Use of NEXRAD to Study Shorebird Migration in the 

Prairie Pothole Region: A Feasibility Study 

By Cynthia P. Melcher, Susan K. Skagen, and Lori Randall 

Open -File Report 2006-1033 


U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

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Suggested citation: Melcher, C.P., Skagen, S.K., Randall, L., 2006, Use of NEXRAD to study shorebird 

migration in the prairie pothole region: A feasibility study: U.S. Geological Survey, Biological Resources 

Discipline, Open-File Report 2006-1033, 8 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 
Contents 

Methods ...............................................................................................................................................................................1


Findings ................................................................................................................................................................................3


Conclusions .........................................................................................................................................................................5


Acknowledgments..............................................................................................................................................................6


References Cited ................................................................................................................................................................6


iii 
Use of NEXRAD to Study Shorebird Migration in the 

Prairie Pothole Region: A Feasibility Study 

By Cynthia P. Melcher, Susan K. Skagen, and Lori Randall 

An essential component of shorebird conservation is identifying, protecting, and 

managing high-priority stopover sites and migration habitats crucial to the long-term persistence 

of migrating shorebirds. Because of the tremendous variability in migrant shorebird occurrence 

patterns in the Prairie Pothole Region of the U.S. (Skagen 1997), it is labor- and cost-intensive to 

locate the majority of sites used heavily by shorebirds in any one migration period. Because 

WSR-88D (Weather Surveillance Radar – 1988 Doppler) or NEXRAD (NEXt generation 

weather RADar) has been useful for locating migrating birds and revealing migration patterns 

and important roosting sites of some species (e.g., Diehl and others 2003, Gauthreaux and Belser 

2003), we undertook a pilot field study to determine wheTHER it also might be feasible to use 

NEXRAD for locating important stopover sites used by migrating shorebirds in the prairie 

potholes landscape. Coordinated efforts to advance the applicability of radar technology to bird 

conservation are underway (Ruth and others 2005). 

Methods 

We selected the Doppler station in Aberdeen, South Dakota (Fig. 1), on which to center 

the pilot study because (1) topographical or other interference in that area is minimal, (2) it is 

within the Prairie Pothole region where large numbers of shorebirds stop over in migration, and 

(3) no-cost housing was provided for the field observer at Sand Lake National Wildlife Refuge, 

which lies within reach of Aberdeen’s Doppler radar beam (i.e., within 70 km of the station). 

Three maps were developed for the project. The first was a small-scale general locator map 

depicting all major roads, cities/towns, Universal Transverse Mercator (UTM) gridlines, and 

federal lands, along with two circles representing 20-km and 70-km distances from the Doppler 

station. (Twenty km is the distance within which there is generally too much ground clutter to 

safely use Doppler for biological purposes, and 70 km is the distance beyond which the Doppler 

beam may be too high to detect birds near their stopover habitat). The second map was a smallscale general locator map depicting all items listed above plus National Wetlands Inventory 

(NWI) wetlands (color-coded by type). The third map featured large-scale sections of the latter 

map for navigation and site identification in the field. 

1 
Fig. 1. Doppler NEXRAD station in Aberdeen, South Dakota. 

The study was conducted via close and constant communication between remote 

personnel monitoring and interpreting NEXRAD images with an on-the-ground observer in the 

Aberdeen Doppler range. We determined which areas to avoid for bird sampling due to 

anomalous propagation (e.g., false echoes resulting from superrefraction of the beam, such as at 

the proximal edges of the Prairie and Missouri coteaus). A crucial criterion for detecting birds in 

NEXRAD images is that they fly high enough to be detected by the NEXRAD beam. The 

2 
beam’s floor (and ceiling) height increases with distance from the Doppler station; thus, birds 

must be flying higher as the observer’s distance from the Doppler station increases. Height, 

however, is relative to elevation, both at the Doppler station and at the observer’s location. For 

example, if the observer detects a bird where the elevation is 200 m higher than it is at the 

Doppler station, and if the beam floor is 300 m at the observer’s location, then a bird would need 

to be flying at >100 m above the ground before radar could detect it. The beam’s floor height 

(relative to the Doppler station’s elevation) ranges from 43.5 m to 308.3 m at 20 and 70 km, 

respectively, from the Doppler station. We did not concern ourselves with the beam’s ceiling 

height (392.6 m to 1529.9 m at 20 and 70 km, respectively, from the station), as shorebirds flying 

that high were not likely visible to the observer. 

