FAQ

Birds are notably volant and move on almost every day of the year. But not all of these movements are indicative of migration. Most bird migration in the contiguous U.S. occurs in narrower windows during spring and fall. BirdCast forecast models are trained on data from early March to early June for spring migration and early August to early November for fall migration, two periods that capture the greatest temporal extent and magnitude of migratory movements and the greatest number of species. Note, a smaller number of species migrates outside of these periods – much earlier in the calendar year than spring, beginning movements as early as January and February; and, so, too, much later in the year, in November and even into December past the winter solstice. Some sparrows and many species of waterfowl and waterbirds are good examples that migrate outside our typical monitoring periods.

Most bird migration occurs at night, a period when it is often impossible to count and to identify passing birds without the use of special tools like radar. Yet, because so many birds migrate at night, characterizing and monitoring the movements are critical, so we focus on reporting during these periods. There is another reason we report at night, and this is that the behavior of the radar beam and its sampling of airspace is usually much more consistent at night – the atmosphere during the day is often characterized by significantly greater turbulence, large numbers of drifting insects, uneven heating of the Earth’s surface and odd atmospheric properties emergent from this unevenness, and many fewer birds aloft and migrating, often making analysis and interpretation of such daytime data more challenging.

​In simple terms, BirdCast ​c​ounts nocturnally migrating birds ​by using algorithms 1) to analyze data detected by weather radars in the form of energy from targets in the atmosphere, 2) ​to remove weather and to retain bird information data, 3) to ​c​overt remaining data characterizing birds’ radar energy ​- with the weather removed – into bird density, and 4) to integrate ​t​he density over space and height​ (because radars do not cover 100% of the atmosphere at all altitudes where birds migrate in the U.S.​).

The primary reason we do not display data from Alaska and Hawaii is about the map graphics we use and the challenge of making a good, concise visualization. The current projection for displaying bird migration data is the contiguous 48 states of the U.S., and although we are seeking a map visual that captures these other areas, this has been a low priority given a very small team of people developing and maintaining the project. Moreover, although there is excellent, and diverse, movement in Alaska captured in WSR-88D data, bird migration on radar in the Hawaiian Islands is generally significantly less intense than any continental location, providing an additional challenges. But, for the record, there are seven weather surveillance radars in Alaska, and another four on Hawaii, all of which detect birds and migration.

There are 32 weather surveillance radars operating in Canada, collecting data very similar to those from the contiguous U.S. already used in BirdCast observations and forecasts. The BirdCast team is actively working to study the possibility of adding Canadian weather surveillance radar data to the BirdCast project’s visualizations and tools. The addition of these data would be invaluable for studying and monitoring the large numbers of birds that breed in boreal forests, taiga, and tundra, areas where human surveys are generally infrequent and highly localized.

Mexico has 14 operational weather surveillance radars, and although access to some version of these data is free, the more raw data that does not exclude biological information are not freely accessible; so, until there is a relationship among necessary partners, we sadly will not be integrating Mexican radar data into the BirdCast system. Filtered data remove all or almost all of the information about birds detected by the radar.

Many local news providers and meteorological outlets of information filter any non-meteorological data (that is, biological data like birds, insects, and bats), or try to filter this, to avoid confusion of anything with weather forecast and observations. Occasionally, bird migration is so intense that the filters do not work, creating a confusing situation for all but the most astute meteorologists and audiences that suggests “rain” in places where skies are clear but intense bird migration is being represented as “rain.”

In recent years, broadcast meteorologists have begun to embrace BirdCast forecast and analysis maps. These have started to appear on nightly weather reporting during prime time broadcasting periods. This is an excellent educational opportunity, and the BirdCast team is grateful for the increasing number of meteorologists using these bird migration data in their local weather updates and interest stories!

Radar operates by sending out radio waves, and when they bump into something like a bird or raindrop, part of the energy bounces back to the radar. Even though a bird is small, modern radar is very sensitive and can pick up even tiny reflections.

Think about it—weather radar can detect things as small as raindrops, and can sense them with enough precision to show whether the rainfall is light, moderate, or heavy. When thousands or even millions of birds are flying high overhead, weather radar certainly detects them.

We can tell radar is detecting birds by their unique flight patterns and speeds, and by comparing radar readings with real‑world observations. For decades, radar operators worked with bird watchers and field scientists to match what the radar detected with actual human observations. In the early days, mysterious radar signals called “angels” were confirmed to be birds once observers reported their sightings at the same time and place. Later, “moonwatching”, counting birds flying across the face of a full moon, helped match nighttime radar signals to actual migrating birds.

Yes, large birds (and large flocks of birds) can obscure data about small birds in radar images, particularly when they are in high densities. Radar doesn’t “see” individual birds in detail the way our eyes do — it detects a combined signal from everything in its beam at a given moment. If a flock of large birds is present, their strong echoes can dominate the signal, making it harder to pick out smaller birds flying nearby or mixed in. This is because large birds reflect more radar energy (they have a larger “radar cross section”), so their signal can overshadow weaker returns from small birds.

BirdCast can detect birds during rain, but accuracy drops as precipitation increases. Radar picks up signals from both raindrops and birds, and heavy rain can mask the weaker echoes from birds, especially small ones. BirdCast uses filtering techniques to remove much of the rain signal, so light rain usually allows reasonable detection, while clear, dry nights give the most reliable migration data.

Weather radar sweeps its beam in a full 360‑degree circle from a central tower. The general radar images always look like circles because the rotating beam detects birds wherever it passes through them at certain heights. (Note: sometimes circular flocks look more like donuts – this is because the radar’s 360-degree sweep also occurs at angles above horizontal, meaning the beam ascends as it travels away from the radar, sweeping higher altitudes with distance and passing through altitudes where there are, first, no or few birds, then more birds, and then beyond to altitudes where birds are rare or not present.)

