More Is not always better: male incubation in two Acrocephalus warblers

This study investigates male and female incubation ability in two monogamous Acrocephalus warblers with overlapping, equally sized territories and similar prey abundance. Given the longer breeding time window of the moustached warbler (A. melanopogon) compared with the reed warbler (A. scirpaceus), the trade-off between the need for biparental care and the cost of inefficient incubation is discussed. Hourly protocols and egg temperature measurements were analyzed with regard to four primary questions: male and female incubation ability, the role of environmental parameters, hatching success and the influence of male incubation on female time allocation. In both species, males increase egg temperature per minute at a slower rate than do females. There is no species difference in the percentage of incubation per hour for males (20%) or females (50%). Ambient temperature influences male incubation only in the moustached warbler during the early season (April) when male incubation correlates with hatching success. The male reed warbler shows daily temporal selectivity throughout the breeding season, increased incubation during rainfall, and no correlation with hatching success. In both species, females receive direct benefits of increased foraging time through male incubation. However, only the female reed warbler adjusts her incubation duration to previous male incubation. Thus, female reed warblers maximise the male component and thereby reduce the total incubation phase with high male effort whereas the incubation phase is increased with above average male effort in the moustached warbler.