Climate influences on avian population dynamics

Predictions of the consequences of the expected changes in climate on bird population dynamics require a detailed mechanistic understanding. Basically, two different hypotheses have been proposed to explain under which period of the year a change in climate will have the strongest effect on fluctuations in population size. The tub-hypothesis proposes fluctuations in population size to be closely related to climate variation during the non-breeding season because in combination with density dependence, the weather conditions determine the number of birds surviving during this critical period of the year. The tap-hypothesis predicts annual variation in population size to be related to the weather during the breeding season because this will influence the inflow of new recruits into the population the following year. We examine the validity of these hypotheses by reviewing studies that have related fluctuations in population sizes to local weather variables or to large-scale climate patterns such as the North Atlantic Oscillation (NAO). We find the tub-hypothesis to be supported in northern temperate altricial birds. In contrast, the effects of weather during the breeding season often affects the population fluctuations of many nidifugous species and species living under and conditions, thus supporting the tap-hypothesis. However, the dynamical consequences of these interspecific differences in the timing of the population limitation cannot be properly understood without modelling and estimating the effects of the density dependence as well as changes in the mean and variance of the relevant climate variables.