This dataset represents the observed and predicted relative bird density during autumn migratory stopover within the Northeast and Mid-Atlantic U.S. as measured by NEXRAD weather surveillance radar during the periods of peak landbird migration (15 August to 7 November) during 2008 through 2014. The dataset also includes measures of land cover characteristics, vegetative productivity, and geographic context used in the models to predict bird stopover use. Observed data are present only in radar-sampled areas (see below for description on how these data are filtered) while predicted data are modeled across the entire Northeast U.S.

The dataset was originally developed as supplemental information for the cooperative agreement (#F13AC00402) between the U.S. Fish and Wildlife Service and the University of Delaware. The data contributed to a larger study to validate NEXRAD radar observations of migrating birds and relate them to bird densities on the ground (for a list of NEXRAD radar stations see: https://www.weather.gov/media/tg/wsr88d-radar-list.pdf). Partners for this project include University of Delaware, Old Dominion University, and USGS.

Please see the accompanying report for details: Buler J. J., J. McLaren, T. Schreckengost, J. A. Smolinsky, E. Walters, J. A. Arnold, D. K. Dawson. 2017. Validation of NEXRAD data and models of bird migration stopover sites in the Northeast U.S. Final Report.

Intended Uses

The dataset is intended to be used by land managers and conservationists looking to identify priority areas for bird conservation by providing estimates of autumn bird stopover use based on weather surveillance radar data.

Description and Derivation

The national network of weather surveillance radars (NEXRAD) detects birds in flight and has proven to be a useful remote-sensing tool for ornithological study. These maps were derived from data collected during Fall 2008 to 2014 by 16 NEXRAD and four terminal Doppler weather surveillance radars (TDWR) in the northeastern U.S. to map and study the spatial distribution of landbirds shortly after they leave daytime stopover sites to embark on nocturnal migratory flights. NEXRAD observations of Vertically-Integrated Reflectivity (VIR) - an estimate of the total amount of reflected cross-sectional area of birds per hectare (i.e., bird density) from 0 to 1750 m above the ground in units of cm2 ha-1 - were summarized across time and aggregated to a spatial resolution of 1 km x 1 km. Observed data (i.e., reflectivity data as measured by radars) have been filtered to exclude areas of partial radar beam blockage, ground clutter (i.e., non-biological reflectivity from objects on the ground) and where radars were unable to detect birds for more than 25% of sampling nights.

This dataset represents input and output variables of Boosted Generalized Additive Models (BGAM) developed to predict bird stopover use throughout the Northeast and Mid-Atlantic U.S. Predicted bird densities increased with increasing hardwood cover at multiple scales and with vegetation productivity. Contrastingly, predicted bird densities decreased with increasing agricultural, emergent marsh and coniferous land cover, but did not change with fraction of urban cover. Bird stopover density increased closer to bright areas and the Atlantic coast. Moreover, interactive effects indicated that migrants were extra concentrated in wooded areas that were both brightly lit and near the Atlantic coast. Large areas of predicted regionally important stopover sites were located along the coastlines of Maine, Long Island Sound, New Jersey, the lower Delmarva Peninsula, within the Adirondack Mountains, Catskill Mountains, and eastern Virginia.

Migrant densities peaked along the Adirondack Mountains early in September, and along the Atlantic coast in late September with the passage of Neotropical migrants. Stopover densities peaked in the most northern extent of Maine and New England states in late October with the arrival of temperate migrants. The maps and ecological understanding produced can help inform conservation planning to protect and enhance stopover sites for migratory landbirds in the future.

Known Issues and Uncertainties

We caution against relying too heavily on our region-wide predictions to assess the relative importance of sites outside of radar-sampled areas. The predicted bird densities within radar-sampled areas agreed quite well with radar-observed densities (explained Deviances of BGAM models predicting mean bird density were around 0.7). However, the accuracy of predicted bird densities elsewhere remains unvalidated. Additionally, the results are not highly precise; they are depicted at a resolution of 1 km by 1 km. Within an area of this size, suitability for migrants may vary dramatically depending on the types of land use and development present. Large, intact, productive hardwood forests that provide an abundance of insects and fruits appear to be particularly valuable to migrants.

Thus, the region-wide map should be viewed as a coarse and preliminary guide for conservation purposes.

NOTE - The data represented were converted to raster for display purposes. The download includes the original shapefile and layer files. In order to view the attributes of a cell using the identify tool, select the record for “Attributes for Migratory Landbird Stopover Habitat Data” (dataset must be turned on). Attribute definitions can be found in the Attachments tab on Data Basin or in the metadata available in the download package.