Résumé
The 'good parent', 'differential allocation', or 'individual optimisation of clutch size' hypotheses all postulate that phenotype-associated breeder characteristics reflect abilities to successfully raise nestlings. Therefore, there will be a statistical correlation with nestling phenotypes (e.g., nestling weight) that indicate survival ability or health. This can occur when breeders with specific phenotypic features are in better physical condition or have access to more resources compared to those with different phenotypic profiles. However, within study populations, parents and nestlings exhibit multiple phenotypic characteristics that do not show consistent correlations across different breeders or broods. Given this, what are the key parental phenotype factors that drive the development of viable nestling phenotypes at the within-population level? We verified 542 results of statistical tests from 119 publications that empirically examined associations between phenotype-associated breeder and nestling characteristics (weight, size, plumage colours, blood profiles, cell-mediated immunity). We focused on non-experimental studies of four well-studied secondary cavity-nesting passerines of which research significantly contributed to the development of theory in the ecological and evolutionary sciences: Great Tit (Parus major, 65 publications), Blue Tit (Cyanistes [Parus] caeruleus; 43 publications), and two flycatcher species (Pied Flycatcher Ficedula hypoleuca, and Collared Flycatcher Ficedula albicollis; 25 publications). Although more than 90 phenotypic characteristics have been measured in nestlings, 54% of the statistical tests focused on nestling weight. In 344 (64%) of 542 statistical outcomes, phenotype-based breeder characteristics did not significantly correlate with nestling phenotypes (P > 0.05). Only 24 (12%) of the 198 statistically significant parent-nestling associations (P < 0.05) involved parental features other than clutch size, brood size, laying date, or hatching date. Our results support previous conclusions in well-studied secondary-cavity nesting passerines that parental phenotypes often poorly correlate with proxies of breeding performance or success. We discuss why the empirical results deviate from theory-based predictions.