Analysis of egg-lay dates of bald eagles as a potential indicator of global climate change

The effects of climate change on many wildlife species is a new area of research. Long-term data sets on raptor populations may hold the key to documenting these impacts from local to ecosystem scales. The Michigan bald eagle (Haliaeetus leucocephalus) population has been monitored continuously since 1961, and virtually all reproductive outcomes are known since 1963. The bald eagle population at Voyageurs National Park, Minnesota has been monitored continuously since 1973. Though these data sets are extensive, little is known about the effects of climate change on egg laying dates in sea eagles (Haliaeetus spp). Understanding these effects could ultimately provide valuable information regarding the overall impact of climate change on entire ecosystems. This preliminary study was conducted to determine the effects of environmental temperature change on the average lay date of bald eagle eggs in MI and MN. In addition, the relationships of change in egg lay date with elevation, water type, latitude/longitude, breeding area, and distance from the Great Lakes were evaluated in the MI study area; the relationships of change in egg lay date with ice-off and lake location were evaluated in the MN study area. In both study areas, the earliest lay dates of nesting pairs were derived utilizing morphometric measurements of nestlings. The MI study period was from 1988-2008, and included 442 nest sites and over 2,300 first egg lay dates. We found both significant and inverse trends in Julian Date for the first lay date that averaged -0.39 d/y over this time period (P<0.0001). Lay dates were further examined at three geographical scales to determine if any trends were evident: Anadromous (AN), Great Lakes (GL), and Inland (IN) breeding areas. AN lay dates averaged -0.95 d/y (P<0.0001), GL lay dates averaged -0.75 d/y (P<0.0001), and IN lay dates averaged -0.32 d/y (P<0.0001) over the study period. The MN study period was from 1989-2009, and included 115 nest sites. We found a decreasing but not statistically significant trend in Julian Date for the first lay date that averaged -0.047 d/y over this time period. After narrowing the MN data set to only nests with 5 or more years of visitation, the trend was still not significant, averaging -0.180 d/y. From the MI data, it is apparent that there may be a correlation between milder winters and earlier lay dates of breeding pairs. Our MN results may suggest that eagles are not responding to climate change, but the reason behind this may be that all MN lakes are inland and the power of the dataset was low. The relatively small area would have needed over 2000 records to show a significant trend. Eagles and raptors may now be warning us of the impacts of climate change, much like they did earlier with impacts of DDT and other environmental pollutants.