A Field Guide to Airfield Birds of North America

UNITED STATES

08.01.2025

Courtesy Story

Naval Safety Command

We’ve all been there. You’re on the final descent, the downwind or smoothly rotating away on takeoff and the sudden “THUD”, “POP” or “POOF” of feathers ricochets off your windscreen. You’ve established aircraft control, noted no damage to the aircraft and comply with your local standard operating procedures. It's time to go home for a full stop. “BASH was low!” your crew exclaims. “It came out of nowhere!” Agreeing, you’re already doing the mental gymnastics of how much paperwork you will need to do for your aviation safety officer once established in cruise.

What really is BASH? It stands for Bird (wildlife) Aircraft Strike Hazard and is a preventive and program management entity with origins and execution in the U.S. Air Force. Notably, BASH itself is not the scale of how many birds are currently present in one area. So what is? Does it even exist?

Most aviators use the Aviation Hazard Advisory System (AHAS) during preflight to check what they believe is avian activity in the local airfield or route locations. Crews will then read the AHAS Risk (Low, Moderate, Severe) and conduct crew risk management (CRM) to determine if the risk can be mitigated. So then how is AHAS risk calculated?

AHAS risk is based on next generation radar (NEXRAD or WSR-88D), Next Generation Bird Avoidance Model (NEXBAM) and the Soaring Bird Forecast Model (SOAR). NEXRAD is a weather radar system with models to predict bird movement every six minutes. NEXBAM is a bird avoidance model showing hourly trends based on five years of NEXRAD data. Finally, SOAR is based on the latest local weather data and the known populations of five different soaring birds every 12 hours. If NEXRAD data is not available, NEXBAM will be used only in conjunction with SOAR as the driving model. Additionally, if SOAR data is missing and the NEXRAD risk is low, the AHAS risk will be based on NEXBAM.

Just as every aviator should be an amateur meteorologist, every aviator should be an amateur ornithologist — if not only to help decipher the acronym alphabet soup from the AHAS Risk explanation above. Understanding basic avian ecology can be an additional safety tool to help mitigate your risk of bird strikes.

Birds, from the class Aves, comprise over 11,000 individual species, totaling over 50 billion worldwide. This is an astronomical number compared to Flight Aware’s average of 12,000-14,000 aircraft in the sky at one time. With the total percentage of flightless birds being .54%, we certainly share the skies with many lofty feathered friends.

To start, let's break down the day and night cycle of birds. Almost all birds are diurnal, meaning most active during the day. Some birds, like the common nighthawk and the chimney swift, are only active during twilight hours, both dawn and dusk. Finally, nearly all Strigiformes members — owls, are nocturnal. Nocturnal animals are active during the night and rest during the day.

Most birds collect food by foraging. It is rare for a bird to forage alone. Groups of birds foraging together are called foraging guilds. This concentration of sometimes hundreds of avians is most active during the early morning and evening hours. This foraging tends to lull during the middle of the day when the sun is at its highest. This is in conjunction with the diurnal habits of most birds.

Airfields are prime foraging locations for both diurnal and nocturnal bird species because large areas of expansive, open vegetation are ideal for insectivores. Seabirds, such as gulls, will forage together on the ground after extensive rainfall on airfields as they have easy access to invertebrates that come to the surface. With large numbers of insects come members of the family Rodentia (rodents). Herons and raptors, although solitary hunters, will frequent airfields to eat rodents. But are these avians on your airfield every day throughout the year? Not always. That’s where studying the concept of migration is essential.

Migratory animals travel from areas of low resources to areas of higher resources to best fit their ecological and productive needs. Most birds in the northern hemisphere move northward during the spring to take advantage of the new resources after a harsh and cold winter. Those birds will move south again once fall arrives. But the distance birds fly during migration isn’t equal across the species board.

The Cornell Lab of Ornithology explains all birds are split into four types of migration based on distance traveled. Avians are split into permanent residents and short-, medium and long-distance migrants. These distances range from changing sides of a mountain to traveling continents. Additionally, North America is split vertically into four major flyways for the medium and long-distance migrants. Think of these as the “avian superhighways” of the sky. This is where bird strikes can become the deadliest.

