When searching for prey, an organism is expected to maximize its energetic gain, while minimizing energy expenditure. To accomplish this, organisms should increase the probability of detecting food patches by altering their movement path relative to the availability of prey [1], [2]. Unlike terrestrial environments, where physical barriers and geography often put constraints on movement, most oceanic environments are less obstructed. As a result, the movements of marine organisms that have high dispersal capability can potentially reflect their assessment of resource distribution and variation.

In the case of pelagic seabirds, like albatrosses and petrels, that nest on remote oceanic islands [3] one would predict that breeding populations on distant islands should maximize energetic gain by exploiting available resources closest to their respective breeding colony [4]. Consequently, regional differences in diet associated with the differences in foraging ranges of a particular population may be observed due to difference in prey distribution. Upon completion of breeding, birds are no longer tied to a nest and so greater population mixing is likely, even for populations separated by several thousand kilometers. Thus, movement patterns during this phase of the life cycle would perhaps be more determined by variations in prey distribution [5], [6], [7].

The at-sea foraging patterns of the highly mobile Laysan albatross (Phoebastria immutabilis) have been well documented for breeding individuals [8]–[13]. Their annual reproductive cycle is marked by several phases: during incubation and early chick rearing stages Laysan albatrosses are central-place foragers, but later, when they are less constrained by nest visitations, Laysan albatrosses forage throughout the North Pacific [8], [11]–[13]. However, it is unknown whether all breeding colonies in the North Pacific exploit the same or similar resources or habitats year round, even though they may be well within their dispersal potential, or if colonies utilize ‘locally’ available resources since previous studies have focused on a single breeding colony [8], [13].

Central place foraging, coupled with huge aggregations of birds at the breeding colonies, likely exerts predation pressure on local prey stocks, and potentially amplifies intra-specific competition at the colonies. This is thought to possibly lead to localized prey depletion around breeding colonies which has been called ‘Ashmoles Halo’ [14]. In order to maximize food availability and minimize intra-specific competition, albatross colonies should be spaced so that the foraging zones of the birds do not greatly overlap. However, in the Hawaiian islands where 99% of Laysan albatross populations breed [10], colonies are not evenly distributed and the inter-colony distances are often much smaller than their maximum foraging range [15]. If prey resources throughout the North Pacific Basin are evenly distributed, then colony-specific foraging radii will overlap widely because the distance between colonies is shorter than the birds foraging radius [8]. However, a more likely scenario given the heterogeneity of the North Pacific, is that Laysan albatrosses should exploit suitable resources closest to their respective colony during the breeding period, even though they are capable of travelling to virtually any foraging ground in the North Pacific.

The variable diet and flexible foraging strategy of Laysan albatrosses (utilizing both scavenging and live capture; [10], [16]) indicate that this species is a generalist. Ironically, the flexible foraging strategy of this species has potentially led to a decrease in their foraging efficiency as they commonly ingest large amounts of plastic, which is in turn fed to their chicks [17]–[20]. However, it is unclear how often Laysan albatrosses encounter or ingest plastic, whether plastic is mistaken for prey, has natural food attached to it, or is consumed to assist in digestion as is sometimes done with pumice [17]–[20]. Unfortunately, plastic ingestion leads to mechanical blockage of the digestive tract, reduced food consumption, satiation of hunger, and potential exposure to toxic compounds. While there have been documented detrimental effects on the growth rates and fledging masses of chicks, it is still unclear what levels of mortality are caused by plastic ingestion [17]–[20]. What is clear is the source of the plastic: there is now so much floating marine debris accumulated in the North Pacific gyre, that is it known as the ‘great garbage patch’ [21], [22], [23]. This patch consists of high densities of floating plastic debris, particularly between 20°–40° N, within a few hundred kilometres of the coast and in the gyre centres, between the tropical and subarctic waters. This area of concentrated debris consists of two accumulations: the ‘Western Garbage Patch’ that occurs off Japan and ‘Eastern Garbage Patch’ residing between Hawaii and California (http://coastwatch.pfel.noaa.gov) [24], [25] that correspond to the locations of two sub-gyres within the North Pacific Gyre [26], connected by a narrower band of marine debris north of the Hawaiian archipelago (Figure 1). Much has been written in the popular press about plastic ingestion by Laysan albatrosses (Figure 2), however, relatively few empirical studies have examined this phenomenon and whether it is species wide, or if this is confined to certain populations.

Here we examine whether Laysan albatrosses nesting on widely separated islands exploit resources closer to their breeding colonies during the breeding season, how their foraging locations change throughout the reproductive cycle, and whether this leads to differences in plastic ingestion represented in their boluses. In addition to the potential of seabird boluses to monitor plastic in our oceans, determining how and where marine organisms come into contact with marine debris could have implications for the design of management strategies that mitigate its environmental impact. To do this, we combined the use of electronic data logging devices to determine at-sea distributions with the collection of chick boluses to evaluate the differences plastic ingestion.

