Effects of nest predation risk on female incubation behavior and offspring growth in great tits

Predation risk is a key driver for the evolution of reproductive strategies and life history traits. In birds, incubation behavior represents one form of parental care where trade-offs between time spent in incubation activities and self-maintenance activities are likely to change in response to predator pressure. This can have strong effects on embryonic development, but is still poorly understood. We investigated the effects of the presence of a nest predator on great tit (Parus major) incubation behavior and the subsequent effects of incubation on nestling morphological traits. We manipulated perceived predation risk using models of short-tailed weasels (Mustela erminea) in combination with great tit alarm calls specific to this predator. Directly after hatching, we swapped whole broods from treated nests with broods from untreated nests to disentangle treatment effects acting during the incubation period from potential carry-over effects on parental care acting on nestlings after hatching. In increased predation risk environments, the number of incubation sessions and recesses, but not their duration, was increased compared to the control group, and the nocturnal incubation session was longer when females were exposed to a predator. Eggs incubated by females under increased predation risk lost more mass over the incubation period compared to the control group. Also, male nestlings hatched from nests exposed to predators were lighter at hatching but were equivalent in weight to their control counterparts at fledging. This suggests that nest predation risk can influence some aspects of incubation rhythm and embryonic development, but has no long-term effects on nestling final body size.