Many animals migrate, and the distance they cover ranges from a few centimeters to thousands of kilometers; some may even cross the entire globe (Dingle 1996). Tracking migrating individuals over large distances is one of the challenges in studying animal migration. Using radar has allowed researchers to examine migration altitude, speed, and wing beat frequency of birds, bats, and even insects during migration (Bruderer 1997).

History of Radar Technology

The potential value of radar for studying animal migration was first discovered during World War II, when "phantom signals" appeared on radar screens that were caused by migrating birds (Buss 1946). Since the 1960s, radar has become a widely-used technique for studying migrating animals (Bruderer 1997). The earlier versions, including WSR-57 (Weather Surveillance Radar - 1957) radar, have been replaced by the Next Generation Weather Radar program (NEXRAD). NEXRAD is also known as WSR-88D (Weather Surveillance Radar 88 Doppler), a Doppler system that replaced the older non-Doppler meteorological radars (Crum & Alberty 1993). NEXRAD is a network of 159 radars covering the US and southern Canada, and it is operated by the US National Weather Service. This updated radar system has both greater power capacity and spatial resolution than the WSR-57 system, resulting in higher sensitivity for migrating animal targets (Russell & Gauthreaux 1998). The NEXRAD Doppler system can determine both the direction and speed of migrants' movements that are traveling both towards and away from the radar (Russell & Gauthreaux 1998, Crum & Alberty 1993). For the purpose of studying migratory animals, a number of different types of radar are being used, some of which are described below.

The Basics of Radar Technology

The term "radar" stands for radio detection and ranging. Electromagnetic waves are emitted from an antenna in pulses that scatter when they hit a new medium with different dielectric properties (the ability of a material to store magnetic and electric energy). Some of the energy from these pulses is reflected back to the radar antenna, where it is received. It is then possible to calculate the distance to the target and its location by using the delay in receiving the echo, the speed of light, the beam width emitted by the antenna, and the position of the antenna. Pulse volumes can affect the resolution of the radar. In general, a smaller pulse volume gives a higher resolution. A smaller pulse combined with a narrower beam allows for the best information regarding the target's position and reduces the odds that several targets will be included within a single echo. In most cases, it is fairly easy to distinguish migrating targets as they produce clear echoes; however, larger groups of birds or bats can take up several pulse volumes and show up as patches of echoes on the radar (Bruderer 1997).

The ability of radar to successfully detect targets and distinguish among them is also affected by the wavelength. A longer wavelength, which corresponds to a lower frequency, is typically less disturbed by environmental factors such as inclement weather. On the other hand, a shorter wavelength, or a higher frequency, is associated with higher noise levels, a greater chance of disturbance by smaller targets, and a smaller range. However, smaller wavelengths have the advantage of being able to be projected in sharper beams by smaller antennas and generally have higher precision (Bruderer 1997).

Advantages and Limitations of Radar in Migration Studies

Radar has several benefits over other techniques to study migration such as visual observations, trapping, and banding, as it works well at altitude and over large distances, is unaffected by the absence of light, and is relatively independent of weather conditions (Cooper et al. 1991, Bruderer 1997). It can therefore provide insights into timing and direction of migratory flight and even measure wing beat patterns of migrants (Bruderer 1997). Radar can, however, only detect individuals that fly within its pre-set range. Any individual that migrates above or below the range of the radar beams will necessarily remain undetected (Russel & Gauthreaux 1998).

Types of Radar Used in Migration Research

There are several different types of radar that are used in migration research. Among the most commonly used are continuous wave, nexrad, harmonic radar, pulse, and Doppler. Pulse radar uses the time between emitting and receiving the radio energy to calculate the distance to the target. Doppler is also a pulse radar, but the shift in wave length within the pulse is analysed using the Doppler shift, which is the change caused by the speed of the target relative to the radar. The advantage of this is the ability to distinguish moving objects from stationary ones. Continuous wave radars also use the Doppler shift, but rather than alternating between transmitting and receiving radio pulses, this type of radar has separate transmitters and receivers (Bruderer 1997). Harmonic radars make use of a rectifier circuit, which can be incorporated into a tag worn by the individual that is going to be tracked. The circuit generates an echo that has exactly half the wavelength of the wave that is being transmitted (Chapman et al. 2011).

