Chapter 4 — Mammals — A Primer

Introduction

Saab 340
Photo courtesy Northwest Airlines

Photo: This Saab 340 struck two deer while landing at a Michigan strike, the engine was held in place only by oil and fuel lines.

As one might expect, mammals are not struck by aircraft as frequently as birds. When mammal strikes do occur, they are confined to runways—with the exception of collisions involving bats. Strikes involving mammals, however, usually inflict significant damage, since the sizes of these animals are, on the whole, greater than those of birds. Yet even small mammals inflict their share of damage; during takeoff and landing, general aviation pilots have on occasion swerved to avoid small mammals, often resulting in damaging runway excursions.

This chapter presents an introduction to mammals: their numbers, distribution and general behaviour. It will provide you with a working knowledge of mammal biology—knowledge that is critical in developing risk-management strategies. Detailed information is provided on some mammal species commonly found at North American airports. For exhaustive research on the matter, refer to publications listed in Appendix E.

Mammals have been the dominant life form for the past 65-million years. During this time, they evolved into a variety of shapes and types ranging from bats, seals, whales and deer to cats, dogs and primates. Mammals share the basic features of all vertebrates but are distinguished from birds, fish and reptiles by two fundamental characteristics: the possession of milk-producing glands—or mammae—and body hair. Other distinguishing features include well developed external ears and a pelvic girdle that permits standing, walking and running in certain species.

As in Chapter 3, we follow the custom of capitalizing the first letter of common names of mammal species (e.g., Red Fox, Coyote, and Grey Squirrel); names of groups of species are not capitalized (e.g., squirrel and deer).

Mammal classification or taxonomy

Mammals belong to the Class Mammalia, which is comprised of three major groups:

  • egg-laying mammals such as the Duck-billed Platypus;
  • mammals that give birth to embryonic young and often have a pouch, such as opossums; and
  • those mammals which gestate inside the mother’s body.

Only the last two groups are found in North America.

Present-day mammals are divided into 18 distinct orders, largely based on differences in gross body structure. Mammals’ taxonomic order also recognizes the kind and number of teeth possessed by the various groups. In North America, there are 10 orders of mammals and more than 100 separate families.

More than 40 percent of mammals belong to the order Rodentia—or make up almost 60 percent of all mammal species. Most rodents are small, secretive and go largely unnoticed. Interestingly, bats are the second largest order of mammals in the world; their 896 species represent nearly a quarter of the world’s mammals.

Mammalian diversity and distribution

Mammals have the lowest species diversity among the world’s vertebrates, comprising about 3,800 species—less than half the number of bird species, and only a fraction of the number of fish species. Mammals are found throughout the seas and continents of the world. The largest concentration of mammal species is found in Central and South America, where 930 species reside. Africa is home to 860 species. In spite of North America’s large landmass, this more recently formed continent is home to only 350 species, and less than 10 percent of the world’s mammals.

The numbers of mammal species and individuals vary considerably around the world. For example, though the number of species in South America is high, these species are typically represented by relatively low numbers of individuals. In contrast, the number of land mammal species in Canada is low at 160, yet are represented by very large local populations. A caribou herd on its calving grounds may number into the hundreds of thousands. Similarly, local deer populations in southern Canada and throughout much of the U.S. often comprise hundreds of individuals each.

Although individual bird species in North America may be found breeding from coast to coast, mammals here tend to be much more restrictive in their distribution. Many species are restricted to specific habitats and are found only in a single province or state. For this reason, problem species at airports vary significantly from region to region.

Species Number of Individuals
Black Bear 1
Red Fox 5-10
Raccoon 50
Striped Skunk 50-65
White-tail Jackrabbit 50-100
Deer Mouse 640-6,400
Eastern Cottontail 3,200-6,400
Woodchuck 3,200-16,000
Meadow Vole 144,000-480,000

Table 4.1 Estimated Average Population Densities of Some Common Mammals in Home-range Areas of 5 Square Miles

As a rule of thumb in North America, the number of mammals found in any local area seldom exceeds 30 species, and only a few of these present a hazard to aircraft. On the other hand, the presence of as many as 80 to 100 species of birds is not uncommon.

Mammalian numbers and population density

Mammalian numbers

The size of continental mammal populations is, for the most part, poorly documented and rarely studied. This may be attributed to the secretive nature of mammals; most are small and nocturnal, making them difficult to observe—unlike birds. Many large mammals occupy a wide home range through which they constantly move, making their detection difficult. In addition, most mammals do not migrate as birds do, and therefore cannot be counted and observed at key points along migration routes. Even so, population estimates for some larger game mammals reach impressive numbers. In Canada, the Alberta population of Mule Deer has been estimated at more than 150,000 animals. The North American Elk population exceeds 500,000 while as many as 300,000 Moose are distributed across the continent. The population of White-tailed Deer across Canada has been estimated at approximately 2.5 million.

