Individual repeatability in dietary specialisation across years and competitive regimes in wild birds

Merit F Pokriefke , Alexander Keller, Alexandra G Cones, Hannah M Scharf, Niels J. Dingemanse

Dietary variation reflects fitness trade-offs shaped by environmental conditions and individual traits. According to optimal foraging theory, individuals should specialise when profitable prey is abundant, yet empirical evidence remains limited. We hypothesised that specialisation is driven by individual personality and competition intensity. Following the pace-of-life syndrome (POLS) framework, we predicted that ”faster” explorers, often securing higher-quality habitats, would show greater specialisation than ”slower” explorers, and that selectivity would in- or decrease with competition intensity. We tested this in sympatric breeding blue (Cyanistes caeruleus) and great tits (Parus major) by combining breeding density manipulations with DNA metabarcoding of faecal samples. By quantifying environmental arthropod abundance, we identified preferred prey as those used proportionally more than available. Contrary to our predictions, specialisation was unrelated to exploration behaviour or competitive intensity. Crucially, however, we found strong individual repeatability in specialisation across years within these relatively short-lived species. While specialisation appears independent of risk-taking personality and experimental densities, this high longitudinal consistency reveals that individuals maintain distinct dietary compositions regardless of environmental or behavioural drivers. These findings call for studies addressing the ecological and evolutionary forces maintaining individualised dietary specialisation in the wild.