Effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens

We used presentations of models to determine the effectiveness of nest defe nce in the Acadian Flycatcher Empidonax virescens against a nest predator ( Blue Jay Cyanocitta cristata) and a brood parasite (Brown-headed Cowbird Mo lothrus ater). Principal components analysis (PCA) of four component variab les sf nest defence (call rate, swoop rate, closest approach and number of adults) generated a measure of overall nest defence (aggression). We determ ined effectiveness of defence by looking for correlations between measures of defence and measures of nest success (nest predation and brood parasitis m). We also determined whether nest defence increased with clutch size, nes tling age and time in the breeding season. Defence against model Brown-head ed Cowbirds did not correlate with levels of parasitism, clutch size, age o f young or time of breeding. There was, however, a strong, but insignifican t, trend for nests with high levels of all measures of defence to suffer le ss from brood parasitism. Aggression, vocalization rate, closest approach a nd number of adults defending against models of predatory Blue Jays correla ted positively with nesting success during the egg sl:age but not the nestl ing stage of the nesting cycle. Aggression, vocalization rate, closest appr oach correlated with clutch size and age of the brood. These results sugges t that nest defence can effectively deter nest predators, but may be less e ffective against brood parasites. Different behavioural components of nest defence may work at different stages of the nest cycle and against differen t nest predators. The components of nest defence that correlated with nest success also correlated with clutch value, a result consistent with hypothe ses on the evolution of nest defence.