Early season morphological correlations with circulating carotenoid concentrations in female (n = 38) and male (n = 33) barn swallows.

Mixed models of total circulating concentrations of carotenoids in males (adjusted R-squared = 0.15) and females (adjusted R-squared = 0.34) in relation to morphology. Variation among breeding sites was controlled for as a random effect in each model. Numerator degrees of freedom = 1, ddf = denominator degrees of freedom vary and are listed below. The negative relationship between throat color and carotenoids indicate that, in both males (Figure 1a) and females, darker individuals have greater concentrations of carotenoids at the start of the breeding season. Note that negative coefficient estimates between color and carotenoid concentrations indicate that less bright (visually darker) individuals have greater circulating concentrations of carotenoids during the early part of the breeding season.