Effect of age, haemosporidian infection and body condition on pair composition and reproductive success in Great Tits Parus major

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Several factors shape lifetime reproductive success, including genetic background, body condition, environmental conditions and ecological interactions such as parasitism. Adults often show higher reproductive success than their young conspecifics, especially in long‐lived bird species, and this may be explained by the cumulative effects of an increase in reproductive experience and the selection of high‐quality individuals from one year to the next. To test whether this pattern also exists in short‐lived bird species, we used 13 years of monitoring data from two Great Tit Parus major populations. The effects of male and female age on several reproductive parameters were analysed in 419 pairs of Great Tits, while accounting for body condition and infection by haemosporidian parasites. Reproductive success was mainly affected by the age‐class of males. Pairs containing a sub‐adult male fledged one‐third fewer chicks than pairs containing an adult male. The difference was not caused by variation in male fertility but could have been caused by better parental care provided by adult birds. In addition to lower reproductive success, first‐year males also had reduced access to mating compared with adult males, suggesting an avoidance of sub‐adult males by females. Nestling body condition was positively correlated with parental body condition, and the body condition of male and female members of breeding pairs was positively correlated. Finally, the number of fledged chicks was mainly affected by the infection status of males. These results temper our previously published results showing an effect of infection on Great Tit reproduction regardless of their sex. In our previous study, and as in most cases, the status of the partner was not taken into account and we show here that this is essential because it can lead to a biased interpretation of the results.