Weather dictates reproductive success and survival in the Australian brown falcon Falco berigora

Abstract

1. We examined the influence of parental age, pair-bond duration, prey size and weather conditions on various measures of reproductive success and survival of a sedentary brown falcon (Falco berigora, Vigors & Horsfield) population that was virtually free from predation and human persecution. 2. Pairs on territories with large prey were more likely to initiate breeding. Females taking larger prey items experienced higher mortality rates when they bred late in the season, whereas those taking smaller prey had a greater probability of survival when laying later in the season - relationships likely linked to opposing seasonal differences in prey availability. 3. Interannual differences were by far the most influential variable assessed, affecting reproductive success and female survival. Pairs on-site in the first year of the study were more successful breeders and had greater survival prospects than those present in the latter two seasons. This pattern corresponds strongly to the observed frequency of heavy rain downpours and implicates these events as the main cause of reproductive failure and mortality amongst adult females. These detrimental effects were due probably to an increased chance of chilling, exposure and starvation for chicks and parents alike. 4. The importance of unpredictable climatic variables in shaping the reproductive success and survival of brown falcons and raptors from other regions indicates that, in the absence of significant nest predation, weather, in this case in the form of heavy rain, may well be the most important ultimate factor influencing long-term reproductive success in this group.