U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3095 

November 2008

Management and Research Applications of Long-range 

Surveillance Radar Data for Birds, Bats, and Flying Insects

Janet M. Ruth, Jeffrey J. Buler, Robert H. Diehl, and Richard S. Sojda

Printed on recycled paper

What are long-range surveillance 

radars?

Long-range surveillance radars include 

weather-surveillance radars, airport-surveillance radars, terminal Doppler weather radars, 

and private Doppler radars. All of these technologies can be used to study bird, bat, and 

insect movement patterns. However, with the 

exception of the weather-surveillance radar 

system in the United States, access to data 

from these radars is currently limited. Therefore, we focus this publication on applications 

of the U.S. weather-surveillance system.

The current nationwide system of more 

than 150 digital Doppler weather surveillance radars, variously known as WSR–88D 

(Weather Surveillance Radar, 1988 Doppler) 

or NEXRAD (NEXt generation RADar), is 

the most well known network of long-range 

surveillance radars. WSR–88D is also the 

long-range radar most frequently applied to 

biological questions. These high-powered and 

highly sensitive radars repeatedly scan a wide 

geographic area at multiple elevation angles 

using a narrow conical beam, providing coarse 

spatial and temporal resolution. WSR–88D 

measures three data products that are useful 

for biological applications: (1) reflectivity, a 

measure of the amount of energy returned to 

the radar from targets in the sampled airspace, 

(2) radial velocity, a measure of mean target 

velocity toward or away from the radar, and 

(3) spectral width, a measure of variation in 

radial velocity. The maximum detection range 

than 100 km (62 mi). The maximum range 

increases when these data are used to answer 

questions about migrating birds at cruising 

flight altitudes. Data can provide information about bird movement patterns at scales 

that range from a few kilometers (Diehl and 

for WSR–88D data typically varies depending 

on the altitude of birds in the airspace (fig. 1). 

When these data are used to answer questions 

related to habitat use (for example, when birds 

are low to or just taking off from the ground) 

the maximum detection range is usually less 

T 

here is renewed interest in using long-range surveillance radar as a biological 

research tool due to substantial improvements in the current network of radars 

within the United States. These improvements include an enhanced capacity to detect biological targets and Doppler capability that enables the measurement of direction and speed of 

targets. Additionally, the digital nature of the radar data, plus the increasing availability of 

inexpensive computing power and geographic information systems, enables a broad range of 

quantitative, spatially explicit biological applications.

The purpose of this publication is to provide a summary of long-range surveillance radar 

technology and applications of these data to questions about movement patterns of birds and 

other flying wildlife based on publications that provide more detailed information (Buurma, 

1995; Gauthreaux and Belser, 2003a, b; Gauthreaux and others, 2003; Diehl and Larkin, 

2005; Larkin, 2005). For simplicity we refer primarily to birds in this publication; however, 

the applications are usually also appropriate for bats and flying insects. The intended 

audience is potential radar-data end users, including natural-resource management and 

regulatory agencies, conservation organizations, and industry.

Figure 1. Height profile of a radar beam in standard atmospheric conditions at the lowest elevation angle (0.5°). 

Solid black lines denote the theoretical beam limits. The tan area denotes the curve of the earth and topographic features. The maximum detection range occurs where the effective radar beam begins to completely 

pass over targets.
Larkin, 2005), to entire continents (Gauthreaux and others, 2003) when combining 

data from an entire network of radars.

Long-range surveillance radar provides 

valuable data that can be applied to addressing biological questions, and upgrades already 

in progress will improve these applications. 

However, it is important to recognize the 

strengths and weaknesses of WSR–88D, 

understand that it is one potential tool in a 

much larger toolbox, and recognize that it 

does not necessarily replace more traditional 

field observations. Validating data from longrange radar through use of complementary 

methods is extremely important. Finally, 

although most long-range radars are maintained and operated by experts in government 

agencies, application of this technology to 

biological issues does require people experienced in analysis to ensure that its results are 

correctly interpreted and applied. The intuitive 

visual appeal of these data (fig. 2) conceals 

the challenging analytical issues that often 

need to be addressed in order to quantify 

the data.

Strengths of WSR–88D

• Well suited to studying bird distributions 

and movements aloft, and their responses 

to factors that affect their distributions and 

movements, across large spatial extents and 

temporal scales.

• Extensive network with individual radars 

identical in design to each other, simplifying 

system-wide applications. Although some 

local parameters do vary by radar, measures of 

reflectivity are standardized across radars.

• Provides data about bird movement 

patterns in remote areas that are difficult to 

access. It may be more cost effective than 

implementing large-scale field data collection, especially for answering questions posed 

over large spatial extents or in cases where 

a relatively small field dataset can be used 

to calibrate or corroborate a more extensive 

radar dataset. In addition, WSR–88D data can 

provide initial information to focus future sitespecific studies and on-the-ground field work.

