Geometry of Visual Recruitment by Seabirds to Ephemeral Foraging Flocks

Using geometric relationships, we calculate theoretical upper (20-30 km) and lower (0.7-6.2 km) limits to horizontal distances over which volant seabirds can be visually recruited to join Type I flocks in the open ocean. These are compared to empirical estimates for recruitment distances, obtained from chumming experiments conducted in the western Atlantic Ocean off the southeastern United States. The product of arrival times and flight speeds for individuals (n = 164) joining 10 flocks indicated that potential recruitment distances were closer to the lower theoretical limits, with a mean distance of 4.5 km when flight time and ground speed were adjusted for detection lags, wind speed, and zigzag flight. Distances and time spans for mutual attraction among seabirds have important consequences for evaluating both intraand inter-specific interactions at the community level, as well as direct implications for sampling independence during distributional surveys that employ consecutive line transects or other sequential counting methods.