Patterns of female nest attendance and male feeding throughout the incubation period in Blue Tits Cyanistes caeruleus

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 similar to 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 degrees C), as expected based on thermal needs of the developing embryos, but also during the coldest periods (2-5 degrees 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.