• Operates continuously across the country. 

A comprehensive dataset of nearly every radar 

sweep, taken every 4.5 to 10 minutes (prior 

to 2008 the frequency was 6–10 minutes), 

for every WSR–88D site dating back to the 

mid-1990s is archived and freely available, 

enabling long-term research across the United 

States.

Limitations of WSR–88D

• Recent improvements (starting in 2008) 

have increased spatial resolution severalfold 

(for example, sampling a 2-dimensional space 

of about 250 m x 250 m at 30-km range [820 

ft x 820 ft at 18-mi range]); data prior to 2008 

are available at lower resolution (about 500 m 

x 1,000 m at 30 km [1,640 ft x 3,280 ft at 18 

mi]). Regardless, the resolution of WSR–88D 

data is still rather coarse and is usually not 

well suited for studying movement patterns at 

finer spatial scales or answering site-specific 

questions. Site-specific applications are best 

addressed using short-range radar systems that 

collect very fine resolution data. However, 

WSR–88D can provide an initial, coarse index 

of activity in places where the radar is suitably 

located relative to the site of interest.

• Target identification remains a challenge in analyzing WSR–88D data. Echoes 

originate from precipitation, dust, ground 

clutter, birds, bats, insects, and other sources. 

Even when nonbiological targets have been 

filtered out, the remaining biological targets 

can be difficult to distinguish from each other. 

Although there are several generally accepted 

means of separating out insectlike targets, the 

data almost invariably include some insects. 

Except in certain circumstances, there is no 

widely accepted way of distinguishing bats 

from birds. The presence of insects or bats in 

“birdlike” radar data can bias analyses of both 

reflectivity and velocity data. It is because 

of this challenge that complementary use of 

other measures is needed to determine target 

identity.

• The locations of WSR–88D radars are 

fixed and may not provide effective coverage for particular areas. Data nearest the 

WSR–88D radar (approximately 10–20 km 

[6–12 mi]) may be unusable due to ground 

clutter caused by physical structures such as 

tall buildings and bridge overpasses. Lack of 

effective coverage may also occur if a location 

of interest is too far from the radar site or the 

radar beam over the location is partially or 

completely blocked by relief in terrain (fig. 1). 

For example, in the Rocky Mountain West, 

substantial portions of the radar coverage 

around some radars are obstructed by mountain ranges, especially at the lowest elevation 

angles.

• WSR–88D is not well suited to detecting 

low-flying birds and is poor at resolving the 

height at which birds fly. 

However, if these limitations are 

addressed and the data are used appropriately, 

WSR–88D data can provide valuable information to managers, regulators, conservationists, 

and researchers.

For what purposes can long-range 

surveillance radar data be used?

Combining the radar-reflectivity and 

radial-velocity data with knowledge of radar 

beam geometry yields quantitative information 

about the density, speed, direction, altitude, 

and location of animals aloft.

• WSR–88D can provide information about 

bird movement patterns at broad spatial and 

temporal scales. This can help us understand 

how migrants respond to weather (hurricanes, 

for example), and physical obstacles (large 

bodies of water, mountain ranges, deserts); 

how bird movement patterns are associated 

with habitat; and how bird movement patterns, habitat use, or numbers are affected by 

management practices at broad spatial scales 

such as wetland restoration or Conservation 

Reserve Program acreage. 

• Where WSR–88D radars provide effective coverage and if there are no radar beam 

obstructions, they can be useful in identifying important migrant stopover sites (fig. 2), 

locating roost sites, and documenting localized movement patterns. This information can 

be valuable in identifying important habitat 

for protection and movement patterns of species of management concern such as waterfowl or blackbirds.

• Data from the entire network of WSR–

88D radars (or some subset) can be used to 

monitor migration patterns at a regional or 

national scale, making it possible to look at 

seasonal and annual variation.

• Archived WSR–88D data can be useful 

for assessing long-term changes in movement 

patterns or bird distributions in response to 

altered landscapes, habitats, and ecological 

communities due to factors such as agricultural practices, climate change, urbanization, 

exotic and invasive species, and habitat loss. 

• WSR–88D data can be used in bird/aircraft collision risk assessment. The U.S. Air 

Force, through its Avian Hazard System, uses 

WSR–88D data to monitor bird activity in 

low-level airspaces.

How much will it cost?

