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Suggestions for Analyzing Bald Eagle Population Data

Every ten years the United States takes a census of all the people living in the country; Americans are required by law to fill out census forms. Too bad the law can't require Bald Eagles in the U.S. and Canada to fill out a census form, too! Then we'd know exactly how many of them are living here with us people.

Even if we can't get precise numbers for such a widespread species, we can get a picture of how healthy eagle populations are on the basis of data collected in various ways during the breeding season, migration, and winter.

Eagle Numbers on Christmas Bird Counts 1900-2000

History Is for the Birds: 101 Years of Christmas Bird Count Data

Probably the longest continuous set of data comes from the Christmas Bird Count established in 1900 and administered by the National Audubon Society. More than 50,000 participants now take part in Christmas Bird Counts. But when the CBC was started by Dr. Frank Chapman, there were nowhere near as many participants. So the apparent low number of Bald Eagles during the first years were is due more to the fact that so little area was covered than to how few eagles there were. But count numbers and participants grew steadily, especially in the 60s and 70s, and yet eagles found on Christmas Bird Counts stayed low.

Try This! Make an Eagle History Timeline
Click on the above graph and print the enlarged version. Using an almanac, encyclopedia, the Internet, and/or other reference materials, find the dates when the following things happened and mark them on the graph to make an Eagle History Timeline.

  • Bald Eagle Act passed
  • Endangered Species Act passed
  • Eagle officially taken off Endangered Species List
  • World War I
  • World War II
  • DDT discovered by Swiss chemist Paul Muller
  • Muller wins the Nobel Prize because DDT is so useful for killing lice on soldiers and for killing mosquitoes that carry malaria
  • Dr. Joseph Hickey determines how much DDT kills half a population of robins
  • Dr. Joseph Hickey organizes a symposium bringing together the top ornithologists in the world to figure out how DDT is destroying Peregrine Falcon and Bald Eagle reproduction
  • Rachel Carson's Silent Spring is published.
  • President Richard Nixon's administration bans DDT for most use in the U.S.

Do events on the timeline seem related to eagle numbers?


Graphing 30 Years of Lake Superior Data

The migration figures below represent the annual seasonal Bald Eagle count totals for an overlook on a migration pathway at the western tip of Lake Superior called Hawk Ridge. This represents just a fraction of North America's Bald Eagles, and daily autumn records have only been kept since 1972, but the numbers correspond nicely with counts at other overlooks, breeding bird censuses, and winter counts (including the Christmas Bird Count). Using the population data from 1972 to the present, make the following 3 graphs to depict the population levels for this once-endangered species.

  • The number of Bald Eagles counted at Hawk Ridge per year.
  • The increase in the number of Bald Eagles per year.
  • The percent increase in the number of Bald Eagles per year.

Changing Eagle Population
1972-Present*
Year # Eagles Year # Eagles
1972

31

1987

166

1973

41

1988

261

1974

93

1989

396

1975

60

1990

422

1976

80

1991

582

1977

68

1992

982

1978

79

1993

1725

1979

86

1994

4368

1980

82

1995

1953

1981

149

1996

3293

1982

207

1997

2407

1983

384

1998

3289

1984

376

1999

3155

1985

159

2000

1919

1986
332
2001
3307
    2002
3837
 
2003
3356


Discussion Questions
  • How do your graphs compare with the corresponding years of the Christmas Bird Count graph?
  • Describe the different visual picture each graph gives you. Was it helpful to graph the data in these different ways? How?
  • Think about what each graph means. What different information does each show you?
  • Which graph do you think is the most revealing?
  • Are there times when the numbers appeared to increase significantly, but the percentage increase was not as pronounced?
  • What questions about the climb in population size do you have?
  • What reasons do you suppose are behind these changes?
  • List all the factors you can which might have caused the population to rise (or fall) at various points.
  • During which decade did the population increase at the fastest rate?
  • What factors do you think could change the numbers at a given location (e.g., weather)? Explain why you think scientists don't pay much attention to year-to-year changes in a count like this, but DO pay attention to graphs with many years of data.

To learn more about Bald Eagle population numbers and the history of this species, see Bald Eagle History.


National Science Education Standards

  • Use data to conduct a reasonable explanation.
  • Think critically and logically to make relationship between evidence and explanations.
  • Scientists develop explanations using observations (evidence) and what they already know about the world.
  • Humans change environments in ways that can be either beneficial or detrimental for themselves and other organisms.
  • Changes in environments can be natural or influenced by humans. Some changes are good, some are bad, and some are neither.

 

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