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

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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.


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Examples of foraging tracks of Socotra cormorants that are closely associated in space and time (white star: Siniya colony; filled circles: dives). Intraflock association between two birds (a–e) or three birds (f); birds 2 and 3 in (a) did not dive
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ece32750-fig-0005: Examples of foraging tracks of Socotra cormorants that are closely associated in space and time (white star: Siniya colony; filled circles: dives). Intraflock association between two birds (a–e) or three birds (f); birds 2 and 3 in (a) did not dive

Mentions: Birds that departed to sea within a few minutes or hours of each other foraged over the same grounds, sometimes associating closely with each other in space and time (Figures 5 and 6). The mean intraflock association distance (IFAD) for eight bird “pairs” (1,784 distances calculated) was 285 ± 235 m (range 8–1,380 m; Figure 7). Maximum flock width on the sea surface was therefore considered to be ca. 1.4 km. However, 87% of the time spent by birds in the flock was spent in IFADs <500 m, suggesting the core of flocks was no wider than 0.5 km. Intraflock associations lasted for an average of 2.2 ± 0.8 hr. During group foraging, birds interspersed their dives with many foraging flights (flights occurring between the first and the last dive of a trip): on average one foraging flight for every 3.7 ± 2.2 dives. Foraging flights lasted 33 ± 29 s (Figure S7).


Group foraging in Socotra cormorants: A biologging approach to the study of a complex behavior
Examples of foraging tracks of Socotra cormorants that are closely associated in space and time (white star: Siniya colony; filled circles: dives). Intraflock association between two birds (a–e) or three birds (f); birds 2 and 3 in (a) did not dive
© Copyright Policy - creativeCommonsBy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC5383476&req=5

ece32750-fig-0005: Examples of foraging tracks of Socotra cormorants that are closely associated in space and time (white star: Siniya colony; filled circles: dives). Intraflock association between two birds (a–e) or three birds (f); birds 2 and 3 in (a) did not dive
Mentions: Birds that departed to sea within a few minutes or hours of each other foraged over the same grounds, sometimes associating closely with each other in space and time (Figures 5 and 6). The mean intraflock association distance (IFAD) for eight bird “pairs” (1,784 distances calculated) was 285 ± 235 m (range 8–1,380 m; Figure 7). Maximum flock width on the sea surface was therefore considered to be ca. 1.4 km. However, 87% of the time spent by birds in the flock was spent in IFADs <500 m, suggesting the core of flocks was no wider than 0.5 km. Intraflock associations lasted for an average of 2.2 ± 0.8 hr. During group foraging, birds interspersed their dives with many foraging flights (flights occurring between the first and the last dive of a trip): on average one foraging flight for every 3.7 ± 2.2 dives. Foraging flights lasted 33 ± 29 s (Figure S7).

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