Table 1. Effects of experimental manipulations on beak colouration, plasma carotenoids, body mass and blood redox state.

Response variables Predictors	Parameter estimates		Model statistics		Standardised effect size
	Mean effect	SE	t	P	b*	CI 2.5%	CI 97.5%
Beak red chroma
initial	0.17	0.10	1.64	0.11	0.13	−0.03	0.28
ROS	−0.26	0.05	−4.81	<0.001	−0.73	−1.04	−0.43
CAR	0.50	0.05	9.38	<0.001	1.44	1.13	1.75
ROS × CAR	0.07	0.11	0.61	0.54	0.19	−0.43	0.80
Total plasma carotenoids
initial	0.08	0.02	4.35	<0.001	0.31	0.17	0.45
ROS	−0.71	0.24	−2.99	0.005	−0.42	−0.70	−0.14
CAR	2.64	0.24	11.08	<0.001	1.56	1.28	1.84
ROS × CAR	0.29	0.48	0.62	0.54	0.17	−0.39	0.74
ZE/tHODE ratio
initial	−0.36	0.25	−1.44	0.16	−0.06	−0.15	0.02
ROS	0.85	0.04	20.10	<0.001	1.55	1.39	1.71
CAR	−0.57	0.04	−14.88	<0.001	−1.04	−1.18	−0.90
ROS × CAR	−0.42	0.09	−4.86	<0.001	−0.77	−1.08	−0.45
OXY
initial	0.67	0.13	5.19	<0.001	0.60	0.37	0.83
ROS	20.56	7.29	2.82	0.007	0.64	0.19	1.11
CAR	−1.15	7.05	−0.16	0.87	−0.04	−0.48	0.41
ROS × CAR	4.13	14.17	0.29	0.77	0.13	−0.77	1.03
8-isoprostane in RBC
initial	0.41	0.09	4.48	<0.001	0.53	0.29	0.76
ROS	1.97	1.04	1.89	0.07	0.43	−0.03	0.90
CAR	1.56	1.06	1.47	0.15	0.34	−0.13	0.81
ROS × CAR	−1.35	2.12	−0.64	0.53	−0.30	−1.24	0.64
Body mass
initial	1.01	0.11	9.62	<0.001	0.83	0.66	1.00
ROS	−0.02	0.31	−0.06	0.95	−0.01	−0.33	0.31
CAR	−0.26	0.33	−0.80	0.43	−0.14	−0.48	0.21
ROS × CAR	−1.44	0.62	−0.62	0.02	−0.75	−1.40	−0.11

Estimates represent coefficients from linear models with oxidative challenge (ROS) and carotenoid intake (CAR) included as factors and pre-treatment (initial) values as covariates. Low and high factor levels were coded 0 and 1, respectively and centred in order to enable the main effects to be properly interpreted without the need to remove the interaction terms from the models. Proportional variables (i.e. beak red chroma and ZE/tHODE ratio) and total plasma carotenoids were normalised using logit and Box-Cox (λ = 0.335) transformation, respectively. Standardised effect sizes are reported as standardised partial regression coefficients (b^*) from the same models with continuous variables z-standardised.