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December 16th count along Wisconsin River totals 123 eagles

 

January 3, 2019

Photo contributed

The following report on eagle counts along the Wisconsin River was written by Jeb Barzen of the Ferry Bluff Eagle Council.

On December 16, 2018, Ferry Bluff Eagle Council volunteers collectively tallied 123 eagles through 34 volunteer hours invested. Though early in the winter, as of yet, this is a reasonably large number of eagles to encounter, especially with such warm conditions. As with most warm periods in winter, the river is wide open and the eagles tend to be located downstream of Sauk Prairie. A minimum of 94 eagles were counted at the Lone Rock Roost and 13 eagles at Ferry Bluff whereas only 3 eagles were seen using Blackhawk Roost (and none in Sugar Loaf). Thus, lots of migrants are here but they are not as visible, feeding on isolated stretches of river and then flying to a relatively isolated roost.

Apropos for this year, Ferry Bluff Eagle Council partnered with Groundswell (a local land trust), the Wisconsin Department of Natural Resources and myriad local businesses to finance the purchase of a 31-acre parcel adjacent to Ferry Bluff State Natural Area (SNA) this summer. The eventual goal of this purchase will be to add this parcel to Ferry Bluff SNA. The data that volunteers have collectively gathered over the decades has been critical in helping evaluate, promote and act upon this conservation opportunity.

At Ferry Bluff SNA the Council was interested in seeing how the hunting season might influence roosting eagles in the new parcel. In relation to winter phenology for eagles roosting at Ferry Bluff Roost our data can be organized into two primary types: synchronous roost counts and radio-telemetry data.

Roost Count Data

There is data from 5 winters where roost counts were completed in the week prior to the gun-deer season and then again after the deer season was completed. In addition to these before/after counts, there will also be a summary of all data collected at Ferry Bluff Roost that relates to counts early in the winter.

For the 5 winters where eagles were counted at Ferry Bluff in the week prior to the Gun-deer season (November 14-19), the average number of eagles noted using Ferry Bluff Roost was 3.8 individuals. Counts collected from mid-December (December 8-19) averaged 18.8 individuals at Ferry Bluff whereas counts collected from late November to early December (November 28 – December 7) averaged 10.1 individuals. Eagle use of roosts in early winter is, therefore, dynamic and changes rapidly in respect to calendar date. Few eagles use the Ferry Bluff Roost just prior to the gun-deer season but that situation can change rapidly as winter progresses, at least in some years. Eagle use of Ferry Bluff following the gun-deer season was more variable and, though it was normally low in early December, it was as large as 28 birds in some years, likely when the night weather is cold.

Radio-telemetry Data

Ferry Bluff Eagle Council trapped 17 eagles in the winters of 2002-2003 and 2003-2004 and attached VHF transmitters to these birds before their release. Eagles were then tracked for the remainder of the winter in which they were caught and then for the one or two subsequent winters that followed. The main purpose was to examine habitat use by eagles in winter but, of relevance in this analysis, it be explored whether or not eagles returned to the Sauk Prairie winter area predictably among years and, if they return, when these birds did so.

The council was able to track 15 of 17 trapped eagles during subsequent winters (88%) so winter philopatry, the tendency of birds to return to winter areas among these long-lived birds, was high. In addition, of the 15 eagles tracked in subsequent winters, 6 returned to the study area before the 9-day, gun-deer season began so migrant eagles are arriving in their winter areas both before and after the 9-day, gun-deer season begins. Roosting behavior, when night temperatures are warm (above 32 degrees), however, differs from eagle roosting behavior when night temperatures are below freezing in that eagles tend to roost in trees located near foraging sites and they do not typically use the communal roosting sites. Ferry Bluff is a communal roosting site. Thus, though eagles often return to the study area before the 9-day, gun-deer season they do not use communal roosts like Ferry Bluff unless temperatures are below freezing. Such cold nights are uncommon in November and may explain the low use of Ferry Bluff Roost in November. Alternatively, eagles could simply be roosting further down river as was found in the roost count in mid-December where 76% of all eagles counted used the Lone Rock Roost.

Conclusion

Photo contributed

Without the dedicated efforts of many, committed among several decades, FBEC would not be able to apply such informative data to the many management decisions that are frequently faced. Currently, and as discussed here, data is being used to develop management guidelines for the new parcel that has been purchased. In addition, data is also being used to minimize the impact of construction within the village of Prairie du Sac by allowing winter outdoor construction activities only when eagle numbers roosting in the Sauk Prairie Area are less than 10 individuals. Similar data are also being used to minimize disturbance to foraging eagles by determining when traffic on the Great Sauk Trail should be diverted to Water Street.

 
 

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