By Brett Mandernack
We began the Decorah bald eagle tracking study in 2011 after RRP’s founder Bob Anderson and I decided to answer the question that followers most often asked him: “Where do the eagles go when they leave Decorah?”
Full Satellite Tracking Paper
We are honored to publish Brett A. Mandernack and Ryan T. Schmitz’s paper on our website. Their paper contains an astonishing level of detail and is well worth the read! Read the full paper here
Phase One: The Eagle Valley Tracking Study
Eagle Valley’s (EV) tracking project began as a partnership with researchers from The Raptor Center at the University of Minnesota. After their field research program ended in 2002, EV took over with the generous financial support of my employer, Kohler Co., and the Kohler Trust for Preservation (KTP). Our initial bald eagle tracking study focused on adult Bald Eagles that wintered in impressive numbers in the EV area of southwest Wisconsin. We published our results in the Journal of Raptor Research in 2012 (J. of Raptor Research, 46(3):258-273 (2012). https://doi.org/10.3356/JRR-10-77.1).
Phases Two and Three: The Decorah Bald Eagle Tracking Study
We were very excited to add the Decorah eagles to our study. We knew very little about post-fledge behavior in eagles from NE Iowa and the Decorah Eagles gave us the unique opportunity to follow fledglings from the same parents over multiple years. Do genetics influence eagle migration/travel behavior? Where do NE Iowa eagles go? Satellite tracking gave us access to the daily details of eagle life and allowed us to compare eagle siblings and changes in travel behavior over time.
How do eagle travel patterns change over time? This question kicked off our third phase of eagle tracking. Immature eagle movements are often more variable and nomadic than those of their older counterparts. Is this change abrupt or gradual? Consistent or not? Clearly, we had more to learn. With Kohler Co./KTP’s continued support, we fitted 10 more immature eagles in the EV area with satellite transmitters, five in early 2015 and five more during the winter of 2015-16.
Co-author Ryan Schmitz and I have summarized our study through February of 2018. Our paper includes sections on hatch, fledge and dispersal dates, migration comparisons, summer and winter range use, home range size, and causes of mortality. We have included a LOT of detail in the individual travels section under “Results”. I realize that this level of depth is not going to be to be appreciated by everyone, so skim, skip sections, take it in “chunks” at your convenience. Find the areas you are curious about and have fun learning about our eagles!
A big thanks to Glenn and Darlene Miller and David Lynch for sharing their photos with everyone!
We fitted seven juvenile eagles from Decorah with satellite transmitters between 2011 and 2017. Our study documented fledging age, post-fledging and dispersal movements, travel behavior of siblings, migration tendencies, fidelity to migration routes and summer and winter ranges, and mortality causes.
We tracked the eagles for periods ranging from 4 days to 3 years. Our average eagle fledging age was 77 days and the fledglings stayed within a mile of the nest for a month following fledge. After a month, most of the eagles began ranging out on exploratory flights, although they returned to the nest occasionally. Their age at dispersal averaged 162 days, or nearly 85 days after they fledged. Two siblings from 2016 exhibited marked differences in dispersal behavior and dispersal for different year siblings varied in timing and direction.
Eagle Tracking Summary
We have confirmed that the adult eagles do not migrate. But four of the fledglings we tracked completed a fall migration from their natal area. One eagle made a long-distance migration of over 850 miles to its northern Ontario summer range and returned to northeast Iowa for the winter. The other three migrated shorter distances (75 to 206 miles) to winter ranges in southeast Iowa. One died there, another returned to northeast Iowa the following spring, and a third began heading north in late February. Their movements were often erratic and unpredictable. Some embarked on long summer or fall journeys north, but did not truly migrate. The eagle that traveled long distances showed increased efficiency in time and miles traveled on subsequent migrations. Overall, the Decorah juveniles exhibited more individual than yearly variability in their summer and winter range sizes.
Four of seven eagles died from electrocution (three) or vehicle collision (one) within their first year. Their survival rate was lower when compared with eagles in other studies, despite limitless food resources and tolerance to human disturbance. Anthropogenic mortality causes (mortality caused or influenced by humans) might temper the high productivity of this suburban site.
We are continuing to study bald eagles to gain more knowledge about travel by eagles in this region. Will their migration tendencies diminish or cease as they approach breeding age? How far will they nest from their natal area? This study gives us the data we need to understand threats posed to suburban-raised eagles and take action to improve their survivability.