Bald Eagles, Illinois

1. Kelsi Potterf
Population Projection of
Bald Eagles: A Comparison of
Techniques

2. Data Origin
• Global Population Dynamics Database
▫ Havera and Kruse (1988)
 Audubon Society [Ground Counts]
 Illinois Natural History Survey [Aerial Counts]
 Elton Fawks [Ground Counts & Aerial Counts]
• Current known population data between 2010-2013
▫ ENF (2013)
 24 hour ground count
• Mid-winter Bald Eagle Counts

3. Study Area
• Mississippi and Illinois River Floodplain, Illinois
• Counts from 1957-1987
• Slightly different between studies

4. Figure 2. Inventory sites for
bald eagles in Illinois conducted
by the Illinois Natural History
Survey, 1972-1987 (Havera &
Kruse 1988). These sites are
indicated by the black hash
marks, bold circles and bold
squares. Counts conducted by
the Audubon Society generally
occurred near chapter locations
(Havera & Kruse 1988)
indicated by the unfilled circles.
Inventory sites of the Fawks
surveys occurred north of the
bold black line to the northern
border of the state as deducted
by locations given by Havera &
Kruse (1988).

5. Figure 1. Bald eagle population size from 1957 to 1987 from each survey method reported by
Fawks, the Audubon Society, and the INHS as provided by Havera and Kruse (1988).

6. Figure 1. Bald eagle population size from 1957 to 1987 from each survey method reported by
Fawks, the Audubon Society, and the INHS as provided by Havera and Kruse (1988).

7. Thoughts
• “Couldn’t you find the actual population size and
compare each method to that?”-comparison of
method accuracy
• Actual population size is only truly known when the
population is closed and when we can obtain a
census
• We accept the data as truth-philosophical approach

8. Goals 1. Do these methods
produce significantly
different results?
2. What are the growth
rates of the population
and potential future
population size given
the data?
3. And how do these
projections differ from
the current known
population size?

9. Data Analysis
1. Frequentist Approach (SAS)
▫ 8 years of common collection (1973-1980)
▫ Student’s T-test
2. Estimation of growth rate and best fit linear
model and average lambda (Excel)
3. Frequentist Approach (SAS)
▫ Population projections from all 3 surveys compared
against the current known population data
▫ Student’s T-test

10. Results/Conclusions
1. Raw data; P and T values conclude-Ha: Fawks ≠
INHS, Fawks ≠ Audubon, Audubon = INHS
2. Trendlines with maximized correlations; each
survey technique predict a different growth model
3. Growth rate predictions from the Fawks study
differ statistically from current known population
sizes
Why?

11. Figure 3. Bald Eagle population growth trends across the 3 survey types projected into the present where the current
known population data is represented by ENF when 8 years of common data are included.
y = 78.3x + 532.39
R² = 0.5335
y = 41.983x + 285.86
R² = 0.4065
y = 48.783x + 303.86
R² = 0.4382
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
PopulationSize
Year
Fawks
Audobon
INHS
ENF
Linear (Fawks)
Linear (Audobon)
Linear (INHS)

12. Figure 4. Bald Eagle population growth trends across the 3 survey types projected into the present
where the current known population data is represented by ENF when all years of data are present.
y = 336.94e0.0495x
R² = 0.7551
y = 23.756x0.9689
R² = 0.2917
y = 27.017x - 7.6913
R² = 0.6523
0
1000
2000
3000
4000
5000
6000
PopulationSize
Year
Fawkes
Audobon
INHS
ENF
Expon. (Fawkes)
Power (Audobon)
Linear (INHS)

13. Discussion
• Not all models are true, but some are helpful.
• Philosophical approach to data collection and
interpretation
• Different estimations of abundance and growth
rate can affect the way we manage a species
• This specific example: determine when we can
delist or predicting when recovery will occur

14. Questions???