Changes in bald eagle (Haliaeetus leucocephalus) breeding phenology and biometrics as a result of environmental change in the Great Lakes Region of North America
Researchers have documented changes in long-term range distributions, migration patterns and timing, and nesting chronology in bird species in response to environmental change. Recent studies suggest that birds are also demonstrating changes in body size in response to environmental change. As part of its recovery, the Michigan bald eagle population has increased in size and productivity, and expanded from a remnant inland population to areas along the Great Lakes shoreline and south to more agricultural and urbanized habitats. In addition, the region has undergone changes in human population and land use, and climate-related variables such as average temperature and duration of ice cover. All things considered, questions arise regarding the effects of such changes on the recovering bald eagle population. We used a two-pronged approach to assess impacts of environmental changes on eagles by evaluating trends in Julian lay date and nestling biometrics from 1988 to 2013 and 1998 to 2013, respectively. Annually, nestlings are banded, aged, sexed, and the following biometric measurements are taken: bill depth, and culmen, footpad, hallux claw, and 8th primary feather length. We used these measurements in combination with an average incubation period of 35 days to determine the lay date of each nestling.
From 1988 to 2013, we observed an inverse relationship between the Julian lay date and time. This relationship was more apparent in breeding areas along the Great Lakes shoreline compared to those Inland that were beyond 8 km of the shoreline. Of 35 breeding areas that had 10 or more years of data, six had significant inverse relationships between lay date and time. We also determined that among those areas, increasing mean temperature had an inverse relationship with lay date during the nesting months of January through April. When we evaluated biometrics from 1998-2013, the data indicated that measurements of nestlings at Inland sites and in Michigan’s Upper Peninsula were decreasing and at a faster rate over time compared to those along the Great Lakes shoreline and those in the Lower Peninsula. Decreases in body size were more apparent in female than in male nestlings. Our results, coupled with a still-growing and expanding eagle population indicate that more than one environmental factor may be responsible for changes in breeding phenology and biometrics. We hypothesize that bald eagle populations may be undergoing phenotypic plasticity in response to several variables, most importantly habitat quality, food availability, and year-to-year climate variation.