Chapter 3 — Birds — A Primer

Red-tailed Hawk
Illustration: Red-tailed Hawk. Weight: 2.5 lbs.
A common bird at Canadian airports.


Birds are the only animals that have feathers, which evolved from reptilian scales, according to current theory. Birds are also distinguished by features that permit them to fly, such as:

  • forelimbs that take the form of wings,
  • the absence of teeth,
  • the absence of a urinary bladder (reducing weight),
  • a light and well-fused skeleton, and
  • a four-chambered heart and warm blood for the high-metabolic energy demands of flight.

This chapter is a general introduction to birds: their types, numbers, distribution and general behaviour. It will provide you with a working knowledge of bird biology— knowledge that is critical in developing an aviation-industry System Safety Approach—and will help optimize related risk-management strategies. Detailed information is provided on some bird species commonly present at North American airports. For exhaustive research on the matter, we suggest you refer to publications listed in Appendix E.

In this book, we adhere to the custom of capitalizing the first letter of bird species’ common names (e.g. Common Snipe and Herring Gull); species’ group names are not capitalized (e.g. gulls and geese).

Bird classification or taxonomy

Taxonomy is the science of the classification of living and extinct organisms. Knowledge of taxonomy will assist in identifying birds and understanding their behaviour, since most field guides, checklists and books present species not alphabetically but in their taxonomic order. The taxonomic order of birds begins with the most primitive species and ends with what is believed to be the most recently evolved order of birds. To explain,

Classification of Boeing 747-400 and a Common Raven
Figure 3.1 Classification of Boeing 747-400 and a Common Raven

Figure 3.1 compares the taxonomic classification of the Common Raven with that of a Boeing 747-400.

From a taxonomic perspective, the world’s birds belong to the Class Aves, divided into 28 orders representing the major groupings of birds. In North America, all bird species belong to one of 20 bird orders. For example, ducks, geese and swans—which share the same features and body form—are in the order Anseriformes, while woodpeckers are in the order Piciformes. Today, more than half the world’s bird species belong to the order Passeriformes. These are perching birds, represented by more than 60 families including most of the common songbirds.

Diverse and distributed species

The exact number of bird species is a topic of ongoing debate among taxonomists, with estimated totals ranging from 8,700 to 10,000 worldwide. The geographic distribution of these species varies considerably; more occupy habitats near the equator, and numbers decline near the north and south poles. The greatest numbers of species are found in Central and South America, where 3,000 species represent nearly a third of the world’s bird diversity. In comparison, only 750 species are found in North America.

The geographic area inhabited by a bird species is called its breeding range. Most species that nest in northern North America are migrants. After the breeding season they travel to more southerly areas—their winter range—before returning north to breed. Therefore, we can expect different birds to populate our airports at various times throughout the year. These include:

  • migrants—birds that stay temporarily during spring and fall;
  • summer residents—birds that breed and raise their young on airport lands;
  • winter residents—birds that spend only their winters at airports; and
  • residents—birds that are present all year long.
Bird numbers and population density

Bird numbers

Even though a geographic location may support relatively few bird species, the numbers of individual birds may still be high. For example, Canada’s extreme north is home to fewer species than South America, and yet the actual number of birds breeding in the far north is astounding. Many millions of waterfowl and shorebirds migrate northward to breed every year.

The total number of birds in the world has been estimated to be in the order of 100 billion. The following list provides bird-number estimates for selected regions in the Northern Hemisphere:

North America 20 billion
United States 6 billion
Germany 200 million
British Isles 180 million
Finland 64 million

Snow Goose
Snow Goose. Weight: 6 lbs. Numerous serious incidents have resulted from aircraft colliding with Snow Geese during the fall migration in North America.

North American populations include well over 100 million ducks of all species, more than three million Snow Geese, over four million Canada Geese, as well as starlings and blackbirds estimated in the hundreds of millions.

Population density

The population density of birds—or number of birds per unit of area—varies considerably among regions and habitats. In general, greater numbers of bird species are attracted to areas offering more diverse food sources; an abundance of food leads to larger numbers of birds.

The distribution and density of birds also changes with the season. In the Northern Hemisphere, bird numbers peak in the summer—after the breeding season—and contribute to the documented annual increase in reported bird strikes in August.

During the breeding season, the concentrations of birds at nesting colonies can be spectacular. In the far north, Snow Geese breed in colonies of up to 150,000 pairs. Large sea-bird colonies comprised of thousands of nesting birds are found along both the eastern and western seaboards of Canada and the United States. Around the Great Lakes—on small islands only hectares in size—gull colonies have been documented as containing over 40,000 breeding pairs.

During migration, birds of some species funnel to and congregate at key staging areas along the flyways. As a result, relatively small areas can become the temporary home to extremely high concentrations of birds, and many airports located along major bird routes suffer a distinct increase in the bird-strike rate during the fall migration period.

Species Weight (lbs)
Canada Goose 7.3-13.8
Sandhill Crane 5.4-8.2
Bald Eagle 8.0-14.1
Common Loon 7.9-9.9
Snow Goose 5.1-6.6
Great Blue Heron 4.1-6.3
Turkey Vulture 3.3
Mallard 1.6-3.5
Red-tailed Hawk 2.3-2.7
Ring-billed Gull 0.83-1.4
Rock Dove (Pigeon) 0.7-0.9
Killdeer 0.19-0.24
American Robin 0.14-0.23
European Starling 0.17-0.21
Red-winged Blackbird 0.06-.18
Snow Bunting 0.07-0.12
Horned Lark 0.07
House Sparrow 0.5-0.07
Barn Swallow 0.02-0.06

Table 3.1 Weights of Some Common North American Bird Species

Bird weights and densities

Bird weights

The heavier the bird involved in a strike, the greater the potential for serious aircraft damage. For example, an aircraft flying at 250 kts and striking a Canada Goose weighing 15 lbs will be subject to an impact force of approximately 57,000 lbs (see Table 12.1).

