When several personality traits covary, they form a behavioral syndrome. Understanding the evolutionary dynamics of a behavioral syndrome requires knowledge of its genetic underpinning. At present, our understanding of the genetic basis of behavioral syndromes is largely restricted to domestic and laboratory animals. Wild behavioral syndromes are mostly inferred on the basis of phenotypic correlations, and thus make the “phenotypic gambit” of assuming that these phenotypic correlations capture the underlying genetic correlations. On the basis of 3 years of reciprocal cross-fostering of 2896 nestlings of 271 families within a pedigreed population, we show that the nestling personality traits handling aggression, breathing rate, and docility are heritable (h2 = 16–29%), and often have a pronounced “nest-of-rearing” variance component (10–15%), but a relatively small “nest-of-origin” variance component (0–7%). The three nestling personality traits form a behavioral syndrome on the phenotypic and genetic level. Overall, the phenotypic correlations provide a satisfactory description of the genetic ones, but significantly underestimate the magnitude of one of the pairwise genetic correlations, which mirrors the conclusion based on domestic and laboratory studies.

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Animal personality is defined as behavior that is consistent across time and context. We here applied a reaction-norm perspective implemented as a random regression phenotypic model (RRPM) to behaviors measured on blue tits Cyanistes caeruleus. During 3 consecutive breeding and winter seasons (2007–2009), a total of 508 wild-caught blue tits were assayed in a standard, artificial setup (a bird cage) for 1) activity, 2) time to escape, and 3) neophobia-related behavior. Activity was found to be repeatable both within and across seasonal contexts, but escape time and neophobia-related behavior were repeatable only in winter. Our RRPM confirmed that this latter finding was due to crossing of the individual-specific reaction norms between the 2 seasonal contexts. Our work illustrates how a behavior measured in a standardized manner may or may not be repeatable across time within a context but not between contexts, depending on the interindividual variation in reaction-norm properties. Our findings suggest that research on animal behavior plasticity can benefit from taking onboard context-specific analyses in a more explicit manner than what is typically done.

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In birds and other taxa, nest construction varies considerably between and within populations. Such variation is hypothesized to have an adaptive (i.e. genetic) basis, but estimates of heritability in nest construction are largely lacking. Here, we demonstrate with data collected over 10 years from 1010 nests built by blue tits in nest-boxes that nest size (height of nest material) and nest composition (proportion of feathers in the nest) are repeatable but only weakly (12-13%) heritable female traits. These findings imply that nest construction may evolve but only if subjected to strong and consistent selection pressures.