Common cuckoo females are not choosy when removing an egg during parasitism

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Abstract

Females of avian brood parasites have evolved various tactics to succeed in their reproductive strategy. Many of these adaptations, for example speed and timing of egg laying or egg mimicry, have been investigated in detail. However, one peculiar habit of parasitic females is still not fully understood—egg removal by adult parasitic females before laying their own egg. Here, we examined 2 hypotheses to explain this behavior in the common cuckoo (Cuculus canorus). Both hypotheses explored if the cuckoo females can inspect eggs in host nests and selectively remove some of them to improve their fitness. First, we investigated the “parasite competition hypothesis,” which proposes that the cuckoo removes an egg from the host nest to get rid of a previously laid parasitic egg from another cuckoo female. Second, we proposed and investigated a new “mimicry improvement hypothesis” stating that the common cuckoo female removes 1 host egg to improve mimicry of its egg in the host clutch and thus increase the chance of acceptance. Randomization tests revealed that cuckoo females are not selective and remove 1 randomly chosen egg from the host nest. We suggest that egg selection behavior could be too costly because it requires time and the cuckoo benefits most from minimizing the time spent at the nest due to aggressiveness of the host.

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