Nest predation avoidance: An alternative explanation for male incubation in Acrocephalus melanopogon

In the monogamous moustached warbler, male incubation changes from predictably variable (it is dependent on ambient temperature and time of day in April) to high average levels across the day (with no predictor variables in May) as the season progresses. In contrast, females contribute the constant incubation component from April to May. This paper investigates possible explanations for the change in male incubation effort involving changing risks to either (1) embryonic survival within the egg, and/or (2) egg predation. Using egg temperature readouts, the probability of reaching the 25°C thermal threshold (below which embryonic development ceases) across the season against the probability of predator sightings 0-15 m from the nest was calculated. The results show an inverse relationship between these two risks. During April, male incubation correlates with egg cold stress and changeovers between males and females occur prior to egg cooling below the thermal stress line. During May, the risk of predation increases. The results show increased predator encounter rates from April to May and active nest defense by the incubating parent. Furthermore, high male incubation reduces brood predation. Selection for reduction of the costs of laying replacement clutches (after predation) is suggested given high male incubation and infrequent male-female changeovers during midday, when egg temperatures are highest, with direct benefits to females of increased foraging. Thus, the shift in male incubation across the season may be explained by minimization of changing risk to offspring survival.