A sex-specific behavioral syndrome in a wild passerine

The direction of covariation in behavioral traits (a behavioral syndrome) is typically the same in males and females, although intersexual differences in life history could lead to intersexual heterogeneity in syndrome structure. We explored whether a behavioral syndrome was the same in both sexes. We recorded 2 metrics of nest defense in wild blue tits Cyanistes caeruleus: 1) Nest defense of a female around the time her eggs hatch (hatching defense), 2) nest defense of the male and female parent when their offspring was 16 days old (nestling defense), and 3) handling aggression. We used repeated records of these behaviors collected on 392 females and 363 males during 2007–2012 in a hierarchical mixed model to separate the between-individual from the residual (co)variances. We find that 1) hatching defense is not repeatable across breeding seasons (but highly repeatable within years). 2) Nestling defense intensity is 38% repeatable in males and females. 3) Contrary to our expectation, females that defended their nestlings more intensively were less aggressive when being handled (negative between-individual correlation), but 4) nestling defense and handling aggression showed a positive between-individual correlation in males although nonsignificantly. These correlations were completely masked on the phenotypic level by a low residual correlation. Individuals may hence display repeatable and correlated aggressive behavior in different contexts, but the direction of this syndrome may differ between the sexes. Intersexual heterogeneity in animal personality and syndrome structure remains poorly understood, and our findings hence urge future work to explicitly consider this heterogeneity.