The incubation stage in avian reproduction could be as costly as the nestling rearing stage. This is particularly true in the case of uniparental incubation, during which both current and future breeding attempts may be compromised. Therefore, the knowledge of the proximate effects that condition the incubation behaviour in free-living bird populations is of great importance in understanding the evolution of avian life history. In this two-year study, we assessed the incubation behaviours of Blue Cyanistes caeruleus and Great Tits Parus major inhabiting the same Mediterranean area in central Spain through the usage of iButton data loggers. It showed that the incubating behaviour of our tit populations resembles that reported in previous studies, but with peculiarities related to living at lower latitudes, i.e. with a relatively low attentiveness and a shorter active day. Both tit species showed very different incubation strategies, with Blue Tits leaving more frequently the nest (Mean ± SE number of off-bouts, Blue Tit = 27.14 ± 0.63, Great Tit = 16.95 ± 0.58) but for shorter periods than Great Tits (off-bout duration, Blue Tit = 8.76 ± 0.22 min, Great Tit = 14.04 ± 0.56 min; on-bout duration, Blue Tit = 22.63 ± 0.60 min, Great Tit = 36.86 ± 0.86 min). Nonetheless, both species provided a similar nest attentiveness, percentage of time of the active day during which the females were actively incubating (Blue Tit = 70.87 ± 0.57%, Great Tit = 70.75 ± 0.83%). Presumably, differences in the cooling rate of clutches, estimated with the iButtons, could be behind the differences in incubation behaviour between species and the greater capacity of Great Tits to adjust their incubation behaviour.
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Vol. 52 • No. 1