Decorah Bald Eagle Tracking Project Introduction
by Brett Mandernack
The Decorah Bald Eagle satellite tracking project began in 2011 after RRP founder and friend Bob Anderson and I decided to collaborate with the objective of answering the question he was most often asked: “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 for the first time, in it’s entirety, on our website. I’ve included the abstract here, but the full paper contains astonishing, comprehensive documentation of the travels of the Decorah eagles that Brett has tracked and is well worth the read! Read the full paper here
To put the Decorah project into perspective, Eagle Valley’s (EV) initial eagle tracking efforts began as a partnership with researchers from The Raptor Center at the University of Minnesota from 1999 through early 2002. Their field research program ended at that time and EV took over the project with the generous financial support of my employer, Kohler Co., and the Kohler Trust for Preservation (KTP) from 2002 through the project’s conclusion in 2014. This initial phase of eagle tracking focused primarily on adult Bald Eagles that wintered in impressive numbers in the EV area of southwest Wisconsin. Results of this study were published 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).
A foundational premise of the study was to better understand travel behavior. After all, how can we understand a species and expect to steward it effectively without this fundamental information? The Decorah eagle study, phase 2, was funded by RRP for the first two birds, D-1 and D-14, and by Kohler Co. and KTP for subsequent eagles. RRP provided tremendous logistical support and education outreach throughout. This phase provided a unique opportunity to examine travels of fledglings from the same parents over multiple years. Is eagle migration/travel behavior genetically influenced? Plus, we had little concrete knowledge of travel behavior of birds fledged from this region. Previous data collection methods of banding, color marking, and short-distance VHF transmitter tracking yield a fraction of the behavior information satellite tracking provides. We now have access to day-to-day details of eagle travels and, if the transmitters function for several years, can document changes in travel behavior, if any, over time.
This question initiated EV’s third phase of tracking, whether immature eagle travel changes over time. It has been documented that immature movements are variable and nomadic. If there is a change, is it abrupt or gradual, consistent or variable, etc.? Additionally, the “revelation” of D-1’s amazing travels inspired us to learn more about immature eagle travels. 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 winter 2015-16.
Co-author Ryan Schmitz and I have provided a summary of Decorah eagle travels through February 2018, at which time D-24 and D-27 were still transmitting data. (This continues to be the case at this writing in late February 2019, so future updates can be expected.) Included in the paper are 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”, because satellite technology enables this level of movement behavior nuances to now be recognized. I also realize 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 those eagles! A big thanks to Glenn and Darlene Miller and David Lynch for sharing their photos with everyone!
To begin to answer the question of where the eagles go, we fitted seven juvenile eagles with satellite transmitters between 2011 and 2017. We 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 to that suburban eagle family.
Eagles were tracked for periods ranging from 4 days to 3 years. Two transmitters continue to send data, though one is intermittent. Average eagle fledging age was 77 days. The fledglings stayed within a mile of the nest for a month following fledge, then most began ranging out on exploratory flights, but returned to the nest occasionally. Eagle age at dispersal from the natal area averaged 162 days, nearly 85 days after they fledged. Two siblings from 2016 exhibited several similarities but also marked differences in dispersal behavior. Dispersal for different year siblings was variable in timing and direction.
The nest camera has confirmed the adult eagles do not migrate. However, four of the fledglings completed a fall migration from the natal area. Spring and fall migration was variable, though similarities were observed. One eagle exhibited long-distance, repeated migration of over 850 straight-line miles to a northern Ontario summer range, then returned to northeast Iowa for the winter. The other three migrated shorter distances (75 to 206 miles) in their first fall from the nest area to a winter range in southeast Iowa. One perished there, another returned to northeast Iowa the following spring, and a third began heading north in late February at the end of the period monitored. Movement behavior of juveniles, in general, is often erratic and unpredictable. Some birds embarked on long summer or fall journeys north from their natal area but were not truly migrating. The eagle that traveled long distances showed increased efficiency in time and miles traveled on subsequent migrations. Overall, the Decorah juveniles exhibited variability in summer and winter range sizes across individuals and, less so, between years.
Four of the seven eagles died from electrocution (three) or vehicle collision (one) within their first year. Survival rate of the Decorah eagles was lower than reported in other studies, despite limitless food resources and tolerance to human disturbance. Anthropogenic mortality causes, those caused or influenced by humans, may temper the higher productivity of this suburban site.
We will continue to track the remaining eagles until transmitter failure to further develop our understanding of travel patterns by eagles in this region. Will migration tendencies diminish or cease as they approach breeding age? How far will they nest from their natal area? We must also realize threats posed to suburban-raised eagles and take action to improve their survivability.