Relationships between Selected Population Phenomena and Annual Bobwhite Age Ratios

Abstract

Relationships between fall age ratios of bobwhites (Colinus virginianus) and certain population phenomena are discussed, based on a computer simulation and 17 years of data from the Carbondale Research Area in southern Illinois. Annual variation in the percentage of juveniles in fall populations on the study area was more strongly correlated with population change from fall to fall and spring to fall than with fall densities per se. Simulation techniques were used to demonstrate the combined effect of changes in proportions of hens successful and survival rates of juveniles and adults on relative population gains from spring to fall. Comparisons were made of the use of young:adult ratios and percentages of juveniles as indicators of these gains. Results of the simulation suggest that variation in juvenile mortality influences percent summer gain more than does variation in adult summer losses. However, the relationship between summer gains and the subsequent percentage of juveniles in the fall is altered by annual variation in adult mortality. In contrast, this relationship is not significantly changed by annual variation in juvenile mortality because the latter exerts an almost equal influence on both relative productivity and fall age ratios. Finally, the relationship between productivity and the proportion of juveniles in the fall is curvilinear, whereas the ratio of juveniles per adult is linearly responsive to productivity, making the latter a preferable index of breeding success. J. WILDL. MANAGE. 38(4):665-673 Over the past 30-plus years, literally hundreds of thousands of wings from harvested bobwhites have been collected for the purpose of estimating sex and age structure of the parent populations (Rosene 1969:385386). There is not, however, unanimity among researchers as to how these data may be interpreted. Proportions of juveniles in fall populations (Pj) are considered indicative of mean annual mortality rates (Hickey 1955:343), but these age ratios do not reflect year-to-year variation in mortality unless the population is stable (Marsden and Baskett 1958:415). More often, fall age ratios are used as indices of productivity and survival the previous summer (Leopold 1945, Speake and Haugen 1960). Age ratios have also been related to fall densities (Lay 1952:23, Reid and Goodrum 1960:244, Speake and Haugen 1960:94) and population change (Legler 1955:17). Stanford (1973:129), though, warned that high young: adult ratios are not always associated with high total production or populations as has often been assumed. Bennitt (1951:32-35), in fact, reported an inverse relationship between annual Pj and fall densities in Missouri. He believed that this negative correlation was due to the phenomenon of inversity (Errington 1945) whereby high reproductive rates (and thus high Pj) were usually associated with low breeding densities, which were often followed by low total production and low fall populations. Similarly, MacDonald (1964:6-7) concluded that highest hen:poult turkey (Meleagris gallopavo merriami) ratios came during years of lowest numbers of hens and thus did not indicate highest total productivity or fall populations. This paper attempts to clarify some of the relationships among annual Pj, young: adult ratios, relative productivity, juvenile and adult survival, population change, and fall densities using computer simulation techniques as well as 17 years of data from a hunted bobwhite population in southern Illinois. This material is a contribution from Project No. 1: Bobwhite Quail Life History J. Wildl. Manage. 38 (4):1974 665 This content downloaded from 157.55.39.186 on Tue, 12 Apr 2016 09:23:09 UTC All use subject to http://about.jstor.org/terms 666 BOBWHITE AGE RATIOS * Roseberry