Figure 3.

The probability of rejecting a coloured foreign egg is shown for (a,b) blackbirds (n = 82) and (c,d) robins (n = 52), with respect to the position of each hosts' own egg colour (see inset eggs above zero on both x-axes) along the (a,c) blue-green to brown and (b,d) purple to green colour gradients (in JNDs). We show a significant logistic (solid line, table 1), Gaussian (dashed, electronic supplementary material, table S2), and Weibull (dotted, electronic supplementary material, table S3) fits. Please note, we plotted all egg rejections, including rejection errors (black dots; n = 2) and foreign eggs falling along both colour dimensions. For comparison, we plotted (c) the mean location (approx. 4 JND on the x-axis) of eggshell coloration along this axis for the robin's heterospecific brood parasite, the brown-headed cowbird (also see figure 2). We illustrate 10 000 resampled slopes from binomial models predicting host behavioural responses (light grey lines); refer to table 1 for the significance of these parameters.