Life-history optimisation in response to climate change

Predicting how climate change will affect populations is one of the major issues facing us today. In many ecosystems, the phenology of one species will influence the optimum timing of life-history events for species at higher trophic levels. In seasonal environments, consumers are selected to time energetically demanding activities, for instance reproduction, to coincide with a short period of favorable conditions, such as the peak in prey availability1,2. However, differential responses to environmental change may lead to phenological mismatches between species and potentially impact on individual fitness (e.g. offspring number) and population demography (e.g. population size)3,4,5. Therefore, determining the optimal annual timing of key life-history events in the context of environmental variation and understanding how closely individuals can track this are crucial for projecting population trends under climate change.
The correct timing of an event relies on the optimal sequence of behavioral decisions preceding it, while considering the associated costs and benefits at each step. The trade-offs between different decisions are influenced by the individual's state, their previous actions as well as the environmental conditions they are faced with6. For instance, woodland passerines, such as tits in the Paridae family, maximize their reproductive success when the provisioning of young coincides with the peak in caterpillar abundance7,8. The optimal timing of the nestling stage is preceded by decisions about reproductive timing and effort including (1) the laying date, (2) the clutch size and (3) the incubation period. These traits are intricately linked through trade-offs and are cued by environmental factors9.10. For instance, in years with higher spring temperatures, females may respond plastically to (1) advance lay date and/or (2) reduce clutch size and/or (3) begin incubation before the last egg is laid in anticipation of an earlier caterpillar peak11,12,13. In this example, the quantity of offspring (smaller clutch size) may be traded for their quality (increased synchrony with resource).
There is substantial variation between individuals in the extent to which they can adjust different aspects of their breeding cycle, thought to be linked to individual condition14,15. A large body of research has explored immune function-related costs of reproductive behavior as a mechanism mediating life-history trade-offs16, though these physiological traits are often difficult to measure and underpinned by complex pathways. On the other hand, plumage quality may be a more easily quantifiable currency underlying the trade-offs between, for instance, reproductive timing and moult rate17,18. Specifically, the insulating capacity of feathers has been suggested to drive the negative impacts of delayed breeding on overwinter survival and future reproductive success19. However, we need to better understand (1) the relationship between feather quality and its insulating properties, and (2) how plumage condition may mediate the effects of both reproductive timing (e.g. lay date) and reproductive investment (e.g. clutch size) on fitness (i.e. future survival and reproduction) during each stage of the breeding cycle.