May PDSI kept forecasting prairie ponds and within-year duck settling in 1997-2026, but the 1998 duck equation under-predicted north-central US ducks by 73% as the US-wide drought response weakened

Question: do the Sorenson et al. (1998) equations, which relate mean May Palmer Drought Severity Index (PDSI) to May ponds and breeding ducks in north-central US survey strata 41-49, forecast the 28 later surveys of 1997-2026? Which link of the climate-wetland-duck chain fails? Data: USFWS Waterfowl Breeding Population and Habitat Survey stratum estimates (release 2026-09-01), NOAA nClimDiv PDSI of the 22 divisions under the strata, and TerraClimate PDSI for Prairie Canada. Method: frozen published equations, frozen early change slopes, 4,000 simulations of a stable relation, block-bootstrap intervals, a stratum panel with year fixed effects, and a split of US ducks into continental total and US share. Results: the replication was close (pond slope 204,043 vs 200,944 per PDSI unit). In 1997-2026 the pond equation kept its year-to-year skill (0.70 vs persistence; stable expectation 0.39 to 0.82), but ponds were 30% above it. Ducks were 73% above their equation, and its year-to-year skill was -0.14 (stable expectation 0.48 to 0.84). Within-year settling of ducks among strata did not change (US elasticity to ponds 0.51 and 0.45; Canada 0.39 and 0.37) and kept its stable skill. Pond responses to PDSI also held in Canada. The failure came from the US share of continental ducks: its response to US PDSI change decreased from 0.143 to 0.037 per unit, and the decrease started in the late 1980s. The continental total did not respond to PDSI, and ponds did not improve Gompertz forecasts of continental growth. Pond scarcity did not explain the change. Spring temperature did not increase and May PDSI became wetter, so the drying premise of the 1998 projection had not occurred. Limitations: one survey system, retrospective test, shared aerial counts of ponds and ducks, incomplete US coverage before 1967, and causes of the share change not tested.