Egg recognition and nestling discrimination in the Crested Myna (Acridotheres cristatellus): Size matters

Most studies exploring abilities of hosts to detect brood parasitism are based on detecting colour and/or pattern differences among parasitic and host eggs or nestlings, while only few were focused on size differences. True recognition and recognition by discordancy are used to explain cognitive mechanisms of host egg recognition; however, only a few studies have found that hosts use recognition by discordancy. This study investigated: 1) whether egg and nestling recognitions in the Crested Myna (Acridotheres cristatellus) are based on size cues; 2) whether the egg cognitive mechanism is recognition by discordancy based on size cues; and 3) whether the longer the experiment time, the higher the egg recognition rate. Our results showed that the Crested Myna uses egg or nestling size as a recognition cue while the egg and nestling colour and patterning are not associated with egg or nestling rejection, thus the cognitive mechanism of egg recognition in the Crested Myna is recognition by discordancy based on egg size cues. Furthermore, there is a rejection delay in time of egg rejection behaviour of the Crested Myna. Therefore, we suggest that the periodicity of egg rejection experiments could be appropriately extended, especially for species with relatively low egg recognition ability.