Midwinter Bald Eagle Survey

Background/History of the Survey

Each January, several hundred individuals count eagles along standard, non-overlapping survey routes as part of a nationwide Midwinter Bald Eagle survey .

Nationwide counts of eagles were coordinated by the National Wildlife Federation from 1979 until 1992, when the Bureau of Land Management's Raptor Research and Technical Assistance Center assumed responsibility for overseeing the count. Responsibility for count coordination shifted to the National Biological Survey (1993-1996) and later to the U.S. Geological Survey (USGS), Forest and Rangeland Ecosystem Science Center, Snake River Field Station. In April 2007, the USGS established a partnership with the U.S. Army Corps of Engineers (USACE) to maintain the long-term, national coordination of the survey, data analysis, and reporting. USACE will assume responsibility for coordinating the national survey, organizing the results, maintaining the long-term database, and jointly compiling, analyzing, and reporting survey data gathered with USGS following previously used methods (Steenhof et al. 2002) in 2010 and 2015.

Initial objectives of the survey were to establish an index to the total wintering Bald Eagle population in the lower 48 states, to determine eagle distribution during a standardized survey period, and to identify previously unrecognized areas of important winter habitat. In 1986, Millsap (Wildl. Soc. Bull. 14:433-440) reported results of the midwinter survey from 1979 through 1986. Beginning in 1984, National Wildlife Federation officials asked participants in each state to count eagles along standard routes to provide data on count trends. Standard survey routes were defined as clearly described areas where eagles had been observed in the past. Federation guidelines stipulated that standard surveys be conducted by the same number of experienced observers using the same method (e.g., fixed-wing, helicopter, boat, vehicle) at approximately the same time of day each year.

Observers conduct surveys on standard routes during the first 2 weeks of January each year, usually on 1 of 2 target days. Most survey participants are employees of state or federal conservation agencies, but private volunteers also participate in the survey. Coordinators from each state are responsible for organizing local counts, enlisting survey participants, and compiling data to eliminate duplicate sightings and overlapping routes. Sizes of survey routes vary from single fixed points to 150 miles. Approximately 44% of the surveys are conducted from vehicles. 18% are conducted from fixed wing aircraft; 8% are collected from boats; and 7% are conducted by helicopter. Due to weather and staffing limitations, not all standard routes are surveyed every year. Twenty-five states identified and began surveying standard routes in 1986; other states did not begin standard surveys until the mid-1990s. Some states stopped participating in the count in the 1990s. The number of states participating each year has ranged from 25 to 41, and the number of standard survey routes per state ranges from 1 to 84.

This web site reports results of an evaluation of data from 1986-2005. The analysis was based on 178,896 observations of eagles during 8,674 surveys of 746 routes in 43 states. Trends were estimated according to procedures outlined by Steenhof et al. (2002): Steenhof, K., L. Bond, K.K. Bates and L.L. Leppert. 2002. Trends in midwinter counts of Bald eagles in the contiguous United States, 1986-2000. Bird Populations 6:21-32.( see full text here).

As a large-scale volunteer effort that developed over many years, the Midwinter Bald Eagle survey has inherent problems. Many reports we received could not be used because of incomplete documentation or inconsistent survey methods. Because survey routes were not randomly selected, we do not know if the standard routes used in this analysis are representative of the contiguous 48 states. Our findings are likely biased towards states and portions of states where agencies and individuals were committed to long-term, consistent data collection. We have assumed that winter counts are a reasonable index to eagle abundance at the areas surveyed during the January sampling period. Trend analyses based on counts as indexes are valid only if the proportion of the population sampled is constant from year to year. The ability to detect eagles on survey routes may vary with many factors, including weather, topography, and vegetation, and we are assuming that errors in detectability are consistent from year to year on a given survey route. We have controlled for variation in detectability by including only those surveys that covered the same area, using the same transportation method each year. Varying ability of individuals to detect and identify bald eagles is likely not as much of a problem in midwinter eagle surveys as it is in Breeding Bird Surveys and other singing-bird surveys. The annual midwinter survey represents a unique source of long-term, baseline data. Unlike nesting surveys, it provides information on both breeding and nonbreeding segments of the population at a potentially limiting time of year. It also provides an opportunity to monitor modifications or threats to habitat at important wintering areas. The count has become a tradition that will likely continue in many states. In addition to providing information on eagle trends, distribution, and habitat, the count has helped to create public interest in Bald Eagles and their conservation.