Related Technology: Sonar

Studying migration in aquatic environments can be particularly difficult. For example, tides, water depth, floating debris, and low visibility hinder studies of migrating aquatic animals by traditional techniques alone. Sonar, a cousin to radar, uses acoustic as opposed to electromagnetic signals to detect targets in aquatic settings. Krumme & Saint-Paul (2003) used sonar technology to research the movements of several species of fish in a mangrove in Northern Brazil. From their acoustic recordings, they were able to successfully characterize migratory movements of several fish species. A distinct migratory pattern was observed using sonar and the authors were able to refute the notion that intertidal fish species are passively moved within their environment. Instead, their data clearly showed that a sizeable amount of the migratory paths were directed against the current in order to assemble in areas rich in resources.

Several studies have used NEXRAD radars to study the migration of birds and bats. For example, Russel & Gauthreaux (1998) examined the movements of purple martins (Progne subis) using NEXRAD with the goal of estimating the number of birds in flight. On the radar screen, their departures could be seen as a roughly circular accumulation that was growing in intensity. The NEXRAD radar also helped to identify a previously unknown roost location (Russel & Gauthreaux 1998).

Diehl et al. (2003) looked at the migratory behaviour of birds flying over the Great Lakes, USA, using NEXRAD radars. Radars are particularly useful for studying patterns of migration over and around the Great Lakes, as the lakes are narrow enough that land-based radars operating simultaneously can obtain a panoramic picture of migrating birds. Using Doppler radar, the authors showed that avian migrants tended to avoid flying over the lakes' surface, and instead, they flew close to the coasts when possible (Figure 1).

The particular advantage of harmonic tags for entomological applications is that they operate passively and therefore do not require a battery; this means that, with modern microelectronic components, they can be made light enough to be carried by medium-sized insects such as honey bees (Apis mellifera) without any obvious effects on behaviour. Their main limitation is that the returned signal contains no identification information. Another constraint is that terrain and vegetation, although not sources of echo, still act as barriers and create shadow areas within which tags are undetectable (Chapman et al. 2011).

Fluctuations in the echo of the radar are correlated with wing beats and can therefore be used to determine the wing beat pattern of individual migrants (Bruderer 1997, Bruderer et al. 2010). These fluctuations are attributed to contraction and dilation of the animal's body that occur during flight. Based on their wing beat pattern, echo signatures of birds can be assigned to different groups of flight style, such as passerine-type, swift, and shorebird (Figure 2).

Before the invention of radar, studying nocturnal migrants was limited to moonwatching, which is estimating the number of birds crossing the disk of a full moon with a telescope (Lowery 1951), and to daytime ground count, but both methods were — not surprisingly — lacking in accuracy. Able (1973) used the earlier radar technology (WSR-57) to accurately determine the altitude of nocturnal migration in a passerine species.

Radar has also been utilized to study the nocturnal migration of insects. Drake (1985) looked at a phenomenon known as the nocturnal low-level jet and its link to the migration tendencies of moths. Nocturnal low-level jets are caused by the Coriolis force acting on the boundary-layer airflow and can be found all around the world, including the Great Plains of the U.S., and the plains region in New South Wales, Australia. Wallin & Loonan (1971) showed that these jets are used by long-distance insect migrants. Using radar, Drake (1985) was able to determine both the number of insects flying and their migration altitude. Initially, density of insects varied very little with increasing altitude, however, as the night progressed past midnight, a very different pattern emerged. After midnight, approximately half of the migrant insects became concentrated into a single layer, following the nocturnal low-level jet. Moths within this jet and within its immediate vicinity travelled at higher speeds than those traveling outside the jet's altitude.