Because extended incubation recesses, where incubating songbirds are away from nests forperiods much longer than usual, occur infrequently, they have been treated as outliers in most previous studiesand thus overlooked. However, egg temperatures can potentially fall below the physiological zero temperatureduring extended recesses, potentially affecting developing embryos. As such, evaluating extended recesses in anecological context and identifying their possible fitness effects are important. With this aim, we used iButtondata loggers to monitor the incubation behavior of female Blue Tits (Cyanistes caeruleus) and Great Tits (Parusmajor) during two breeding seasons in central Spain. We classified incubation recesses as extended if they weremore than four times the mean recess duration for each species. Extended incubation recesses occurred morefrequently in 2012 when females exhibited poorer body condition. Female Blue Tits had more extendedincubation recesses than female Great Tits and, for both species, more extended recesses occurred at thebeginning of the breeding season. Both nest attentiveness and average minimum nest temperature decreasedwhen at least one extended recess occurred. Incubation periods averaged 4 d longer for nests where femaleshad at least one extended recess, potentially increasing predation risk and resulting in lower-quality nestlings.Overall, our results suggest that extended recesses may be more common among songbirds than previouslythought and that, due to their effects on egg temperatures and attentiveness, they could impose fitness costs.
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