Laura Martinez Anton, Karine Delord, Christophe Barbraud, Cécile Ribout, Timothée Bonnet 

Phenotypes are changing in many wild populations, largely in response to environmental changes due to human activities. Phenotypic change can be driven by several mechanisms, with contrasted consequences for the persistence of populations. Identifying those mechanisms is key to understand current responses to human pressures and to predict the future fate of populations.
Here we attempt to disentangle the causes of the increase in bill length observed in the population of wandering albatross breeding on La Possession Island, Crozet Archipelago, over the course of 60 years. Taking advantage of long-term monitoring, morphological and pedigree data, we build a model that estimate changes due to demographic structure, plastic responses to several key environments, selective disappearance within generations, and genetic change.
We found that changes in sex-ratio caused a decline in bill length that opposes the phenotypic change and adds an extra ca. 25% of change to explain. Bill length was highly repeatable and was almost fixed after growth within an individual. However, bill length covaried with age among individuals, in part due to selective disappearance filtering out shorter bill lengths. Despite selective disappearance we did not identify a significant contribution of genetic change. In contrast, we identified an important contribution of phenotypic plasticity, in particular in response to the Southern Annular Mode, which relates to the distribution and strength of wind in oceanic regions used for foraging. In the end, we could explain about half the increase in bill length through demographic and plastic mechanisms. The demographic response is most likely transient and will not continue on the long-term, while the plastic response could be quickly reversed in parallel to environmental variables driving plastic changes. Phenotypic change accrued so far is likely not stable but is adaptive and given bill length high heritability, bill length has the potential to evolve adaptively in the future.