Binary dietary responses of northern gannets Sula bassana indicate changing food web and oceanographic conditions

Interactions between seabirds and their prey are shaped largely by the dynamics of the marine ecosystems in which they are embedded. Physical oceanographic processes can drive the distributions of ectothermic and planktonic prey and hence their availability to marine birds. Owing to the complex nature of these processes and interactions, the signal-to-noise ratios of avian indicators of prey conditions are variable, often low and further degraded (buffered) by seabird behaviour and life-history features. Cairns (1987, Biol Oceanogr 5:261-271) detailed seabird responses operating over a range of temporal scales to variation in food supplies, and suggested that interval relationships might be developed between avian responses and environmental variation. While this may be possible in some instances, it appears unrealistic in most instances to expect interval relationships between seabird responses and prey conditions that are often nonlinearly related. The present paper focuses on binary data (e.g. breeding success versus failure) derived from seabirds that can provide robust information about major shifts in prey and oceanographic conditions and that are particularly informative when accumulated over decadal and large ocean scales. Inter-annual and decadal variations in specific and nominally categorized (warm-versus cold-water) prey landings of northern gannets Sula bassana at a large oceanic colony in the NW Atlantic reflect shifts in pelagic food webs induced by changes in regional sea surface temperature. Binary patterns emphasize decadal shifts in food webs and yield predictive indication of systemic change.