Effect of parasite egg size and quantity contrast of parasite-host eggs on recognition and rejection mode of Green-backed Tits

In the co-evolutionary system of avian brood parasitism, egg recognition and rejection are common and generally act as effective anti-parasitic adaptations by hosts. Accordingly, most studies have examined the role of parasite egg colors and markings in detection cues in hosts; however, studies focusing on the effects of egg size and quantity are relatively scarce and have mostly concentrated on the hosts of parasitic cowbirds. Here, we studied the egg recognition behaviors of a potential host of the Common Cuckoo (Cuculus canorus)—the Green-backed Tit (Parus monticolus), to determine: (1) whether the host uses the sizes of parasite eggs and/or the quantity contrast between parasite and host eggs as cues for recognition, (2) whether the host employs the template or discordant recognition mechanism for egg recognition, and (3) whether the size and quantity of parasite eggs affect the egg rejection modes of the hosts. The results indicate that Green-backed Tits did not use parasite egg size as a primary cue for egg recognition. We hypothesized that both visual and tactile detection might be involved in egg recognition by Green-backed Tits and suggest further studies after controlling for nest light conditions. Differences in egg quantity between parasites and hosts were not used as cues for egg recognition because the hosts employed the template mechanism rather than discordance to recognize parasite eggs. However, both the relative sizes and quantity of parasite and host eggs significantly affected the rejection modes of parasite eggs, and larger or more parasite eggs triggered higher probabilities of nest desertion in hosts.