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Background/History of the Survey
Each January, the U.S. Geological Survey, Forest and Rangeland Ecosystem
Science Center's Snake River Field Station (SRFS) coordinates the
Midwinter Bald Eagle Survey, in which several hundred individuals count
eagles along standard, non-overlapping survey routes.
Nationwide counts of eagles were coordinated by the National Wildlife
Federation from 1979 until 1992, when the Raptor Research and Technical
Assistance Center (now SRFS) assumed responsibility for overseeing the
count. 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-seven 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 38 to 49, and the number of
standard survey routes per state ranges from 1 to 80.
This web site reports results of an evaluation of data from 1986-2000. The
analysis was based on 101,777 observations of eagles during 5,180 surveys
of 563 routes in 42 states. (See 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.) for a
full explanation.
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.
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