LAUSR

Animals are thought to adjust the intensity of their parental defense to fitness benefits associated with their current reproduction. They should be more defensive when they breed early in the season, when they produce more young, or as their young get older. Beyond these plastic changes, associations among traits could be explained by consistent among-individual differences. Separating the mechanisms at the origin of behavioral phenotypic variation is essential to understand the determinants and evolution of parental care. During four breeding seasons, we investigated whether variation in Canada goose (Branta canadensis) nest defense resulted from individual short-term plastic adjustments to reproductive stage within breeding seasons, plastic adjustments to brood value across breeding seasons, or from long-term among-individual differences. We thus estimated the among- and within-individual relationships between nest defense, clutch size, and laying date. Female and male nest defense behaviors were highly repeatable in time (R = 0.72 and 0.67 on the short term). At the among-individual level, we found no significant association between female nest defense and life history traits, but the more defensive males were mated with the more fecund females. Female and male nest defense was not adjusted to annual variation in laying date nor in clutch size. However, females adjusted their nest defense within each breeding season and became more defensive when approaching egg hatching. Thus, Canada goose nest defense behavior is partly explained by plastic adjustments, but also by consistent among-individual differences which indicate the evolutionary potential of nest defense behavior.Significance statementAn association between traits at the phenotypic level can be partitioned into association at the among- and within-individual levels. This allows to better understand covariation of traits resulting from both among-individual differences and phenotypic plasticity. We observed 141 Canada goose pairs to identify what affected the intensity of their nest defense behavior. We found that geese were very consistent in their nest defense behavior and that female fecundity was related to male personality. Females also became more defensive as their young got older. Analyzing different sources of variation in parental behaviors enables a better understanding of their evolution. Our study also demonstrates the importance of studying nest defense in both sexes even in species where only females incubate eggs, as both sexes can invest in nest defense.