Mammalian population density

In any local area, mammal population densities are usually high. Many species of rodents, such as voles and mice, can reproduce at formidable rates; 6 to 8 litters of young in a single season is not uncommon. As a result, the number of voles in the grass fields around an airport can easily reach the tens of thousands. Average densities of the Meadow Vole range between 45 to 150 animals per acre. During population peaks, this figure can go as high as 400. Table 4.1 provides average home-range density estimates for a number of common mammal species.

Species Weight (lbs)
Moose 800-1100
Elk 400-1000
Black Bear 200-600
Mule Deer 70-450
Barren Ground Caribou 150-400
White-tailed Deer 90-400
Pronghorn Antelope 70-150
Coyote 15-50
Raccoon 10-30
Red Fox 8-30
Woodchuck 5-14
European Hare 6-12
Whitetail Jackrabbit 5-10
Striped Skunk 2-5
Snowshoe Hare 3-4
Eastern Cottontail 2-4

Table 4.2 Weights of Some Common North American Mammals

Mammalian weights

The range of mammal body sizes and weights is much greater than among birds, because the vast majority of mammals live terrestrial or aquatic lives and are not restricted by the demands of flight. The North American Pygmy Shrew is the smallest mammal, with a body length of less than 2 inches and a weight of only 0.1 ounce. The largest land mammal is the African Elephant, standing as high as 11 feet and weighing up to 15,000 lbs. The largest animal on the planet is the Blue Whale, 70 to 80 feet in length and weighing up to a staggering 390,000 lbs. Apart from these extremes, the majority of mammals are smaller than the common house cat, and weigh less than a pound.

In North America, hoofed mammals and large carnivores are the largest mammals, and pose significant hazards when they roam onto active runways. Table 4.2 presents the weights of mammal species considered hazardous to aircraft in North America.

Mammalian senses

The degree to which mammals see, hear, smell and taste varies considerably. These variations are directly related to an animal’s environment, way of life and role as either predator or prey.

For example, deer—constantly on the lookout for predators—have far better vision than moles, whose eyes have adapted to life in dark underground tunnels. We are all familiar with the fact that dogs have a much better sense of smell than we do—and hear sounds inaudible to the human ear. Many species have highly developed senses of hearing, smell and vision—unlike birds that have evolved primarily with only a keen sense of vision.

Vision

All mammals—including humans—detect light in the same spectral range. Mammals cannot see ultraviolet or infrared light. To the best of our knowledge, except for humans and other primates, mammals do not recognize colour.

The retina of the human eye is composed predominately of cone cells, responsible for sharpness of vision and detection of colour. In contrast, the retina of the non-human mammalian eye is almost entirely composed of rod cells, which register black, white and grey. While limiting colour detection, these rod cells also afford enhanced night vision—many mammals see as well at night as we do in daylight hours.

The lack of retinal cone cells in non-primate mammals results in poor visual acuity; in response, these animals have adapted to detect motion. Mammals may not detect the presence of a human provided he or she remains motionless; however, non-human mammals can detect the slightest movement—even the blinking of a human eye. Predators such as the wolf and Coyote have a visual acuity similar to humans. They have eyes that face forward, providing binocular vision for depth perception. Most prey mammals have poor vision, but are highly sensitive to the detection of movement. The bulging eyes on the sides of their heads provide primarily monocular vision throughout a range of almost 360 degrees, enabling detection of movement and danger on all sides.

Hearing

Non-primate mammals possess well-developed hearing; their inner ears are similar in both structure and function to the human ear. Human hearing is sensitive to sounds between 40 Hz and 20 kHz. Dogs and other canids—such as the Coyote and wolf— can hear frequencies as high as 30 to 40 kHz. Deer are believed to hear frequencies as high as 30 kHz. Bats, which emit sounds for echolocation of insect prey, can detect frequencies as high as 100 kHz, although it is unknown whether the detection of sound at these high frequencies constitutes hearing as we understand it.

Apart from their ability to perceive sound frequencies beyond our range of hearing, many mammals have external ears proportionately larger than those of humans. Larger ears provide significantly more reflective surface, directing sound waves to the inner ear for collection and detection of the faintest sounds. Mammals also have the ability to move their ears—often independently—to search for and track sounds.

Smell

Humans and birds see the world; other mammals smell it. Of all the senses, smell is the most highly developed in mammals. Their environment is rich with odours, informing them of the presence of danger, food and family. Studies have shown that deer cannot recognize their own fawns by sight—they rely on scent recognition. Though humans can identify hundreds of different odours, we may never appreciate the scope of other mammals’ sense of smell.

In humans, the nose is associated with breathing, but for most mammals its primary function is that of olfaction, or smell. This sense detects minute amounts of chemical particles that trigger responses from chemoreceptors located in the mucus-covered epithelium membranes lining the nasal passageway.

