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.
A real-time, online checklist program, eBird has revolutionized the way that the birding community reports and accesses information about birds. 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 one of the largest and fastest growing biodiversity data resources in existence, with more than 100 million 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 will be a cornerstone for the BirdCast project, providing near-real time information on species-specific migration patterns for comparison with radar and eBird data.
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.