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Wisconsin State Natural Areas Program Fourmile Island Rookery (No. 41)

Artificial nesting platforms at Fourmile Island

Photo by Joshua Mayer



Within Horicon Wildlife Area, Dodge County. T12N-R16E, Section 19. 25 acres.



A narrow island located near the center of Horicon Marsh, Fourmile Island is forested with large oak, elm, basswood, aspen, and cottonwood, which are used by a variety of colonial waterbirds. For over 50 years, the island harbored one of the largest colonial waterbird rookeries in the state with hundreds of nesting pairs of primarily great blue heron, along with egrets, black-crowned night heron and double-crested cormorant. Damage from a 1984 windstorm, along with Dutch elm disease and the effects of heron guano have reduced the available nesting habitat in recent years. Most nesting is currently limited to artificial nesting platforms on the east side of the island. Fourmile Island Rookery is owned by the DNR and was designated a State Natural Area in 1965.


Driving directions

Note: The island is closed to all public use from April 1-August 24. Contact the State Natural Areas Program for more information.


Fourmile Island Rookery is owned by:

  • WDNR


Site objectives

Manage the site as a heron rookery and a breeding bird conservation site. Natural processes, especially guano deposition, determine the structure of the above ground natural communities. Augmentation of habitat by constructing nesting poles appropriate for use by heron species is a desirable management practice. Provide opportunities for research and education.

Management approach

On the island, the native dominant tree species (primarily maples and oaks) are managed passively. Death of trees and shrubs due to guano deposition may occur as long as herons use the site.

Site-specific considerations

  • Construction of nest poles adjacent to the guano-killed tree area permits gradual recovery and long-term use of the island as a rookery.
  • Access is prohibited during heron nesting season (April 1 through August 24).
Last revised: Thursday, October 11, 2018