The evolution of egg size in the brood parasitic cuckoos

Abstract

We compared genera of nonparasitic cuckoos and two groups of parasitic cuckoos: those raised together with host young (“nonejectors”) and those in which the newly hatched cuckoo either ejects the host eggs or chicks, or kills the host young (“ejectors”). Nonejectors are similar to their hosts in body size and parasitize larger hosts than do ejectors, which parasitize hosts much smaller than themselves. In both types of parasite, the cuckoo's egg tends to match the host eggs in size. To achieve this, nonejectors have evolved a smaller egg for their body size than have nonparasitic cuckoos, and ejectors have evolved an even smaller egg. Among ejector cuckoo genera, larger cuckoos have larger eggs relative to the eggs of their hosts, and the relationship between cuckoo egg volume (mass of the newly-hatched cuckoo) and host egg volume (mass to be ejected) did not differ from that predicted by weight-lifting allometry. However, comparing among Cuculus cuckoo species, the allometric slope differed from the predicted, so it is not clear that egg size is related to the need to give the cuckoo chick sufficient strength for ejection. Comparing the two most speciose ejector genera, Chrysococcyx cuckoos (smaller and parasitize dome-nesting hosts) lay eggs more similar in size to their host's eggs than do Cuculus cuckoos (larger and parasitize open cup–nesting hosts). Closer size-matching of host eggs in Chrysococcyx may reflect the following: (1) selection to reduce adult body mass to facilitate entry through small domed nest holes to lay, and (2) less need for a large egg, because longer incubation periods in dome-nesting hosts allow the young cuckoo more time to grow before it need eject host eggs.