By Jeb Barzen
On Jan. 3 we counted 64 eagles through 26 hours of volunteer effort. A count of 64 is a low number for early January, but only one less individual than what we counted on Dec. 29, 2019, and on Dec. 31, 2017. The last time our counts were relatively high in early January was during the winter of 2014-15 when maximum eagle numbers were much higher than now.
Are the number of wintering eagles along the Lower Wisconsin River declining? If so, why? We know that the population of occupied eagle territories in Wisconsin is still increasing. From our telemetry project, we also know that most eagles that wintered along the Lower Wisconsin River came from territories located in Wisconsin, the Upper Peninsula of Michigan and northeastern Minnesota. Our rising number of occupied nesting territories in Wisconsin should, therefore, result in a consistently rising numbers of eagles wintering along the Lower Wisconsin River if population size is the only determinant. A constant rise in eagle numbers does not appear to be unfolding.
Alternatively, many have argued (though few have provided data) that food supplies in winter will determine the number of eagles occupying a particular area during winter. Does the low number in our eagle counts mean that winter food supplies are lower now than they were in 2014-15? That is a possibility and we have found a relationship between the number of eagles found wintering along the Lower Wisconsin River and the amount of gizzard shad found in the river during the preceding summer. But food abundance does not equal food availability, meaning that, even if food is present it may not be available to eagles because of extensive ice cover.
Another hypothesis may also be relevant. Has the location of some traditional, communal winter eagle roosts changed over the years and we have failed to discover (and thus count) them so far?
The numbers of eagles counted at Sugar Loaf, Fox, McDonald, Jones Slough and Leland Roosts have been very low for several years now. The lower use of these roosts by eagles could be due to a lower population or due to new roost traditions developing. We stopped counting Jones Slough this year because there haven’t been more than four eagles counted in that roost for the past two winters. Counting no eagles in the cold is hard to do for too many years in a row. For the past three winters Sugar Loaf, Leland and Fox Roosts have had low numbers of eagles during most days counting occurred, but high counts (more than 15 eagles) occurred at least once a winter. We have thus been able to continue counting these roosts. At the other extreme, for the last 25 years, McDonald Roost has only been used occasionally by a few eagles in some winters. Since some roosts have had low counts overall but high counts at least once in the winter, it is unlikely that low population numbers are causing the low counts overall.
Finding new roost locations has occurred in the past but not lately. When we started roost counting in 1988, we only knew of Ferry Bluff Roost. Soon thereafter, Blackhawk and Sugar Loaf were added to our list of roosts. Lone Rock is the last major roost that we have added (2002) that continues to host eagles now. Perhaps it is time to start conducting surveys for new roosts.
Interestingly, if roost use is shifting due to circumstances unrelated to overall eagle populations in the Midwest, then we might see some interesting phenomenon. If a declining food base is the problem then eagles should leave the Lower Wisconsin River completely. But, if food abundance is not a problem, but roost disturbance is, then we would possibly find new roosts being occupied as traditional roosts are abandoned. Roost disturbance is known for Ferry Bluff. People were again seen on top of Ferry Bluff during the last count in January.
These distinctions are important because they point to very different management actions. Additional research is needed.
2020-21 ROOST COUNT
The following table tracks the bald eagle population along the Lower Wisconsin River from Lone Rock and Spring Green to Sauk Prairie during this winter’s first roost counts conducted by Ferry Bluff Eagle Council volunteers. To volunteer, call (608) 544-2107.
Roost Dec. 6 Dec. 20 Jan. 3
Blackhawk 0 5 3
Sugarloaf 0 0 10
Ferry Bluff 1 9 22
Fox 0 0 0
McDonald 0 0 0
Lone Rock 55 72 22
Leland 1 2 2
Ederer/Been 2 2 5
Total 59 90 64
Jeb Barzen is president of the Ferry Bluff Eagle Council, ferrybluffeaglecouncil.org.