Additionally, when large numbers of birds take off from a roost, such as swallows or blackbirds leaving a nighttime roost at dawn,  they spread out in all directions from that location. As the radar sweeps around, it picks up these outward‑moving flocks, which show up as an expanding ring on the radar display. This is called a “roost ring”.

Although advances in radar technology have aided in our differentiation of birds from other fliers, more innovation is still necessary before we’re able to distinguish unique species of birds. Without verification from other data sources, radar data cannot determine specific species in clouds of migrating birds, and its strength lies in the quantification of higher level general patterns that apply to many species.

Even if large numbers of birds are migrating over your county, you may not see them for several reasons. Most migratory songbirds travel at night and, on average, fly more than 350 meters above ground level across the contiguous United States, which is well above the range of human vision in the dark. Although experienced birders can detect their presence by identifying nocturnal flight calls, most migrants pass overhead unnoticed. In addition, birds migrating over an area are not necessarily stopping there to rest or refuel. Whether they stop depends on local habitat quality, food availability, weather conditions, and where they are in their migration schedule.

It’s almost like pilots running preflight checklists for departure, migratory birds also take in local weather information to make decisions for night-to-night migration. Wind, temperature, precipitation, cloud cover, and many other weather conditions all impact their migration decision. Especially, a night with tailwinds can assist birds to cover longer distances with less energy expenditure. For small long-distance migrants, making the right migration decision is key to their survival. Therefore, the number of migrating birds can fluctuate a lot depending on the current and previous nights’ weather conditions.

Migratory birds take on seasonal journeys to track the seasonality of resources across the earth. Taking the flight at the right timing is an important decision. They are relying on their internal biological clock, as well as external environmental conditions as cues. Climate change has led to warmer temperatures and earlier springs in many regions, altering the timing of optimal conditions for migrants. To keep pace with the “green wave,” when food resources peak, many species are shifting the timing of their migration, or phenology. In addition, species’ ranges may shift as birds adjust where they breed or overwinter. BirdCast provides long-term, standardized monitoring of nocturnal bird migration, making it a powerful tool for quantifying changes in migration timing, intensity, and spatial patterns at broad scales over multiple decades.

Birds are able to use the night sky (e.g., the rotation of the stars) to help with their orientation during migration journey, in addition to the magnetic field. Additionally, there are fewer predators at night, the atmosphere is more stable than the daytime condition (i.e., free from turbulent thermals), and it’s relatively cooler at night to prevent overheating. 

The bright skyglow from an overlit city will confuse and disorient migratory birds to take detours from their migration journey, possibly because it makes them hard to see the stars. Even on clear sky nights, which previously thought to be less impacted by artificial light at night, birds can gather in large numbers to a strong light source and get trapped in a city with collision risks everywhere (e.g., National 9/11 Memorial & Museum Tribute in Light). Another possibility is that birds are attracted to the insect prey that are flying towards light, as seen in some nighthawks. 

Yes! Light pollution is harmful to a wide variety of wildlife by mimicking, masking, or confusing natural light signals and species’ circadian rhythms. Artificial night lighting can also unfairly benefit predators and invasive species and disrupt food webs.

Light pollution can have the following impact on species and ecosystems:

• mistimed activity, growth or breeding
• disturbed sleep and circadian rhythms
• disorientation and poor navigation
• attraction to artificial lights
• encounters with new predators
• reduced survival and reproduction.
• benefiting invasive species (cats, foxes and cane toads take advantage of artificial lights to feed).
• dividing and disconnecting suitable habitat
• reducing pollination by nocturnal animals
• disrupting food webs and nutrient webs

Turning lights out does more than save birds and protect other wildlife, it saves energy and money! The Environmental Protection Agency highlights energy as the largest operating expense for commercial buildings. Reducing energy use by shutting off lights for migration season makes environmental sense and fiscal sense. 

Turning lights out at night can also have health benefits to humans! By encouraging natural circadian rhythms, we can improve our sleep quality which is associated with a number of health benefits.

Lights out campaigns offer a critically important opportunity to reduce and to eliminate an enormous contributing source of nocturnal and diurnal hazards. Turning lights out does more than reduce risk of collision for birds, and helps preserve a critical ecosystem and resource for all kinds of wildlife.

However, collisions cannot be prevented by turning lights out alone. To prevent collisions, glass must be made visible to birds. To learn more about how to achieve this, visit the Bird Collision Prevention Alliance’s website.

There are many benefits to turning your home and business’ lights out at night all year round, as explained in the first two questions listed in this section. However, this is certainly not an all-or-nothing positive action. To protect the most migratory birds, try to commit to turning your lights out during the full period of both fall and spring migration (August 15 – November 30, March 1 – June 15). If this is still too long, turn your lights out during peak migration in your city, or commit to year-round light reduction methods like automatic shades and motion-sensors.

Most buildings are vastly over-lit: only decorative and unnecessary lighting are the primary sources of light to be turned off. Lights that are legally required for building, public and aviation safety must be kept on. Additionally, research has linked reduced light pollution with health and wellness benefits like decreased disruption of sleep and circadian rhythms.

Many groups coordinate Lights Out efforts across the U.S. – check out a list here.

Light pollution isn’t the only threat to birds. Birds don’t understand the concept of glass as an invisible barrier that can also be a mirror. They take what they see literally: Glass appears habitat they can fly into, whether that habitat is reflected or visible through glass. The American Bird Conservancy has developed a suite of resources to reduce glass collisions by creating bird friendly buildings. Check out the resources here.