Using Post-Release Stratification to Detect Heterogeneity in Mallard Survival

Abstract

We evaluated post-release stratification (after banding) developed by Schwarz et al. (1988) to improve managers' capability to detect and account for heterogeneity in survival among mallards (Anas platyrhynchos) recovered along the upper Mississippi River (UMR). We used band recoveries from 4 banding reference areas in the Prairie Pothole Region spanning 25 years and stratified them by 4 recovery areas in the Central and Mississippi flyways: UMR, UMR-vicinity (UMRV), lower Mississippi River (LMR), and southern plains (SP). We validated the assumption of fidelity to migratory routes and applied post-stratification to estimate recovery and survival rates for mallards recovered in each stratum. Average distance between geographical centers, or centroids, of direct and indirect recoveries was 246.7 km, supporting the assumption of migratory fidelity (P > 0.05). A model with heterogeneous recovery was selected in a majority of tests among strata, due in part to unequal sizes of the 4 recovery strata. However, in most (54.7%) cases survival was estimated to be homogeneous among these strata; only 10.9% of tests among strata rejected (P < 0.05) homogeneous survival in favor of heterogeneous survival. Estimates of homogeneous survival of mallards recovered along the UMR ranged from 52.8% for after-hatch-year (AHY) females banded in southwestern Manitoba to 72.4% for hatch-year (HY) males banded in southwestern Saskatchewan. Survival estimates for the UMR and UMRV recovery strata were not higher (P ≥ 0.06) than estimates from the LMR and SP recovery strata in most cases. Estimated survival was as much as 48.6% less between HY females recovered on the UMR and the LMR, but survival differences averaged 11.1% among all the cases when heterogeneous models were selected. Post-release stratification applied at this regional scale often reduced samples to insufficient size to detect heterogeneity among strata. Biologists should not use evidence provided by this stratified analysis of mallard survival to justify large-scale habitat protection of the UMR.