Bird sizes cover a considerable range. A tiny hummingbird weighs no more than an ounce, while a large flightless ostrich weighs up to 300 lbs. The vast majority of birds, however, weigh less than a pound. Table 3.1 shows the range of weights for some common bird species in North America.

An examination of the chemistry and physics of flight for the largest bird species— including pelicans, swans and albatrosses—indicates that 30 to 40 lbs is the maximum weight at which birds can achieve flight. Even then, many of these species rely heavily on wind currents and updrafts to provide lift for soaring and gliding.

Large flocks of starlings create significant hazards at many airports
Large flocks of starlings create significant hazards at many airports.
Weight: 0.2 lbs. Density: 0.85 g/cc.

Although emphasis is placed on the management of larger birds at airports, small birds are also hazardous to aircraft. This is particularly true for small flocking species; simultaneous multiple strikes by these birds can equal the impact of one large bird. For example, the jet-engine ingestion of seven European Starlings is equivalent, based on weight, to that of one Ring-billed Gull.

Bird density

Recent investigations into the effects of bird ingestion by jet engines suggest that potential damage may be dictated more by the density of a bird—the ratio of bird weight to its volume—than its weight. For example, the Laughing Gull weighs only a third of the larger Herring Gull, but has a higher density—0.7 g/cc as opposed to 0.602 gm/cm3. Perhaps this is why starlings are often referred to as ‘feathered bullets.’

Bird senses

Birds are equipped with the same sense organs as humans; they generally hear, see, taste, smell and feel in the same range as we do. A number of wildlife-management devices on the market today (see Chapter 8) are said to be effective in targeting bird senses through the use of various chemicals, sounds, vibrations and visual cues. When considering such devices, follow this simple rule of thumb: if you cannot hear, feel, see or taste it, neither can birds.


Birds as a group have one of the most highly developed senses of vision in the animal kingdom. The importance of this sense is best illustrated by the larger size of a bird’s eyes relative to other animal groups. For example, heads of both humans and starlings represent about one-tenth of total body weight. The starling’s eye, however, represents 15 percent of its head weight, compared to less than 1 percent in humans.

The structure of a bird’s eye is similar to that of a human eye. Though birds of prey and those that inhabit open country enjoy vision highly superior to that of humans, studies have shown that for the most part birds have vision that is similar to humans. Birds can see roughly as far as humans can, and are able to see near and far with equal acuity.

There is no evidence to support the theory that some birds can see polarized light, or recognize ultraviolet wavelengths to assist in migration or foraging. Birds also see in the same light-frequency range as we do. They can distinguish various tints and shades of colour.


The sense of hearing in birds is well developed. The inner ear functions in essentially the same way as a human’s. Optimum hearing occurs in the frequency range of 1 to 5 kHz. While it is believed that some bird species can detect low-frequency sound—sometimes referred to as infrasound—most birds cannot hear high-frequency sounds of 10 kHz or above. Large birds, such as waterfowl and birds of prey, cannot hear high-frequencies above 6 or 8 kHz. Songbirds are able to hear in a range of frequencies narrower than those in which a human can hear. Generally, however, if you can’t hear a sound, chances are birds can't either.


For birds, the sense of touch is concentrated primarily in their feet and bills— the areas that are not feathered. Their feet detect feelings of cold, heat and pain. Many birds have a highly developed sense of touch in their bill, which they employ when capturing and manipulating food. Experiments on birds indicate they have an acute sense of taste, but they have fewer taste buds than mammals. Some bird species are insensitive to bitter, sweet, or sour tastes—a fact to consider when choosing chemical deterrents in the airport environment.


Of all senses, smell is generally the least developed in birds, although their detection and discrimination ability varies considerably. Some species have poor abilities, while others have some of the best-documented scent capabilities of any terrestrial vertebrates.

Bird behaviour

Flight characteristics, feeding habits, reproduction, social interaction, migration and predator avoidance are all aspects of bird behaviour, knowledge of which is essential when determining effective wildlife-management techniques.

Food Type Species or Family
Flying Insects Swallows, goatsuckers, flycatchers
Insects in trees
and shrubs
Cuckoos, woodpeckers, jays, chickadees, nuthatches, thrushes, vireos, warblers, blackbirds, tanagers, finches, sparrows
Insects in grass fields
and pond edges
Ducks, geese, rails, plovers, sandpipers, Common Snipes, gulls, American Kestrels, larks, crows, starlings, blackbirds
Worms Gulls, Common Snipes, crows, robins, blackbirds, starlings
Aquatic vegetation/ insects Grebes, ducks, geese, rails
Berries Grouse, pheasants, thrushes, thrashers, waxwings, blackbirds, starlings
Grass Ducks, geese
Fish Herons, cranes, osprey, eagles, terns, gulls, sea birds, kingfishers
Frogs Herons, bitterns, cranes
Mice/voles Cranes, gulls, accipiters, harriers, buteos, owls
Small birds Accipiters, buteos, falcons, owls, turkey, grouse, pheasants, pigeons, doves, finches
Seeds Sparrows, longspurs, Snow Buntings
Crops (corn, grains) Ducks, geese, turkey, grouse, pheasants, pigeons, doves, crows, blackbirds, longspurs, Snow Buntings
Garbage Gulls, crows, ravens, magpies, blackbirds, starlings
Carrion Vultures, eagles, crows, ravens, magpies

Table 3.2 — Food Types and Associated Birds


Birds are attracted by food, so knowing what they eat and controlling food availability is key to successful bird management.

