Figure 6.

Analysis of an intraflock association (presented in Figure 4a) between Socotra cormorants (white star: Siniya colony; filled circles: dives). (a) Path taken by the flock: the journey comprised 1) a southwesterly outbound phase involving drifting at the sea surface (b) followed by prospective foraging (c), 2) a southeasterly foraging phase with serial diving (d), and 3) a northeasterly inbound phase. (b) Raft formation: the flock, constituted by one or several rafts, was joined separately by birds 27 and 31. Birds drifted at the sea surface following a northeasterly coastal current (birds 27 and 31 drifted for 43 and 12 min, with a speed of 0.41 and 0.39 m/s and a bearing of 80° and 60°, respectively). (c) Prospective group foraging: Birds 27 and 31 closely associated within the flock. After a short dive bout (starting 15:09), this phase was characterized by flights alternating with short periods at the sea surface (e). Surface periods were presumably intended for exploration by birds of the water column for fish, either visually by submergence of the head or by shallow diving, as in bird 31. (d) Intensive group foraging: birds dived serially within one long bout. The dark circle represents the hypothesized maximum width of the flock at the sea surface (f). After the last dive, birds flew back to the colony simultaneously at an average speed of 44 km/hr (no stopover). (e) Dive profiles of birds during the prospective (c) and intensive (d) group‐foraging phases. Temperature profiles (gray lines) indicate the position of flights (for detail, see Figure S2). Synchronous diving was not detected in these two study birds, although they clearly dived in a coordinated manner. They carried out a comparable number of dives (94 and 115 dives by birds 27 and 31, respectively), which became shallower as the flock progressed toward the shore, suggestive of benthic diving, in accordance with bird dive depth and local bathymetry (a). Benthic dives were occasionally interspersed with shallower pelagic dives. Furthermore, birds undertook a comparable number of short flights between dives: 23 and 27 flights by birds 27 and 31, respectively, each lasting 0.5 ± 0.5 min. (f) Distance between birds within the flock: between the first and the last dive of the trip, birds were distant by 341 ± 196 m on average (range: 27–1,030 m)