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The most common BCI divides body mass by wing chord length (flattened wing from wrist to longest primary tip). Higher values indicate better body condition within a species.
BCI = Mass(g) / Wing_chord(mm)Modern studies prefer scaled mass index, which regresses mass against tarsus length or wing chord for the population. Residuals from the regression indicate condition.
SMI = mass × (L₀/L_measured)^b, where b is regression slopeBCI correlates with visual keel fat scores (0–5 scale) but is less subjective. A BCI below the species 5th percentile suggests poor condition requiring intervention.
Condition = BCI relative to species-specific reference rangeUpdated: July 2026
A song sparrow captured during banding with mass 22.5 g and wing chord 68 mm.
→ BCI: 0.331; within normal range (0.28–0.38 for adults)
A red-tailed hawk presenting thin with prominent keel, mass 850 g, wing chord 340 mm.
→ BCI: 2.50; below healthy range (2.8–3.4) — refeeding protocol indicated
A yellow-rumped warbler showing pre-migratory mass gain, 15.8 g mass, 67 mm wing.
→ BCI: 0.236; elevated above breeding-season average — normal migration preparation
BCI reference ranges are species-specific and often sex-specific. A healthy BCI for a hummingbird bears no relation to a raptor's normal range.
Wing chord must be measured flattened with moderate pressure, wrist to longest primary tip, to the nearest 0.5 mm. Bent or folded measurements introduce error.
Dry feathers only. A full crop can add 5–15% to mass in seed-eating birds. Note crop status and time since last feeding in records.
Avian body condition index (BCI) is a morphometric ratio used by ornithologists and wildlife rehabilitators to assess nutritional status without invasive fat scoring. The ratio of body mass to structural size indicators helps detect emaciation or obesity in captured birds.