Distant colonies of Laysan albatrosses utilized similar oceanographic productivity regimes in widely separated areas, suggesting that this vagile animal is able to assess resources on large scales. For both colonies, similar associations with productivity and depth domains indicate that oceanographic characteristics of suitable foraging areas are part of an albatross's search image when they are trying to locate prey [8], [11], [13]. However, it is their ability to identify suitable areas closest to the colony when they are breeding which suggests that their assessment of resource variation also includes an assessment of the tradeoff between self-maintenance (longer trips) and chick provisioning (shorter trips) [8], and thus leads to foraging segregation during the early part of the reproductive cycle when they are required to return to the breeding colony frequently [5], [28].

During the non-breeding phase when Laysan albatrosses are not constrained by the time and energy required for breeding, there is a larger degree of overlap in foraging distributions between distant populations. This suggests that the energy required to reach the foraging grounds becomes less important than the total energy available on the foraging grounds themselves. Another possible explanation is that the colony on Oahu, which is a ‘new colony’ comprised almost entirely of immigrants from other colonies [29], [30] has immigrants from Kure Atoll, and that the bi-modal distribution in the non-breeding season may actually reflect Kure-born birds returning to their ancestral non-breeding foraging grounds. In either case, the results indicate that Laysan albatrosses are able to assess resource variation and alter their foraging strategies accordingly throughout their reproductive cycle as has been shown for other species [5], [6], [7], [31], [32], [33].

The fact that Laysan albatrosses from both colonies ingest plastic, unfortunately, suggests that the core areas where albatrosses prefer to forage contain substantial amounts of floating debris that are consumed. Previous studies have shown that the highest concentration of marine debris occurs in the spring and early summer months when the NPTZ moves south due to shifts in wind and weather changes [21]. This corresponds to the chick rearing season in Laysan albatross [10]. The finding that birds breeding on Kure Atoll fed their chicks, on average, ten times more plastic than birds breeding on Oahu suggests that putative Western Garbage Patch where the majority of Kure birds foraged may in fact be a just as much of a threat to marine life as the frequently discussed Eastern Garbage Patch. Furthermore, every bolus examined from Kure Atoll contained multiple pieces of fishing paraphernalia, while only two boluses on Oahu contained any evidence of fishing line or tools (despite recreational fishing adjacent to the breeding colony on Oahu), suggesting that the threat from fisheries not only comes from bycatch for this species but also from the consumption of fishing gear. It is unclear whether the Western Garbage Patch contains more trash than the Eastern Garbage Patch, or if the size and composition of the pieces are easier for the birds to ingest compared to those found in the Eastern Garbage Patch in addition to the finding that albatrosses from Kure spend a greater proportion of time foraging in this area.

Previous studies on seabirds have shown that comparisons of plastic accumulations within a species should have consistent biases when different populations are relatively synchronous in their breeding cycle, which Laysan albatross are. Studies on Northern Fulmars (Fulmarus glacialis) found that regional differences in foraging/nesting locations led to differences in plastic ingestion [34], [35]. While the most likely explanation of why the plastic loads differ between the two colonies examined in this study is due to seasonal differences in at sea distribution, it is possible that competition may be playing a role. There are approximately 75 Laysan albatross pairs on Oahu which may experience less intraspecific competition for prey than the 3,900 pairs breeding on Kure Atoll. The slightly smaller foraging area of Oahu-nesting birds during early chick-rearing stages might be an evidence for this. Since the amount of natural food in the boluses of both colonies is the same, adults on Kure Atoll may cope with this pressure by being less selective and thus misidentify more plastics for food. Further research identifying potential rates intraspecific competition between individuals in these two colonies will help to resolve this question. Additional research to determine the size, location(s) and contents of these floating debris patch(es) in the North Pacific would also greatly enhance our ability to determine their impacts on marine life.

Environmental heterogeneity in marine, as well as terrestrial systems, affects animal movements on a range of scales [5], [6], [7], [31], [32], [33], [36]. Our results demonstrate that Laysan albatrosses are able to assess prey availability over large scales and make foraging decisions based on the energy required to reach feeding grounds throughout their breeding cycle. While the preferred foraging locations during the early breeding season are different for each population, these predators appear to seek out common ecological characteristics, leading to the use of similar habitats by albatrosses from the two colonies in the non-breeding season when they are not constrained by breeding. Studying movements at a smaller scale in relation to resource distribution, and at multiple colonies over multiple years will be crucial to fully understand scale-dependent adjustments and the ultimate foraging distribution of these animals. Future studies of foraging behavior would also benefit from not only monitoring plastics ingested, but also quantifying natural diet to ascertain regional differences. The large range and potentially long-term retention of ingested plastic indicate that albatross may prove to be a useful species for sampling marine debris and other pollutants in the North Pacific Ocean [35], and as such, efforts should be made to continue monitoring their plastic ingestion and foraging patterns.