The origin and function of odours in island birds
Thesis DisciplineEvolutionary Biology
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The field of avian chemical ecology is very much in its infancy, but there has undoubtedly been a surge of interest in recent times for the study of odour in birds – it has even recently been termed “the New Black of avian communication”. I sought to add to the field by carrying out a study on various aspects of the origin, development and function of odours in island birds. I first explored the ontogeny of preen wax production and the development of a chemical signal in nestlings of 5 native and introduced species of New Zealand birds. I did not find significant differences in the start of preen wax production in native vs. introduced species, mainly due to the highly distinctive preen wax development pattern shown by the European starling (Sturnus vulgaris). However, I did discover that nestlings modify the composition of their preen wax over the course of the nestling phase, and the compositional change follows different trajectories in native and introduced species. Multivariate analysis of the preen wax profiles revealed significant variation at all levels examined (status, species and nest). This is consistent with the hypothesis that different evolutionary histories are reflected in the preen wax profiles of birds, and this is discernible from a very early age. I next examined the effect of diet on the chemical profile of a generalist bird widespread in the South Pacific, the silverye Zosterops lateralis. Diet is often cited as a possible confounding effect in studies comparing composition of chemical signals between sexes, seasons, breeding and developmental stages, and among different species, but the effect of diet on chemical profiles has rarely been tested directly. I used a supplemental feeding experiment with tallow fat but this change in diet did not affect preen wax composition in silvereyes, and there was no change in preen wax composition among sampling periods. This supports the hypothesis that preen wax composition in silvereyes is determined by endogenous mechanisms and is not affected by day to day variation in diet. I then explored the differences in chemical profiles of “inbred” vs. “hybrid” South Island robins Petroica australis, living sympatrically on Motuara Island, where an experimental genetic rescue translocation was carried out in 2008. This setup allowed me to investigate the influence of inbreeding and geographic origin on the chemical profiles of a native New Zealand species, without facing confounding effects derived from the birds living in different habitats. I detected a significant effect of status (“inbred” vs. “hybrid”) on chord-transformed chemical profiles, but found no correlation between chemical profiles and heterozygosity, or between chemical and genetic distances. These results suggest that South Island robins preen wax could encode a geographic signal rather than a heterozygosity signal. Finally, I present the results of a broad comparative study that I undertook in collaboration with Aude Thierry, comparing chemical profiles of a number of passerine species across the South Pacific which are phylogenetically related but have very different evolutionary histories, having either evolved in Australia, under pressure from a number of olfactory-searching predators, or on islands were such predators were absent (New Zealand and New Caledonia). We found that island birds showed a significant loss of complexity in the non-volatile components but a significant increase in the diversity of compounds in the volatile part of their wax profile. This suggests birds on islands produce preen waxes, and thus odours, that are likely to be more conspicuous than those of continental species. This is concordant with our expectation, given what we know of their evolutionary history with regards to coexistence with predators, and points to the possibility that some form of “olfactory crypsis” is at work in the continental Australian birds, but is not present – or has been lost – in island birds. Overall, my study confirms that the current interest in avian chemical communication is well warranted as there appears to be a wealth of information encoded in avian chemical signals, and we are only just starting to explore and understand the biological relevance of it, both at the intra- and inter- specific level. This could have repercussions on conservation strategies and in general practices of bird handling, and will allow us to gain an understanding of a communication system that, if ubiquitous across the animal world, has thus far been overlooked in the avian class.