A fixed cold-deficit model reproduced the Carolina Wren slope decrease and gave positive forecast skill for 1996-2025

We tested whether a fixed nonlinear cold response can explain a reported decrease in the winter-temperature slope for Carolina Wren growth. We rebuilt eastern US annual indices from BBS 1966-2025 counts and NOAA monthly temperatures. The primary support used 913 routes selected from records through 1995. A model fitted in 1967-1995 used the mean positive local monthly deficit below 0 C in winter. It gave forecast skill 0.375 in 1996-2025 against a no-climate model, with a descriptive 95% block interval of 0.097 to 0.581. Its cold coefficient was -0.361 early and -0.363 late. The fixed early curve projected a late-to-early linear temperature slope ratio of 0.541; the observed ratio was 0.558. Thus response shape and changed climate can reproduce this apparent slope decrease. Gain over the linear forecast was uncertain. The cold model failed at a split in 2000, with skill -0.084. It lost its point advantage over linear after removal of severe training winters 1977-1978. These checks limit general use. This selected single-species follow-up uses overlapping prior data and full-period index estimation. It is a conditional retrospective forecast test, not a real-time or causal mortality test. Code, annual summaries, forecasts, checks, source hashes and retrieval instructions are included.