Birds can be grouped into four categories based on their diets and feeding habits. Most birds are insectivores—insect feeders. The second largest group feeds on various parts of plants, including seeds and berries. Carnivores feed on animals like fish, small invertebrates, small birds and mammals. Finally, some species of birds are omnivorous—they thrive on both plants and animals.

Table 3.2 associates these groups of birds with their food sources. Use the table to determine which bird species might be attracted to your airport.

Most bird species eat a limited variety of foods; some consume only one food type such as a specific seed, fish or insect. Certain species such as gulls, crows and some waterfowl are less particular—their diets include a wide range of foods, both plant and animal. Their generalized diet allows these species to take advantage of local and seasonal foods where and when they can be found. For example, many species of gulls can switch quickly from feeding on fish at a waterfront to feeding on worms in recently plowed fields.

Most birds tend to feed individually within a territorial space or home range, from which they exclude others of the same species. When a food supply is abundant, however, birds will concentrate in large numbers and tolerate one another’s presence. Some bird species are referred to as flock feeders; they routinely feed in groups made up of their own kind, or in mixed-species flocks. Among other benefits, flock feeding offers safety in numbers; with more eyes to detect predators, individual birds can spend more time feeding rather than watching for potential threats. Common flock-feeding birds include waterfowl, gulls, blackbirds, starlings, doves and pigeons. Feeding flocks can contain hundreds or thousands of birds—a severe hazard in an airport environment.


Bird reproduction tends to include a typical set of behaviours:

  • establishment of a breeding territory,
  • courtship and mate selection,
  • nest building and egg laying, and
  • brooding and caring for young.

Well over 80 percent of the world’s birds are monogamous, forming a single mated pair during the breeding season. In North America, the vast majority of bird species breed in mated pairs and raise their young in well-established and well-defended territories. Spring is a time of territory establishment and heightened bird activity as males actively defend their territories from all challengers.

As the days grow longer, breeding begins. In North America, the primary breeding season begins in April and is usually completed by the end of July. During this period, many species attempt to rear more than one set of young; some raise as many as three broods. If eggs are lost to predators, most species will re-nest.

A diverse selection of bird species—including herons and egrets, swallows and swifts, gulls, terns and other sea birds—breed in dense colonies. Once a colony is established, adult birds and their adult offspring return to the same colony year after year— colonies that can remain active for hundreds of years.

These dense bird colonies feature individual territories no larger than the space occupied by the nest and the brooding adult. This high-nest density results in enormous congregations, sometimes populated by tens of thousands of birds, as well as colony-growth rates that can be exponential in nature. Colonies can grow from just a few birds to hundreds or thousands in a short time. As an example, a Ring-billed Gull colony near Toronto, Canada, grew from 10 pairs to approximately 80,000 pairs in a span of only ten years. Similar dramatic growth in new colonies has been recorded elsewhere in the world.

Feeding territories are established well away from the colony; the distance birds travel between the two locations can be considerable. Round trips of 10 km or more are not unusual for many colonial species.

Needless to say, nesting colonies near airports create serious hazards. When a colony of Laughing Gulls was established near JFK International Airport in New York City, authorities had to move quickly to implement a comprehensive and aggressive wildlife-management plan that included lethal control.

Bird Behaviour that can create aviation hazards

Bird flight

Daily and migratory flight activities are prime causes of bird strikes, so understanding when, where and how birds fly is one of the key factors in determining exposure to, and probability and severity of, the hazard they create.

Most birds flap their wings to move forward and attain lift. Smaller species fly at moderate speeds between 10 and 20 mph. Larger birds such as waterfowl can maintain flight speeds of more than 40 mph, although high speeds make significant energy demands and are generally avoided. During migration, birds take advantage of tail winds at various altitudes to significantly increase their speeds, sometimes achieving radar-detected ground speeds of more than 60 mph.

Bird-flight altitudes

The majority of day-to-day movements occur between 30 and 300 feet above ground level (AGL). Little regular activity occurs above 1,000 ft AGL, so it’s not surprising that over 80 percent of reported bird strikes occur when aircraft are below that level; the vast majority of strikes are suffered below 300 ft AGL.

One of the highest altitude bird strikes on record involved a Boeing 747 that struck a large bird flying over the West African coast at 37,000 ft above sea level (ASL), but highaltitude bird activity generally occurs only during migration. At that time, birds attain greater heights either to take advantage of winds aloft or to pass over obstacles such as mountain ranges. Migrating Bar-headed Geese have been reported above the summit of Mount Everest, and typically cross the Himalayas at altitudes up to 30,000 ft ASL. A flock of swans migrating from Iceland to Western Europe was reported by a pilot at just over 27,000 ft ASL. Mallards have been reported at 21,000 ft, and Snow Geese have been reported at 20,000 ft. While the altitudes of most migrating birds tend to be much lower, documented average migration altitudes are impressive. Radar observations during peak migration movements in Europe have shown that the majority of migrants flew between 5,000 and 7,000 ft AGL, with a lower limit of 1,600 ft and an upper limit of 11,500 ft.

Bald Eagle
Bald Eagle. Weight: 11 lbs. The high altitude soaring flight of raptors such as eagles and vultures puts them out of reach of most wildlife management techniques.