According to the Federal Aviation Administration’s (FAA) published Wildlife Strikes to Civil Aircraft in the United States 1990-2023, both raptors and waterfowl, with the addition of pelicans, had the highest correlation of human fatalities after a bird strike. They additionally had the highest number of human injuries after a bird strike. High periods of bird movement correlate with high numbers of bird strikes.

Waterfowl, such as snow geese and Canada geese, migrate south deep into the United States in the winter to forage in spacious fields of alfalfa, wheat and clover crop. In addition to their normal diet of insects, these birds will eat germinating vegetation. While foraging in large guilds of several hundreds to thousands, some flocks will break off into smaller entities to “hop” fields to find more natural resources. Airfields are a perfect alternative to supplement avian hunger and are frequently visited by migratory birds.

Returning to our definition of AHAS Risk, NEXRAD, while primarily a Doppler weather radar, works by targeting and identifying migratory birds, bats and sometimes insects, populating meteorology screens as “clutter”. There are 159 NEXRAD stations per the National Oceanic and Atmospheric Administration. Each station, varying in capacity, can collect clutters of birds up to 140-290 miles away. This data is not necessarily a real-time warning of individual birds at individual airfields, but a focus on flocks. Large flocks of birds may appear as curved crescents on NEXRAD screens and are like those of clouds. Operators are unable to tell species of bird from NEXRAD readings alone.

According to the FAA’s published Wildlife Strikes to Civil Aircraft in the United States 1990-2023, 80% of the top 10 species of bird strikes are permanent residence and short-distance migrant bird species. The outlier species of birds are both from the swallow family; the barn swallow and cliff swallow, both of whom are long distance migrants. Swallows create cup-shaped nests high above the ground using grass and mud on both vertical and horizontal surfaces, as well as inverted corners. They habitually choose aircraft hangars due to high overhangs and protection from predators and natural elements. Therefore, swallows are a nuisance to many airports and airfields.

Swallows are also one of the many species protected by the Migratory Bird Treaty Act of 1918. This federal law prohibits (without a waiver) hunting, taking, capturing, killing or selling those species listed as migratory birds. This law does not differentiate between live or dead birds and includes bird parts such as feathers, eggs and nests. This law was updated in 2004 with the Migratory Bird Treaty Reform Act delineating this protection as only for native migratory bird species whose occurrence in the United States results from natural biological or ecological conditions. In 2023, the top 10 species of birds most likely to be involved in bird strikes are protected by the Migratory Bird Treaty Act (except one), despite in most areas being permanent residence or short-distance migrant species. This also includes the more deadly bird species upon bird strike, such raptors and geese.

The one unprotected outlier in the top 10 likely bird strikes is the European Starling, an invasive species to North America since the 1870s. European Starlings create massive flocks, also called murmurations, and are the most common at the end of the day to return to their nightly roosting sites. Murmurations have been recorded to range in size from a few hundred to millions of birds. Yes, you read that right, millions. The highest recorded murmuration of European Starlings was estimated to be over 6 million individual birds.

In Europe, avian murmurations are relayed as Bird (Density) Notams (BIRDTAMs). BIRDTAMs, while not an official International Civil Aviation Organization term, is used throughout Northern Europe and is recognized by the European Aeronautical Information System Database and as its own Aeronautical Fixed Telecommunication Network address. BIRDTAMs originated in the military services to provide information for bird strike risks. In conjunction with radar activity and predicted bird movement, BIRDTAMs are the only bird strike prediction service including real time, in-person human observation. Specific countries participating in the creation of BIRDTAMs and have airspace represented in prediction models include Belgium, the Netherlands, Germany, Denmark, Sweden and Poland.

Returning to our AHAS risk definition and re-visiting the SOAR Model, these models are based on the latest local weather data and the known populations of five different soaring birds over a 12-month period. Not much is published online other than the above definition on how this model is calculated. So what can we as amateur ornithologists do to help mitigate risk on short notice?

A soaring bird is defined as an avian using the rising air currents to maintain flight; they rarely flap their wings. These avians use columns of heat from the ground, called thermals, to help generate lift. This list includes birds of prey, passerines (ravens), cranes, storks and seabirds. Note that many birds on this list are solitary or paired hunters, many only congregating during mating season. Seabirds are the exception and can be guild foragers. Thankfully, many of these birds range in size (wingspan) from two feet to eight feet. This makes them somewhat easier to recognize and “see and avoid” tactics can be established.

