Self-referent phenotype matching is a poor predictor of egg rejection by American Robins

Many hosts of obligate brood-parasitic birds use variation in the coloration and pattern of eggshells to identify and reject foreign eggs from their nests. However, egg-rejection behavior of several hosts is not tightly predicted by the modeled output of overall avian-perceivable chromatic differences between foreign and host eggs. This demands a re-investigation into the proximate sensory cues and perceptual processes used for egg recognition. One plausible mechanism is that rejection by hosts depends on the relative stimulation of one (or a combination) of their photoreceptors when comparing their own and parasitic eggs (i.e., self-referencing). To test this, we placed dark-blue egg models (with a known ~ 50% rejection rate) in nests of American Robins (Turdus migratorius), which are occasional hosts of Brown-headed Cowbirds (Molothrus ater). We found that relative metrics of individually repeatable size or shape and overall avian-perceivable differences in color between American Robin eggs and model eggs did not explain the probability of an experimental egg being rejected. However, despite a limited R² value, hosts were significantly more likely to reject a model egg when the estimated contrast between their own egg and the model egg was greater for the blue (SWS) photoreceptor. These results are consistent with female American Robins using a simple physiological algorithm that would confer widespread benefits by reducing the likelihood of rejecting their own eggs, while still eliminating parasitic eggs because of their dissimilar novel coloration.