Source-sink dynamics of a recovering population of bald eagles in the aquatic ecosystems of Michigan, USA

Background/Question/Methods

The bald eagle (Haliaeetus leucocephalus) population in Michigan has undergone a significant recovery following the ban of organochlorine pesticides by the Environmental Protection Agency. Population productivity however, is not consistent throughout the state of Michigan. Great Lakes breeding areas along the shorelines of lakes Superior, Michigan, Huron, and Erie have historically experienced less productivity than inland breeding areas because of higher concentrations of contaminants. High quality inland areas were the most productive until the 1990’s. Great Lakes breeding areas, most notably Lake Michigan and Lake Huron, are now more productive than inland breeding areas. Despite increases in productivity, it is unknown whether Great Lakes breeding pairs are experiencing chronic residual reproductive effects from contaminant exposure. To assess the reproductive fitness of Great Lake shoreline versus inland breeding pairs, we used productivity data from 1981-2010 collected throughout the state of Michigan. We used two parameters; length of site occupancy and decadal success rate. We calculated length of site occupancy as the mean number of years one breeding pair occupied one breeding area between breeding pair changeovers. We calculated the decadal success rate as the number of years that young were produced per number of years that the breeding area was occupied. 

Results/Conclusions

Length of site occupancy and decadal success rates significantly decreased in lakes Michigan and Huron shoreline breeding pairs from 6.84 to 5.61 years and 7.04 to 4.72 years, respectively. This is in comparison to inland breeding pairs, which remained constant at 7.35 years. This indicates that the length of time that Great Lakes shoreline breeding pairs are reproductively capable has shortened. In addition, the decadal success rates of lakes Michigan, Huron, and Superior have decreased from 45.2% to 17.7%, 46.4% to 20.4%, and 42.0% to 31.7%, respectively, during the time period. Inland breeding areas remained relatively constant at 46.9%.  This decrease in reproductive fitness may be the result of in ovo, or early developmental exposure, leading to ‘second generation’ effects, or chronic lifetime exposure. The low reproductive fitness indicated in our results, yet high productivity of Great Lakes breeding areas could be suggestive of a high turnover rate within the Great Lakes breeding population from inland areas, Wisconsin, Minnesota, or Canada. This constant turnover of breeding pairs may overshadow any underlying effects or decreases in Great Lake shoreline reproductive fitness caused by environmental contaminants.