Chemical detection occurs both inside the nose and on the nose surface—outside the nostrils. Chemicals in the air dissolve and are detected by surface receptors on this wet portion of the nose.

The keen sense of smell in mammals is truly amazing. Coyotes and wolves often locate voles under a deep layer of snow by smell alone. Deer and Bighorn Sheep also use their sense of smell to locate food under snow. Large ungulates—such as deer, Moose and Elk—do not possess sharp vision, and often depend on the detection of a predator’s scent to become alerted to danger. Under ideal conditions, bloodhounds can follow a scent that is two-weeks old. Small mammals with poorly developed hearing and vision—such as voles and mice—depend almost entirely on smell to survive in their environments.

Taste

Experiments on mammals indicate they have an acute sense of taste. Like humans, mammals can detect taste only as being either sweet, sour, bitter or salty. The human ability to sense flavours is in fact more dependant on what we smell than what we think we taste. This is probably true for other mammals as well, and that may confound efforts to achieve an appealing balance between taste and smell in the development of chemically altered mammal-deterrent foods—we simply don’t know what tastes good or bad to different species of mammals.

Touch

For mammals, the sense of touch is primarily concentrated in skin not covered by fur—the nose, tongue and pads of the feet. Mammals have tactile sensors located throughout their skin that detect the sensations of warmth, cold, touch, pressure and vibration. Unlike birds, the sense of touch in mammals is important to communication. Tactile stimulation such as licking, nuzzling, grooming and nipping is an important aspect of various social behaviours including mating and nurturing.

Mammalian behaviour

Collectively, mammals show a diverse and complex array of behaviours that vary with the season, time of day, environmental conditions and species.

Periods of activity

The majority of mammals are nocturnal—they are active at night. The presence of tracks and droppings are often the only clues that mammals inhabit an area. Identifying these clues and determining which mammals occupy an airport environment is critical in reducing potential hazards, since more than 60 percent of reported mammal strikes occur at night. Some mammals—including rabbits, hares and deer—are most active during the early morning and evening periods. They spend midday—and night—at rest. Other mammal species such as squirrels and large herbivores are active only during the day.

A number of factors can influence mammals’ activity patterns. For example, the abundance or scarcity of food sources will extend feeding activity beyond its normal periods. In the fall, many mammals increase the time they spend feeding to build up energy reserves for winter. During mating periods, both males and females are often active for prolonged periods. Mammals tend to be less active when weather is unfavourable, although these periods of forced inactivity are often followed by marked increases in activity.

Feeding

Mammals are generally placed into four groups according to their eating habits:

  1. Carnivores (meat)
  2. Herbivores (vegetation)
  3. Insectivores (insects)
  4. Omnivores (generalists having a highly varied diet)

Approximately 80 percent of mammal species are herbivorous, living on leaves, shoots, roots, twigs, buds and seeds. Many mammals are attracted to airport environments by grass fields, and by trees and shrubs often found growing at airfield perimeters. Most herbivores feed on specific types of vegetation, so eliminating or controlling these food sources can be a primary management method. For example, deer activity can be reduced through removal of shrubs and early succession-forest habitat that provide browse. Similarly, grass-management programs that control broad-leaf cover and seed production can reduce small mammal populations.

Carnivores are the second most common group of mammals living in airport environments, and are attracted by the presence of small mammals. The presence of Coyotes and foxes indicates healthy populations of small mammals including voles, mice, rabbits and hares. In such cases, the management of prey populations is often the best means of reducing predator numbers.

Species Number of Individuals
Black Bear 80 sq. miles
Raccoon 1  sq. mile
Wolf 100 to 300 sq. miles
Coyote 50 to 100 sq. miles
Fox 1 to 4 sq. miles
Moose 1 to 2 sq. miles
White-tailed Deer 0.0625 to 0.469 sq. miles
Snowshoe Hare 0.016 sq. miles
Red Squirrel 0.003 to 0.008 sq. miles
Meadow Vole 0.0008 to 0.001 sq. miles

Table 4.3 Home Range Sizes of Some Common North American Mammals

Behaviour that can create aviation hazards

Mammal behaviour that is hazardous to aviation is sub-divided as follows:

  • behaviour that creates direct and indirect threats to aviation, and
  • behaviour that creates other aviation hazards in the airport environment.

Mammalian behaviour that creates direct and indirect aviation hazards

Movements

Mammals do not roam randomly; their daily activities occur within well-defined home ranges and territories. There is great variation in the size of these home ranges, which are key in determining local-population densities. Generally, the home-range size is correlated to species size; larger mammals are more mobile and require greater food resources, so they occupy more territory. Table 4.3 presents typical home-range sizes for groups of mammals sharing similar diets but with varied body size.

