Risk-taking by incubating rufous bush robins Cercotrichas galactotes: season-dependent incubation stage effect

Risk-taking by incubating birds is commonly assumed to increase with reproductive stage due to increased nest value, and also with time of the breeding season due to decreased renesting opportunity. Nonetheless, the potential for these two factors to interact has not been given much importance. However, the extent to which risk-taking may increase with reproductive stage could be expected to vary with season. We investigated this issue using data on rufous bush robins (Cercotrichas galactotes) that we experimentally flushed from their nests at different stages of incubation and at different dates in the breeding season. We found that flushing distance of incubating birds decreased significantly with both incubation stage and season. More interestingly, we also found that incubation stage and season interact in shaping risk-taking the behaviour of incubating birds. As the breeding season progresses, and renesting opportunity declines, the effect of incubation stage on incubating parent risk-taking decreases. Overall, our work once again underlines the great complexity of behavioural and ecological factors and processes that affect risk-taking by nesting birds. In particular, it stresses the need to consider the interaction between reproductive stage and season when dealing with nest-defence behaviour in birds.