Methodology
This study was conducted at the Queen's University Biological Station north of Kingston, Ontario. The study site was Barb's Marsh, a 2-ha marsh dominated by willow (Salix spp.) and cattail (Typha spp.).
The front half of the marsh was searched for nests 3 times weekly from early May to the end of July 1981. Nests were marked by placing a numbered piece of flagging tape on vegetation approximately 1 m from the nest. Thirteen territories of males were mapped during the first three weeks of May. A random-number generator was used to select 7 experimental territories, leaving the remaining 6 as controls. Female removals occurred between 1800 and 2100 on 23 and 25 May and 6, 9, and 18 June 1981. On each of these days all females that could be associated with nests in the experimental territories were shot using a 12-gauge shotgun. Nests of incubating females were left as a test to ensure that the eggs were unattended on subsequent nest checks. If a removed female had nestlings, they were removed and killed in ether.