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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2005.01.005 Byline: Arild Johnsen, Kaspar Delhey, Emmi Schlicht, Anne Peters, Bart Kempenaers Abstract: 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. Author Affiliation: Max Planck Institute for Ornithology, Seewiesen, Germany Article History: Received 17 February 2004; Revised 7 May 2004; Accepted 3 February 2005 Article Note: (miscellaneous) MS. number: 8012R

Most bird species exhibit biparental care, but the type of care provided by each sex may differ substantially. In particular, during the incubation phase in passerines, females perform most or all of the incubation, while the male cares for the brood indirectly by feeding the female. However, detailed descriptions of this male investment during the incubation period are missing. Here, we quantitatively describe female nest attendance and male incubation feeding throughout the ~ 14‐day incubation period in a population of Eurasian Blue Tits Cyanistes caeruleus breeding in nestboxes. Males and females progressively increased their daily activity at the nest over the incubation period. The amount of day‐time incubation, measured as the proportion of the active day (time interval between first nestbox exit in the morning and last entry in the evening) a female spent inside the nestbox, varied between 52 and 60% with an average of 55% per day. The frequency of male incubation feeding varied between 0 and 74 times per day with an average of 12 feeds per day. Both male feeding rate and female nest attendance were highest in the morning and declined rapidly throughout the day. Females were more likely to be off the nest during the warmest periods (15–21 °C), as expected based on thermal needs of the developing embryos, but also during the coldest periods (2–5 °C), presumably due to the energetic needs of the female. This was despite the fact that males fed their females more often at the nest when ambient temperatures were low. Females that received more feeds incubated more and their off‐nest bouts were shorter after a feed. The observed variation in female incubation and in male feeding rate was not linked to individual age or to variation in measures of reproductive success. However, direct observations showed that in some pairs a substantial amount of feeding by males occurred outside the nestbox. This suggests that the true male investment might have been underestimated, here and in previous studies. [ABSTRACT FROM AUTHOR]