Individual Variation in Nest-Defence Behaviour and its Link with Survival and Reproduction in the Long-Lived Alpine Swift

Abstract

Given the limitations in resources that individuals can allocate to life‑history traits, trade-offs are expected to occur between survival and reproduction in the wild. According to the pace‑of‑life syndrome (POLS) hypothesis, these trade-offs may also be accompanied by changes in behavioural traits, such as the degree to which individuals defend their offspring, because such behaviours carry risks that reduce survival while increasing reproductive success. The POLS hypothesis predicts that individuals with a 'fast' lifestyle should take more risks (e.g., stay with their offspring to defend them) and, in return, achieve higher reproductive success but have a shorter lifespan than individuals with a 'slow' lifestyle.

Using a dataset with more than 20 years of measurements from a Swiss population of Alpine swifts (Tachymarptis melba), I investigated how nest defence behaviours change over the course of an individual's lifespan and how these behaviours influence survival and reproductive success. This long‑lived bird breeds mostly in urban colonies in Switzerland, where it may be exposed to aerial and terrestrial nest predators. Both parents incubate and raise their nestlings, allowing us to quantify nest defence behaviour based on their reaction to a human intruder approaching the nest. Responses ranged from flying away, to remaining motionless on the nest, to showing aggression toward the intruder.

Analyses of intra-individual variation in nest defence behaviour across age revealed that breeding female Alpine swifts were more likely to stay on the nest and defend their offspring compared to males. Both sexes became less likely to fly away as they aged, up to around 10 years old, after which this effect reached a plateau. This pattern suggests increased investment into offspring survival as individuals age, until reaching a peak, or alternatively, that adults learn to be more defensive after the onset of reproduction. However, analyses of the relationships between reproductive traits (clutch initiation date, clutch size, brood size at fledging, and lifetime reproductive success) and nest defence behaviours in male and female Alpine swifts showed no evidence of correlations. This suggests that nest defence behaviour has minimal detectable effects on nestling survival and adult fitness in our study system and is therefore unlikely to be under selection.

Overall, my study highlights variations in behaviours between and within individuals of a wild bird species living close to humans: nest defence behaviour varied with age, and females showed higher levels of defence behaviour. However, as there is no evidence that thisvariation confer fitness advantages, the idea that natural selection accounts for the observed natural variation in behavioural traits in this population is not supported. Therefore, this study provides no support for the POLS hypothesis, which suggests that nest defence behaviours covary with life history traits such as reproductive success and survival.