North-South Gradient in Survival Rates in Midcontinental Populations of Mallards

Abstract

I used band recovery data to test for the existence of a north-south gradient in survival and recovery rates for midcontinental populations of mallards (Anas platyrhynchos) during 3 time periods (196270, 1971-78, 1979-84). Mean annual survival rates for adult males and females were significantly associated with mean banding latitude (P < 0.0001 and P < 0.0004, respectively) and time period (P < 0.0001 and P < 0.001, respectively). Survival rates for both adult males and females were higher in the north and lower in the south. Because individuals from the northern regions migrate farther, the cost of migration in terms of survival was lower than some other factor(s) that may cause the gradient. The relationship between mean banding latitude and mean annual recovery rates was marginally significant for adult males (P = 0.043) but not for adult females (P = 0.183), suggesting that the gradient was not due to differential harvest pressure. At present, the north-south gradient in survival rates cannot be explained but may be caused by similar north-south clines in predation, habitat degradation, land use, and/or agricultural practices. J. WILDL. MANAGE. 54(2):206-210 Predation, habitat, agricultural practices, migration distance, and harvest pressure vary throughout the range of the mallard. Accordingly, survival and recovery rates would be expected to vary over time and geographic area. These differences have, however, been very difficult to demonstrate. Anderson (1975) observed an east-west cline in mean survival and recovery rates, with eastern Canada, the Great Lakes, and western mid-Atlantic regions having lower mean survival and higher mean recovery rates than the eastern Canadian and United States prairies. Recently, Caswell et al. (1987) reported an east-west cline of increasing mean survival and decreasing mean recovery rates for 197984 in prairie Canada and suggested that the cline may be due to higher harvest in the east. Because no reports of a north-south gradient in mean annual survival and recovery rates have been demonstrated, I tested for a north-south gradient with band-recovery data for mallards. I thank all federal, state, and private biologists who have banded mallards. I thank J. D. Nichols for assistance with the analysis, J. E. Hines and D. S. Chu for computer assistance, and J. E. Hines for plotting the figures. I also thank D. R. Anderson, J. D. Nichols, G. W. Pendleton, and D. W. Sparling for reviewing the manuscript.