Within and between population variations of incubation rhythm of great tits Parus major

Abstract

Uniparental incubation frequently means that eggs remain unattended for periods where the incubating bird is foraging out of the nest. The determination of incubation rhythms (i.e., the length and temporal pattern of incubation sessions) and the factors which affect them are therefore important to understand life-history trade-offs. We described the incubation rhythm and its temporal variation of a southern European great tit Parus major population, and review previous studies to check for latitudinal trends. In the studied population, females were active (from first exit in the morning to last entrance in the evening) 12.5 h per day, performing incubation sessions (on-bouts) of 26 min and recesses (off-bouts) of 12 min. Thus, they were incubating around 67% of their active day, or 83% of the whole day. Attentiveness (% of time incubating) increased throughout the incubation period, due to shorter off-bouts. The active day was longer as the number of daylight hours increased. We show for the first time in a bird species that attentiveness was constant along a latitudinal gradient ranging from Norway to Spain. Females spend a higher proportion of the daylight hours out of the nest as latitude decreases, compensating incubation time during the longer nights. Off-bouts were shorter in central European populations, increasing towards the north and the south, while on-bouts showed no latitudinal variation.