Home-range movements vary by species. Many carnivorous species move constantly throughout their home range in search of prey. Other species make local movements between different habitats within their home range, responding to local and seasonal changes in abundance of specific food types, or specific breeding habitat requirements. During breeding season, the search for a mate may extend a male’s typical home range. Many small rodent species are amazingly static animals, moving less than a few hundred yards in the course of their daily activities.

A number of mammals, particularly larger ungulate species such as deer, undertake seasonal migrations. Knowledge of these movements helps wildlife-management personnel reduce the hazards of larger mammals. Caribou, which inhabit the far north, undertake extensive migrations between summer and winter ranges. Some herds travel thousands of miles each spring and fall. Many local White-tailed Deer populations undertake migrations to yard-up in well established and protected areas during deep-snow winters. Depending on local conditions, these movements can cover more than a hundred kilometres. A review of five-year deer-strike data in the U.S. shows 45 percent of all strikes occur in the fall, when many local deer populations are on the move to wintering areas.

Social behaviour

Mammals exhibit complex social behaviour in all aspects of their lives. Studied extensively over the past 30 years, knowledge of this behaviour forms much of the scientific literature on mammals, and provides valuable information for airport wildlife-management personnel—specifically in relation to the way individual mammals associate. Some live in small loose groups; others form well structured herds and packs, or live in highly organized colonies.

The majority of North American rodents live solitary lives within their territories. In contrast, a few species of rodents—marmots, ground squirrels and prairie dogs—are colonial and live communally in large numbers. Colonial rodents often live in dens and burrows, which members of the colony build and defend collectively. A prairiedog town, with its complex network of burrows, tunnels and entrances, can cover several hundred acres and be inhabited by hundreds of individuals grouped into discrete blocks. Both the Columbian and Richardson’s Ground Squirrel live in small colonies of 20 to 30 individuals. The large, undisturbed grass fields of airports are attractive to such colonies. Once established, these colonies can cause a number of problems at an airport, interfering with grass-management programs, chewing and damaging electrical cables, undermining runways and taxiways, and attracting both bird and mammal predators.

Ungulates, such as deer, Elk and Caribou, live in groups and herds varying from three to several hundred animals. The White-tailed Deer and Mule Deer are the most common herding species in most parts of North America. Mule Deer are typically more gregarious than White-tailed Deer. They live in small mixed-age groups of two to 20 individuals throughout the year. White-tailed Deer tend to be solitary throughout much of the summer; however, during the late fall and winter they may form large herds that number into the hundreds. In areas where food resources are limited, protected grass fields and small woodlots at airports can attract large numbers of deer. For example, Chicago’s O’Hare and Toronto’s Lester B. Pearson International airports are located in areas that are highly urbanized; both airports have reported deer herds of as many as 50 animals.

Regardless of their size, deer herds are significant hazards in an airport environment. Controlling this hazard is a delicate balance between passenger-safety and wildlife-conservation concerns. Increasing public awareness of the hazard posed by deer is necessary before effective management measures can be undertaken.

Mammalian behaviour that creates other aviation hazards

Gnawing

Rodents are distinguished by two pairs of specialised, chisel-like incisors used to gnaw and clip vegetation, twigs, bark and seeds. Growing throughout an animal’s life, these teeth require constant use to maintain their sharpness. The front face of the tooth is harder than the back, but wears faster through gnawing, creating a sharp, chisel edge. The need to chew leads many rodents to gnaw instinctively on such hard materials as wood, plastic and even soft metals, and often poses a threat to airfield lighting cables, fixtures and to interiors of buildings and aircraft. For airports, which support large populations of small mammals, damage costs caused by gnawing can be significant.

Burrowing

Digging and burrowing behaviour—common to many mammal species—is a cause for concern in airport environments. Some mammals, such as Coyotes, foxes and wolves, dig and occupy dens solely for the purpose of rearing young. Groundhog, ground-squirrel and prairie-dog burrows provide nesting sites, shelter for sleeping and protection from predators. The den of a single groundhog can have a number of entrances and tunnels; a well-established den site may feature a tunnel system more than 45 feet in length. Ground squirrels excavate complicated multi-entrance burrows that are a maze of galleries, blind passages and chambers. These tunnel systems range between 10 and 60 feet long.

Burrowing activity threatens grass-management programs at airports, interfering with cutting blades and the wheels of cutting machinery. Burrowing can also cause the collapse of runway and taxiway shoulders.

Mammalian behaviour towards aircraft

Many factors can alter a mammal’s behaviour toward aircraft including:

  • mammal species,
  • time of year,
  • weather conditions,
  • age and condition of the mammal, and
  • the mammal’s experience with aircraft and airport environments.

