Abstract
Many bird species have been observed shifting their laying date to earlier in the year in response to climate change. However, the vast majority of these studies were performed on non‐threatened species, less impacted by reduced genetic diversity (which is expected to limit evolutionary response) as a consequence of genetic bottlenecks, drift and population isolation. Here, we study the relationship between lay date and fitness, as well as its genetic basis, to understand the evolutionary constraints on phenology faced by threatened species using a recently reintroduced population of the endangered New Zealand passerine, the hihi (Notiomystis cincta). A large discrepancy between the optimal laying date and the mode of laying date creates a strong selection differential of −11.24. The impact of this discrepancy on fitness is principally mediated through survival of offspring from hatchling to fledgling. This discrepancy does not seem to arise from a difference in female quality or a trade‐off with lifetime breeding success. We find that start of breeding season depends on female age and average temperature prior to the breeding season. Laying date is not found to be significantly heritable. Overall, our research suggests that this discrepancy is a burden on hihi fitness, which will not be resolved through evolution or phenotypic plasticity. More generally, these results show that threatened species introduced to restored habitats might lack adaptive potential and plasticity to adjust their phenology to their new environment. This constraint is also likely to limit their ability to face future challenges, including climate change.
Highlights
Phenology is the study of the timing of life‐history events for individuals, populations and species
We investigate the evolutionary aspects of laying date in the hihi using a extensive data set from a threatened bird species, which includes data on nest lay date, fledgling recruitment, survival and individual fitness
Phenology is a key feature in adaptation to climate change for a broad spectrum of species (Parmesan & Yohe, 2003) and an important aspect of the long‐term conservation of threatened species (Rosemartin et al, 2014; Wadgymar, Cumming, & Weis, 2015)
Summary
Phenology is the study of the timing of life‐history events for individuals, populations and species. Some of the environments within which hihi were reintroduced are currently recovering from heavy degradation (e.g., almost complete land clearing), by contrast with the only native population consisting predominantly of mature forest (Makan, Castro, Robertson, Joy, & Low, 2014) This means that introduced populations are not expected to be at evolutionary equilibrium with these growing, immature forests, possibly resulting in a strong mismatch between their phenology and resource availability (Gienapp et al, 2008), reducing population fitness. If this is the case, the currently increasing additional pressure from climate change might pose a threat to their conservation in these environments. We use long‐term data on a pedigreed wild population of hihi to explore variation in laying date, its genetic basis and the selective pressure upon this variation, in order to determine whether (a) lay date is under stabilizing selection with an optimum of fitness, and if so, whether the observed lay date matches this fitness optimum, and (b) if a discrepancy with this optimum occurs, whether this trait has enough adaptive potential (i.e., trait heritability) to evolve in response to selection
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