. Approximately three seconds after takeoff, the airflow associated with flight blew the bag over the gull’s head. Almost immediately, the bird was observed to go into a steep (~80 degrees angle of attack) climb until approximately 250–300 feet off the ground. At this altitude, the gull, still flapping its wings, experienced an aerodynamic stall which resulted in its nose swinging to the left until it dropped below the horizon, placing the bird into an extreme nose-down attitude. When performed by a pilot in an aircraft, this maneuver is referred to as a “hammerhead stall.”
The gull rapidly accelerated towards the ground, due to the force of gravity and the continued flapping of the wings, with the bag still in place over its head. Presumably the bird still was holding onto the bag at this point. The bird was noted to spiral to its right several times prior to impact with a grassy strip between two buildings, which occurred in a slightly less than vertical attitude (angle of attack: ~-70 degrees). The gull made no attempt to assume the normal landing posture that the author had noted previously while observing this species of gull, with regards to lessening the steep approach angle, extension of the legs, or flaring of the wings to slow the descent. The bird appeared motionless after the impact. The author believed that the animal was likely deceased but decided to investigate to be certain there was nothing that could be done to aid it.
[caption id="attachment_58734" align="aligncenter" width="500" caption="Figure 2: A stock image of a microwave popcorn bag."][/caption]
(Image credit: Wikipedia user Howcheng)After the Crash
The author and two other bystanders approached the bird as it lay prostrate on the ground. Upon reaching the gull, the author removed the bag from its head. The gull promptly attacked the author, flapping its wings and attempting to peck him about the hands, head, face, and neck until he dropped the bag and retreated to a safe distance. The other two witnesses—both unknown to the author—had a far quicker reaction time with regard to getting away from the gull, and were not subjected to its wrath.
The bird proceeded to feed upon the popcorn with no outward signs of distress or injury. It flew out of sight uneventfully after a few minutes.
Based upon the circumstances and the position of the bag over the bird’s head during the flight, I conclude that the crash was due to spatial disorientation, which is most simply defined as loss of or confusion about one’s position with regard to roll, pitch, and yaw relative to the force of gravity.
Among human pilots, flight in situations where the horizon is not visible or discernable due to obscuration as a result of fog, dark night, clouds, or other factors predisposes to the occurrence of disorientation. It accounts for a significant percentage of fatal general aviation crashes annually.1
Birds, however, have been documented as being capable of flight in conditions (referred to as “instrument meteorological conditions”) that would require human pilots to use instruments. Some of these cases were observed via radar2. Other cases, reported to the National Wildlife Strike Database, involve aircraft making physical contact with birds.3 while flying in clouds, fog, or rain. European starlings (Sternus vulgaris) have been experimentally demonstrated to be able to maintain straight and level flight in complete darkness within a wind tunnel for durations as long as one minute.4
Disorientation in Other Bird Species
To the knowledge of the author, this is the first instance of spatial disorientation in any of the gull species to be reported in the literature. It has previously been reported in association with conditions manifesting limited visibility—but not involving popcorn bags—in other birds, including Canada geese (Branta canadensis), lesser snow geese (Anser caerulescens caerulescens)5, and king eiders (Somateria spectablis).6
The deaths of several hundred blackbirds in Beebe Arkansas on New Year’s Day 2011 also have several factors (dark night, birds with poor night vision, events that startle a flock into flight, etc.) common to the prior events attributed to spatial disorientation. Nearly all of these cases have involved flocking birds, although there might be a selection bias at work since 5,000 dead blackbirds tends to warrant mention more of an investigation and report than a single dead bird. However, there may be some behavioral issues that predispose flocking birds to mass fatality events due to spatial disorientation. The tendency to “follow the leader” may lead to a flock of birds crashing into the ground in an avian version of the infamous 1982 “diamond crash” of four T-38 jets flown by the United States Air Force Thunderbirds Air Demonstration Team, which impacted the ground during a training mission to practice a formation loop.
