Eagle Roost Count 1-17-10

gengberg January 19th, 2010

Hello all:Our count yesterday went off well even though there were not many eagles to count. We collectively saw 59 birds as compared to 101 (originally reported as 98 birds) birds on January 3.Eagle Tree Small Originally I reported this total as 98 eagles but counters at Fox Roost saw an additional three birds that were not likely seen elsewhere so these birds were added to the count after the summary was sent out.

The ice cover on the river has remained extensive between Jan. 3 and Jan. 17. Even though some pools are currently open, especially near bridges, the open pools are still relatively small for eagle foraging. As before, under conditions where ice cover is great down river eagles normally concentrate in the immediate Sauk Prairie environs where open water persists even in the coldest weather. Though 23 eagles were seen perched on Eagle Island December 19, 2009, eagle numbers for the upper stretch of the Lower Wisconsin have not remained high even though ice cover has. If this trend continues through the remainder of this winter this would be the third year in a row where eagle numbers have been low at times where ice cover down stream is extensive.

Of course, why eagle numbers have not concentrated in the Sauk Prairie area is not yet completely understood. Fish numbers, especially shad numbers are down in the Lower Wisconsin River so the absence of eagles upriver could be explained by the absence of a primary food source.Eagle Island 1 Small If true, however, the number of eagles located down river has been normal or higher than normal for the past three winters when the river was open from Prairie du Sac to Lone Rock. Low fish populations are occurring throughout the entire stretch of river and not just in the up river stretch so eagle distribution does not completely correlate with fish numbers. This phenomenon keeps alive the hypothesis that eagles are reluctant to concentrate in the Sauk Prairie area, at least for a long period of time. If eagles are still adjusting the condo development near Eagle Island then perhaps the large number of eagles perched on Eagle Island in late December, but not thereafter, indicates that disturbance is still a problem but something the birds will slowly acclimatize to.

This winter, eagle numbers along the Baraboo River have also been high. More than 85 eagles were counted during the late December Christmas Bird Count in the Baraboo area, almost twice the normal count. Most of the counted birds were located along the Baraboo River near Circus World Museum. The Baraboo River has had it’s three dams removed in the last decade and shad populations have exploded in the last year. Perhaps eagles normally using the Sauk Prairie area under extensive ice conditions shifted their distribution to this open area of the Baraboo River that now contains shad. With our radio tracking project, eagles captured in the Sauk Prairie area did use night roosts in Devil’s Lake and at Wisconsin Dells and did feed along the Baraboo River but not near the Circus Museum so this sort of shift is well within the winter behavior repertoire of birds in the Sauk Prairie population.

To investigate this hypothesis, Sharon Reilly organized some volunteers and attempted to count eagles during our roost count on Sunday. They saw 3 adults and 1 immature but did not locate the night roosts. Even so, they surveyed the area thoroughly enough to conclude that the large number of eagles along the Baraboo River may have moved out of the region by now. Unfortunately this finding is confounded by the increasing amount of disturbance in the Circus World Museum area as people try to see the birds up close.

The DNR also conducted it’s mid-winter aerial survey on January 6, 2010 and counted only 19 eagles along the stretch of river from Prairie du Sac to Lone Rock, the same stretch where we counted 76 eagles on January 3, 2010. This FBEC total excludes the off-river night roosts that would not be covered by the DNR survey which follows the river only and is conducted during the day. The Baraboo River is not counted on the DNR aerial survey either. Eagle numbers may have dropped along the Wisconsin River quickly following our January 3 count. Thus the question as to why eagle numbers have been low in mid-winter remains.

Though data from our collective surveys are as yet inconclusive, they are helpful in solving this puzzle. If future shad populations recover on the Lower Wisconsin River, eagle numbers should again concentrate in the Sauk Prairie area if food is the primary driver determining the distribution of eagles along the Wisconsin River during times when ice cover is extensive. If shad populations recover and eagle numbers do not rebound to numbers seen in the first half of this decade then other factors may also be important in determining eagle numbers. Certainly breeding numbers of eagles in the Upper Midwest continue to increase, even if more slowly. The eagles wintering in the Sauk Prairie area come from breeding areas located in the Upper Peninsula of Michigan, northern Wisconsin, and northeast Minnesota so there is no evidence that the overall population is declining.

Among other factors that may influence winter eagle distribution in southern Wisconsin is human disturbance from development. Fortunately the DNR continues to conduct their fish surveys in the Lower Wisconsin River and we continue to collect roost count data. FBEC is also developing some research focus on disturbance itself so that we can better understand how or when people and eagles co-exist. Since we do not have active graduate students doing more intensive research that currently focuses on eagle movements we must rely on our more general survey data and our developing disturbance research to understand these complicated relationships between eagles, people and other aspects of their environment. As the monitoring effort is less intensive our data collection must be more extensive. Your continued efforts greatly assist that effort. Thank you.

Jeb

Count Results:

Adult Imm. Unkn. Total

Blackhawk 1 1

Sugarloaf 12 7 3 22

Ferry Bluff 2 8 10

Fox 0

McDonald 0

Jones Slough 1 1

Lone Rock 2 8 10

Big Hollow 1 1 2

Ederer/Been 1 1

Leland 8 4 12

TOTAL 28 12 19 59

Volunteer Hours: 36

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