There is little scientific documentation concerning mammal behaviour towards aircraft. Anecdotal information is also lacking; many mammal/aircraft encounters occur at night when pilots are unable to observe fright-flight behaviour, for example.

Mammalian evolutionary and adaptive behaviour to aircraft

Unlike birds, most mammals are wary of human presence. This is particularly true of larger mammals such as deer, bears, wolves and Coyotes. Mammals respond with freeze behaviour when startled by noise or motion, remaining still to limit their own detection as they locate the source of danger. Flight behaviour follows as animals escape by running in straight lines away from perceived threats; they seem to know instinctively that attempting escape before a threat is identified may cause them to blunder into the source of danger itself.

Yet mammals that pose strike threats at airports are not innately afraid of aircraft or vehicles. Mammals adapt to almost any sound or motion, and quickly habituate to aircraft noise and movements. In national parks for example, deer, Moose, Elk and bears frequently forage undisturbed along busy highways and rail-lines, accustomed to the intense activity found there.

Mammalian behavioural responses are unpredictable

The behaviour of mammals toward aircraft is unpredictable; it varies with mammal species and maturity of individual animals.

Lear 60
Photo courtesy Brian Blackley, Troy Messenger

Lear 60 that was destroyed as a result of a deer strike while landing at Troy, Alabama, in January 2001.

Data show deer as the mammal most frequently struck at airports. Given their agility and wariness, their susceptibility seems puzzling, but startled by the noise and caught in landing lights of oncoming aircraft, deer freeze behaviour often spells their doom—before they are able to locate the source of danger and escape, aircraft overtake them. These mammals exhibit a mesmerised behaviour when looking directly into a strong light source at night; they often remain frozen for a long period of time before moving off, perhaps because the glare blinds them to motion behind the light.

Hunters often use tree stands to hunt White-tailed Deer, maintaining that deer do not detect movements more than three metres above the animal’s line of sight. It may be that White-tailed Deer do not typically look upward to scan for danger since few natural predators attack them from above. Yet even in areas prowled by Mountain Lions—which pounce from trees and rock ledges—Mule Deer and Blacktailed Deer are more likely than Whitetails to look upward in search of danger.

The dynamic nature of mammal populations

Most mammal populations remain stable at or near a habitat’s carrying capacity from year to year. Apart from annual population fluctuations—high in late fall, low in early spring—dramatic shifts in mammal numbers are rare. Unlike birds, which are highly mobile and capable of moving quickly in or out of an area, mammals tend to be limited to movements within local areas in which they were born. In addition, competition between similar mammal species results in well-defined territorial boundaries, preventing establishment of new populations outside existing home ranges.

Some species of mammals show cyclic changes in population numbers. For example, the Snowshoe Hare, lemming and some species of voles undergo dramatic oscillations in population densities. These fluctuations follow a cyclic pattern over a span of years—from extreme lows to extreme highs, followed by a population crash, which usually results from either exhaustion of food resources or spread of disease. The difference between population highs and lows can be extreme. For example, studies of the Snowshoe Hare have shown population lows of one individual per square mile, and extreme highs of 3,400 individuals per square mile. Fluctuations in Meadow Vole populations occur over three- to four-year periods; densities rise from 15-45 animals per acre to peak highs of 400 animals per acre. Not surprisingly, predator populations rise and fall with those of their prey; however, predator numbers rarely reach the same dramatic highs and lows.

Other non-cyclic changes in population numbers of mammals can also occur as a direct result of either a sudden abundance or shortage of food. Populations can increase when extended periods of favourable weather lead to abundances of food. Numbers can also rise when animals are attracted to areas providing short-term availability of food. Periodic and extreme shortages of food can result in mass movements of animals; Black Bears in search of food roamed into suburban Ottawa communities during the mid-1990s after the failure of fall berry crops. In areas with significant winter snowfall, two or three consecutive mild winters can produce a dramatic rise in deer numbers, which are controlled primarily through winter mortality; with an abundance of food, fewer die.

Mammals are able to expand their ranges only when predator or competitor pressures are relaxed, or when new habitat becomes available. These changes tend to take place over large geographic areas in which scattered local populations expand gradually. For example, the Coyote expanded its range northward and eastward into Canada beginning in the early 1900s. This slow spread of the Coyote’s range was directly linked to the disappearance of its predator and competitor—the wolf. As a result, the Coyote’s range and numbers continue to grow in Canada. Similarly, deforestation— coupled with diminishing wolf populations—has allowed the White-tailed Deer to increase its presence throughout eastern Canada and the U.S.

Some mammals have recently increased their range and numbers as a direct result of human efforts. Large game species such as the White-tailed Deer, Elk and Moose have been subjects of introduction programs since the early 1900s. Habitat-management programs and the establishment of nature reserves and parks have greatly benefited local game-animal populations. Finally, reduced trapping activities and the elimination of some pest-management programs have drawn many species back to their historic ranges and resulted in resurgence in local-population numbers.

