Effects of temperature and food on incubation behaviour of the northern mockingbird, Mimus polyglottos

Avian incubation behaviour is thought to be influenced mainly by ambient temperature and food availability. Field studies, however, have generated contradictory results; there is little agreement about the relative importance of food and temperature and how different components of incubation behaviour are affected by them. To date, no studies have manipulated both food availability and nest temperature in a controlled experiment, making it impossible to assess any potential interaction between food and temperature. We experimentally increased both food availability and ambient temperature during incubation in the northern mockingbird. Our results show that both food availability and temperature influence incubation behaviour. Increasing food availability enabled females to spend more time on the nest and in self-maintenance activities when off the nest. Increasing nest temperature caused females to spend less time incubating and to make more trips to and from the nest. When both food and temperature were increased, their effects on incubation time offset each other. These changes in incubation patterns had little effect on fitness, although embryo mass was lowest in the treatment in which only heat was increased, suggesting that heat may stress embryos, but not when extra food is also provided. Perhaps the reason previous studies have yielded contradictory results is that food and temperature offset each other in complex ways that could obscure their individual effects. Indeed, our experiment shows that food and temperature both affect avian incubation behaviour, but that different trade-offs apply to each environmental factor.