PSEUDO CONCORDE FALLACY IN THE WILLOW TIT

The predictions of optimal parental investment theory were tested by s tudying nest defence decisions of willow tits, Parus montanus. As pred icted, parental defence intensity increased with offspring age, female parents defended early and middle-season broods more vigorously than late ones and the annual proportion of mobbers correlated significantl y with annual means of clutch and brood sizes. Against the predictions , defence intensity was not adjusted to the quality (weight, size and condition) or quantity of offspring, which was also confirmed by brood size manipulation experiments. In this respect, willow tits seemed to commit the so-called Concorde fallacy. A suggested explanation for th ese non-adaptive responses is that a trait to determine the brood valu e continuously is not needed, since breeding success is high and predi ctable every year. In addition, in this population parents with reduce d and enlarged broods had the same fitness as those with control brood s. Therefore, parents behaved optimally when they did not change their defence intensity according to the manipulations. Thus, this lack of response to unnatural variation in a minor component of lifetime repro ductive success could be considered a pseudo Concorde fallacy. These r esults suggest that parents are able to predict the forthcoming breedi ng success early in the season, and that this prediction holds quite p recisely, with parental investment levels being relatively equal regar dless of clutch and brood size.