Soaring and gliding

Other bird-flight behaviours such as gliding, soaring and towering also pose a threat to aircraft. Towering is the slow circling flight that birds engage in as they harness rising parcels of warm air. Towering, soaring and gliding are often used in combination; the bird takes advantage of rising thermals of air—towering to effortlessly gain altitude—and then uses the gained altitude to soar aloft and then glide down. Soaring and gliding flight are energy-efficient behaviours typical of larger bird species—such as condors, vultures, eagles, hawks, storks, gulls and pelicans— that travel long distances as they hunt and migrate.

In bird-hazard assessment, soaring flight is important for a number of reasons:

  • Towering conditions are often found at or near airports. Open and flat, airfieldscontain large expanses of concrete and asphalt which re-radiate stored heat, creatingideal conditions for the development of local thermals. As a result, towering birds—particularly hawks and vultures—often concentrate and circle above airfields.
  • Soaring birds tend to make their daily movements at greater altitudes than otherbirds. During ideal thermal conditions, hunting hawks and vultures can maintainaltitudes greater than 1,000 ft AGL. Soaring also permits these birds to cover morelateral distance, as the activity allows them to save energy. As a result, these speciesrange over a much greater airspace in and around airports—vertically and horizontally—raising their profile as bird-strike hazards and putting them out of the reach of many wildlife-management techniques. Studies of gull movements to andfrom landfills found that flapping-flight movements occur at under 300 ft AGL—while birds are more likely to glide at altitudes over 1,300 ft. Birds save energy bytowering to gain altitude over the landfill before moving off to roosting sites.
  • During the migration period, large concentrations of hawks and vultures congregatein areas such as mountain ranges and coastlines—areas that offer dependablethermals and updrafts. In the late morning—along North American migrationcorridors—boils of hawks and kettles of vultures each containing hundreds andthousands of birds are not uncommon. Under ideal conditions, these birds can ridethermals to altitudes at which they can no longer be seen from the ground.

Daily bird activities

More than 90 percent of all bird species are diurnal; they remain active during the day and sleep at night. Some birds such as owls and nighthawks are nocturnal—primarily active at night. Peak diurnal activity usually occurs in the morning, beginning before sunrise and stretching to approximately 11:00 a.m.

Great Horned Owl
Great Horned Owl. Weight: 3.5 lbs.
The nocturnal activity of birds such as owls results in bird strike rates that can be higher at night than during the day.

Unfortunately, aircraft movements at airports typically increase at these times, and bird-strike data tends to show a sudden increase starting at around 07:00 a.m. But there is also growing evidence to suggest that strikes per 10,000 aircraft movements are in fact higher at night—a reminder that wildlife management must be undertaken around the clock.

Daily bird activity is lowest at midday. There is often a second activity peak in the late afternoon and early evening when birds again move between feeding and roosting sites.

During midday, most birds spend their time resting or loafing, preening and avoiding predators. Some birds—such as gulls and waterfowl—will congregate at loafing sites where, in the safety of numbers, they rest and watch for predators. At many airports, loafing gulls are a common problem. They make regular flights from feeding sites to the airport and spend their idle time in the safe, open expanse of the airfield before moving on to their roosting site in the early evening.

The sleeping and resting activities of birds are referred to collectively as roosting. Most birds sleep throughout the night, choosing to rest alone in sheltered areas such as dense foliage, cavities or tangled undergrowth. Depending on latitude and time of year, sleep periods can vary from four to eight hours. Even when asleep, birds open their eyes every few minutes, alert to potential danger.

Loafing gulls
Loafing gulls. Dramatic increases in some gull populations, resulting largely from poor municipal wastemanagement practices, has created one of the most serious North American flight-safety issues.

Following the breeding season, some birds roost communally. Flights to communal roost sites are either direct or made in stages. In most cases, the same roost and preroost sites are used every year. The number of birds that congregate at roost sites can be enormous. Roosting waterfowl and gulls on inland lakes often number into the thousands. Fall and winter flocks of roosting starlings, crows and blackbirds in the southern United States have been estimated in the millions; the weight of these flocks can be enough to break branches in trees in which they roost.

Daily bird activity is greatly influenced by local weather; short-term forecasting of bird movements based on weather conditions is an important aspect of an effective wildlife-management program. Birds are generally less active during extreme heat or cold, rain or snow, mist or fog. In these conditions, birds significantly limit time spent feeding and moving about. In contrast, bird activity can show a marked increase immediately before and following rain showers. The rain drives insects out of trees, and brings worms and other invertebrates above ground. A burst of feeding activity follows. After a summer rain at many airfields, runways must be cleaned—they become slick with worms and attract hundreds of birds that flock to the sudden surfeit of food. Pools that develop after rainfall also provide much needed water for drinking and bathing.

Wind speed and direction can also affect the daily movements of birds. High winds generally reduce bird movements and flight altitudes as birds hug the ground. Wind Loafing gulls. Dramatic increases in some gull populations, resulting largely from poor municipal wastemanagement practices, has created one of the most serious North American flight-safety issues.

Worms on the runway
Worms on the runway. Large numbers of worms that emerge onto runways after rain events create an attractive food source for birds such as Ring-billed Gulls.

direction can alter the time and direction of daily movements to and from roosts, feeding and loafing sites. Many bird species use different feeding and loafing sites based on local wind conditions.

On the move: bird migratory activity

The vast majority of North American bird species (60 to 80 percent, or more than five billion birds) migrate each fall to the southern U.S., Mexico—and as far as Central and South America—only to make the journey back in the spring. During these migration periods, enormous numbers of birds move across the entire North American continent. There is no question that migration periods (September to October and April to May) are times when there is great risk of a serious bird strike.

