The daily catch: Flight altitude and diving behavior of northern gannets feeding on Atlantic mackerel

Abstract

Predators utilize a variety of behavioral techniques to capture elusive prey. Behavioral flexibility is essential among generalist predators that pursue a diversity of prey types, and capture efficiency is expected to be intense during the breeding season for parents that engage in self- and offspring-provisioning. We studied the foraging behavior of parental northern gannets in the northwestern Atlantic (Gulf of St. Lawrence) when they were feeding on Atlantic mackerel almost exclusively. Data-loggers recorded short (mean duration: 6.3s), high speed (inferred vertical speeds of up to 54.0m*s−1, equivalent to 194km*h−1), and shallow dives (mean depth: 4.2m; maximum: 9.2m). Dives tended to occur in bouts, varying between 0.3 and 4.6 per hour (mean=1.6). During foraging, overall flight heights ranged from 0 to 70m, with no clear preferences for height. Most plunge-dives were initiated at flight altitudes of 11–60m (mean±SE=37.1±2.8m; range 3–105m except for 1 of 162 dives that was initiated at the sea surface). Dive depth and flight altitude at plunge-dive initiation were positively and significantly correlated, though it appears that low flight altitudes were sufficient to reach dive depths at which mackerel were present. Almost all dives were V-shaped indicating that a high acceleration attack is the most effective strategy for gannets feeding on large rapid-swimming prey such as mackerel that owing to thermal preferences does not occur below the thermocline and are thus well available and essentially trapped in the water depths exploited by northern gannets.