Our initial approach was for the observer to locate flocks (or individuals) of birds flying 

within the NEXRAD beam, determine the species, height, origin/direction of flight, number of 

birds in the flock, the observer’s location, wind speed/direction, date and time of observation, 

and any other potentially pertinent data (e.g., other factors that could affect NEXRAD images, 

such as smoke from a fire, airplanes, etc.). The observer used a Davis Turbo Meter Electronic 

Wind Speed Indicator to measure wind speed at the ground and a Laser Atlanta Optics 

rangefinder to measure distance from the observer to the flock (angle between observer and bird 

was also measured for subsequent calculations of flock heights). A handheld compass was used 

to determine flight directions. Observations were recorded on audio tape and then later entered 

into a spreadsheet. 

We compared the observer’s data to NEXRAD images captured simultaneously (i.e., 

same time and date) at the observer’s location to examine the radar images for useful 

information. While traveling throughout the Aberdeen Doppler range, the observer also reported 

sites currently in use by foraging or resting shorebirds (as well as sites used by flocks of other 

wetland species, such as ducks, waders, gulls, and swallows); later, NEXRAD images were 

scanned for evidence of shorebirds flying to and from those sites. In addition, NEXRAD staff 

directed the observer to locations where NEXRAD images indicated possible shorebird activity. 

Findings 

 Field work took place from 16–25 May 2005. It was immediately apparent that most 

shorebirds making their daily flights between foraging sites and roosting sites do not fly high 

enough to be detected by the NEXRAD beam. Rather, most shorebirds flew remarkably close to 

the ground, often barely skimming over the habitat surface. Birds were high enough to be in the 

NEXRAD beam only when flocks gained flight elevation for apparent long-distance migration 

movements; observations of those occurrences were rare. 

The American Golden-Plover (Pluvialis dominica) provided an exception. The observer 

discovered large flocks of these birds roosting and/or feeding in fallow (or planted but not yet 

sprouted) fields (Figs. 2a, 2b), and often discovered these flocks making local flights at fairly 

high elevations (depending on location relative to the Doppler station, sometimes high enough to 

be detected by the beam). The observer checked the plover roosting sites throughout the 10-day 

observation period, and on several occasions, witnessed large migration lift-offs as well as daily 

foraging flights. However, it was determined later that none of their flights were large enough to 

creatE a distinct pattern at the 1o

 x l km spatial scale of the radar image (i.e., whether one bird or a 

flock of 1000 is in flight, the radar imagery still displays reflectivity at a 1o

 x l km spatial scale.) 

Accompanying this report is an animated Microsoft Power Point® presentation that captures one 

3 
A 

B 

. 

Fig. 2. Two types of fallow fields used as stopover roosting and foraging sites by 

American Golden-Plovers west of Aberdeen, South Dakota, 25 May 2005. The arrow in 

Fig. 2A points to one of ~50 plovers using a fallow corn field, and the circled area 

includes a group of plovers using a fallow wheat field. Overall, approximately 600-800 

plovers were using fallow fields in the immediate area. 

4 
of the larger liftoffs of American Golden Plovers on 25 May 2005. Appendix 1 describes the content of 

the images. 

The observer recorded flights of other species (e.g., American white pelicans [Pelecanus 

erythrorhynchos], Franklin’s gulls [Larus pipixcan], horned larks [Eremophila alpestris]) that were high 

enough to be detected by radar. A data file containing information on nearly 600 flights of >40 bird 

species, including GPS coordinates and estimated height, exists and is available upon request. In future 

work, some of these observations may be useful to compare with archived radar data to form reference 

data for identifying these species in flight using NEXRAD. 

Clusters of apparent avian activity often aligned with landscape features. In our study area, large 

reflectivity values indicating significant bird activity were detected regularly north and east of the 

Aberdeen Doppler station. Upon investigation, it was determined that the area in question had an 

unusually high density of shelterbelts (relative to the overall region’s shelterbelt density) and riparian 

woodlands; thus, it is quite likely that the high reflectivity values in that area were due to passerine 

migration activity. 

NEXRAD staff also identified radar images that indicated flights of many birds using certain 

wetlands or wetland complexes, and, on several occasions, these findings led the observer to large 

numbers of shorebirds foraging and/or roosting at those wetlands. There was no way of knowing 

whether the radar images that led the observer to those sites were of shorebirds, ducks, or other 

waterbirds (or even swallows); however, by inspecting other wetlands of similar type and size within 

the same relative area, the observer often located even more shorebirds. In general, ducks, gulls, and 

swallows flew much higher than shorebirds as they made daily foraging flights, and may have 

accounted for much of the avian activity detected by NEXRAD during the study. It is likely that these 

species were depicted in the images that led the observer to sites hosting many shorebirds. 

Conclusions 

Techniques for interpreting Doppler radar data are not yet able to address the original purposes 

outlined in this study (i.e., distinguishing shorebirds from other “biologicals” on NEXRAD images, and 

using these images to locate important stopover sites in the Prairie Pothole landscape). Our 

recommendations for future investigations regarding applications of NEXRAD information are 

presented below. 