Many species of small songbirds migrate slowly on a broad front, moving a few hundred miles every few days. However, the study of waterfowl migration has identified specific flyways through which the majority of the migrating birds pass. In North America, four distinct flyways have been identified. They are the:

  • Atlantic and Pacific coastlines;
  • Mississippi Flyway, wherewaterfowl from the interior of Canada and the U.S. movesouthward along the Mississippi River Valley; and
  • Central Flyway, where birds from the western interior of Canada and the U.S.follow the central prairie regions along the Rocky Mountain foothills.

Important resting and feeding sites are found along these flyways, and are home toconcentrations of thousands of waterfowl.

Seasonal changes in day length cause birds to prepare for migration instinctively. The onset of favourable weather conditions is usually the trigger that sets birds on their way north. Each year, a species' arrival at and departure from a specific geographical area can occur within the same week of the same month. This predictability is crucial; knowing when high concentrations of birds will migrate allows ATS providers, wildlife-management staff and pilots to prepare for the increased numbers.

Depending on the species and type of navigation method used, migration flights occur day and night. Many species migrate at both times; however, soaring species that depend on air thermals are restricted to daytime migration. Other species use a cruise-climb strategy to gain altitude during long flights, flying higher as their fuel load decreases. The ground speed of birds varies with speed and direction of winds aloft, as well as the species' airspeed capability. Most songbirds travel at ground speeds of 20 to 30 mph. Shorebirds and waterfowl travel at 30 to 50 mph.

Weather plays a significant role in bird migration. Birds do not generally travel through rain, fog, mist, high winds or heavily overcast skies. At these times, birds remain downed until conditions improve. The best conditions for fall migration are created by strong tail winds behind cold fronts where birds will move along flyways en masse. These waves can be predicted by weather conditions, offering the opportunity to forecast bird movements over airports located within migration corridors.

Bird behaviour towards aircraft

Are birds naturally afraid of aircraft? The answer to that seemingly simple question is complex. Many factors can alter a bird’s behaviour toward aircraft including:

  • bird species,
  • time of year,
  • weather conditions,
  • age and condition of the bird, and
  • the bird’s experience with aircraft and an airport environment.

To date, few scientific investigations have been undertaken to study bird reactions to aircraft. Most current information is anecdotal and has been reported by pilots and airfield staff, although research has recently been initiated to acquire a better understanding.

Evolutionary and adaptive behaviour

Through natural evolution, birds have learned to respond quickly to animals that prey upon them. Birds are genetically programmed to avoid and escape predators. Since birds have not evolved with aircraft as predators, they are not naturally programmed to be frightened by or avoid them. If aircraft did cause alarm, bird strikes would be much less of a problem.

Birds are naturally cautious of new or unfamiliar objects in their environment but, as long as these objects do not cause harm, birds also quickly habituate to them. Evidence suggests that airport birds have adapted to their surroundings, learning that aircraft are not a threat. The sight of birds feeding and loafing along busy runways— apparently oblivious to noise and movement—is a familiar one. Many involved in airport-wildlife management believe these smart airport birds are not a hazard as these animals have learned to stay out of the way. However, there is little—if any— documented evidence to suggest that this is true.

Bird behavioural responses are unpredictable

When considering bird behaviour toward aircraft, the most important thing to remember is that it is unpredictable. Bird behaviour varies with individual species, maturity of individual birds and any threats facing the animals. Generally, birds that feed and loaf on airfields either ignore aircraft threats, avoid busy runways in advance of approaching aircraft, or respond with fright or panic flight as aircraft approach.

Young and migrating birds unfamiliar with airport environments seem more prone to panic flight. Adults of the same species may completely ignore aircraft. In panic flight, starlings and shorebirds form dense flocks, then undertake erratic movements that result in thick and extremely hazardous congregations of birds crossing paths of arriving and departing aircraft.

The response of birds in flight is also highly unpredictable and varies greatly by species. Typically, birds undertake simple manoeuvres to escape the path of aircraft. Gulls often attempt to out-fly aircraft rather than move away at right angles to aircraft flight paths. Hawks and eagles will occasionally attack aircraft rather than avoid them.

Gulls on runway
Gulls on runway. The biomass contained in a small flock of gulls is more than any currently operating jet engine is designed to withstand.

While straight and level aircraft flights are relatively easy for birds to anticipate, avoiding aircraft in climb-descent or turns seems to be more difficult. Recent studies indicate some birds view aircraft as immobile objects, much like trees or buildings, and attempt to turn slowly away from the threat at a perceived safe distance.

When encountering aircraft, a number of birds free-fall, folding their wings and diving. This behaviour has been documented to be common among a number of waterfowl species, but has also been noted in others. This free-fall behaviour has led the U.S. Air Force to evaluate a manoeuvre in which pilots undertake a steep climb when directly confronted with birds. It is a manoeuvre that may put the aircraft in a marginal flight profile should it strike them, can easily lead to an over-stressed airframe and, most importantly and perhaps unadvisedly, assumes consistent behaviour by the birds.

Remember: the behaviour of birds toward aircraft is highly unpredictable. Additionally, modern aircraft are more vulnerable due to higher flight speeds and larger engine inlets.. Combine these factors with significant reductions in aircraft noise levels, and it’s easy to understand that birds often have little time to react. Therefore, to reduce bird-strike risks, efforts must focus on limiting the numbers of birds in and around airfields.

