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:

1. 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.
2. Subadult eagles - Juvenile eagles arrive just after adult eagles, and their percentage of the whole rises throughout the winter.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.

Back to Dances with Eagles

*Along the "upstream" and "downstream" stretch of river, eagles were counted weekly along the Skagit River.

Chart and text summarized from Long-term Trends of Bald Eagles in Winter on the Skagit River, Washington, Journal of Wildlife Management, 65(2):290-299.