Products, Data, and Interpretation

Background Materials

Weather surveillance radar detects far more than weather! See this post from 2018 by Carley Eschliman and Kyle Horton to learn more. Additionally, we posted further cursory tutorials on radar ornithology background and application here in 2013. For information about aeroecology more generally, proceed to this recent special issue and this perspective.

Migration Forecast Maps (est. 2018)

Migration forecasts come from models trained on the last 23 years of bird movements in the atmosphere as detected by the US NEXRAD weather surveillance radar network. In these models we use the North American Mesoscale Forecast System (NAM) to predict suitable conditions for migration occurring three hours after local sunset. Warmer colors correspond to more intense bird migration. These maps also show precipitation (outlined and shown in grayscale) forecasts. Note, areas forecast to experience precipitation and bird migration may overlap, and predictions for migration intensity may be highly variable in these locations. Support for this research comes from Leon Levy Foundation, Edward W. Rose Postdoctoral Fellowship, and the Marshall Aid Commemoration Commission. These models were made possible by additional research at University of Massachusetts Amherst and University of Oklahoma. The BirdCast project, a collaboration among Cornell Lab of Ornithology, Colorado State University, University of Massachusetts Amherst, and Oregon State University, was funded by grants from the National Science Foundation and Leon Levy Foundation. Citation for forecast models: Van Doren, B. M. and K. G. Horton. 2018. A continental system for forecasting bird migration. Science 361:1115-1118. doi: 10.1126/science.aat7526.

Citation for imagery: Van Doren, B. M. and Horton, K. G. Year/s of migration forecast map image. BirdCast, migration forecast map; date and time (from forecast image lower right corner). Cornell Lab of Ornithology, Colorado State University and Oxford University. Date/s of access or download.

Live Migration Maps (est. 2018)

Real-time analysis maps show intensities of actual nocturnal bird migration as detected by the US weather surveillance radar network between local sunset to sunrise. All graphics are relative to the Eastern time zone. The yellow line moving east to west represents the timing of local sunset. Areas with lighter colors experienced more intense bird migration. Orange arrows show directions to which birds flew. Green dots represent radar locations for which data are available; red dots represent radar locations with no data available. Note that many radars in mountainous areas (e.g. the Rockies) have obstructions that restrict radar coverage, providing the appearance of no migration where migration may be occurring. Support for this research comes from from Leon Levy Foundation, NASA, Edward W. Rose Postdoctoral Fellowship, and Amazon Web Services. The BirdCast project, a collaboration among Cornell Lab of Ornithology, University of Massachusetts Amherst, and Oregon State University, was funded by grants from the National Science Foundation and Leon Levy Foundation.

To cite live migration map graphics, please use the following syntax: Dokter, A. Year/s of live migration map image. BirdCast, live migration map; date and time (most easily accessible from image file name/s). Cornell Lab of Ornithology. Date/s of access or download.

Forecast and Analysis Discussions

Prior to 2018, weekly posts on the BirdCast site focused on forecasts and analyses of bird migration occurring in four BirdCast regions. Beginning in spring 2018, the BirdCast team changed the format and content of these posts. However, we have maintained an archive for previous years’ postings on the site. Additionally, from 2015-2017 the BirdCast team posted sporadic Traffic Reports, all of which are archived on the site. These represented the trial phase of our automated forecast and live bird migration maps, which are now live as of spring 2018.

BirdCast Regions

For migration forecasts and analyses, we use the following divisions to represent the continental US. We use these regions because their component states have many migration patterns in common and because their component states’ proximity make for logical groupings. Without doubt, finer scale delineation based on analyses of the details of migration patterns might yield more meaningful and representative biogeographic patterns, and our regions are not meant to replace such analyses’ findings (when we get to do them!).

West — Washington, Oregon, California, Idaho, Nevada, Utah, Arizona, Montana, Wyoming, Colorado, New Mexico

Great Plains — North Dakota, South Dakota, Nebraska, Kansas, Oklahoma

Upper Midwest and Northeast — Minnesota, Iowa, Missouri, Wisconsin, Michigan, Illinois, Indiana, Kentucky, Ohio, Pennsylvania, West Virginia, Virginia, New York, New Jersey, Delaware, Maryland, Vermont, Massachusetts, Rhode Island, Connecticut, Maine, New Hampshire

Gulf Coast and Southeast — Texas, Arkansas, Tennessee, Louisiana, Mississippi, Alabama, Georgia, Florida, North Carolina, South Carolina

Species on the Move (pre-2018)

Beginning in Spring 2015 we will be presenting more detailed information about when we expect species to pass through the four BirdCast regions in the continental United States. These are not complete lists, but they highlight many species on the move. The values below are calculated from the past 12 years of eBird data. The following are interpretations of these dates:

Noticeability We categorized (using *,**,***,****) how likely birders are to notice a given species’ arrival or departure based on eBird reports.