The dynamic nature of bird populations

In any local area or region, bird numbers and species diversity can vary considerably. Some of these variations are natural and occur annually during migration or following the breeding season. Other changes are less obvious and are often related to gradual habitat changes, to which bird species respond by altering their range or local distribution. At other times, birds respond quickly to environmental changes. These rapid changes may come when birds respond to either sudden changes in important resources—such as food and nesting habitats—or to changes in predator and competitor populations. The dynamic and adaptive nature of bird populations demands that airport wildlife-management programs be equally responsive.

Urban Geese
Urban Geese. Weight: 15 lbs.
FAA/USDA data show that damage occurs to 64% of jet engiunes struck by geese. The FAA estimates that if current trends continue, the probability of a major goose-strike incident resulting in uncontrolled fire or loss of two or more engines will double in the next 10 years.

Landscape alteration can change the distribution, diversity and abundance of birds. In general, bird species that inhabit mature forests, wetlands and riparian habitat are now less abundant. Species that rely on open fields, scrubland and young forests are more numerous than in the past. Over the past 50 years, urban sprawl has created artificial habitats to which a number of bird species have adapted. Many of these urban species—including crows, starlings, pigeons, Canada Geese and, more recently, gulls—have shown significant increases in their populations at both local and regional levels. Species such as Canada Geese and blackbirds have adapted to feed on agriculture crops. Gull populations have boomed with the increase in both the size and number of landfills and other waste-disposal facilities in North America and Europe. Landfills have also changed wintering ranges for these species, providing reliable year-round food sources.

Direct human intervention—through species introductions and conservation efforts—has also led to increases in both the number and distribution of some bird species in North America. More than 200 species of birds have been introduced to various regions of North America with varying degrees of success. Two species—the House Sparrow and European Starling—are examples of highly successful introductions. Common and widespread, the Rock Dove is a hybrid of several feral domestic European pigeons that were released or escaped into the wild during the 1600s. More recently, the relocation of nuisance Canada Geese has become an issue. Nuisance birds captured in urban areas are released in areas still willing to accept the birds. This is of great concern when these relocated birds take refuge in proximity to airports, as small populations quickly increase and become flight-safety hazards.

Finally, in response to the loss of natural habitats—particularly wetlands—many governments and non-governmental agencies have initiated conservation efforts. Over the past 30 years, millions of acres of wetlands have been created across North America. Marsh-habitat areas that traditionally supported low numbers of waterfowl can be easily enhanced; increasing waterfowl numbers often result, rapidly propelling bird populations into the hundreds and thousands. The establishment of reserves and conservation areas can also result in a sudden rise in bird numbers. Birds are quick to find areas that provide reliable food, shelter and protection.

Readily adaptable to human environments

Though it’s widely accepted that human activities—such as the destruction of natural habitats—have a significant negative impact on birds, many species have proven

Agricultural Food Sources
Grain crops, vegetable crops
Plowing and harvesting activities
Crop storage and transfer areas
Feed lots, manure piles
Orchards and vineyards

Other Food Sources
Landfills and waste transfer stations
Food waste composting sites
Abattoirs and fish processing plants
Sewage treatment lagoons
Fast food restaurants, malls,
Fairgrounds, parks and sport facilities
Golf courses
Breeding Sites
Fence and hedgerows
Buildings, including roof tops
Poles/lighting structures/hydro towers
Quarries and borrow pits
Reservoirs and stormwater ponds Landfills Human made lakes/wetlands/impoundments Conservation areas/refuges/sanctuaries Airports

Loafing/Roosting Sites
Parks, parking lots, sport fields
Buildings ledges, roof tops
Reservoirs/stormwater ponds
Human-made lakes
Golf courses

Table 3.3 Features that are Attractive to Birds in the Human Environment

adaptable to human environments. Their presence has increased in rural, suburban and urban areas where birds find new feeding and nesting opportunities, as well as safety from predators. Table 3.3 presents some of the environments to which birds have successfully adapted.

Several common factors help identify species that are successful in exploiting human environments:

  • Successful species are often generalists in regard to food requirements. Birds such asgulls, crows, House Sparrows, blackbirds and starlings feed on a variety of fooditems and delight in the smorgasborg provided by agriculture crops, restaurantwaste and landfills.
  • Flocking birds are known to take advantage of food availability that results from farmingactivities such as spring plowing and planting, summer haying, and fall harvest.
  • Human activities have increased the availability of nesting habitat for some speciesof birds, providing a variety of locations suitable for nesting and rearing young.
Weight (lbs)
Great Black-backed Gull
Glaucous Gull
Herring Gull
Ring-billed Gull
Iceland Gull

Table 3.4 Weights of Some North American Gull Species

These locations include buildings, bridges, quarries, sewage lagoons, storm-water ponds, piers, hedgerows, parks, malls and golf courses. It’s not surprising that airports have become prime bird locations—they offer abundant food sources, water for drinking and bathing, safe open space for both feeding and resting, perching opportunities and a wide variety of nesting opportunities.

Bird species that commonly create flight-safety problems

Bird-strike data from around the world indicates that hundreds of different species of birds have been struck by aircraft. Nevertheless, a review of strike data from airports across North America consistently indicates that some species are more likely to be struck.


