Résumé
Seabird proxies have the potential to act as useful and cost-effective indicators of the state of the marine environment. Seabird time-activity budgets, in particular, reflect short-term changes in prey conditions. We tested an automated technique for long-term continuous recording of Cape gannet, Morus capensis, time-activity budgets using coded very high frequency (VHF) transmitters allowing for simultaneous monitoring of a large sample of study birds. Radiotransmitters attached to leg-rings had no impact on adult foraging trip and nest attendance durations, breeding success or chick growth. Furthermore, frequencies of nest attendance and foraging trip durations estimated by the VHF logging system were no different to those estimated from hourly direct observations. Using time-depth recorders, the relationship between the time that birds rested on the sea surface in relation to foraging trip duration was assessed. Trip duration during chick rearing was clearly an accurate proxy for foraging effort. The VHF monitoring system provides a simple method of accurately assessing the time-activity budgets of colonial seabirds, which can be expanded to a range of other colonially breeding taxa. In the case of seabirds, this approach can potentially provide sensitive, real-time indicators of prey abundance for fisheries management.