Tracking Bird Migration with Radar
Radar is traditionally thought to be used in tracking weather or airplanes, however, it can also be used to track wildlife. Virtually since the creation of radar (which stands for radio detection and ranging), ‘angels’ have been detected on radar screens, which turned out to be migrating birds, bats and insects (Brown 1999). Many advancements have been made since the 1930’s when radar was first brought into use that have refined radar imagery. With the advent of computers, microelectronics and other technological advances, radar imagery has been fine tuned to be able to provide greater details in the location, direction, and density of biological targets (birds, bats, and insects).
Over the past few decades, ornithologists have begun using radar as a means of tracking and monitoring bird migration (Peckford and Taylor 2008). Radar imagery from NEXRAD (next generation radar) systems has been used by meteorologists in forecasting precipitation, however, in the 2000’s this radar imagery has been made available online for public use. This imagery also detects migrating birds, and can be used by birders and ornithologists to watch for bird migratory movements. This article will explain the details of how to interpret NEXRAD radar in order to watch bird migration events and even predict when and where to expect birds the following day.
Radar uses pulses electromagnetic radiation to detect objects within its field of vision (Skolnik 1990). Radiation is emitted from a dish or antenna, these pulses will bounce off any object in their path and are reflected back towards the antenna or dish where they are detected. These detections or ‘echoes’, are then interpreted by a computer and presented on a screen for visual interpretation, the radar image.
The amount of time measured between emitting a pulse of radiation and receiving the reflection can be used to calculate the distance the object is located away from the radar unit. Repeatedly sending out pulses of radiation makes it is possible to observe the movement of objects (i.e. precipitation, birds) over time. The number and size of objects that radar detects is measured in the dBZ-scale (Skolnik 1990). While this sounds complex it essentially translates to the intensity of the objects moving through the radar’s field of view, when considering rain; drizzle is typically 1-8 dBZ, a moderate rain is 9-45, and a heavy rain would be over 45.
To the untrained eye it may be difficult to separate precipitation from migrating birds, but with a little training and experience identifying birds can be accomplished. One of the first things to take into consideration is which radar imagery to follow; the NEXRAD radar imagery shown on US websites such asIntellicast will show bird echoes, however, the radar imagery provided by Environment Canada does not.
When looking for bird echoes it is helpful to know what to expect. Generally, birds do not migrate during precipitation, and even light precipitation also will mask any bird echoes. So when first trying to identify birds on radar images it is best to start on clear nights with no expected precipitation to cause any confusion.
Bird echoes typically do not have an organized pattern in the same way precipitation does, as they typically appear more as a ‘cloud’ around the radar station that moves slightly (Figure 1).
Figure 1. Bird migration across southwestern British Columbia and the Puget Sound area of Washington.
Precipitation and weather systems typically have a more organized look, with ‘blotches’ of more intense precipitation moving across the landscape as a unit, typically southwest to northeast in the spring and northwest to southeast in the fall (Figure 2).
Figure 2. Precipitation moving across southern British Columbia and the Puget Sound area of Washington.
Movements generally take place around waterbodies, or if they do they are typically focused around the shortest point in crossing, such as between Metchosin on Vancouver Island and Port Angeles, Washington (Figure 3).
Figure 3. Birds migrating across from Vancouver Island to Washington.
The colours that represent birds will change depending on the intensity of the movement. A larger migration event would be represented by colours higher on the dBZ scale as reflected on the legend of any radar imagery. Dr. Sid Gauthreaux and Carol Belser have recently developed a methodology for quantifying bird migration through radar (Table 1).
Table 1. Relationship between bird densities and the dBZ scale.
|dBZ||Bird Density (birds/km³)|
When to Look
Migration in the Pacific Northwest and across North America typically occurs from March-May and August-November. Some movement does happen outside of these periods, but this covers when most of the movement happens and so these periods are when to watch radar for bird migration.
Birds migrate throughout the day, however, the majority of smaller birds tend to migrate at night when conditions are more favourable for migration (Newton 2010). Most birds tend to take flight between 20-40 minutes after sunset and land between 10-30 minutes before sunrise. Migration tends to occur earlier in the night, with birds landing well before sunrise, however depending on the preceding conditions (several nights of precipitation) heavy movements may occur all night long. The Intellicast website usually shows a loop of radar imagery between 40 – 90 minutes in length, so by checking around an hour after sunset, it is possible to watch as birds take flight in the evening (Figure 4). Checking the radar imagery repeatedly over a night, it is possible to then gauge the intensity of the night’s movement.
Figure 4. Birds beginning migration on October 15, 2011, note the precipitation along the bottom of the screen.
After becoming familiar with how migrating birds appear on radar imagery, and knowing when to check the imagery, it is possible to predict what conditions may be for finding birds on the next day.
Recommended Radar Websites
Brown, L. 1999. Technical and Military Imperatives: A Radar History of World War 2. Taylor and Francis, Oxon, United Kingdom.
Skolnik, M. 1990. Radar Handbook. McGraw-Hill Professional, New York, New York.
Newton, I. 2010. The Migration Ecology of Birds. Academic Press, Waltham, Massachusetts.
Peckford, M.L. and P.D. Taylor. 2008. Within night correlations between radar and ground counts of migrating songbirds. Journal of Field Ornithology 79: 207-214.