Mammalian adaptations to the human landscape

In North America, the vast majority of mammal species have not adapted well to the increasing presence of humans which, over the last 200 years, has resulted in significant and dramatic reductions in the numbers and distribution of some of the continent’s mammals. It is sometimes difficult to credit the historic accounts of early settlers and explorers that described a coast-to-coast abundance of bears, wolves, Cougars, large game animals and fur-bearers. Displaced and exterminated by human activity, many large mammals occupy present-day ranges that are but a fraction of what they once were. Lumber and agricultural activities eliminated habitats of many species now found only in remote areas and wilderness parks. Many smaller rodents and carnivores considered a direct threat to agricultural interests were subjected to extensive and prolonged extermination programs that greatly reduced their range and population size.

Only a few mammals have proven adaptable in rural, suburban and urban environments— animals such as the park squirrel, country deer, skunk and Raccoon. Three factors have contributed to the success of some mammals in today’s human landscape:

  1. An increase in favourable food resources
  2. An increase in suitable habitats
  3. Population increases and range expansions resulting from relaxed hunting and trapping activities, and an absence of natural predators and competitors.

A number of mammals have benefited from the spread of agriculture, which has increased open country habitat and provided new sources of food. Many crops—such as grains, vegetables and fruits—provide a new and abundant food source for a number of mammals. Pastures and hay fields provide habitats for some small mammals, which were restricted historically in both their abundance and range by the dominance of forests. The rural mosaic of abandoned scrub fields, crop and pasture lands, hedgerows and woodlots provide ideal habitat for a diverse number of species including Coyote, fox, rabbit, hare, Woodchuck, vole and White-tailed Deer. Some mammals such as skunks, Raccoons, bats and squirrels have been particularly successful in exploiting the human landscape, and are now common in suburban and city environments. Some larger mammals such as deer and Coyotes have greatly benefited from removal of natural predators and competitors from the environment. In many parts of Canada and the U.S., deer and Coyotes are now often more abundant in rural and suburban areas than in their natural habitats. As their populations grow, many species that are no longer subjected to population control are now re-establishing themselves in rural and suburban areas.

Feature Benefit Mammal Species
Grain and vegetable crops Direct food source Rabbits, hare, squirrel, deer, Woodchuck, Raccoon
Pasture lands and hay fields Direct food source

Increase in small prey for predators Increase in habitat
Rabbits, hare, ground squirrels, deer, voles

Fox, Coyote, Badger, skunk Voles, mice, moles, rabbit, hare, Badger, Woodchuck, ground squirrels
Mosaic of hedgerows and woodlots Increase in habitat Fox, Coyote, rabbits, Woodchuck Raccoon, skunk, deer
Orchards and berry farms Direct food source

Increase in habitat
Deer, Raccoon, skunk, bear, rabbits, mice, voles Voles, mice, rabbits, Woodchuck, skunk
Landfills and food waste Direct food source Fox, Coyote, bear, skunk, Raccoon, rats, mice
Buildings Shelter Raccoon, skunk, mice, rats, bats, tree squirrels
Old shrub fields Increase in habitat Fox, Coyote, skunk, Raccoon, Woodchuck, deer, rabbit, hare, voles, mice
Reservoirs, ponds, channels and ditches Increased habitat Muskrat, Beaver, Raccoon
Harvested/managed forests Increased food/habitat Deer, Moose, Elk
Conservation/refuge areas Increased habitat Most mammals
Wildlife management, hunting/trapping/control programs Reduce pressure on populations Coyote, fox, Beaver, Muskrat, deer, hare, ground squirrels, Woodchuck, Raccoon

Table 4.4 Features in the Human Environment that are Attractive to Mammals

Though mammals as a group have not exploited the human landscape as successfully as birds, some have clearly benefited—and tend to be among those species most often encountered in the airport environment (Table 4.4).

Mammals that commonly create flight-safety problems

Wildlife-strike data indicates that a number of mammal species have been struck by aircraft in North America. Some, such as deer and Coyote, are directly involved in collisions with aircraft. Others, especially voles, tend to be indirectly involved, attracting predators such as foxes, hawks and owls which may be directly involved in collisions. The following sections present some species directly and indirectly involved in collisions.

