Antipredator cautiousness and colouration in the Eurasian blackbird Turdus merula and song thrush Turdus philomelos

Abstract

Predation is an important driver of the evolution of animal colour and behaviour. More colourful animals are theorised to be under higher risk of predation due to being more conspicuous. However, empirical data suggests conspicuous colouration is often unrelated, or even negatively related, to predation risk. One explanation for this counterintuitive pattern is that conspicuously coloured individuals reduce their risk of predation by behaving cautiously (the compensation hypothesis). Studies of birds typically find the more conspicuously coloured sex (usually males) to be more cautious, supporting the compensation hypothesis. However, an alternative hypothesis is that female birds are less cautious than males because they have higher caloric requirements associated with reproduction. Obtaining calories usually comes at the expense of cautiousness, so female birds may need to be less cautious than males to meet their caloric requirements (the caloric requirements hypothesis). The primary aim of my thesis is to test the compensation and caloric requirements hypotheses in the Eurasian blackbird Turdus merula and song thrush Turdus philomelos. I did this by measuring the bill and feather colouration of these birds using colour photography and spectrometry (respectively) and testing for a relationship between colouration intensity and cautiousness during both the breeding and non-breeding seasons. My main metrics of cautiousness were vigilance while foraging and escape behaviour when approached. I found strong evidence for the caloric requirements hypothesis. Female blackbirds were less vigilant in the breeding than the non-breeding season, and female blackbirds with redder bill colouration were generally less cautious. My evidence for the compensation hypothesis was mixed. Song thrushes were more cautious than blackbirds, despite being less conspicuous. Female blackbirds tended to be less cautious than males, which could support either hypothesis. Blackbirds with more intense colouration were sometimes more cautious, which could support the compensation hypothesis, but this was not consistent across metrics of cautiousness. Overall, my results more strongly support the caloric requirements hypothesis than the compensation hypothesis, but individual antipredator behaviour is complex and may be influenced by both. My results also suggest the caloric requirements hypothesis is a potential mechanism for the evolution of sexual dichromatism in birds.