The costs associated with using WSR–

88D data to address bird management and 

conservation questions will vary substantially, 

depending on the research question. Providing comparative estimates is beyond the scope 

of this publication. However, we will attempt 

to summarize the major cost components 

involved. There is free access to WSR–88D 

data, but these data are difficult to parse and 

analyze, expertise is scarce, and the process 

can require substantial database-storage space 

and complex computing capacity. Therefore, 

the major expenses for a research project 

using WSR–88D data are similar to those 

for other remote-sensing data applications, 

including: 

• Labor for screening, interpreting, and 

analyzing data

• Support to acquire, maintain, and(or)

update large, complex computing capacity 
• Possible collection of ancillary field

data for calibration or corroboration of radar 

measures 

• Development of new methods to

reduce known sources of bias in radar measures and(or) apply computing techniques 

(for example, artificial intelligence) to replace 

current labor-intensive analyses 

The end user can choose a low-cost 

approach, which produces a cursory, qualitative result that may effectively answer simpler 

types of research questions, or a higher-cost 

approach to provide more rigorous, quantitative answers, which requires more complex 

analyses, field-data collection, and(or) 

development of new methods of analysis. It 

is crucial that the end user clearly define the 

research or management questions in order to 

know where along this continuum the answers 

to their questions lie. In general, the cost of 

research increases with the size of the area 

or length of time being studied or with the 

complexity of the question being asked.

What methods can be used to 

calibrate or validate long-range 

surveillance radar data?

The importance of using complementary 

methods to calibrate or validate long-range 

radar data can not be overemphasized. Gathering such data enables us to verify the interpretations of long-range surveillance radar data. 

This may involve:

• Coordinated use of portable short-range 

radars, including vertically-pointing, fixedbeam radars

• Integration with landcover maps (for 

example, Landsat) and other geospatial data 

layers

• On-the-ground methods such as thermal 

imagery, acoustic monitoring, mist netting/

bird banding, bird surveys (point counts), 

visual observations

• Telemetry

• Expert knowledge about bird (and other 

biological target) distribution, behavior, and 

timing of migration, to further inform radar 

data that are not species or taxa specific

How can my agency/organization 

take advantage of this technology?

If you think radar technology might be 

useful in addressing your management, regulatory, conservation, or operating question, 

you should become an “educated consumer.” 

Read up on the materials provided on the 

Radar Technology Web site (http://www.fort.

usgs.gov/radar) and other resources before 

contacting potential collaborators to discuss 

your information needs and the technologies 

available to address those needs. We do not 

suggest that managers, regulators, or conservationists need to become experts in accessing 

and analyzing WSR–88D (or other radar) 

data; there are many opportunities to work 

with collaborators who have this expertise.

What information do I as a 

manager or regulator need to 

provide?

Collaboration and communication 

among the team made up of end users (managers, regulators, conservationists, industry), 

biological researchers, and experts in accessing, analyzing and interpreting radar data is 

crucial to developing the approach that truly 

answers the questions being asked by the end 

users. The most important thing for potential 

users to do in preparing to talk with a scientist 

about radar applications is to identify their 

highest priority issue and develop a clear and 

specific description of the priority management, regulatory, conservation, or operating 

question(s) that they are trying to answer. The 

more details a user can provide about exactly 

what information is needed and why it is 

important, the easier it will be for a potential collaborating scientist to help design a 

research project using the appropriate radar 

technology and(or) other methods that will 

answer his or her questions. For example, do 

you want to know the relative abundance of 

birds using a stopover habitat? The passage 

rates over a given area? The prevalence of 

roosts near an airport? The foraging behavior 

of waterfowl? Do you need to know what species they are? What is the spatial and temporal 

extent of the question? Is your question sitespecific? What is the nature of the landscape 

(homogenous, mountainous) involved? 

Whom can I contact with 

questions? Who are my potential 

collaborators?

There are numerous opportunities for 

partnership in developing projects to apply 

radar technologies to address management, 

regulatory, and conservation questions. Some 

potential contacts are provided on the Radar 

Figure 2. (Left) Radar reflectivity of birds during the onset of nocturnal migration near Corpus Christi, Texas, on May 14, 2005, at 7:34 p.m. CST. A white dot marks the 

location of the radar. Different colors indicate differences in the density of birds aloft, where purples and pinks represent high densities. Spatial variation in reflectivity 

(color) shortly after birds take flight reflects how birds were distributed in the landscape just prior to flight. Using this association, data from these radars can identify 

important stopover habitat. (Right) Radial velocity of birds on the same date about 2.5 hours after the onset of migration. Blue indicates movement toward the radar, red 

away, showing that overall direction of flight is almost due north.
Key References 

Several of these publications are available online (titles bolded) through the Radar 

Biology Bibliographies (http://www.fort.usgs.

gov/radar/bibliography.asp) on the Radar 

Technology Web site.

Buurma, L.S., 1995, Long-range surveillance 

radars as indicators of bird numbers aloft: 

Israel Journal of Zoology, v. 41, p. 221–236.