Species involved directly in wildlife strikes

Deer

  • Nearly 70 percent of all reported mammal strikes in North America involve deer, making this animal the greatest mammal hazard. More than 40 deer strikes are reported annually in North America—many resulting in significant aircraft damage.
  • Of the two North American species of deer—Mule Deer and White-tailed Deer— involved in mammal strikes, the White-tailed Deer is the greater hazard due in part to its wider distribution.
  • The White-tailed Deer has adapted well to the human landscape. Populations in many rural and suburban areas have increased significantly due to lack of natural predators, absence of hunting and availability of food. In some areas, populations are reaching such high densities that starvation is the primary control factor.
  • Both species of deer show a tendency to migrate as much as 100 miles to winter feeding grounds in herds of varying size.
  • At airports, deer are attracted by broad-leaf vegetation, grasses, and crops— particularly clover and alfalfa; they also browse on shrubs and young trees. Woodlots and forested ravines provide safe cover and resting areas. In suburban settings, airports may be home to concentrations of deer, providing the only source of food and cover.
  • In rural areas, deer are attracted to grain crops, orchards, early-age deciduous woodlots and plantations of spruce and pine that provide ideal winter cover. Farmingarea hedgerows are often used as corridors between feeding and resting areas.

Coyote

  • Coyotes are second only to deer as the most hazardous mammal at North American airports. Between 1992 and 1996, 35 Coyote strikes were reported in the U.S.— 11 percent of all those involving mammals. Coyotes are attracted to airport environments by the availability of small mammals such as voles, rabbits, hares and Woodchucks. Airfields that support Woodchuck and Badger populations also provide denning sites for Coyotes.
  • Though often mistaken for wolves, Coyotes are smaller and have more slender bodies; they resemble medium-sized dogs. Over the past 50 years, this species has expanded its range throughout north-eastern U.S. and eastern Canada.

Extensively damaged Beech 1900
Extensively damaged Beech 1900 as a result of striking a White-tailed Deer at Latrobe, Pennsylvania, in December 1996.

  • The basic social unit includes the mated pair and pups, but in winter they will form packs to hunt larger game such as deer. Coyote packs usually comprise related family members, and include 4 to 10 animals.
  • The Coyote is intelligent and highly suspicious of people. It is an adaptable animal and is one of the few mammal species able to adjust to and thrive in rural and suburban environments. In settled areas, it prefers a landscape of open grasslands, woodlots, ravines and agricultural fields.
  • Coyotes can be active at any time of day, but are primarily nocturnal.
  • Females deliver pups in enlarged dens often originally created by Woodchucks and Badgers. The hunting territory surrounding a den may be as large as 12 miles in diameter. Females return to the same breeding territory each year.

Red Fox

  • Though considered minor hazard, Red Fox are involved in some reported strikes each year in North America. They are attracted to airport environments by availability of voles, rabbits and hares. Red Fox will also feed on garbage.
  • There are five species of fox in North America; the Red Fox has the widest distribution and is by far the most common. The Red Fox is relatively small—its body is not much larger than that of the average house cat.

Deer and Coyote
Deer and Coyote. These are the two most frequently struck mammals in North America.

  • The family is the basic social unit through at least half the year—from mating in early spring until pups disperse in late summer. After this period, animals are solitary until next mating season. Foxes usually modify abandoned Woodchuck burrows to serve as dens, but they will occasionally excavate their own.
  • The Red Fox favours varied habitats in suburban and rural areas. Over the past few hundred years the species has become particularly abundant in rural areas, attracted by a mixture of small woodlots, open fields and hedgerows.
  • The Red Fox is an omnivorous and opportunistic feeder and will eat almost anything it can catch. During late summer and fall, fruits, berries and insects make up the bulk of its diet. In winter, meat is its primary food. Small mammals like voles, Woodchuck, squirrels, Muskrats, rabbits and hares form its principal prey. Red Foxes will also scavenge carrion and feed at garbage dumps.
  • Red Foxes are most active at night but may hunt during the late afternoon and early morning. They may travel up to five miles on a single hunting trip.
  • The average fox density in agricultural areas is approximately two animals per square mile, yet Red Fox population cycles are subject to regular 8- to 10-year fluctuations in which peak densities may reach 25 animals per square mile.

Species indirectly involved

Rabbits and Hares

  • Contrary to popular belief, rabbits and hares are not rodents but belong to the family Leporidae in the order Lagomorpha. Though rabbits resemble rodents (the order Rodentia), there are a number of anatomical differences that separate the two orders.
  • Hares differ from rabbits in their larger body sizes, and longer ears and hind legs. Rabbit young are born naked, blind and helpless, whereas the young of hares are born with body hair, eyes open and the ability to run soon after birth.
  • North America is home to 15 species of rabbit and hare. The most widely distributed and most common at airports include the Snowshoe Hare, Whitetail and Blacktail Jackrabbit and Eastern Cottontail.
  • All species inhabit open fields and meadows and are common in rural landscapes. They are attracted to airfields by an abundance of field weeds and forbs. Crops such as clover and alfalfa are particularly attractive. Fencerows, shrub-covered ravines, ditches and small woodlots around airports provide excellent cover.
  • Rabbits and hares are most active during early evening and morning hours, although some activity occurs at night.
  • All species are extremely prolific breeders, producing three to four litters a year, four to five young per litter. Local populations can suddenly and dramatically increase. Some species—such as the Snowshoe Hare—can undergo dramatic population fluctuations. Densities can change from lows of only a few individuals per square mile to peaks of thousands per square mile in just a few years.
  • Rabbits and hares are a minor hazard. Only a few strikes are reported each year in North America; however, rabbits and hares attract other animals that constitute a greater risk in airport environments—predators such as foxes, Coyotes, hawks, owls and eagles.

