A comparative analysis of evolutionary changes in island birds
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The evolutionary pressures of islands are often considered to be quite different to those found in continental systems. The insular flora and fauna and their characteristics that have resulted from this unique suite of pressures have puzzled scientists for centuries. This thesis uses the comparative approach to examine how island passerine birds differ from continental birds. Birds on islands like New Zealand have had an evolutionary history free from mammalian predators, in contrast to continental European species that co-evolved with mammals. Given this difference I examined how island birds differed from continental birds in three ways: sound, smell and appearance. I first looked at differences in the begging vocalisations of New Zealand nestling birds and compared these to the vocalisations of nestling introduced European birds. I expected that introduced species should produce less conspicuous calls given their co-evolutionary history with mammalian predators, while New Zealand birds should have comparatively more conspicuous begging calls. In fact, the calls of the two groups of birds were relatively similar. I then looked at the differences in the volatility ("smell") of preen waxes between native New Zealand species and introduced European species. I tested the prediction that New Zealand birds, which did not evolve with predatory mammals that located prey by smell, should produce preen waxes that do not function as 'olfactory crypsis' as found in continental birds. As found previously, introduced species adopt an 'olfactory crypsis' regime in by producing less volatile waxes during incubation. In contrast, most native species showed no shift in wax volatility, with one species even becoming more volatile in the breeding season, supporting a role of predation risk in the evolution of bird odours. Finally, I conducted a survey of evolutionary changes in appearance between insular and continental birds across a variety of isolated island groups, I compared changes in size, mass, wing length, bill size, carotenoid pigmentation, melanin pigmentation and 'dullness'. I found that island species were significantly larger, with larger bills than their continental counterparts. I also found that carotenoids decreased, and melanin pigmentation significantly increased, resulting in 'duller' island species. The reasons for these changes are not clear but highlight how island environments must differ in selective pressures from that encountered by birds on the continents. Overall my findings confirm that island species differ from continental species, and this extends to both their appearance, and somewhat surprisingly, their smell. My work raises a number of additional avenues for research, including a need to investigate the causes of changes exhibited by island birds.