Parental phenotypes and breeding performance: a review of non-experimental investigation in well-studied Western palearctic tits and flycatchers

Numerous hypotheses predict associations between phenotype-associated breeder traits and reproductive success. In secondary cavity-nesting passerine birds, which have been investigated most, up to more than 90 parental characteristics have been linked to at least one field measure of breeding performance. However, within study populations, different parental traits, such as clutch size, first-egg date, egg size, nest architecture, plumage colour, adult morphometry, or song performance, are often weakly correlated across female and/or male breeders. Furthermore, many of these studies have been conducted outside the theoretical framework of the ‘Individual Optimisation Hypothesis’ of clutch size (IOH). IOH predicts that: (i) females with larger clutches will have larger broods at hatching and fledging compared to those with smaller clutches; and (ii) clutch size, which is adaptively adjusted to the parents' ability to rear nestlings, should always maximize the percentage of eggs producing fledglings so that variation in clutch size becomes disconnected from variation in breeding success. In this paper, we present the first detailed review of implications of IOH for parental characters other than clutch size. Our review covered 188 non-experimental studies and 1074 statistical results that examined how parental traits influence breeding success in Western Palearctic Great Tits (Parus major), Blue Tits (Cyanistes spp.), and flycatchers (Ficedula spp.). Clutch size explained one third of the variation in brood size at hatching and fledging within study populations. However, most parental characteristics associated weakly with the number of hatchlings or fledglings, likely because they did not correlate with clutch size. Overall, parental traits were poorly correlated with the proportion of hatchlings and fledglings per egg. We discuss why intraspecific variation in phenotype-associated breeder traits is often disconnected from interindividual differences in breeding success, and highlight the importance of underexplored research problems in avian breeding biology.