Squirrels

  • The squirrel family is one of the largest families within the order Woodchucks, marmots, ground squirrels, prairie dogs and tree squirrels.
  • Of the tree squirrels, the Red Squirrel, Fox Squirrel and Grey Squirrel are the most common and widespread. All species are arboreal and terrestrial and live in a variety of woodland habitats. Nests are typically located in trees but they will also use artificial structures such as poles, towers, buildings and machinery as nesting sites. They eat everything from seeds, nuts and buds to flowers and mushrooms. These species have adapted well to urban and rural environments and can be found in small woodlots, parks, hedgerows, windbreaks, and all kinds of landscape plants.
  • Although there are no documented cases of aircraft collisions with either tree or ground squirrels, these mammals can become indirectly involved by attracting larger predatory birds and mammals to airport environments. Both species can also cause problems at airports by gnawing on cables and wires, and by nesting and storing food in buildings, maintenance equipment and parked aircraft. Ground squirrels’ vast burrow systems can interfere with grass-maintenance operations.
  • There are more than 15 species of ground squirrels, the prairie dog among them. Many species are restricted in their range, found only in parts of one province or state. Most inhabit well-drained open grass plains where they excavate elaborate networks of tunnels and many entrances. They eat leaves, seeds and crop plants.
  • North America is home to five species of marmots, the largest ground-dwelling squirrels. The Woodchuck—or groundhog—is the best known, ranging across Canada and most of the eastern United States. A large Woodchuck can measure two feet in length and weigh 14 pounds. These animals inhabit well-drained fields, pastures and fencerows. Primarily grazers, they eat vegetative parts of grasses, field weeds and young field crops. Dens and burrows are large elaborate structures; piles of earth often form at the entrances.
  • Woodchucks are a minor hazard—only a few strikes are reported each year in North America; however, these animals attract direct-hazard mammals and birds to airport environments. Their burrows significantly inhibit mowing operations, and can lead to the collapse of runway and taxiway shoulders. Woodchucks also gnaw wires, damaging airport communications and lighting systems. Their abandoned burrows provide denning and nesting sites for a variety of other mammal species such as foxes, Coyotes, skunks and Raccoons.

Voles

  • Voles are often mistaken for field mice, but have shorter tails, smaller ears and larger, more robust bodies.
  • More than 20 species of voles live in North America. Many inhabit dense grassy fields where they feed on such plant matter as leaves, stems, roots, fruits, seeds and flowers. The Meadow Vole has the widest distribution and is the species most often found in airports.
  • Voles are rarely seen and are best evidenced by their extensive system of grass tunnels that measure about 1.5 inches in diameter; their grass-ball nests range in size from 6 to 8 inches in diameter.

Woodchuck
Although not often considered a hazardous species, small mammals such as Woodchucks can undermine runways and taxiways with their burrowing activities.

  • Under ideal conditions, Meadow Voles can breed year-round; populations can increase quickly. Local populations cycle over a span of three to four years, peaking at hundreds of animals per acre.
  • Meadow Voles are a primary food source for many predatory species of mammals and birds. Many species of hawks and owls rely on voles for as much as 80 percent of their diets. Voles are also a staple diet of the fox and Coyote.

Beaver and Muskrat

  • Beavers and Muskrats are aquatic mammals, never found far from water. Both species inhabit rivers, lakes, creeks, marshes, swamps and ditches. Though rarely directly involved in collisions with aircraft, they can be an indirect hazard.
  • Through the construction of dams, beavers create lakes, ponds and wetland habitats that attract many species of hazardous wildlife, particularly waterfowl, shorebirds and raptors.
  • Beaver dams can cause flooding of runways and taxiways. Their dams also raise water tables, causing frost heaves beneath runways and taxiways.
  • Muskrats attract predatory mammals and raptors. Although not often considered a hazardous species, small mammals such as Woodchucks can undermine runways and taxiways with their burrowing activities.

beaver
The large areas of standing water that result from beaver activity can create an attraction for hazardous birds such as waterfowl.

  • As a result of tunnelling activities, beavers and Muskrats also cause problems at airports by damaging and undermining the integrity of drainage ditches and the banks of streams and creeks.
  • Due to a decline in demand for fur, both species have shown dramatic population recovery in former ranges. The suburban and urban presence of these species is increasing throughout much of southern Canada and the U.S.
Date modified: