Temporal spacing of breeding activity in the natterjack toad, Bufo calamita

The reproductive behaviour of the toad Bufo calamita was studied in a large population in the northern Rhineland, Germany. Toads bred at four spatially separated but neighbouring areas including 34 breeding ponds. Three methods were applied to monitor quantitatively the intensity of breeding and migratory activity: capture-mark-recapture, a mechanical tracking device, and radio-telemetry. In 1987 a twofold temporal spacing of breeding activity was observed: (1) division of the reproductive period into early, main, and late breeding periods; (2) subdivision of each breeding period into several short calling periods. Since weather conditions were favourable during most of the reproductive period, environmental factors contributed little (23.4%-41.0%) to the temporal variation in breeding intensity as shown by multivariate statistics. The period of vocalization (April to August) included the activity of three distinct male groups. The first group called from April to mid-May (2 calling periods), the second from May to early July (5 calling periods) and the third from mid-July to August (2 calling periods). Within a calling period most males stayed near their calling sites and migratory activity was low. Extensive movements were detected when a new group of males immigrated from the feeding regions to the breeding areas and males that had been calling previously emigrated, at least partly. However, exchange of males among the neighbouring breeding choruses were clearly audible. It is suggested that the energetic costs of vocalization impeded continuous calling activity in individuals and accounted for the periodical variations in chorus size. The short-time variations of breeding activity therefore probably reflect the physiological limits of toads in a situation where the weather permits continuous reproductive activity.