The Problem
The Solution
Our Approach

THE PROBLEM

     More than half the songbird species that breed in North America are migratory, and many of these are classified as "neotropical." That is, they travel to subtropical and tropical regions of the Western Hemisphere to winter. Every year, millions of songbirds migrate tremendous distances between breeding grounds in North America and wintering grounds in the tropics. 
     The energy required to complete the northbound or southbound phase of migration is considerable. Small migrating songbirds generally cannot store enough energy to fly nonstop between wintering and breeding destinations. Consequently, they must stop frequently along the migration route to rest and replenish energy stores.
     The availability of suitable "stopover" habitats along the migration route that can provide the food resources necessary for birds to accumulate energy quickly and safely is critical to their ability to complete migration successfully. Because migratory songbirds must stop at series of locations en route to their seasonal destinations, they are especially vulnerable to alterations in the landscape. If one of these stopover areas is destroyed or degraded, it may compromise a bird's ability to reach its goal. 

THE SOLUTION

     One of the most exciting developments in avian conservation is the use of NEXRAD to map stopover areas used by migrating songbirds. Radar data collected as migrants depart from stopover sites can provide very strong evidence for the location of important resting and refueling areas. 
     Combining radar data with Geographic Information System (GIS) data provides a method for determining the frequency of stopover site use and the relative the number of birds using those sites. By combining radar data with data on land use and land cover types, possible links between the use of stopover sites and specific habitat types can be determined. 
     These data can be used to compare and rank the relative importance of particular stopover sites. Given the likelihood that not all stopover sites can be protected, understanding which sites are most important is essential to developing effective land protection and management strategies.

NEXRAD image (above) from DIX, 12 Sep 2001, 1101 PM EDT (13 Sep 2001, 0301 GMT) showing extensive bird migration event.	NEXRAD image (above from DIX, 12 Sep 2001, 0724 PM EDT (13 Sep 2001, 0301 GMT) showing the beginning of the migration event shown above.

OUR APPROACH

• During spring and fall, and spring 2002, we collected radar data at 6-10 min intervals from the DOX and DIX radar stations.

• We identified nocturnal migration events using radar data generated approximately three hours after sunset (see example, above left), and followed the guidelines previously described to discriminate between bird migration and other atmospheric phenomena like precipitation or insects.

• For nights with migration, we imported radar images generated 20-40 min after sunset into ArcView® 3.2, a GIS software program. We used these images because this is the approximate time many songbirds start their nocturnal migrations (see example, above right). 

• The imported migration “departure” images were layered (see below) to create composite radar images. 

• We used composites to examine bird density (lower left) and frequency of stopover site use (lower right) in the coverage area of each station, for each season.

• Using ArcView, we merged composites of bird density and site use frequency to identify areas with the highest values (90th percentile) for both elements. 

• Composite images were overlaid onto New Jersey land-use/land-cover data provided by Rutgers University's Center for Remote Sensing and Spatial Analysis (below) to compare habitat composition (e.g., forested wetland, forested upland) in important stopover site areas, with areas not identified as important.