An imposter in the nest: Coevolution between the little bronze-cuckoo and its host, the large-billed gerygone

Interactions between avian brood parasites and their hosts are one of the most suitable model systems for studying coevolution. Brood parasitic cuckoos lay their eggs in the nests of other bird species, called hosts. In most cuckoo species, the cuckoo chick evicts or outcompetes the host's own brood and becomes the sole occupant of the nest. Therefore, parasitism by cuckoos is highly costly to hosts, selecting for the evolution of host defences. In turn, host defences select for counter-adaptations in cuckoos, giving rise to a coevolutionary arms race. Many hosts have evolved the ability to recognize and reject mimetic cuckoo eggs, yet fail to discriminate cuckoo chicks. Theory predicts that hosts should not evolve to recognize and reject cuckoo chicks via imprinting because of the risk of mistakenly imprinting on a cuckoo chick in their first brood and thereafter always rejecting their own chicks. However, recent studies have found that the large-billed gerygone (Gerygone magnirostris), a host of the little bronze-cuckoo (Chalcites minutillus), accepts non-mimetic cuckoo eggs and rejects the cuckoo nestlings. This unique coevolutionary pathway between Australian cuckoos and their hosts has generated much recent discussion in the literature about why cuckoo-host interactions evolve along several different coevolutionary trajectories. In order to address this question, it is necessary to explore the arms race at all stages of the breeding cycle, yet few studies of brood parasites have done this. The general aim of this thesis is to understand how the reciprocal interactions between one of the virulent avian parasites little bronze-cuckoos and large-billed gerygones have evolved across all breeding stages. I begin with a general introduction to coevolution between cuckoos and their hosts, and an introduction to my study system. In the next two chapters, I focus on defensive strategies in a common host of the little bronze-cuckoo, the large-billed gerygone, throughout the breeding cycle. I begin by examining three frontline defences: nest structure, nest placement and mobbing behaviour (Chapter 1). I test whether nest architecture or position influences the rates of parasitism or predation, and whether gerygones have evolved cuckoo-specific mobbing behaviour or vocalizations using model-presentation experiments. In Chapter 2, I explore defensive behaviour of large-billed gerygones against little bronze-cuckoos at the egg stage. I test the clutch dilution hypothesis by investigating whether accepting parasite eggs increases reproduction in a multiply parasitized host. Moreover, since large-billed gerygones never reject cuckoo eggs, I investigate whether they have any other form of egg rejection, such as nest abandonment, using field-based observation data. The aim of Chapters 3 and 4 is to determine whether hosts use visual or vocal cues to discriminate little bronze-cuckoo chicks. In Chapter 3, I use experimental manipulations to test whether gerygones recognize and reject little bronze-cuckoo nestlings based on true template-based recognition of visual cues, or whether they use recognition-free contextual cues. In Chapter 4, I evaluate whether cuckoo begging calls provide a potential additional cue for discrimination of cuckoo chicks by large-billed gerygones, and whether little bronze-cuckoos mimic the begging calls of their hosts. Using a cross-fostering experiment, I then test whether begging calls are plastic, allowing exploitation of multiple hosts, or fixed, potentially constraining the cuckoo to a specialist strategy