Flexible parental care: Uniparental incubation in biparentally incubating shorebirds

Recent findings suggest that relative investment of females and males into parental care depends on the population’s adult sex-ratio. For example, all else being equal, males should be the more caring sex if the sex ratio is male biased. Whether such outcomes are evolutionary fixed (i.e. related to the species’ typical sex-ratio) or whether they arise through flexible responses of individuals to the current population sex-ratio remains unclear. Nevertheless, a flexible response might be limited by evolutionary history when one sex loses the ability to care or when a single parent cannot successfully care. Here, we demonstrate that after the disappearance of one parent, individuals from 8 out of 15 biparentally incubating shorebird species were able to incubate uniparentally for 1-19 days (median = 3,N= 69). Such uniparental phases often resembled the incubation rhythm of species with obligatory uniparental incubation. Although it has been suggested that females of some shorebirds desert their brood after hatching, our findings indicate that either sex may desert prior to hatching. Strikingly, in 27% of uniparentally incubated clutches - from 5 species - we document successful hatching. Our data thus reveal the potential for a flexible switch from biparental to uniparental care.

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

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.

Two kinds of timing in circadian incubation rhythm of ring doves

Ring dove males share incubation with females by sitting on the nest for a block of time in the middle of the day. The timing of nest exchanges was studied by experimentally delaying the onset, in the morning, of the male's sitting bout. Such delays induced a concomitant but smaller delay in the offset of the male's sitting bout in the afternoon. The female, however, approached the nest to start her bout of sitting at the usual time in the afternoon even though her previous sitting bout had ended later than usual. These findings, together with data on interactions between the pair at the afternoon exchange, suggest that the behavior of the sitting male reflects an interval timing mechanism initiated with the onset of his sitting bout. In contrast the attempts to regain the nest by the nonsitting female probably reflect a circadian oscillation.