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Table 2 Linear mixed model results for each morphometric measurement that can be used to estimate age of Western Bluebird nestlings

From: A field guide for aging passerine nestlings using growth data and predictive modeling

Developmental measurement Model output Equation for estimating age 95% CI of coefficients t-value (df) Valid range (days)
Mass (g) R2 = 0.96, P < 0.001 Age = 0.43x − 0.68 0.424–0.439 114.3 (372.8) 0–10
Tarsus length (mm) R2 = 0.95, P < 0.001 Age = 0.64x − 3.1 0.625–0.648 112.7 (369.1) 0–10
Total culmen (mm) R2 = 0.97, P < 0.001 Age = 0.14x2 + 1.33x − 3.67 x2: 0.102–0.173 x2: 7.58 (132.3) 0–15
x: 0.962–1.69 x: 7.13 (132.3)
Culmen from tip of nares (mm) R2 = 0.96, P < 0.001 Age = 0.26x2 + 1.47x − 3.82 x2: 0.205–0.318 x2: 9.12 (402.8) 0–15
x: 1.02–1.93 x: 6.39 (406.2)
  1. Final equations were obtained from the model coefficients. Mass and tarsus measurements should only be used to estimate ages 10 days old and younger. Both culmen measurement techniques can be used to estimate age to day 15. For the quadratic models, the 95% CIs, t-values, and degrees of freedom (df) are listed for each of the coefficients (x2 and x) in the equation
  2. CI confidence interval