Bald eagles have overcome many challenges to their sustainability as a population. Service biologists are studying climate change effects on Michigan’s eagles. Photo: Tim Kaufman.
More than a half century of research has shown that bald eagles along Michigan’s shorelines and rivers are gradually beginning to nest earlier each season -- a potential indication of this iconic species’ response to changes in climate in the upper Midwest.
Bald eagles have overcome many challenges to the sustainability of their populations -- from loss of habitat to contamination of their food sources by pesticides and environmental contaminants.
National legislation banning the use of contaminants such as DDT and PCBs, coupled with habitat restoration in key portions of the eagle’s range, has resulted in a comeback for this beautiful raptor. More than 750 bald eagle pairs today fly the skies of Michigan, up from only 50 to 60 breeding pairs just half a century ago.
In 1961, University of Michigan graduate student Sergej Postupalsky began documenting bald historic and current eagle nesting sites and collecting banding data for the existing population in Michigan. Today, eagle research in Michigan spans state and federal agencies and academic institutions.
The result is more than 40 years of data on nesting, behavior, productivity, survival and overall population dynamics for bald eagles. It is safe to say the bald eagles of Michigan are among the most documented and well-monitored birds in North America.
More recently, Dave Best, fish and wildlife biologist at the U.S. Fish and Wildlife Service’s (Service) East Lansing Ecological Services Field Office, and Bill Bowerman, from the University of Maryland, have been studying bald eagles as indicators of water quality in the Great Lakes watershed of Michigan. The two have seen a trend in coastal bald eagle nesting patterns that may point to the effects of the changing climate.
A biologist bands a bald eagle in Michigan. More than four decades of research and study have made Michigan’s bald eagles among the most documented and well-monitored birds on the continent. Photo: Tim Kaufman.
"When Sergej first started banding eagles in 1961, nestlings were banded in mid-June,” Bowerman said. “In the past few years we've been able to start banding in early May. This is almost a six-week change in nesting chronology; that is significant.”
Their research has determined that from 1988 to 2006, eagles nesting along the Great Lakes shorelines in Michigan initiated egg laying an average of 0.7 days earlier each year, and those nesting along Great Lakes tributaries began 0.9 days earlier. These are the highest such rates recorded to date. Analysis of data from eagles inhabiting Michigan’s interior did not show significant changes in nesting behavior.
The Great Lakes tend to moderate the climate along Michigan’s shoreline and rivers; with less snow and ice cover, temperatures have a tendency to stay warmer. However, interior portions of Michigan are less buffered from pronounced changes in temperature, which may contribute to the insignificant changes in nesting dates for birds in Michigan’s interior.
Dave Best of the Service’s East Lansing Field Office holds an eaglet in preparation for sampling and banding. Photo: Kim LeBlanc.
“While the numbers may seem small when you just compare two consecutive years, long-term data from more than 2,300 nest records shows that eagles nesting along the Great Lakes and along large rivers in Michigan are laying eggs up to two weeks earlier on average now compared to 19 years ago,” Best said.
What does earlier nesting mean for the bald eagle?
“We believe that the reduced duration and extent of ice cover on the Great Lakes due to climate change has led to earlier access to foraging areas along the lakes, which has triggered earlier initiation of egg laying by nesting eagles,” Best said. “Earlier nesting has not yet resulted in any change in productivity, suggesting that the eagles have adjusted to climate change so far.”
Still, biologists are thinking about all of the ways climate change may impact eagles and similar raptors. Less ice cover earlier in the year may mean birds have easier and longer access to foraging opportunities. Longer feeding times increase the chances for contamination in birds.
“Although contamination has decreased, there’s still a lot out there in biota and sediment,” Best said. "This is something we will need to monitor and evaluate any potential impacts."
Today, bald eagles are well-known members of Michigan’s landscape. The Service and its conservation partners are invested in fish and wildlife research, and in pooling resources with the rest of the scientific community to better understand climate change impacts.
“The data collected in Michigan are some of the most long-term, unique data for a bird species,” Best said. “Working collaboratively with many partners we can analyze past, present and potential threats to native, iconic species like the bald eagle. This helps us to understand how they will adapt, and it allows us to develop effective ways to conserve and protect them in a changing climate."
Like many of the most successful conservation efforts, the study of Michigan’s bald eagles is a part of a larger, collaborative effort.
“We have worked to form an international coalition, with researchers from 19 countries on four continents to improve our understanding of the impacts of climate change on eagles,” said Bowerman said.
Climate Change Focus: Adaptation
Author: Ashley Spratt, Midwest Region External Affairs Office, 612-713-5314, Ashley_spratt@fws.gov
Contact: Chuck Traxler, 612-713-5313, Charles_traxler@fws.gov