Final Report of the NALCC funded project “Identifying Important Migratory Landbird Stopover Sites in the Northeast”.
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. We used datacollected during Fall 2008 to 2014 by 16 NEXRAD and four terminal Doppler weather radars(TDWR) in the northeastern U.S. to map and study the spatial distribution of landbirds shortlyafter they leave daytime stopover sites to embark on nocturnal migratory flights. Given observedvariability in the precise timing of migratory exodus, we developed a new method to sample theonset of migration at the point of maximum rate of increase in bird densities aloft to consistentlysample exodus across radars and days.
The mean linear trend in aggregate stopover densities of migrants indicated a 4% declineper year from the 2008 baseline density (29% decline over the seven years). Regionally, coastalVirginia and Maine had the steepest declines. The steepest increases in migrant densities acrossyears occurred within the Delmarva Peninsula and in coastal Connecticut.
We used NEXRAD observations to develop models to predict potentially importantstopover sites throughout USFWS Region 5. Observed NEXRAD data were positively correlatedto observations from TDWR and NASA’s S-Band Dual-Polarimetric Radar (NPOL), though not strongly. Predicted densities increased with increasing hardwood cover across multiple scalesand with vegetation productivity. Contrastingly, predicted densities decreased with increasingagricultural, emergent marsh and coniferous land cover, but did not change with fraction ofurban cover. Stopover density increased closer to bright areas and the Atlantic coast. Moreover,interactive effects indicated that migrants were more concentrated in forested areas that wereboth brightly lit and near the Atlantic coast. Large areas of predicted regionally importantstopover sites were located along the coastlines of Maine, Long Island Sound, New Jersey, thelower Delmarva Peninsula, within the Adirondack Mountains, Catskill Mountains, and easternVirginia.
We also created maps of classified stopover use during bimonthly periods and at multiplescales.Migrant densities peaked along the Adirondack Mountains early in September, and alongthe Atlantic coast in late September with the passage of Neotropical migrants. Stopover densitiespeaked in the most northern extent of Maine and New England States in late October with thedeparture of temperate migrants.
Ground surveys conducted at 48 forested sites within the Delmarva Peninsula andTidewater Virginia during Fall 2013 and 2014 revealed that nocturnal migrant densities pooledacross species and for 14 individual species, after accounting for temporal phenology in theirpassage timing, were related to factors operating at multiple scales including food resources(primarily arthropod abundance in understory) and understory shrub density at a patch scale, andlatitude and proximity to the Atlantic coast at a regional scale.
We integrated field survey and radar data to estimate relative stopover duration and toidentify stopover functional types among 45 sites that included data from a past study near theGulf of Mexico. We identified four functional types spanning the gradient of short rest stops torefueling stops with variable duration of stopover in relation to food abundance. The MidAtlanticsites were dominated by rest stops near coastal areas and lacked quick refueling stopsdue to low overall food abundance. The maps and ecological understanding produced can helpinform conservation planning to protect and enhance stopover sites for migratory landbirds in thefuture.
