Bird species were detected within five years after 58% of three-survey absences on 183 Swedish routes, with equal route weights and the same observer per window

How often does a bird species enter a survey record again after repeated annual zero counts? We used Swedish fixed-route line counts from 1996–2025. Each selected window had nine annual surveys, one recorded observer, and survey dates within a 14-day range. Species had a positive count before one, two, or three consecutive zero counts. We then measured detections in the next five surveys. The five-year return rate after three absent surveys was 57.6% with equal route weights. The descriptive 95% spatial block interval was 54.9–59.8%. This result used 4,499 episodes, 203 species, and 183 routes. Rates after one and two absent surveys were 77.1% and 66.0%. After three absent surveys, equal species weights gave 45.9%. Requiring detections in all three prior years gave 83.0% in a smaller selected set. Date and non-overlap checks kept the primary rate near 57–58%. An independent calculation from the raw archive confirmed every primary episode and point estimate. The package preserves source data, code, provenance, figures, and all checks. The result describes later detections. It cannot separate missed birds from movement, local extinction, or recolonization. The future observer filter makes this a retrospective audit, not a live forecast rule.

bird monitoring · recorded absence · later detection · Swedish Bird Survey · survey continuity · reproducible data