Positive covariation between current reproduction and subsequent performance in a raptor: Is the devil in the details?

The theory about reproductive trade-offs suggests that as reproduction is costly, individuals should trade current reproduction against future reproduction or survival, leading to within-individual negative covariation between current reproduction and future performance. Despite clear predictions at the individual level, within-individual negative covariations do not always translate into negative covariations at the population level: the devil may be in the details. For instance, if one sex only exhibits negative covariations between current reproduction at t and performance at t + 1, the covariation at the population level may be null or positive. Similarly, ignoring age effects may prevent the detection of negative covariations between current reproduction and subsequent performance at the population level. For a monogamous species with biparental care of young, a negative covariation between current and subsequent demographic performance is expected, similar for both sexes, but potentially stronger for the oldest senescent individuals. Here, we take advantage of a long-term individual monitoring of male and female little owls (Athene noctua) and state-of-the-art capture-mark-recapture models to assess covariation between vital rates between two consecutive years at the population level. When analyzing all individuals together, we found a positive covariation between reproduction in year t and survival or reproduction in year t + 1, regardless of sex, indicating that individuals with a high reproductive success in a given year tend to survive and reproduce better in the following year. This is an important finding because such positive covariations between demographic rates at the population level may overestimate the population growth rate. Looking more closely at individuals of known age within the population, we found evidence for age-specific expression of reproductive trade-offs. Population-level covariation between current reproduction and subsequent demographic performance may thus mask more complex patterns of covariation between vital rates at finer scale.