Interactions between nesting birds and invertebrates are a common, yet poorly understood ecological phenomenon. Many of these types of interactions are close and prolonged, and therefore potentially critical to one or both of the species involved in the interaction. However it is unusual for the nature of the interaction to be evaluated in a manner that reveals the impact of the relationship on both parties to the interaction. This study examines two relationships between nesting birds and invertebrates, both of which involve the hooded parrot (Psephotus dissimilis), a small grass parrot that inhabits the tropical savannas of northern Australia. ¶ The field-work for this project was conducted over two parrot breeding seasons in 2006 and 2007 near Katherine, Northern Territory, in the Australian dry tropics. In chapter II, I present data on the breeding biology of the hooded parrot as background for the study that follows. Nest building commenced in January, with peak activity in February and the last chicks fledged in April. Fifty three active nests were located. The mean number of eggs laid per nest was 4.5 (s.d. ± 0.9), of which 3.0 (± 1.79) hatched and 2.0 (± 2.0) fledged. Clutches were laid asynchronously over a period of a week and chicks remained in eggs for 18.6 (± 1.95 days). Chicks were 29.2 (± 2.9) days old when they fledged from the nest. These data are typical for Australian parrots. ¶ In chapter III, the unusual nature of the parrot’s nest site is examined. Many species of bird nest in natural cavities or those they excavate. Whilst cavity nesters as a whole experience increased nesting success, the greatest success is experienced by species that can excavate their own nests. Certain arboreal cavity nesters, such as woodpeckers, require extensive morphological adaptation for this behaviour, but this has not occurred in Australia, despite competition among birds and a suite of arboreal mammals for naturally occurring cavities. Some species, however, have adapted their behaviour to make use of substrates that are not as hard as wood. Hooded parrots excavate nests in terrestrial termitaria, releasing them from competition for limited arboreal cavities. However, I show that only termitaria with a high level of termite activity, and which are more than two metres tall, are suitable, and that the parrots exhibit a strong preference for the cathedral mounds of Nasutitermes triodiae. Nests placed in highly active mounds had a significantly higher success rate than those in mounds where activity was somewhat lower, suggesting that the behaviour is adaptive. ¶ The thesis then shifts focus from the parrot to its nest symbiont, first describing the species involved in the interaction in chapter IV, and then its behaviour in the nests of hooded parrots in chapter V. Trisyntopa neossophila sp. n. (Lepidoptera: Oecophoridae) was reared from the nest of the hooded parrot and described using morphological characters. Aspects of its biology are reported and similarities to the biology of Trisyntopa scatophaga found in the nests of the golden-shouldered parrot (Psephotus chrysopterygius) are discussed. The possibility that a moth was associated with the extinct paradise parrot (Psephotus pulcherrimus) is considered in the light of the phylogenetic relationships between the parrots. ¶ Trisyntopa neossophila is an unusual moth whose breeding cycle is shown to be closely synchronised with the hooded parrot. T. neossophila is one of three coprophagous, nest dwelling moths in the genus Trisyntopa. True coprophagy is rare in the Lepidoptera, although some species occasionally consume faeces to gain rare nutrients. T. neossophila lays its eggs in the nest of hooded parrots so that larvae hatch in synchrony with the hatching of the parrot’s eggs. The larvae spend their larval period in the nest and exclusively consume the excrement of the nestling parrots. When the parrot chicks fledge, the larvae move to the walls of the nest cavity to pupate, emerging the following wet season to repeat the process during the next parrot breeding season. ¶ With a description of the ecology of both species involved in the nesting symbiosis, chapter VI reviews the literature surrounding other interactions between nesting birds and invertebrates. A large number of birds are shown to nest in, or in close proximity to, structures made by invertebrates and avian nesting material provides a reliable shelter for many invertebrate species. However, the nature of such relationships has rarely been experimentally demonstrated. I propose that in order to understand the nature of these relationships they need to be explored within the theoretical framework of community ecology. Putative commensal and parasitic relationships have all been documented in the bird/invertebrate nesting literature, yet researchers, with few exceptions, repeatedly overlook the impact that these relationships are having on the invertebrate, at best assuming the nature of its impact, but more often ignoring its impact entirely. Here I present a framework for formulating hypotheses to ensure that the nature of the relationship can be identified. Only by explicitly stating the level of organisation at which the experiment is to occur (individual or population), identifying the net cost or benefit of the interaction, the range of conditions under which such costs or benefits would apply and the spatial and temporal context in which they apply, can an investigator expect to recognise and describe the often complex nature of these relationships. ¶ While parasitic and commensal relationships between nesting birds and invertebrates are commonly reported, mutualisms between birds and invertebrates have not been reported. Despite this, candidates for this type of relationship exist. Chapter VII uses the framework outlined in the literature review (chapter VI) to experimentally examine the relationship between the hooded parrot and Trisyntopa neossophila. By manipulating the populations of moth larvae in a sample of hooded parrot nests, we sought to establish the impact of the relationship on each species. The moth depends on the parrot for provision of shelter and a reliable food source. The parrot however, was neither benefited nor harmed by the interaction in terms of short term reproductive output or chick growth, although differences between the experimental and control nests were noted. The relationship between the hooded parrot and T. neossophila, at least during the study period, is therefore concluded to be commensal. ¶ Collectively, the chapters of this thesis explore the complicated interactions between species. The dependence of the moth on the parrot and the parrot on the termite, demonstrate the importance of understanding interactions between species in a manner that reveals the impacts of the interactions, the range of conditions under which they would apply and the level of organisation at which they apply, as outlined in chapter VI. The dependence of the animals in this study on each other makes them more vulnerable to extinction than previously thought. Whilst this may not be immediately significant for the hooded parrot/T. neossophila system, which is thought to be secure, the ecologically similar system on the Cape York Peninsula, involving the golden-shouldered parrot and its nest attendant moth Trysintopa scatophaga, is vulnerable to extinction and subject to intensive management to ensure its persistence. This study brings new information to the management of the golden-shouldered parrots and urgently recommends increased protection for Trysintopa scatophaga.