Migrants Begin Arriving — We expect migrating individuals of this species’ to begin arriving around this date.

Rapid Migrant Influx — We expect migrating individuals of this species to be increasing rapidly around this date.

Peak — We expect this species’ occurrence in the region to peak around this date.

Rapid Migrant Departure — We expect migrating individuals of this species to be departing around this date. (We may show Rapid Depart dates even if some individuals of a species remain in the region, as long as there is a detectable departure from other individuals.)

Last Migrants Depart — We expect the last migrating individuals of this species to be departing the region around this date.

Note that sometimes we will not provide all dates for a given species in a given region. This may occur if arrival/departure occurs largely outside of the spring months or if a species lingers in the region before or after migration.

Migration Amounts (pre-2018)

For ease of describing bird densities and migration traffic from weather surveillance radar (WSR-88D in particular), BirdCast has chosen to divide movements among four different classes. We provide a qualitative description, a relative measure in decibels of Z (dBZ) of the energy returned to a radar, and a bird density based on previous work by Gauthreaux et al. (1999). Additionally, see this article published previously.

Little to no migration — Minimal migration: < 5 dBZ — fewer than 59 birds per cubic kilometer

Light — Light migration: 5-10 dBZ — approximating 59-71 birds per cubic kilometer

Moderate — Moderate migration: 10-20 dBZ — approximating 71-227 birds per cubic kilometer

Heavy — Heavy migration: 20-30 dBZ — approximating 227-1788 birds per cubic kilometer

Very Heavy — Extreme: >30 dBZ — more than 1788 birds per cubic kilometer (this intensity of movement occurs rarely, perhaps at certain individual radar stations a handful of times each migration season)


Bird migration is a spectacular global phenomenon that has long captured the attention of human observers; even Aristotle mentions witnessing bird migration in his writings. But it wasn’t until the turn of the 20th century that ornithologists realized the magnitude of migration that occurred at night.

Over the last century, technological advances have created breakthroughs in radar, acoustic, electronic, and optical technologies that have allowed finer-scale exploration of the ecology of bird migration.

  • On certain nights hundreds of millions of birds are aloft, migrating across North America
  • Birds generally take off 30-45 minutes after sunset.
  • Some birds fly all night when conditions are good and land just before dawn the next morning.
  • In some places, some birds continue migration in nonstop flights of 60-100 hours that span oceans and continents!
  • Migration altitude varies by species and by local weather and topographical conditions, ranging from just 10s of meters to several kilometers above the ground.

Direct observation of nocturnally migrating birds is difficult. Most of the information gathered so far has been limited to a handful of large species capable of wearing tracking devices. But other sources of data provide partial information about migration, which when taken together can provide insight into migration at a scale previously unimaginable. These sources include a continental-scale network of volunteer bird watchers (eBird), flight calls of nocturnally migrating birds captured by acoustic monitoring stations, and clouds of migrating birds detected at night by WSR-88D weather radar stations.

Data Sources

Weather surveillance radar

Since the first units were placed along the Gulf Coast in the 1950s, ornithologists and birders have become increasingly aware of the power of using radar as a tool for understanding bird migration. Presently an operational network of over 140 weather surveillance radars (WSR-88D) provides coverage of the atmosphere above the continental US. In addition to detecting and depicting meteorological phenomena, this radar network can be used to watch and to track the movements of birds. Radar is an excellent tool for determining where flights are occurring, how many birds are aloft, and in what direction and speed they are moving. This technology is not effective for identifying species, but in conjunction with acoustic and eBird data, it can provide a unique insight on movements of birds at the continental and regional scales.


A real-time, online checklist program, eBird has revolutionized the way that the birding community reports and accesses information about birds (see this for more information). Launched in 2002 by the Cornell Lab of Ornithology and National Audubon Society, eBird provides rich data sources for basic information on bird abundance and distribution at a variety of spatial and temporal scales. eBird’s goal is to maximize the utility and accessibility of the vast numbers of bird observations made each year by recreational and professional bird watchers. It is amassing the largest and fastest growing biodiversity data resources in existence, with hundreds of millions of observations recorded so far.

Flight calls of nocturnally migrating birds

Flight calls are typically simple, single syllable vocalizations (usually less than one quarter of a second in duration) produced by birds during sustained flight, particularly during nocturnal migration. These calls are species specific, and they differ from other calls and songs typical of many species. At present, monitoring of flight calls is the only reliable method for identifying the species composition of nocturnal migration while it is occurring. Such monitoring is now a cornerstone for a sister project, BirdVox, which seeks to apply the power of machine learning techniques to providing automated, near-real time information on species-specific migration patterns for comparison with radar and eBird data.