Abstract
Nest defense is an essential behavior in birds that protects nests from predation. Defensive behavior often varies among individuals, yet how hemispheric lateralization, a phenomenon commonly existing in vertebrates and invertebrates, shapes this behavior in the wild remains poorly understood. In this study, we investigated the nest defense behavior in incubating female Japanese Tits (Parus minor) in a free-living wild population, with a focus on whether this behavior was associated with footedness, a behavioral indicator of hemispheric lateralization that was measured using their foot-use preference while grasping food items against a perch. Based on simulated nest intrusions under standardized procedures, we found that their active nest defense increased as the breeding season progressed, consistent with the hypothesis that the value of the clutch increases later in the breeding season when opportunities for re-nesting decline. Importantly, the results showed that as the distance from the nest to the nearest building increased, left-footed females were more likely to show active defense, whereas right-footed females showed a non-significant tendency toward reduced defense. Given the fitness consequences of nest defense, these findings provide field evidence for the adaptive significance of lateralization and advance an integrative view that connects potential neural specialization, environmental context, and parental strategies.