How do brood-parasitic cuckoos reconcile conflicting environmental and host selection pressures on egg size investment?

Brood-parasitic cuckoos lay their eggs in the nests of other birds and thereafter abandon their young to the care of the host. Thus, all maternal investment is restricted to investment in the egg. Optimal investment at this stage is likely to have a large impact on maternal reproductive success. Many bird species optimize the size of their eggs to suit both the prevailing environmental conditions and the number of individuals that will provide care to the chicks. However, relatively few cues are available to avian brood parasites to facilitate optimal investment in their eggs. Moreover, optimization of egg size to suit environmental conditions or the social structure of the host group may be constrained by stronger selection for egg mimicry, which reduces the likelihood of detection and rejection of foreign eggs by the host. We aimed to test how the conflicting selection pressures of (1) selection for large eggs in harsh environmental conditions versus (2) rejection of large eggs by hosts interact to influence the size and shape of Horsfield's bronze-cuckoo, Chalcites basalis, eggs. Using a sliding window approach to investigate periods of climate sensitivity, we show that climatic variables did not predict egg size. Nor did cuckoos modify their egg size or shape in relation to the group size of the host (superb fairy-wren, Malurus cyaneus). Conversely, host defences did appear to have influenced cuckoo egg morphology: eggs that were relatively short and round had a higher probability of being abandoned by the host than eggs that were long and narrow. This suggests that host defences are the overriding selection pressure on cuckoo egg morphology, and, unlike their hosts, cuckoos do not adapt their eggs to the prevailing climate or host group size.