Figure 3. Host behaviour in relation to the proportion of foreign eggs and the level of colour difference between eggs.

Foreign eggs were more likely to be rejected the more they differed in colour from the host’s own eggs. However, this effect was dependent on their relative proportions in the nest. As foreign eggs increased in number, hosts required a greater difference in colour to reject them. Colour differences are discrimination units or ‘just noticeable differences’ (JNDs), where a JND of <1.0 means that two objects cannot be discriminated, and values between 1.00 and 3.00 should be difficult to discriminate. Boxplots show median bars and interquartile ranges; whiskers show outer quartiles. There were 79 trials with eggs accepted and 90 trials with eggs rejected (see Results).