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Group foraging in Socotra cormorants: A biologging approach to the study of a complex behavior

View Article: PubMed Central - PubMed

ABSTRACT

Group foraging contradicts classic ecological theory because intraspecific competition normally increases with aggregation. Hence, there should be evolutionary benefits to group foraging. The study of group foraging in the field remains challenging however, because of the large number of individuals involved and the remoteness of the interactions to the observer. Biologging represents a cost‐effective solution to these methodological issues. By deploying GPS and temperature–depth loggers on individuals over a period of several consecutive days, we investigated intraspecific foraging interactions in the Socotra cormorant Phalacrocorax nigrogularis, a threatened colonial seabird endemic to the Arabian Peninsula. In particular, we examined how closely birds from the same colony associated with each other spatially when they were at sea at the same time and the distance between foraging dives at different periods of the day. Results show that the position of different birds overlapped substantially, all birds targeting the same general foraging grounds throughout the day, likely following the same school of fish. There were as many as 44,500 birds within the foraging flock at sea at any time (50% of the colony), and flocking density was high, with distance between birds ranging from 8 to 1,380 m. Birds adopted a diving strategy maximizing time spent underwater relative to surface time, resulting in up to 72% of birds underwater in potential contact with prey at all times while foraging. Our data suggest that the benefits of group foraging outweigh the costs of intense aggregation in this seabird. Prey detection and information transmission are facilitated in large groups. Once discovered, shoaling prey are concentrated under the effect of the multitude. Fish school cohesiveness is then disorganized by continuous attacks of diving birds to facilitate prey capture. Decreasing population size could pose a risk to the persistence of threatened seabirds where group size is important for foraging success.

No MeSH data available.


Related in: MedlinePlus

Different phases of Socotra cormorant foraging trips and associated bird velocity. (a) Structure of a typical foraging trip, including the outbound phase (blue), the foraging phase (pink), and the inbound phase (green). The white star corresponds to Siniya colony. Stars along the outbound path mark places where birds landed on the water surface (no stopover during the inbound phase). Circles correspond to dives; in this example, the bird carried out one prospective dive during the outbound phase and 52 dives during the foraging phase. (b) Average bird velocity (linear distance between the starting and ending points of the phase/phase duration) for the different trip phases (***P < .001). (c) Average bird velocity during the outbound phase as a function of trip departure time (y = 2.7x − 3.43, R2 = .22, P < .0001, n = 47) and frequency distribution of trip departure time (gray vertical bars, n = 47)
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ece32750-fig-0003: Different phases of Socotra cormorant foraging trips and associated bird velocity. (a) Structure of a typical foraging trip, including the outbound phase (blue), the foraging phase (pink), and the inbound phase (green). The white star corresponds to Siniya colony. Stars along the outbound path mark places where birds landed on the water surface (no stopover during the inbound phase). Circles correspond to dives; in this example, the bird carried out one prospective dive during the outbound phase and 52 dives during the foraging phase. (b) Average bird velocity (linear distance between the starting and ending points of the phase/phase duration) for the different trip phases (***P < .001). (c) Average bird velocity during the outbound phase as a function of trip departure time (y = 2.7x − 3.43, R2 = .22, P < .0001, n = 47) and frequency distribution of trip departure time (gray vertical bars, n = 47)

Mentions: Foraging trips were typically composed of an outbound commuting phase from the colony to the foraging ground, a foraging phase with intense diving activity and an inbound commuting phase from the last foraging dive back to the colony (Figure 3a). The outbound phase lasted longer (1.3 ± 0.7 hr) than the inbound phase (0.9 ± 0.5 hr) (df = 73, t = 2.11, P = .038), while the foraging phase lasted 1.7 ± 1 hr on average. Birds landed more often on the sea surface during the outbound (4.1 ± 4.8 landings per trip) than during the inbound (1.6 ± 4.9 landings per trip) phase. They also dived occasionally during the outbound phase (but not during the inbound phase): such dives were considered to be prospective dives, as opposed to the foraging dives characterizing the foraging phase (Figure 3a). Prospective dives were present in 49% of trips (6 ± 10 dives per trip), amounting to 0.05% of all dives carried out by birds (foraging dives added to 97 ± 50 dives per trip). Birds spent more time at the sea surface and underwater during the outbound phase (42.3 ± 25.9%) than during the inbound phase (17.8 ± 17.2%) (df = 73, t = 5.99, P < .0001), a proportion that was 82.8 ± 10.8% during the foraging phase. Bird instantaneous flight speed was significantly slower during the outbound phase (45.4 ± 7.1 km/hr) than during the inbound phase (48.6 ± 6.1 km/hr) (df = 73, t = −2.4, P = .018), while it was an average of 34.7 ± 6.0 km/hr during the foraging phase. As a result of these differences, bird velocity was 11 km/hr slower on average during the outbound than during the inbound phase (df = 73, t = −4.92, P < .001) (Figure 3b). Bird velocity during the outbound phase increased linearly with trip departure time (Figure 3c).


