Short and long-term consequences of early growth environment in the long-lived Alpine swift: insights into viability selection and hormesis

With the increasing environmental pressure on wild organisms due to global changes, it becomes essential to better understand the consequences of these changes at the individual and population levels. Changes in early growth environment are particularly concerning because of their short-term effects on individual growth and phenotype, as well as long-term effects on individuals that survived to adulthood in the population. We experimentally tested for short- and long-term effects of early growth conditions by manipulating brood size in the long-lived Alpine swift (Tachymarptis melba). Enlarging or reducing broods by one nestling is known to alter sibling competition and how much food is provided per capita, with consequences on resource allocation between growth and self-maintenance. We manipulated 45 pairs of broods between 2003 and 2006 and followed the offspring throughout their lives until the last observation in 2019. Nestlings from reduced broods grew faster despite having higher ectoparasite loads per individual, were larger in size and heavier close to fledging, fledged earlier, and had a higher probability of survival up to fledging in comparison to nestlings from enlarged broods. There was no significant difference between treatments in the probability of offspring being recruited in adulthood in the population, in their adult morphology and parasite load, and in age at first reproduction and lifespan. However, females, but not males, from enlarged broods showed higher lifetime reproductive success, explained by their production of bigger clutches. We discuss two complementary hypotheses that can account for short-term negative effects and long-term positive effects of early growth environment on the life history trajectory of this long-lived bird. First, the higher mortality early in life in enlarged broods may have filtered out low-quality individuals, thus leading to the recruitment of higher-quality individuals raised in enlarged broods (viability selection hypothesis). Second, a mild stress early in life has been suggested to prepare individuals to better deal with stress in adulthood (hormesis hypothesis), which may account for the higher reproductive performance of adults raised in enlarged broods.