What is not established in the SOAR definition is what populations of which five soaring birds are reported. We can research what birds we are most likely to see and mitigate risks based on the current weather, our new bird knowledge and the time of day and month. Let's use Naval Air Station (NAS) Whidbey Island, Washington, with conditions similar to a clear morning in April with little precipitation during the previous week as our example.

To begin, identify what free bird resources you have quickly at your disposal. Cornell Lab of Ornithology, Ebird, Audubon Societies and ornithology nonprofits can publish checklists for local birders to enjoy. For Whidbey Island I can see from a published checklist my Soar Birds in April can include the Mew Gull, GWxWestern Gull (hybrid), Great Blue Heron, Osprey, Northern Harrier, Bald Eagle, Red-tailed Hawk, Great Horned Owl and the Raven. Additionally, all of our migratory waterfowl have returned home to Canada since it is springtime.

It is morning, so our local birds are out foraging. We can almost guarantee to have Gull activity near the water to find food due to the proximity of NAS Whidbey Island to the Pacific Ocean. The Gulls would be more concentrated near the water, as they cannot forage on the ground due to the lack of precipitation. The Great Horned Owl is nocturnal so we don’t expect to see it in the morning. Ravens tend to live in wooded areas, not open fields, so is another avian nonplayer. The base itself does not have superior wetlands near the runways so the Great Blue Heron may be an infrequent guest. This leaves our four birds of prey remaining; each with a history of bird strikes.

On taxi you could anticipate a single Northern Harrier or Red-tailed Hawk in the adjacent fields looking for rodents, both soaring low over the ground and blending in seamlessly to the background. Bald Eagles tend to only forage for carrion, the decaying flesh of dead animals and will soar to wait for primary hunters, such as the aforementioned Northern Harrier or Red-tail Hawk to complete a kill first before swooping in. Finally, like the Gulls, the solitary Osprey will only be found in vicinity of water. Ospreys soar between 30 to 130 feet over the water before diving in at 50-plus miles per hour to complete their aquatic kills.

In this example, and based on our AHAS risk definitions, the above bird activity would probably constitute AHAS risk: Low. There are no migratory birds reported, guilds are not appearing on NEXRAD and individual birds of prey dominate specific territories while soaring. What is not mentioned in the scenario are the permanent residence birds, individually they can easily travel to forage without being picked up on weather radar. If you are lucky, Tower may make a 99 call which states there are now birds in the vicinity. Try this example at your home airfield and explore different months and times of day. You may be surprised by what you learn about your local avian ecology. This begs the question, is your bird strike risk ever just Low? Never, I would argue. It only takes one bird to potentially down an aircraft.

The next time you “Clear Left, Right and Above” on taxi and takeoff, think additionally what you are also looking for on the airfield other than other humans, vehicles and aircraft. When landing on a new airfield, practice a chart study of the surrounding area. Are there surrounding low-lying fields? Is there an abundance of agriculture? What time of year is it? Am I about to enter a common migratory bird zone? What about a wildlife refuge or a nature preserve? Has someone passed a pilot report or is the Automatic Terminal Information Service (ATIS) blaring about birds? All these factors matter in your preflight and in-flight planning to best mitigate your bird strike risk. Additionally, ask if your airfield specifically uses a 3D avian radar. This is reliant technology and greatly increases the accuracy of bird reporting.

Use the AHAS risk definition to your advantage. Teach and brief your crews on what to look for using the AHAS website instead of requiring one-risk adjective on a single line in your Naval Air Training and Operating Procedures brief. BASH is only the name of the preventive program, and doesn't explain the risk itself and should be addressed accordingly. I am not advocating for every aviator to take courses in ornithology at the graduate level, but do recommend each aviator has a fundamental knowledge of avian ecology to help mitigate operational risk. If this level of information helps crews create more informed decisions in their CRM process regarding reported AHAS risk and ATIS BASH reporting alone, then this article is a success. After all, is not a bird in the hand worth two in the bush?

• Date Taken: 08.01.2025
• Date Posted: 12.11.2025 11:43
• Story ID: 553577
• Location: US