7 minute read
Moderate Bird Activity: An Aerial Pivot to Nature’s Drone
By LT Justin “Toto” Davis, USN
Advertisement
In last quarter’s Rotor Review, we broke down the mechanics of a bird strike from a bird’s perspective, dissecting how a seemingly avoidable incident like a bird strike occurs. In that article however, large generalities were made in discussing specific bird characteristics by lumping the whole of the bird class into two large groups based on body size and maneuverability. This article aims to parse those groups down into smaller categories, identify some representative species in each category, and highlight their characteristics which make them threat worthy.
Appearing across nearly every operating environment and employing a wide range of capabilities, these naturally occurring drones if you will are an inherent threat to us at all times and at all altitudes. Each species is specially adapted to its own specific environment which means that each species also has its own set of advantages and disadvantages we can exploit to avoid a strike altogether.
Birds Analysis
Appearing in order from the most hazardous to helicopters to the least, the following group descriptions provide a general overview of the bird group and then anchor down on key aspects of their physical characteristics and behavior that make them a threat to us. Hopefully this helps you to predict when and where a strike may occur based on your location, the time of year or day it is and the altitude at which you’re flying. As always, keep in mind that this list is not all inclusive.
Vultures
Vultures are routinely ranked as the most hazardous bird group when it comes to bird strike risk. This is primarily due to their size, distribution, and altitude at which they fly. There are two species of vultures found throughout North America year-round, Turkey Vultures and Black Vultures. Both are known for their large body size, expansive wingspans (up to six feet) and relatively large body mass (up to 6 pounds for Black Vultures). In terms of striking an object of a similar body mass, smacking into a vulture at helicopter speed would be like hitting an adult cat, an average sized pumpkin, or two copies of Leo Tolstoy’s War and Peace.
As for their flight behavior, both species predominantly utilize thermals to glide and soar in the search of food AKA carrion. This soaring behavior is what makes them so hazardous as they usually take flight in the afternoon, catching and riding thermal updrafts for hours at about 1,600 feet AGL. The Black Vulture in particular has even been observed to soar at altitudes of 650 feet AGL and below, mainly in the late afternoon to evening timeframe. Given their size, distribution, and soaring behavior, it’s easy to see why vultures are ranked quite high in terms of BASH hazard.
Geese and other Waterfowl
Geese. Canadian Geese. Snow Geese. It doesn’t matter the flavor, this group of birds are large, extremely tolerant of human activity, have high population densities, and routinely squat in urban/suburban areas where food is plentiful yearround. These qualities make them a very high BASH risk. Just their size alone, close to 15 pounds for male Canada Geese, make them a considerable threat as striking one would be akin to hitting an airborne cast iron skillet, gallon of milk, or bowling ball for comparison. Not to mention, that large body size only hinders their ability to maneuver away from an aircraft should one get too close.
When it comes to flight behavior, geese pose the highest risk for helicopters when they attempt to take flight from a field in response to an advancing aircraft flying at a low altitude. Low flying helicopters are unfortunately quite capable of stirring up animal life when in low-level flight. In terms of migratory flight altitudes, Canada Geese have been observed to fly at altitudes between 1,000 to 3,300 feet. These migration routes take them South from Canada in the winter to the southern portions of the United States then return North in the spring. With that said, increasing proportions of geese populations do remain in locations year-round if food is available.
Birds of Prey (Eagles, Hawks, Owls, etc.)
For a carnivorous group of birds that is united by hooked beaks, talons, and superb eyesight there is a great deal of variety in terms of their body size, lifestyle, and flight behavior. Birds in this group, for example, can vary in weight from mere ounces for American kestrels to up to 14 pounds for Golden eagles. Despite this variety, those species that stand out as BASH hazards tend to share some of the characteristics of the other groups, specifically a higher body mass and a tendency to soar on thermals in search of food. One stand-out species in particular is the Bald eagle which routinely soars on thermals that put them at risky altitudes similar to that of vultures though usually higher, up to 10,000 feet AGL. Additionally, the Red-tailed hawk, being the most abundant and widespread of this group, is a particularly noteworthy threat. Despite the birds in this group having remarkable eyesight and aerial maneuverability skills they just can’t seem to stop hitting aircraft.
Sea Gulls
17 species of gull inhabit the conterminous US. That’s 17 ways to have a strike. Mainly found along coast lines and forming large flocks near concentrations of food, most species pose a threat due to their attraction to human disturbed environments. Gulls average around three pounds but a wellfed Great Black-backed gull living life along the Atlantic coastline can push the scales at four pounds. In terms of flight behavior, Ring-bill gulls, which are by far the most abundant throughout the United States, routinely migrate in large flocks at 1,300 feet AGL or higher during the migrating seasons in the fall and spring. Their poundage coupled with their tendency to fly in flocks at helicopter altitudes makes them a particularly noteworthy threat.
Passerines (Perching Birds)
Passerines are the perching birds. The little brown birds. The flocking birds. Named after the arrangement of their toes, what they lack in size they make up for in sheer numbers. While the Common raven may weigh up to 3 pounds, most birds of this group will register mostly in ounces. With that said, a single chickadee alone wouldn’t pose a significant risk but when a murder of crows congregate in a good-sized flock, as they are known to do, the threat is very real.
If there are two notorious species in the Passerines group they are the Red-winged blackbirds and European starlings as both species share the nasty tendency to flock in large numbers and fly at low altitudes. Red-winged Blackbirds for example routinely migrate at altitudes of 330 feet AGL and below, often flying under 160 feet AGL. European starlings are known to forage at altitudes below 330 feet and migrate at altitudes of over 3,300 feet, covering every altitude helicopters typically operate at. A great fact regarding European starlings is that they are actually native to Great Britain and were introduced to the US in 1890 by a one Eugene Schieffelin, an American amateur ornithologist. From the initial group of about 60 individuals he released into Central Park, the current starling population now exceeds 200 million, much to the dismay of thousands of indigenous bird species and pilots everywhere. Thanks Eugene.
Conclusions
The bird threat we face is ubiquitous, growing in population size, and not at all adapted to the threat we pose to them either. As our respective populations continue to grow and our operations continue to expand, the overlap between birds and humans will become more increasingly more complex and entrenched, inevitably leading to more strikes in the future. Flying in bird-free skies would be the ideal scenario but unfortunately for us, that’s not the reality. The reality is that we share the skies with ancient, well-adapted, flying dinosaurs that have no idea what a helicopter is or how much of a threat it is to them. So until we educated birds to stay to stay away from us or exterminate the skies of their presence we will have to stay aware of their presence. It’s a predicament that can be made easier to deal with if we develop some understanding of how, when, and where birds fly. You won’t overhaul a mission over a the possible presence of sea gulls in your operating area for example but you may at least be on a heightened alert if you take into account the time, altitude, and location you’ll be working in. Just remember, birds aren’t real… until you hit one
References
1. Pfeiffer, Morgan B., Blackwell, Bradely F., DeVault, Travis. L. (2018). Quantification of avian hazards to military aircraft and implications for wildlife management. PLOS ONE 13, 11
2. Jung, Jacob F., Fischer, Richard A. (2018). Regional Asessments and Life-history Investigations of Problematic Birds on Military Airfields. U.S. Army Engineer Research and Development Center Environmental Laboratory
