Abstract
Recent studies have documented that little penguins (Eudyptula minor) associate at sea, displaying synchronised diving behaviour throughout a foraging trip. However, previous observations were limited to a single foraging trip where only a small number of individuals were simultaneously tracked. Consequently, it is not known whether coordinated behaviour is consistent over time, or what factors influence it. In the present study, breeding adults were concurrently instrumented with GPS and dive behaviour data loggers for at least 2 consecutive foraging trips during guard and post-guard stage at two breeding colonies (London Bridge and Gabo Island, south-eastern Australia) of contrasting population size (approximately 100 and 30,000–40,000, respectively). At both colonies, individuals were sampled in areas of comparable nesting density and spatial area. At London Bridge, where individuals use a short (23 m) common pathway from their nests to the shoreline, > 90% (n = 42) of birds displayed foraging associations and 53–60% (n = 20) maintained temporally consistent associations with the same conspecifics. Neither intrinsic (sex, size or body condition) nor extrinsic (nest proximity) factors were found to influence foraging associations. However, individuals that departed from the colony at a similar time were more likely to associate during a foraging trip. At Gabo Island, where individuals use a longer (116 m) pathway with numerous tributaries to reach the shoreline, few individuals (< 31%; n = 13) from neighbouring nests associated at sea and only 1% (n = 1) maintained associations over subsequent trips. However, data from animal-borne video cameras indicated individuals at this colony displayed foraging associations of similar group size to those at London Bridge. This study reveals that group foraging behaviour occurs at multiple colonies and the pathways these individuals traverse with conspecifics may facilitate opportunistic group formation and resulting in foraging associations irrespective of nesting proximity and other factors.
Highlights
In a highly dynamic environment, predators are faced with two main foraging pressures: finding prey before starving; and the metabolic cost of capturing and processing prey [1]
Foraging associations were observed in > 90% of instrumented individuals at London Bridge (LB) during both guard and post-guard stages, significantly more than at Gabo Island (GI) where only 18 and 32% of instrumented birds associated for each stage respectively (Table 2)
Individuals at LB associated with multiple instrumented individuals throughout their foraging trips, this was consistent across all breeding stages
Summary
In a highly dynamic environment, predators are faced with two main foraging pressures: finding prey before starving; and the metabolic cost of capturing and processing prey [1]. Group foraging can assist in the capture of large prey, where individuals coordinate efforts to subdue prey [4], and small prey, where individuals further concentrate aggregations of prey, thereby increasing capture capacity and reducing metabolic costs [5,6]. When prey is unpredictably distributed, large foraging groups encounter more opportunities due to an increased ability to detect prey. Within-group competition may decrease the net prey intake per individual when prey is limited [11]. In such instances, where group size exceeds the optimal foraging limit, competition for resources may outweigh the benefits of group foraging, which can lead to the dissolution of coalitions [12,13]. A degree of sociality is required between conspecifics for individuals to cooperate in groups
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