Investigating the first stages of coevolution between the Pacific koel and its newest host,the red wattlebird

Abstract

Avian obligate brood parasites lay their eggs in the nests of other bird species and never provide their own parental care. This behaviour is a model example of coevolution and while multiple studies and reviews have discussed the different types of adaptations and counter-adaptations hosts and brood parasites evolve, there have only been a handful of empirical studies focused on how quickly coevolution can occur in a host-brood parasite system. Additionally, little is known about the early stages of brood parasite and host coevolutionary interactions. Understanding the rates of coevolution between brood parasites and their hosts is an important step in uncovering aspects about the process of speciation, determining which traits represent true genetic change and can aid in conservation decisions of endangered potential hosts, especially as brood parasites expand their breeding ranges with environmental changes. I investigated these issues by capitalising on the recent exploitation of the Red Wattlebird (Anthochaera carunculata) by the brood-parasitic Pacific Koel (Eudynamys orientalis). I conducted a literature review on factors that influence the rate of coevolution between avian obligate brood parasites and their hosts and performed observational and experimental studies at two sites where wattlebirds have experienced different durations of parasitism: Sydney (parasitism for 38-86 years) and Canberra (parasitism for 8-33 years). I determined that host switching can pose challenges for both the host and brood parasite, as parasitised wattlebird nests fledged significantly fewer young than unparasitised wattlebird nests, but fledged similar numbers of wattlebird and koel young. The koel’s later breeding season relative to the wattlebird’s and the koel’s poor timing of egg laying may have contributed to the low success of koel eggs. Mobbing experiments demonstrated that naïve hosts can learn to recognise a brood parasite within 33 years or less, but the speed at which this defence spreads throughout the population may be constrained by low parasitism rates. Egg rejection experiments indicated that more than 38 years is required for egg ejection to evolve in wattlebirds, as they only ejected model eggs at an extremely low rate in Sydney and Canberra, while two older hosts showed high levels of ejection at both sites. Lastly, I found evidence that the koel likely evolved mimicry of the eggs of one of its old hosts, which allowed it to exploit several other host species, including the wattlebird, due to all of its hosts having similar egg morphology. My results support the many theoretical models which agree that at least 30 or more years is required for egg ejection alleles to spread throughout a population. This process may be slowed because wattlebird eggs appear so similar to koel eggs, making it more difficult for wattlebirds to recognise parasitic eggs. However, I discovered that naïve hosts without specific anti-parasite traits can still utilise generalised defences, such as mobbing, in order to reduce the impact of brood parasitism, and that host switching can also be difficult for the brood parasite, as it may not be well-adapted to the new host’s breeding season or behavioural habits.