Male sexual attractiveness and parental effort in blue tits: a test of the differential allocation hypothesis

When the reproductive value of a breeding attempt is related to attributes of the breeding partner, an individual is expected to allocate more resources to parental care when mated to a high-quality partner. We tested predictions of the differential allocation hypothesis, by experimentally increasing and decreasing male blue tit, Parus caeruleus, sexual attractiveness and recording subsequent measures of male and female parental effort during the chick-feeding period. We used marker pens, to create two distinct male phenotypes: one more attractive phenotype with a shift in peak reflectance towards the ultraviolet (UV) part of the spectrum (UV+) and one less attractive phenotype with a shift towards the human-visible part of the spectrum (UV-). There was no significant difference in absolute or relative female feeding rate with respect to treatment. However, there were significant interaction effects between treatment and female age on female feeding rate, indicating that 1-year-old females provisioned more when mated to a UV+ male than a UV- male. UV- males fed their chicks at a higher rate than UV+ males, but there was no significant difference between the groups in total feeding rate. Females contributed less to nest defence relative to their mates when they were mated to UV- males, whereas the opposite was true for females mated to UV+ males. The behavioural responses did not translate into differences in measures of reproductive output. Our study suggests that male phenotypic appearance at the chick-feeding stage influences female decisions about level of parental effort.