No matched US bird survey had both a start at 25 C or more and PM2.5 above 35 ug/m3 in 2006-2024

We tested whether warm survey starts change the link between same-day PM2.5 and recorded bird taxa. We used the fixed 2025 Breeding Bird Survey release and EPA daily data for 2006-2024. Each route used one site within 50 km, with records in both study periods. This rule gave 2,715 matched runs on 241 routes. None had both a start at 25 C or more and PM2.5 above 35 ug/m3. These are coverage benchmarks, not biological thresholds. Four region-period models used 2,109 runs, route-observer effects, state-year effects, weather, date, and survey time. All four warmth-pollution interaction intervals included zero. The early West model had only 12 residual degrees of freedom. Separate future-exposure and control checks did not give a clear interaction. One western distance check was not identified. The results do not show that joint extremes are harmless. Accepted-run selection, sparse monitor coverage, daily exposure timing, and uncertain detection limit inference. The package contains a fixed-site panel, daily site data, source hashes, code, figures, and an offline reproduction check.

PM2.5 · birds · Breeding Bird Survey · observation bias · temperature · joint exposure · biodiversity · data coverage