Use of Social Indices to Predict Reproductive Success in Canvasbacks

Abstract

We correlated temporal changes in social groupings of canvasbacks (Aythya valisineria) breeding near Minnedosa, Manitoba, with an independent estimate of hen success during 1974-80. Roadside counts of pairs, lone males, and flocked males were made along transects at 5-day intervals, normalized to percentages to allow comparisons among years, and plotted to obtain measurements of selected areas between and under the curves. An estimate of hen success was regressed on these selected graph areas each year to derive a predictive equation. Graph areas (social indices) determined from temporal changes in the proportion of pairs, lone males, and flocked males correlated (r2 = 0.69-0.93) with hen success. This technique avoids the need for pair counts, nest searches, and brood counts and provides managers with a useful index to evaluate local management practices and to predict yearly production in time for setting hunting regulations. J. WILDL. MANAGE. 54(1):66-72 With onset of egg laying and incubation, female ducks become less visible, and observed social groups change from predominantly paired males and females to mostly lone and flocked males (Dzubin 1969). LeBret (1961) found that hatching dates of mallards (Anas platyrhynchos) could be calculated from the time pairs began to break up and lone males first began to appear. Dzubin (1969) found that when lone male mallards first comprised over 10% of pairs, females were beginning egg laying, and the first appearance of 2-3 males in a group indicated the start of incubation. Olson (1964) reported that canvasback sex ratios in early June provided a useful index to ensuing production. Therefore, if we assume observed males to be paired with nesting females and if there is minimal immigration or emigration in the local population, then seasonal changes in observed social structure of the population should be related to some measure of female reproductive success. Traditional methods of estimating reproductive success in ducks require detailed information on the size of the breeding population and the number of young produced for any given area. However, in addition to being time conI Present address: U.S. Fish and Wildlife Service, Office of Migratory Bird Management, Laurel, MD 20708. This content downloaded from 157.55.39.84 on Tue, 07 Jun 2016 05:24:06 UTC All use subject to http://about.jstor.org/terms J. Wildl. Manage. 54(1):1990 CANVASBACK BREEDING INDEX * Serie and Cowardin 67 suming and labor intensive, these methods must overcome a wide spectrum of potential biases, such as weather, breeding phenology, bird mobility, behavior, and changing observation rates based on vegetative growth (Diem and Lu 1960, Dzubin 1969, Sauder et al. 1971, Sorenson 1978, Sugden and Butler 1980). Alternative methods depend on use of radios (Cowardin et al. 1985) and marking techniques (Trauger 1971) to track individual females and assess reproductive success, but they are also subject to bias (e.g., radio malfunction, marker loss, and behavior modification). In spite of these special problems, accurate estimates of annual productivity are integral to the management of continental duck populations and are necessary to formulate harvest strategies. Therefore, development of new, more rapid, and reliable methods of indexing annual breeding success are needed for setting annual hunting regulations and assessing management practices on local areas. We described temporal changes in social groups of canvasbacks during the breeding period and investigated the potential for using social indices to predict reproductive success. This method of indexing productivity was first described by J. R. Serie (J. R. Serie, 15th Semin. Breeding Biol. of Waterfowl, Delta Waterfowl and Wetland Res. Stn., Manitoba, 1980) and was later adapted and modified by Hochbaum et al. (1987). J. H. Stoudt initially established roadside transects near Minnedosa, Manitoba, and we thank D. L. Trauger for his encouragement to periodically record canvasback social groups. Field assistance was provided by L. Deede, H. A. Doty, T. Fondell, T. Jessicoff, D. H. Johnson, J. H. Noyes, F. Roetker, and L. Vanderholf. We are grateful to M. G. Anderson and the Delta Waterfowl and Wetlands Research Station for their assistance. In particular, we are indebted to T. L. Shaffer for statistical analyses. We thank T. G. Hanson for typing the manuscript and J. E. Austin, G. S. Hochbaum, C. E. Korschgen, R. B. Oetting, A. B. Sargeant, and T. L. Shaffer for review of manuscript drafts.