Cracking egg investment: Maternal investment in cuckoos and their hosts

Abstract

Maternal investment is important for the growth and survival of offspring. Maternal investment is influenced by the environment of the mother, including both the physical environment (the climate), and the social environment (the number of individuals that will care for the offspring). Females may vary their investment in offspring relative to environmental conditions to optimise the trade-off between investment in current and future offspring. This thesis examines how environmental factors affect maternal investment and the impact of maternal investment strategies on offspring, using the superb fairy-wren Malurus cyaneus and a brood parasitic cuckoo that exploits them, the Horsfield's bronze-cuckoo Chalcites basalis, as a model system. In the second and third chapters of my thesis, I used superb fairy-wrens as a study species. Superb fairy-wrens are an ideal species to investigate maternal investment as their egg size varies according to the climatic and social conditions. The second chapter of this thesis presents a field experiment that explored the mechanism underlying adaptation of egg size to the environment. Using a food supplementation experiment, I ascertained that food availability is a limiting factor in egg investment in superb fairy-wrens. The third chapter investigated the impact of maternal investment in egg size on chicks, through a whole-clutch cross-fostering experiment. I found that egg investment had a positive effect on chick growth, larger eggs size buffered chicks against extreme conditions, and females breeding in groups tailored their post-natal investment to their pre-natal investment. In the fourth and fifth chapters of my thesis I investigated an atypical method of maternal investment using the primary brood parasite of superb fairy-wrens, the Horsfield's bronze-cuckoo. Brood parasitic cuckoos are an interesting case study for maternal investment as their entire investment is in their eggs, which they dump in the nest of a host to be raised exclusively by the host. In addition, cuckoos face selection pressure from their hosts, because raising cuckoos is costly for hosts and selects for host defences against parasitism, including rejection of foreign eggs. Thus, cuckoos face the competing selection pressures of maximising investment in their offspring, whilst minimizing detection of the foreign egg by the host. The fourth chapter of this thesis explores these competing selection pressures on cuckoo egg investment and concludes that the need to evade detection by hosts exerts the strongest selection pressure on Horsfield's bronze-cuckoo egg morphology. Moreover, the size of cuckoo eggs doesn't show the same fine-tuned adaptations to climate and social circumstances as their hosts. Another complicating factor for migratory Horsfield's bronze-cuckoo maternal investment is coordinating arrival at the breeding site with the breeding season of their host. Climate change has made this relationship more precarious by increasing the likelihood of phenological mismatches between cuckoos and their hosts. Chapter 5, using a long-term dataset of cuckoo and host breeding dates, found little evidence of a phenological mismatch between Horsfield's bronze-cuckoos and their superb fairy-wren hosts. However, trends suggest there is potential for a mismatch in the future and negative consequences for cuckoo breeding success associated with this mismatch. Show more