Sex- and Age-Ratio Bias of Cannon-Netted Geese

Abstract

Age ratios of Canada geese (Branta canadensis), derived from catches in cannon traps, are of questionable value unless time on bait is taken into account. Findings were based on examination of 2,879 geese caught in 46 timed catches at Swan Lake National Wildlife Refuge in 1961 and 1962. The proportion of adults in the catches increased with length of time on bait. No significant differences were found in sex ratios as time on bait increased. This study was designed to determine whether any measurable sexor age-ratio biases are obtained in the cannon-net trapping of Canada geese. It has been speculated that the period of time geese spend on the bait at the trap sites, before the catch is made, may influence sex and age ratios obtained. Correct interpretation of band recoveries is important in analysis of population dynamics. One problem that plagues wildlife research is the difficulty of obtaining random samples from wild populations. If sex and age ratios from trap samples contain biases of unknown magnitude, we cannot accurately determine flock composition or predict productivity and turnover from these samples. Until we know the magnitude of bias involved, we cannot make corrections and must assume that present trap samples and band recoveries are indicative of the populations. This study was carried out at Swan Lake National Wildlife Refuge in north-central Missouri during October, November, and December of 1961 and 1962. Financial assistance was supplied by the Missouri Conservation Commission and an Edward K. Love Fellowship from the University of Missouri. Dr. W. H. Elder, Rucker Professor of Zoology at the University of Missouri, guided the study. I am also grateful to the following: Dr. Donald H. Hazelwood, Department of Zoology, University of Missouri, for advice on statistics; Richard Vaught, Waterfowl Biologist, and Fred Veach, Area Manager, Missouri Conservation Commission; Leo Kirsch, Manager, and Howard Thornsberry, Wildlife Technician, Swan Lake National Wildlife Refuge. Richard Thompson, from the Savannah National Wildlife Refuge, Georgia, and Earl Cunningham, of the Carolina Sand Hills National Wildlife Refuge, South Carolina, also assisted with observations and handling of geese for several days during the fall of 1962. MATERIALS AND METHODS Determination of time on bait before each catch necessitated observation of the bir s from the time they came near the trap site until the actual shot was fired for the catch. The two trap sites used during this study were located on opposite sides of Swan Lake. The points of concealment and detonation control were an old silo, now an observation tower, and a storage barn. The silo is about 400 yards from one site, the barn only 30 yards from the other. Nets and cannons were laid out parallel to the shoreline in conventional fashion 'Contribution from the Missouri Cooperative Wildlife Research Unit, U. S. Fish and Wildlife Service, Wildlife Management Institute, Missouri Conservation Commission, Edward K. Love Foundation, and the University of Missour , cooperating. 2 Present address: Bureau of Sport Fisheries and Wildlife, 403 Perlstein Bldg., Beaumont, Texas.