  • There are 45 gull species worldwide, including 23 in North America. Weights of somegull species are provided in Table 3.4. Males are on average heavier than females.
  • Gulls are a common problem at many airports in North America and Europe.Where reported strikes identified the species, more than one third involved gulls.
  • Factors that contribute to the gull problem include size, flocking behaviour,relatively slow flying speed and their preference for airport environments as bothfeeding and loafing sites.
  • Gulls adapt to the human environment exceptionally well. They are quick to adjustto new food sources such as those found in agricultural sites, feed lots and landfills.Gulls also take advantage of urban food sources provided by garbage cans, fast-foodrestaurants and malls. The use of rooftops as gull-breeding and roosting sites appearsto be spreading in heavily urbanized areas—especially in the Great Lakes area ofNorth America.
  • Gulls are food generalists. They eat a wide variety of both plant and animal foods,which are readily available at airports.
  • Several North American gull species have increased steadily in number over the past50 years, and the increase in gull numbers at landfills—particularly during thewinter—is well documented.
Species Weight (lbs)
American White Pelican 9.9-30
Mute Swan 3.2-16.5
Tundra Swan 14-21
Canada Goose (the "maxima" race)* 11.0-16+
Canada Goose (the "interior" race)* 6.8-10.4
Canada Goose (the "Canadensis" race)* 7.3-13.8
Snow Goose 5.1-6.6
Brant 1.9-4.0
American Black Duck 1.6-3.5
Mallard 1.2-3.8
Northern Pintail 1.3-2.4
Gadwall 1.4-2.3
American Widgeon 1.1-2.3

* There are several subspecies of Canada Goose; each has its own name.

Table 3.5 Weights of Some North American Waterfowl Species

  • Movements among feeding, roosting and loafing sites follow well-established andpredictable flight lines. Gulls consider airport environments relatively safe, andchoose them as loafing or pre-roost sites. Gull flights typically occur below 300 ftAGL, yet towering behaviour is common over feeding sites. Feeding flights of over30 km are not uncommon.
  • Gulls usually breed in densely packed colonies that may comprise many thousandsof birds.
  • Most gull species are migratory. In Canada, southern migration begins in lateSeptember and October and is usually completed by November. Spring migrationbegins in March and is usually well underway by April. Adult birds typically arriveat the breeding colonies by the end of April.


  • Nearly 160 species of waterfowl are found worldwide; North America is home to62 species.
  • As a group, waterfowl are among the world’s largest flying birds (see Table 3.5).
  • Large size combined with flocking and migration behaviour make waterfowlparticularly hazardous to aircraft operations.

Mallard Ducks
Mallard Ducks. Weight: 3 lbs. Many serious incidents have resulted from strikes with ducks that frequent ponds and grain stubble near airports.

  • Waterfowl are attracted to common airport features such as ponds, wetlands,ditches and grass fields. Temporary ponding of water from spring snow melt andprolonged rains also attracts these birds.
  • After feeding, flocks of geese and ducks commonly loaf within the confines of an airfield.
  • During spring and fall migration, many thousands of waterfowl concentrate infarm fields, wetlands and nature reserves that may be located near airports, creatinga significant hazard to aircraft operations.
  • Recently, the numbers of some waterfowl species—Mallards and Canada Geese inparticular—have increased substantially in the U.S. and Canada. Between 1985and 1997, the North American Canada Goose population increased by 54 percent,and is now estimated at over two million birds.
  • Their extended winter range into the North has been attributed to their adaptabilityto changing food sources and nesting locations—behavioural changes that haveresulted in ever-increasing, year-round resident populations at many airports.
  • Relocation programs have contributed to rising waterfowl populations, as these animalsare moved to locales that offer fresh food sources and relative safety from predators.

Doves and Pigeons

  • North America is home to 16 species of doves and pigeons. The majority live in thesouthern U.S. and Mexico. The Domestic Pigeon (Rock Dove) and MourningDove range widely throughout most of the U.S. and southern Canada.
Species Weight (lbs)
Rock Dove (Pigeon) 0.7-0.9
Ringed Turtle-Dove 0.3-0.4
Mourning Dove 0.2-0.4

Table 3.6 Weights of Some North American Dove Species

  • Doves and pigeons are medium-sized birds (see Table 3.6) that frequent open areas;all species of this group have adapted well to rural and urban environments.
  • All dove and pigeon species feed on grains, small seeds and fruits. Gregarious innature, they feed and roost in flocks of varying size depending on time of year andlocation.
  • The Rock Dove is commonly found in urban areas where it frequents roofs, ledges,bridges and parking garages. Like other dove species, the Rock Dove commonlyfeeds on seeds found in farm fields, feed lots, bird feeders, grain elevators, flourmills, railway yards and docks. During the fall and winter period, large feedingflocks gather at abundant food sources—particularly following harvests of cerealcrops and grains. In cities, the Rock Dove also feeds on a variety of food itemsincluding bread, nuts, fruits, chips and French fries.
  • Doves and pigeons require grit (particles of small gravel and sand) to aid in thedigestion of seeds and grains.
  • Airports attract doves and pigeons with food sources, water and grit found alongroads, taxiways and runways—particularly during snow removal. Airfields andassociated buildings and structures provide loafing and roosting areas.
  • Buildings, hangars and parking garages provide nesting sites for Rock Doves, whichoften nest in close proximity to other breeding pairs once ideal nesting habitats are

Dove and Pigeon
Dove and Pigeon.

Species Weight (lbs)
Bald Eagle 9.1-11.8
Gyrfalcon 2.1-4.4
Turkey Vulture 2.5-3.5
Red-tailed Hawk 2.3-2.7
Rough-legged Hawk 1.7-2.7
Peregrine Falcon 1.4-2.1
Northern Harrier 0.65-1.66
Broad-winged Hawk 0.93-1.1
American Kestrel 0.24-0.26

Table 3.7 Weights of Some North American Raptor Species

found. These birds can produce several broods during spring and summer breeding periods. The species can also breed during the winter period when a reliable food supply is available. Under ideal breeding conditions, Rock Dove populations can grow dramatically; a few pairs can produce hundreds of birds in a very short period of time. (See Appendix 3.1 for information on diseases associated with some common urban birds such as Rock Doves.)


