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

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

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
Total culmen (mm)	R² = 0.97, P < 0.001	Age = 0.14x² + 1.33x − 3.67	x: 0.962–1.69	x: 7.13 (132.3)	0–15
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
Culmen from tip of nares (mm)	R² = 0.96, P < 0.001	Age = 0.26x² + 1.47x − 3.82	x: 1.02–1.93	x: 6.39 (406.2)	0–15

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
CI confidence interval

ISSN: 2053-7166