Long-term Bald Eagle Research on the Skagit River
 Research
Summary
Who do you think plays in the Skagit River Bald Eagle Band?
North Cascades long-term monitoring 'recording studio' has collected
data on bald eagles since the 1970s. Researchers have a good idea
who is in the band.
Members of the Band
In music, each member of a band affects every other member, and
without each part, the music would not sound the same. In scientific
terms, the members of an ecosystem are called variables. Scientists
look at each variable that affects the Skagit River ecosystem to
understand more about the eagle's role. Scientists have discovered
the following variables in the Skagit River Eagle Band:
- Eagles - A small number of bald eagles live along the Skagit
all year. The bulk of the wintering eagles arrives in the Skagit
in November. Their population peaks during the first three weeks
of January and most are gone by the end of February.
- Subadult eagles - Juvenile eagles arrive just after adult eagles,
and their percentage of the whole rises throughout the winter.
- Chum salmon on the Skagit River - Eagles are thought to come
to the Skagit and other areas to feed on chum salmon runs that
peak at the same time as the eagles numbers. Notice how chum and
eagle population lines dance together from year-to-year before
1989 in the graph above.
- Chum salmon in nearby rivers - The health of salmon runs on
nearby rivers—miles from the Skagit—affect regional
eagle populations. When salmon runs are down on nearby rivers,
the Skagit River may take up the slack because of the Skagit's
good salmon run.
- River discharge - Skagit River flooding varies each winter affecting
salmon spawning and eagles. During the first ten years of the
study, fewer eagles were detected during years of frequent winter
flooding. Flooding may affect the number of chum in the river,
and/or the ability of eagles to catch them or reach the carcasses
of chum that have spawned and died.
- Recreational use - A new member of the Skagit River Eagle Band
has entered the band: humans. Although tribes have lived in the
region for thousands of years, only very recently have we dammed
the river, fished the rivers from motorized boats and used industrial-sized
nets. Scientists seek to find out how we and our modern tools
influence the overall melody.
- Unknown players - What undiscovered variables do you think may
affect eagles in the Skagit River Watershed?
Rhythm and Tempo
Place your mouse over the graph above. Before 1989 the peaks and
valleys of each of the lines are well synchronized (the data follows
the pattern together). When chum salmon were abundant in the Skagit,
there were also many eagles. Notice that in 1989 the tempo changed
dramatically. Numbers of downstream* eagles climbed while chum counts
on nearby rivers fell dramatically. In the meantime, chum counts
along the Skagit stayed about the same. Why did eagles suddenly
appear on the Skagit? Researchers hypothesize that since chum counts
of four nearby rivers fell, eagles moved to the Skagit for food.
Ultimately, scientists search for higher-level answers so we can
better understand how to limit our impact on natural systems. What
caused this change in tempo? Why did chum populations drop dramatically
in 1992, and recover? Who's leading this band anyway?
 Subadult
Members
Young generations are important to all populations. Invariably
the young ones will take the lead when older ones die. Scientists
study juvenile populations to learn about the health and reproductive
rate of a population. Percentage of juveniles can also indicate
species-wide behavior changes.
Notice the graphs at right. Scientists are finding that subadult
bald eagles are becoming a smaller percentage of the overall population
on the Skagit and other rivers over the past 20 years.
What do you make of these trends?
New Members of the Band
Other members have entered the band who were not members until
recent times. Humans have started playing, and sometimes we are
not playing in the same key. By studying how humans affect the rivers
where eagles live, scientists hope to learn how we can turn our
noise into harmony.
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