Artificial nest structures at Fourmile Island rookery at Horicon Marsh

Weekly News article published: April 14, 2009 by the Central Office

HORICON - The replacement of 59 artificial nesting structures at Horicon Marsh will help ensure that great blue herons will remain a mainstay at the marsh, which annually attracts about 250 bird species and is considered a Globally Important Bird Area, as well as being designated a Wetland of International Importance.

The 32,000-acre Horicon Marsh in Dodge County is the largest freshwater cattail marsh in the United States. Its southern 11,000 acres are managed as a State Wildlife Area by the Department of Natural Resources while its northern 21,000 acres are managed as a National Wildlife Refuge (exit DNR) by the United States Fish and Wildlife Service.

This past February, Department of Natural Resources wildlife staff under the supervision of wildlife biologist Brenda Hill, replaced 59 great blue heron artificial nesting structures at the Fourmile Island Rookery State Natural Area located within the marsh. The island, the largest of 36 small islands throughout the marsh, covers about 15 acres and is closed to public access from April 1st to August 30th to limit nesting disturbance.

Heron rookery poles
DNR crews replace poles for heron nesting structures.
WDNR Photo

Nesting structures were first erected during the winter of 1992-1993 on utility poles ranging from 40 to 65 feet high. The nesting platforms were placed at the top to simulate tree branches. Each pole contains eight nesting structures. Structures had to be placed adjacent to the island because of the lack of suitable trees remaining on the island.

heron nesting platform
Great blue heron nesting platforms
WDNR Photo

At one time, Fourmile Island contained 800 to 1,000 nesting pairs of birds that translated into 2,000 adults and another 2,000 young during the peak of the nesting season. These birds ate mostly fish and frogs and other small animals caught along the waters edge. Much of this food ended up on the forest floor at Fourmile Island as the birds came back to feed their young and rest among the trees. All this food became nutrient rich and highly acidic fertilizer in the form of bird droppings or guano and changed the island soil chemistry. The high concentration of acidic soil stressed the trees and made them more susceptible to diseases, as well as prevented any new trees from growing. Oak wilt has also been a problem on the marsh islands.

Dutch elm disease, oak wilt, wind storms and changing soil chemistry all led to the placing of a series of utility poles for nesting structures adjacent to the island during the early 1990s to compensate for the loss of Fourmile Island's nesting trees. In 2008, there were 37 nesting pairs on or adjacent to Fourmile Island. This spring the birds have already begun arriving to the new and improved nesting structures and started building their nests.

Historically, Fourmile Island was home to great egrets, double-crested cormorants, black-crowned night herons and great blue herons, but over time and with the destruction of nesting trees, great blue herons became the predominant nester. Many of the trees fell victim to Dutch elm disease during the 1960s and 1970s and eventually fell down. A July 1984 windstorm toppled nearly 80 large trees.

Besides DNR Dodge County wildlife staff and volunteers, the structures were constructed and replaced with help by donations from RH Equipment & Services of Mayville, Horicon Hardware Hank, the Horicon Marsh Bird Club and the Wisconsin Department of Natural Resources Bureau of Endangered Resources.

FOR MORE INFORMATION CONTACT: Brenda Hill, Wildlife Biologist, Horicon, 920-387-7882


Last Revised: Tuesday, April 14, 2009