To continue efforts to develop specific shorebird “signatures” from radar images, one needs to 

select Doppler stations that cover sites used by large numbers of shorebirds at times when few other 

species are likely to occur there, or when other species would be flightless (e.g., early southbound 

migration of shorebirds coincides in many regions with periods when waterfowl are flightless due to 

wing molt). Candidate sites include American Reservoir near Idaho Falls, Idaho; Bear River Migratory 

Bird Refuge near Salt Lake City, Utah; and Lake McConaughy in west-central Nebraska. By acquiring 

NEXRAD images of shorebirds that are not “contaminated” by flights of other bird species, one could 

possibly examine flight velocities as a means of distinguishing shorebirds (or individual species of 

shorebirds) from other groups of birds. Radar ornithologists and entomologists have been able to 

differentiate flights of birds from insects on the basis of velocities. Once shorebird “signatures” are 

developed, the information theoretically could be used to detect shorebirds in NEXRAD images 

elsewhere. 

It may be feasible to use radar data to develop landscape- and climate-based models to identify 

areas of importance to waterbirds in general. By overlaying existing landscape data with liftoffs from 

radar images, we may be able to correlate bird use with landscape structure (elevation change, wetland 

5 
cover and density, wetland types, grass cover, etc.) in dry and wet years. This could allow us to 

distinguish year-to-year shifts in bird distribution between wetland landscapes and give insight into the 

temporal dynamics of water conditions and habitat suitability for shorebirds. 

Acknowledgments 

The Prairie Pothole Joint Venture and the U.S. Geological Survey provided the funding to conduct this 

feasibility study. We thank William Schultz and the Sand Lake National Wildlife Refuge for housing 

and logistical support. Diane Schneider prepared the locator and NWI maps for the field work and 

image interpretation. Robb Diehl and Wylie Barrow provided consultation and guidance; they also 

provided interpretation of images as NEXRAD staff. 

References Cited 

Diehl, R.H., Larkin, R.P., and Black, J.E., 2003, Radar observations of bird migration over the Great 

Lakes: Auk, Vol.120, p. 278-290. 

Gauthreaux, S. A., Jr., and Belser, C. G., 2003, Radar ornithology and biological conservation: Auk, 

Vol. 120, p. 266-277. 

Ruth, J.M., Barrow, W.C., Sojda, R.S., Dawson, D.K., Diehl, R.H., Manville, A., Green, M.T., Kreuper 

D.J., and Johnston, S.,2005, Advancing migratory bird conservation and management by using radar: 

an interagency collaboration: Open-File Report 2005-1173, Fort Collins, Colorado: U.S. Geological 

Survey, Fort Collins Science Center, 12 p. 

Skagen, S.K., 1997, Stopover ecology of transitory populations: the example of migrating shorebirds. 

Ecological Studies, Vol. 125, p. 244-269. 

6 
Appendix 

Animated NEXRAD Image for a liftoff of American Golden Plovers, 25 May 

2005, near Mina, South Dakota, USA. 

The following link is to a Microsoft PowerPoint® file (Appendix Mina_AGPL.ppt) contains 

animated versions of the two slides of NEXRAD imagery. 

http://www.fort.usgs.gov/products/publications/21596/Appendix_Mina_AGPL.ppt 

2005 May 25 

18:55 – 21:03 CDT 

Observation sites 

Slide 1: This image depicts the exodus of American Golden Plovers from stopover habitat as 

captured by the Aberdeen, South Dakota radar on 25 May 2005. Ground-based observations indicated 

that most of the bird activity occurred between 19:05 and 19:30 CDT (00:05–00:30 GMT). The 

animation runs from 18:55–21:03 CDT (23:55–2:03 GMT) in 5–6 minute increments. Rain is moving 

through the image from the northwest toward the southeast. Dark blue and green hues near the Mina 

observation sites represent the greatest numbers of birds. 

7 
Rain 

Observation sites 

Slide 2: Reflectivity data (left) generated by American Golden Plovers departing from stopover 

habitat near the Aberdeen, South Dakota radar on 25 May 2005 at 19:15 CDT. Dark blue and green hues 

represent the greatest numbers of birds. Rain appears at the bottom of the image. The radial velocity 

data (right) shows the speed and direction of movement. Green hues indicate movement toward the 

radar and red hues indicate movement away from the radar. The echoes near the observation sites are 

indicative of biological activity because (1) the rain in this area appeared in multiple elevational sweeps 

while the reflectivity near the sites is only in the lowest elevational sweep, (2) the radial velocity data 

show that rain is moving towards the southeast, while targets near the sites are moving north, and (3) the 

spectrum width for the targets near the Mina observation sites is in the 0-6 range. 

8