Diehl, R.H., and Larkin, R.P., 2005, Introduction to the WSR–88D (NEXRAD) for 

ornithological research, in Ralph, C.J., 

and Rich, T.D., eds., Bird conservation implementation and integration in 

the Americas, Proceedings of the Third 

International Partners in Flight Conference, 

Vol. 2: Albany, Calif., USDA Forest Service, Pacific Southwest Research Station, 

General Technical Report PSW–GTR–191, 

p. 876–888.

Eastwood, E., 1967, Radar ornithology: London, UK, Methuen Publishing Ltd., 278 p.

Gauthreaux, S.A., Jr., and Belser, C.G., 2003a, 

Overview—Radar ornithology and 

biological conservation: Auk, v. 120, p. 

266–277. 

Inside photo: McCown’s Longspurs (Calcarius mccownii). Photo: Bill Schmoker

Back photo: Hoary bats (Lasiurus cinereus). Photo: J. Scott Altenbach

Gauthreaux, S.A., Jr., and Belser, C.G., 

2003b, Bird movements on Doppler 

weather surveillance radar: Birding, v. 35, 

p. 616–628. 

Gauthreaux, S.A., Jr., Belser, C.G., and van 

Blaricom, D., 2003, Using a network of 

WSR–88D weather surveillance radars to 

define patterns of bird migration at large 

spatial scales, in Berthold, P., Gwinner, E., 

and Sonnenschein, E., eds., Avian migration: Berlin, Germany, Springer-Verlag, p. 

335–346.

Some Additional Information and Online Resources

Radar data are archived at the National Climatic Data Center (NCDC). They can be 

downloaded free with the NEXRAD Inventory Search Tool and viewed using the Weather 

and Climate Toolkit. The Toolkit can also export data from its unique binary format to 

a variety of GIS-friendly formats (shapefile, tif). Links to several free visualization and 

analysis products are provided below. If you choose to delve into these resources, you will 

be confronted by terminology (for example, Level II and Level III data) that is beyond the 

scope of this publication. You will either need to do extensive research or consult with 

experts to understand the terminology, analyses, and interpretation.

Unfiltered Real-Time Radar Images

Current images and loops — 

http://radar.weather.gov 

Current images and regional composite loops — 

http://whirlwind.meteor.wisc.edu/~wxp/nids/nids.html

Images archived for up to a week — http://www.rap.ucar.edu/weather/radar/

WSR–88D Data Information and Archive from the National Climatic 

Data Center (NCDC)

NCDC Radar Resources Web site — 

http://www.ncdc.noaa.gov/oa/radar/radarresources.html

NEXRAD Inventory Search Tool — http://www.ncdc.noaa.gov/nexradinv/

Radar Data Access Page (download, order, visualize, real time) — 

http://www.ncdc.noaa.gov/oa/radar/radardata.html

Free Radar Data Viewing Software

Unidata Integrated Data Viewer — http://www.unidata.ucar.edu/software/idv/

NOAA’s Weather and Climate Toolkit — http://www.ncdc.noaa.gov/oa/wct

Technology Web site by going to the Radar 

Workshop Participants List (http://www.fort.

usgs.gov/radar/workshop_participants.asp)

or by going to information on USGS Radar 

Research (http://www.fort.usgs.gov/radar/

USGSResearch.asp) or Related Resources 

(http://www.fort.usgs.gov/radar/resources.

asp); however, this information is by no 

means comprehensive. There are scientists 

working for Federal agencies, universities, 

conservation groups, and private contractors 

with whom you can collaborate. An internet 

search using combinations of words such as 

“radar,” “WSR–88D,” “NEXRAD,” “bird,” 

“avian,” and “ornithology” will produce additional information and potential collaborators. 

However, application of radar technology to 

wildlife-resource issues remains a developing 

science requiring peer review. The importance 

of being an educated consumer can not be 

over-emphasized. The USGS does not generally endorse other particular organizations 

or products for WSR–88D radar analysis; 

however, we do collaborate extensively 

with others and we have a duty to promote 

the best science available. End users should 

inform themselves about the technology, 

clearly identify their management or research 

questions, evaluate the programs, products, 

and peer-reviewed publications produced by 

potential collaborators, talk with scientists and 

ask probing questions. You are welcome to 

contact the authors of this document, or any 

of the scientists listed among the workshop 

participants. 

Larkin, R.P., 2005, Radar techniques for 

wildlife, in Braun, C.E., ed., Techniques for 

wildlife investigations and management, 

6th edition: Bethesda, Md., The Wildlife 

Society, p. 448–46

Contact Information:

Janet M. Ruth

U.S. Geological Survey

Fort Collins Science Center

Arid Lands Field Station

janet_ruth@usgs.gov