Adaptive Management of Double-crested Cormorants on Four Brothers Islands. Technical Report #67.

Capen, D.E., Bryant, D. University of Vermont and The Nature Conservancy, Adirondack Chapter. (2012). Adaptive Management of Double-crested Cormorants on Four Brothers Islands (Technical Report No. 67). Grand Isle, VT: Lake Champlain Basin Program

Double-crested Cormorants nest on Four Brothers Islands, a preserve owned by The Nature Conservancy, in densities that threaten standing trees and other species of nesting birds. Control activities on other Lake Champlain islands where the species has nested have concentrated the lake-wide nesting population almost entirely on the Four Brothers islands. This report presents findings from 4 years of study, 2008-2011, during which numbers of cormorant nests on the four islands averaged about 3800 per year. The Nature Conservancy, UVM researchers, and government agencies cooperated to conduct experimentalpopulation control designed to reduce numbers of nesting cormorants on the two islands with remaining tree cover, without causing dispersal of these birds to other islands in Lake Champlain where they are not wanted. The design of the experimental management approach was to treat nests on two islands with corn oil, preventing eggs from hatching, and to determine success of egg oiling by measuring four indicators: (1) nesting pairs of cormorants on the two experimental islands; (2) nesting success of herons and egrets on these islands; (3) rates of dispersal of cormorants to and from the experimental islands; and (4) numbers of cormorants nesting or attempting to nest in other locations on Lake Champlain. An additional product from the work is a decision model that will guide development of management options for cormorants as part of an ongoing process for a Lake Champlain Colonial Waterbird Plan.