Red does not always outperform black: morph-specific behavioural variation in response to environmental changes

Abstract

Variable selection pressures on individuals contribute to the occurrence of distinct phenotypes within species. Rapidly changing environmental conditions due to climate change are likely to alter these selection pressures and hence the current balance between phenotypes within species. In birds, colour polymorphism has been related to alternative behavioural strategies, potentially underlying different responses to extreme environmental conditions between colour morphs. Here, we examined how red- and black-headed Gouldian finches, Erythrura gouldiae, responded to experimentally induced heatwaves of different intensities by measuring their feeding behaviour, their response to novelty and their behavioural reactivity. As red-headed birds have been described as physiologically more plastic than black-headed birds, we expected them to alter their behaviour more strongly in response to heatwaves. Even though black-headed males fed overall less than other birds (thereby suggesting lower energy requirements), differences in feeding behaviour between morphs remained unaffected by thermal conditions. However, birds’ response to novelty varied between morphs under variable thermal conditions. Indeed, before thermal treatment, black-headed birds, irrespective of their sex, spent less time near a novel object than red-headed birds, thereby suggesting lower novelty interest. However, because only red-headed birds reduced the time spent close to a novel object during heatwaves, differences in novelty interest between morphs disappeared under these thermal conditions. These results suggest that behavioural plasticity in response to thermal conditions differ between colour morphs of Gouldian finches, thereby showing that behavioural differences between morphs are not static. How such behavioural adjustments within species affect the balance between colour morphs in natural populations facing extreme environmental conditions remains to be determined.