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A major question in behavioural ecology is why behaviour, physiology and morphology are often integrated into syndromes. In great tits, Parus major, for example, explorative males are larger (vs. smaller) and leaner (vs. heavier) compared to less explorative individuals. Unfortunately, considerable debate exists on whether patterns found in specific studies are replicable. This debate calls for study replication among species, populations and sexes.We measured behavioural (exploration), physiological (breathing rate) and morphological traits (body mass, tarsus length, wing length, bill length) in two species (great vs. blue tits Cyanistes caeruleus), two populations (Forstenrieder Park vs. Starnberg) and two sexes (males vs. females). We then tested whether the same pattern of integration characterized all unique combinations of these three biological categories (hereafter called datasets).We used a multi‐year repeated measures set‐up to estimate among‐individual trait correlation matrices for each dataset. We then used structural equation modelling to test for size‐dependent behaviour and physiology, size‐corrected (i.e. size‐independent) behaviour‐physiology correlations and size‐corrected body mass‐dependent behaviour and physiology. Finally, we used meta‐analyses to test which structural paths were generally (vs. conditionally) supported (vs. unsupported).We found general and consistent support for size‐dependent physiology and size‐corrected body mass‐dependent physiology across datasets: faster breathers were smaller but heavier for their size. Unexpectedly, condition‐dependent behaviour was not supported: explorative birds were neither leaner, nor was this relationship heterogeneous across datasets. All other hypothesized patterns were dataset‐specific: the covariance between size and behaviour, and between behaviour and physiology differed in sign between datasets, and both were, on average, not supported. This heterogeneity was not explained by any of our moderators: species, population or sex.The specific pattern of size‐ and condition‐dependent physiology reported for a unique combination of species, population, and sex, thus generally predicted those in others. Patterns of size‐ or condition‐dependent behaviour (i.e. 'personality'), or behaviour‐physiology syndromes reported in specific datasets, by contrast, did not. These findings call for studies revealing the ecological background of this variation and highlight the value of study replication to help understand whether patterns of phenotypic integration reported in one study can be generalized. [ABSTRACT FROM AUTHOR]

There is considerable debate about the occurrence of assortative mating between phenotypic traits measured within natural populations. Meta‐analyses have implied that assortative mating occurs generally in natural populations, but recent work indicates these conclusions largely result from biased data. Specifically, estimates of phenotypic correlations between mating partners do not solely result from nonrandom associations between individual‐level traits of partners but also from other biological processes (joint phenotypic plasticity, indirect genetic effects), methodological practices (observer bias) and other unexplained residual correlations (e.g. correlated measurement error). This paper puts this critique to test. First, we estimated the overall phenotypic correlation between phenotypic traits of mating partners for a wild population of great tits. Second, we estimated various key variance components to reveal the extent to which phenotypic correlations between partners resulted from assortative mating, reversible plasticity, social partner effects and methodological practices. We performed our analyses for a range of phenotypic traits (body mass, breathing rate, exploration behaviour, wing and tarsus length) to derive general conclusions not hinging on the specifics of the traits involved. Our analyses support the conclusion that patterns of assortative mating exist at first glance but occur because of the biasing effects of correlated residuals likely caused by a combination of phenotypic responses to unknown environmental factors or measurement error—not because of intrinsic patterns of assortative mating. [ABSTRACT FROM AUTHOR]