  • Raptors are diurnal birds of prey that are found worldwide. There are six familiesof raptors inhabiting North America:
    • vultures (3 species),
    • hawks (15 species),
    • eagles (2 species),
    • kites (5 species),
    • falcons (6 species), and
    • harriers (1 species).
  • Raptor species commonly found in and around North American airports includethe Turkey Vulture, Red-tailed Hawk, Swainson’s Hawk, Northern Harrier, BaldEagle and American Kestrel, as well as the Rough-legged Hawk during the winter.
  • Most species are attracted to open grass fields at airports—home to small mammalssuch as voles, ground squirrels, gophers, rabbits and hares. The abundance ofprominent perching sites also makes airfields attractive. Flat open expanses providegood conditions for development of strong local thermals that enable soaring flight.
  • Their large size (see Table 3.7), and high-altitude soaring and towering flight makethese birds extremely hazardous to aircraft.

American Kestrel
American Kestrel. Weight: 0.25 lbs.

  • During migration, raptors prefer mountain ranges and coastlines where warm-airthermals and wind updrafts can be found. Migrating waves of raptors comprisingthousands of birds can be encountered during ideal soaring and towering conditions.
  • Normally, raptors are specific in their food requirements, preferring small mammalsand birds. Smaller species will also feed on insects and shellfish. Vultures and eaglesare commonly attracted to landfills because of their scavenging behaviour, andfalcons are particularly attracted to areas that support populations of shorebirds.
  • During the breeding season, individual pairs require large feeding territories and aretherefore well spaced throughout their breeding range.
  • With a few exceptions, both sexes have similar plumage, but immature birds can bestrikingly different in colour.
  • Some species have shown population declines over the past few decades and areprotected under various wildlife regulations. As a result, specific identification iscritical when raptor management is required.
  • Turkey Vulture numbers have increased in recent years, and their habitat range nowextends into southern Canada. The United States Air Force believes the greatestbird-strike risks to their aircraft are posed by Turkey Vultures.
Species Weight (lbs)
Eastern Meadowlark 0.16-0.23
Common Grackle 0.22-0.28
European Starling 0.17-0.21
Yellow-headed Blackbird 0.09-0.19
Red-winged Blackbird 0.06-0.18
Rusty Blackbird 0.18-1.0
Brown-headed Cowbird 0.07-0.13

Table 3.8 Weights of Some North American “Blackbird” Species

Starlings and Blackbirds

  • In North America, the name blackbird is often misused. It properly refers to a diversenumber of species including:
    • Eastern and Western Meadowlarks,
    • Common Grackles,
    • Red-winged Blackbirds,
    • Yellow-headed Blackbirds,
    • Brewer’s and Rusty Blackbirds,
    • Brown-headed Cowbirds, and sometimes
    • European Starlings and American Crows.
  • Species commonly found in airport environments include Red-winged Blackbirds,Yellow-headed Blackbirds, Common Grackles, Brown-headed Cowbirds andEuropean Starlings. As shown in Table 3.8, they are generally small in size, usuallyweighing less than 0.25 pounds.
  • These species are significant hazards due to their flock-feeding and roostingbehaviour. Based on a five-year average (1992 to 96) of reported bird strikes in theU.S., blackbirds, including starlings, were involved in 13 percent of all strikes—second only to gulls.
  • All species in this group are attracted to short-grass and cash-crop fields, pastures,barn yards, feed lots, grain-storage bins and feed-transfer facilities. They eat insects,grains, seeds and fruits.
  • This species is attracted to standing open water and wetland habitats commonlyfound at airports. The starling often nests in holes and cavities in airport buildings.
Species Weight (lbs)
Snow Bunting 0.07-0.12
Lapland Longspur 0.05-0.07
Horned Lark 0.07

Table 3.9 Weights of Some North American Smaller Song Bird Species

  • After the breeding season, these birds form large pre-migratory flocks that caninclude thousands of individual birds. During migration—and throughout theirwinter range—large concentrations of birds feed in grain fields. The number of birdsthat congregate in late-summer, fall and winter roosts can number into the millions.
  • Starlings and blackbirds have adapted well to rural and urban human environments.Many species have extended their winter ranges by taking advantage of abundantagricultural crops throughout North America. Some species—especially starlings—have adapted to feeding at landfills, dumps and compost facilities.

Horned Larks, Snow Buntings and Lapland Longspurs

  • These small (see Table 3.9), sparrow-like birds inhabit open grasslands and fields.In North America, they breed in northern Canada and Alaska.
  • During the winter, large mixed-species flocks gather throughout the U.S. andsouthern Canada. Lapland Longspurs and Snow Buntings are the most widespread;their flocks can be made up of thousands of birds.
  • During the winter, these species inhabit open fields where they feed on seeds and dryfruits. They favour fields where vegetation and seed heads are exposed above the snow,as well as plowed spaces and fields recently spread with manure. Large flocks willreturn repeatedly to the same fields as long as a good food supply remains available.

Horned Lark and Snow Buntings
Horned Lark and Snow Buntings.

  • Airfields are popular with these species because of seed availability. When fields arecovered with deep snow, the bare grass edges of plowed runways are particularlyattractive, providing a source of seeds and grit.
  • The tendency of these species to move unpredictably as highly synchronized, denseflocks makes them potentially hazardous to aircraft operations.
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