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

Sex and morph differences in age-dependent trait changes in a polymorphic songbird

There is growing evidence of important variation in how animals age, in particular in how the expression of traits changes with age among different species and populations. However, less is known about variation within populations, which may include variation in ageing patterns between different types of individuals (e.g. sexes or distinct polymorphisms) and between different types of traits (e.g. general traits versus those used in social signalling contexts). We used 6 years of longitudinal data to examine age-related changes in trait expression in a captive population of Gouldian finches (Erythrura gouldiae), a socially monogamous songbird with genetically determined colour morphs that differ in behaviour and physiology. We contrasted ageing patterns of different types of traits (social signalling vs. size-related) in both sexes and in two colour morphs, using a mixed model approach to account for both within- and between-individual effects. We found pronounced sex differences in how social signalling traits change with age, showing a quadratic pattern in males, but not changing with age in females. In contrast, we observed no sex-specific ageing patterns in size traits. We also found subtle morph differences in how size-related traits changed with age, with black morphs stable or increasing with age while red morphs showing a decline with age. Finally, we found an interesting sex by morph interaction in one important social signal (headband width). These results highlight the importance of using within-individual approaches to understand ageing patterns across types of individuals (sex, morph, etc.) and the need for further research on the ageing patterns of traits that may experience different selective pressures.

Birds of different morphs use slightly different strategies to achieve similar reproductive performance following heatwave exposure

Responses to extreme climatic events may differ between individuals of distinct morphs which differ in life-history strategies, resulting in climate change 'winners' and 'losers' within species. We examined the reproductive performance and carry-over effects on offspring of black- and red-headed Gouldian finches Erythrura gouldiae after exposure to simulated heatwaves of moderate or severe intensity. We expected black-headed pairs' reproductive performance to decline after the severe heatwave because only the condition of black-headed females deteriorates during such a heatwave. Supporting the fact that Gouldian finches of different morphs use alternative reproductive strategies, we found that black-headed females initiated egg-laying a month earlier than red-headed females after experiencing a severe heatwave. We also found that this severe heatwave resulted in shorter spermatozoa in males irrespective of their morph. Despite these effects associated with heatwave intensity, the overall reproductive performance of both morphs was not affected by this factor, which was possibly due to an increased nestling provisioning rate by parents after exposure to the severe heatwave. However, offspring still bore the cost of parental exposure to the severe heatwave, as they showed a reduced condition (lower plasma antioxidant capacity and transient lower breathing rate) and higher oxidative damage (at least in fledglings with black-headed parents). These results suggest that inter-morph phenotypic variability in the Gouldian finch does not result in clear differences in reproductive performance following heatwave exposure, despite basal phenotypic differences between morphs. Whether animals using alternative reproductive strategies are, in the end, differently affected by climate changes will likely depend on the capacity of their offspring to recover from altered developmental conditions.

Morph- and sex-specific effects of challenging conditions on maintenance parameters in the Gouldian finch

Intraspecific discrete polymorphism is associated with the use of alternative life-history strategies, reflected by distinct reproductive or copying strategies in individuals of different morphs. Yet, morph-specific costs and benefits related to different life-history strategies remain unclear. Here, we examined in the polymorphic Gouldian finch (Erythrura gouldiae) whether markers of somatic maintenance (body mass, oxidative status and telomere length) differed between red- and black-headed birds under energetically demanding conditions (during heatwaves of different intensity, and during moult or breeding following heatwaves). During heatwaves, red-headed birds showed a homogeneous response, as males and females initially tended to gain mass and had reduced plasma hydroperoxide levels (a marker of oxidative damage) irrespective of heatwave intensity. In contrast, black-headed birds showed a stronger and more heterogeneous response, as black-headed males gained mass at the beginning of the thermoneutral heatwave and showed stable oxidative status, whereas black-headed females lost mass and tended to show higher hydroperoxide levels at the end of the thermocritical heatwave. Following heatwaves, we found morph-specific oxidative costs owing to moult or reproduction, with oxidative markers varying only in black-headed birds. Again, oxidative markers varied differently in black-headed males and females, as plasma antioxidant capacity decreased in moulting or breeding females, whereas males showed higher hydroperoxide levels with larger broods. For the first time, our study highlights that within polymorphic species, some individuals appear more vulnerable than others when coping with energetically demanding conditions. In the context of climate change, such differential effects may ultimately alter the currently observed balance between morphs and sexes within natural populations.