Aggressive Ural owl mothers recruit more offspring

Animals are thought to adjust their behavior optimally to any given environment. So-called behavioral syndromes, or consistent patterns of behavior across environments, contradict this assumption of unlimited plasticity. We studied nest defense aggressiveness of female Ural owls (244 females with 482 breeding attempts) breeding in a highly variable environment created by fluctuations in the abundance of their main prey (field and bank voles) across years. Ural owls were more aggressive when voles were increasing in density as well as when the Ural owls had large brood sizes and laid early in the season. Aggressive nest defense was highly repeatable between breeding attempts (r = 0.52 +/- 0.05 standard error), but individuals also differed in their plasticity (the extent to which they adjusted the level of their aggression to the varying food conditions). Fierce nest defenders produced more recruits to the local breeding population, but a female's survival was not affected by her intensity of nest defense. A path analysis revealed that nest defense aggressiveness, rather than its correlates vole abundance, brood size, or laying date, best explained offspring recruitment. Our findings provide an ultimate explanation for the Ural owl's extremely aggressive nest defense.