Prothonotary Warblers were seen for the first time on the upper river in a Park Service survey. (Photo by )
Bald eagles and other bird populations are generally healthy on the St. Croix and nearby rivers, although the St. Croix lags behind when compared to an incredible population explosion in recent years on the Mississippi River in the Twin Cities area.
According to the National Park Service, eagle numbers more than doubled at the Mississippi National River and Recreation Area between 2007 and 2014, from around 20 active territories in 2007 to 49 in 2014.
Reproduction rates on the St. Croix are below the “healthy population threshold” of one young bird per territory. But the Great Lakes Inventory and Monitoring Network newsletter says “periods of low productivity” should be expected at such a northern park due to less abundant prey and greater chances of severe weather during nesting.
The Park Service does aerial surveys of nests to determine population numbers. Scientists also take blood samples from young eagles on their nests to determine pollutants, as described in an article this summer. Eagles on the upper St. Croix were found to have the lowest level of contaminants of any others in the study, which covered several National Park units in the region.
In other news, a survey of birds on the St. Croix found a couple unexpected species. Retired Park Service biologist and bird expert Robin Maercklein found a warbler species that he had previously only known to live south of Osceola. This year, he found Prothonotary warblers at two locations near the mouth of the Sunrise River. He also heard the first Red-headed woodpecker on the survey since 1982.
Download the Great Lakes Inventory and Monitoring Network’s newsletter on its website.