Group foraging in Socotra cormorants: A biologging approach to the study of a complex behavior
Different phases of Socotra cormorant foraging trips and associated bird velocity. (a) Structure of a typical foraging trip, including the outbound phase (blue), the foraging phase (pink), and the inbound phase (green). The white star corresponds to Siniya colony. Stars along the outbound path mark places where birds landed on the water surface (no stopover during the inbound phase). Circles correspond to dives; in this example, the bird carried out one prospective dive during the outbound phase and 52 dives during the foraging phase. (b) Average bird velocity (linear distance between the starting and ending points of the phase/phase duration) for the different trip phases (***P < .001). (c) Average bird velocity during the outbound phase as a function of trip departure time (y = 2.7x − 3.43, R2 = .22, P < .0001, n = 47) and frequency distribution of trip departure time (gray vertical bars, n = 47)
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ece32750-fig-0003: Different phases of Socotra cormorant foraging trips and associated bird velocity. (a) Structure of a typical foraging trip, including the outbound phase (blue), the foraging phase (pink), and the inbound phase (green). The white star corresponds to Siniya colony. Stars along the outbound path mark places where birds landed on the water surface (no stopover during the inbound phase). Circles correspond to dives; in this example, the bird carried out one prospective dive during the outbound phase and 52 dives during the foraging phase. (b) Average bird velocity (linear distance between the starting and ending points of the phase/phase duration) for the different trip phases (***P < .001). (c) Average bird velocity during the outbound phase as a function of trip departure time (y = 2.7x − 3.43, R2 = .22, P < .0001, n = 47) and frequency distribution of trip departure time (gray vertical bars, n = 47)
Mentions: Foraging trips were typically composed of an outbound commuting phase from the colony to the foraging ground, a foraging phase with intense diving activity and an inbound commuting phase from the last foraging dive back to the colony (Figure 3a). The outbound phase lasted longer (1.3 ± 0.7 hr) than the inbound phase (0.9 ± 0.5 hr) (df = 73, t = 2.11, P = .038), while the foraging phase lasted 1.7 ± 1 hr on average. Birds landed more often on the sea surface during the outbound (4.1 ± 4.8 landings per trip) than during the inbound (1.6 ± 4.9 landings per trip) phase. They also dived occasionally during the outbound phase (but not during the inbound phase): such dives were considered to be prospective dives, as opposed to the foraging dives characterizing the foraging phase (Figure 3a). Prospective dives were present in 49% of trips (6 ± 10 dives per trip), amounting to 0.05% of all dives carried out by birds (foraging dives added to 97 ± 50 dives per trip). Birds spent more time at the sea surface and underwater during the outbound phase (42.3 ± 25.9%) than during the inbound phase (17.8 ± 17.2%) (df = 73, t = 5.99, P < .0001), a proportion that was 82.8 ± 10.8% during the foraging phase. Bird instantaneous flight speed was significantly slower during the outbound phase (45.4 ± 7.1 km/hr) than during the inbound phase (48.6 ± 6.1 km/hr) (df = 73, t = −2.4, P = .018), while it was an average of 34.7 ± 6.0 km/hr during the foraging phase. As a result of these differences, bird velocity was 11 km/hr slower on average during the outbound than during the inbound phase (df = 73, t = −4.92, P < .001) (Figure 3b). Bird velocity during the outbound phase increased linearly with trip departure time (Figure 3c).

View Article: PubMed Central - PubMed

ABSTRACT

Group foraging contradicts classic ecological theory because intraspecific competition normally increases with aggregation. Hence, there should be evolutionary benefits to group foraging. The study of group foraging in the field remains challenging however, because of the large number of individuals involved and the remoteness of the interactions to the observer. Biologging represents a cost&#8208;effective solution to these methodological issues. By deploying GPS and temperature&ndash;depth loggers on individuals over a period of several consecutive days, we investigated intraspecific foraging interactions in the Socotra cormorant Phalacrocorax nigrogularis, a threatened colonial seabird endemic to the Arabian Peninsula. In particular, we examined how closely birds from the same colony associated with each other spatially when they were at sea at the same time and the distance between foraging dives at different periods of the day. Results show that the position of different birds overlapped substantially, all birds targeting the same general foraging grounds throughout the day, likely following the same school of fish. There were as many as 44,500 birds within the foraging flock at sea at any time (50% of the colony), and flocking density was high, with distance between birds ranging from 8 to 1,380&nbsp;m. Birds adopted a diving strategy maximizing time spent underwater relative to surface time, resulting in up to 72% of birds underwater in potential contact with prey at all times while foraging. Our data suggest that the benefits of group foraging outweigh the costs of intense aggregation in this seabird. Prey detection and information transmission are facilitated in large groups. Once discovered, shoaling prey are concentrated under the effect of the multitude. Fish school cohesiveness is then disorganized by continuous attacks of diving birds to facilitate prey capture. Decreasing population size could pose a risk to the persistence of threatened seabirds where group size is important for foraging success.

No MeSH data available.


Related in: MedlinePlus