Investigating sex-specific crypsis as a driver of sexual dichromatism in titipounamu (Acanthisitta chloris)

Abstract

While sexual selection is often the evolutionary force behind sexual dichromatism in birds, natural selection acting on one or both sexes can also drive sex differences in colouration. This can occur if males and females use different foraging habitats with different background environments, requiring sex-specific plumage colouration to be cryptic against their respective foraging backgrounds and thus reduce predation risk. Sexual dichromatism in titipounamu (Acanthisitta chloris) could be driven by sex-specific needs for crypsis against their respective foraging backgrounds; previous research found that the green males spend more time foraging in amongst leaves and small branches, whereas the brown females spend more time foraging on trunks. However, that research focused on a population in atypical, primary succession forest and only considered titipounamu colour from a human visual perspective. Green plumage might also be less cryptic than is often assumed due to its ultraviolet reflectance and signalling properties. To determine whether this crypsis hypothesis holds true for titipounamu in their typical habitat and from the visual perspective of their avian predators, I investigated 1) sex differences in titipounamu foraging behaviour in a complex, native forest; 2) compared titipounamu colour to their background environment using relevant visual perception models and calibrated digital photography; and 3) whether either sex was compensating for more conspicuous colour through increased anti-predation behaviours through focal bird nest observations. Despite finding some sex differences in where titipounamu forage, I found no differences in how likely either sex was to be observed foraging against green or brown backgrounds. I also found that neither sex was cryptically coloured in their natural habitat from an avian visual perspective and could be distinguished from their background environment. While I found some evidence that titipounamu are cryptically patterned, this did not differ between sexes nor background substrate. Lastly, neither sex displayed more anti-predation behaviours at the nest, suggesting that neither sex is compensating more than the other for conspicuous colouration. Thus, my results suggest iii that it is unlikely that titipounamu sexual dichromatism is driven by the need to be cryptic against different background environments. Rather, it could be driven by various aspects of sexual selection. This thesis highlights the importance of studying cryptic colour from the perspective of potential predators and challenges assumptions about crypsis, sexual dichromatism and green colouration in birds.