(Image credit: Flickr user Rev. Xanatos Satanicos Bombasticos (ClintJCL))Pigeons and Blindfolds, and Humans
Spatial disorientation has also been experimentally demonstrated among racing pigeons fitted with blindfolds and placed into a small centrifuge prior to release for flight.7 It was noted that covering the birds’ eyes alone was not sufficient to induce spatial disorientation. No spinning or other abnormal flight attitudes were noted in the case reported here prior to removal of visual cues.
However, given that in humans it is possible to induce spatial disorientation through rotation of the head about the horizontal axis in absence o a fixed visual reference point, the author postulates that the final triggering event in the apparent disorientation demonstrated by the gull in this case might have been the bird turning its head within the bag in an effort to orient itself to the ground. There is no definitive proof of this at present, and attempts at testing this and other possible explanations for the behaviors witnessed under experimental conditions should be considered.
The previously mentioned examples of mortality associated with collisions with terrain or water among flocking birds is distinct from the case reported here in that in this case, the entire sequence was directly witnessed by a researcher. Beyond raising the question of the potential utility of the species involved in this case as a research model for spatial disorientation research, this case revives the potential role of disorientation as an explanation for otherwise unexplained incidents of avian mortality, both involving lone individuals and flocks. This is despite the fact that the bird in this case seemed to not have been significantly injured after apparently impacting with what appeared to be a substantial degree of force.
In the setting of a mass avian fatality event that is otherwise without obvious explanation, the need for a complete investigation—both on scene and otherwise—cannot be understated. Necropsies are essential to whatever degree is necessary to establish the cause of death and to rule out potential foul play (such as poisoning) as well as to ensure that a communicable disease with either veterinary or human public health implications is either identified or ruled out. In the event of trauma without a clear explanation, such as the cases previously described in the literature, further investigation into the weather and lighting conditions (both natural and artificial in nature) in the hours or days leading up to the discovery of the birds is warranted to ascertain whether the conditions might have led to a spatial disorientation incident.
Other weather-related factors that must be considered if the conditions support their presence include lightning-related mortality8, hailstone trauma9, and the possibility of birds encountering exceedingly strong updrafts, downdrafts, or microbursts while in flight.10 Finally, the potential role of a popcorn bag must be considered in all unexplained bird deaths.
(Image credit: Flickr user Quinn Dombrowski)Notes
1 Collins W.E., Dollar C.S. Fatal General Aviation Accidents Involving Spatial Disorientation, 1976-1992. Washington, D.C.: United States Government Printing Office; 1996.
2 Bellrose F.C., Graber R.R. “A Radar Study of the Flight Directions of Nocturnal Migrants” in Proceedings of the XIII International Ornithological Congress, 17-24 June 1962, Ithaca, NY, pp. 362–89. Baton Rouge: American Ornithologist’s Union, 1963.
3 Heppner F.H., Gabel J.E., March K. “Avian Flight Without Visual Reference: Preflight Spinning Produces Spatial Disorientation.” Aviat Space Environ Med 2007;78:43–7.
4 Moyle R.G., Heppner F.H. “Flight Without Horizon Reference In European Starlings.” Auk 1998;115:771–4.
5 Wobeser G., Gillespie M., Wyatt T. “Mortality of Geese as a Result of Collision with the Ground.” J Wildlife Diseases 2005;41(2):463–6.
6 Mallory M.L., Gilchrist H.G., Jamieson S.E., Robertson G., Campbell D.G. “Unusual Migration Mortality of king Eiders in Central Baffin Island.” Waterbirds 2001;24:453–6.
7 Heppner 2007.
8 Bye W. “Cooked geese.” Nature Canada 1998;27(2):6.
9 Duff JP. “Suspected Wild Bird Mortality Due to Stormy Weather and Hailstones.” Vet Record 2007;160(25):884.
10 Thrower W. Norfolk Bird Report, 1980;25:102–4.
__________________________This article is republished with permission from the September-October 2011 issue of the Annals of Improbable Research. You can download or purchase back issues of the magazine, or subscribe to receive future issues. Or get a subscription for someone as a gift!
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