A junk-food hypothesis for gannets feeding on fishery waste

Worldwide fisheries generate large volumes of fishery waste and it is often assumed that this additional food is beneficial to populations of marine top-predators. We challenge this concept via a detailed study of foraging Cape gannets Morus capensis and of their feeding environment in the Benguela upwelling zone. The natural prey of Cape gannets (pelagic fishes) is depleted and birds now feed extensively on fishery wastes. These are beneficial to non-breeding birds, which show reduced feeding effort and high survival. By contrast, breeding gannets double their diving effort in an attempt to provision their chicks predominantly with high-quality, live pelagic fishes. Owing to a scarcity of this resource, they fail and most chicks die. Our study supports the junk-food hypothesis for Cape gannets since it shows that non-breeding birds can survive when complementing their diet with fishery wastes, but that they struggle to reproduce if live prey is scarce. This is due to the negative impact of low-quality fishery wastes on the growth patterns of gannet chicks. Marine management policies should not assume that fishery waste is generally beneficial to scavenging seabirds and that an abundance of this artificial resource will automatically inflate their populations.     

Predator–prey interactions: why do larger albatrosses eat bigger squid?

The relationship between predator sizes and prey sizes is well documented for terrestrial but rarely for marine ecosystems. We show that wandering albatrosses, the biggest albatross species, feed on larger cephalopod prey than those consumed by smaller albatrosses (grey-headed and black-browed albatrosses). This reflects differences in timing of breeding, foraging ecology and their feeding methods. Wandering albatrosses breed later in the year, during the austral winter, than smaller albatrosses (therefore catching older squid) and forage most of the year in Antarctic open waters, sub-Antarctic, subtropical and tropical waters, overlapping minimally with the smaller albatrosses' foraging range while breeding. Also, wandering albatrosses mostly scavenge whereas smaller albatrosses feed more on live prey. Prey ecology may also play a key role because many squid species might experience post-spawning mortality during the austral winter, becoming easily available to wandering albatrosses. Spawning in winter can be linked to predator avoidance (i.e. reduction in mortality in winter by avoiding pelagic predators) and would allow squid larvae to develop and take advantage of the high productivity (i.e. Antarctic phytoplankton bloom) in spring and at the beginning of summer. Thus, aspects of prey and predator ecology may combine to generate observed differences in prey size.     

Behavioral responses to encounter of fishing boats in wandering albatrosses

Animals are attracted to human food subsidies worldwide. The behavioral response of individuals to these resources is rarely described in detail, beyond chances of encounters. Seabirds for instance scavenge in large numbers at fishing boats, triggering crucial conservation issues, but how the response to boats varies across encounters is poorly known. Here we examine the behavioral response of wandering albatrosses (Diomedea exulans), equipped with GPS tags, to longline fishing boats operating near their colony for which we had access to vessel monitoring system data. We distinguish between encounters (flying within 30 km of a boat) and attendance behavior (sitting on the sea within 3 km of a boat), and examine factors affecting each. In particular, we test hypotheses that the response to encountered boats should vary with sex and age in this long‐lived dimorphic species. Among the 60% trips that encountered boats at least once, 80% of them contained attendance (but attendance followed only 60% of each single encounter). Birds were more attracted and remained attending longer when boats were hauling lines, despite the measures enforced by this fleet to limit food availability during operations. Sex and age of birds had low influence on the response to boats, except the year when fewer boats came fishing in the area, and younger birds were attending further from boats compared to older birds. Net mass gain of birds was similar across sex and not affected by time spent attending boats. Our results indicate albatrosses extensively attend this fishery, with no clear advantages, questioning impacts on foraging time budgets. Factors responsible for sex foraging segregation at larger scale seem not to operate at this fleet near the colony and are not consistent with predictions of optimal foraging theory on potential individual dominance asymmetries. This approach complements studies of large‐scale overlap of animals with human subsidies.     

Orientation in the wandering albatross: interfering with magnetic perception does not affect orientation performance

After making foraging flights of several thousands of kilometers, wandering albatrosses (Diomedea exulans) are able to pinpoint a specific remote island where their nests are located. This impressive navigation ability is highly precise but its nature is mysterious. Here we examined whether albatrosses rely on the perception of the Earth's magnetic field to accomplish this task. We disturbed the perception of the magnetic field using mobile magnets glued to the head of nine albatrosses and compared their performances with those of 11 control birds. We then used satellite telemetry to monitor their behavior. We found that the ability of birds to home specific nest sites was unimpaired by this manipulation. In particular, experimental and control birds did not show significant differences with respect to either foraging trip duration, or length, or with respect to homing straightness index. Our data suggest that wandering albatrosses do not require magnetic cues to navigate back to their nesting birds.     

From the Eye of the Albatrosses: A Bird-Borne Camera Shows an Association between Albatrosses and a Killer Whale in the Southern Ocean

Albatrosses fly many hundreds of kilometers across the open ocean to find and feed upon their prey. Despite the growing number of studies concerning their foraging behaviour, relatively little is known about how albatrosses actually locate their prey. Here, we present our results from the first deployments of a combined animal-borne camera and depth data logger on free-ranging black-browed albatrosses (Thalassarche melanophrys). The still images recorded from these cameras showed that some albatrosses actively followed a killer whale (Orcinus orca), possibly to feed on food scraps left by this diving predator. The camera images together with the depth profiles showed that the birds dived only occasionally, but that they actively dived when other birds or the killer whale were present. This association with diving predators or other birds may partially explain how albatrosses find their prey more efficiently in the apparently ‘featureless’ ocean, with a minimal requirement for energetically costly diving or landing activities.     

Does prey capture induce area-restricted search? A fine-scale study using GPS in a marine predator, the wandering albatross

In a patchy environment, predators are expected to increase turning rate and start an area-restricted search (ARS) when prey have been encountered, but few empirical data exist for large predators. By using GPS loggers with devices measuring prey capture, we studied how a marine predator adjusts foraging movements at various scales in relation to prey capture. Wandering albatrosses use two tactics, sit and wait and foraging in flight, the former tactic being three times less efficient than the latter. During flight foraging, birds caught large isolated prey and used ARS at scales varying from 5 to 90 km, with large-scale ARS being used only by young animals. Birds did not show strong responses to prey capture at a large scale, few ARS events occurred after prey capture, and birds did not have high rates of prey capture in ARS. Only at small scales did birds increase sinuosity after prey captures for a limited time period, and this occurred only after they had caught a large prey item within an ARS zone. When this species searches over a large scale, the most effective search rule was to follow a nearly straight path. ARS may be used to restrict search to a particular environment where prey capture is more predictable and profitable.     

Avian daily foraging patterns: Effects of digestive constraints and variability

Summary Birds show a typical daily pattern of heavy morning and secondary afternoon feeding. We investigate the pattern of foraging by a bird that results in the lowest long-term rate of mortality. We assume the following: mortality is the sum of starvation and predation. The bird is characterized by two state variables, its energy reserves and the amount of food in its stomach. Starvation occurs during the day if the bird's reserves fall to zero. The bird starves during the night if the total energy stored in reserves and the stomach is less than a critical amount. The probability that the bird is killed by a predator is higher if the bird is foraging than if it is resting. Furthermore, the predation risk while foraging increases with the bird's mass. From these assumptions, we use dynamic programming techniques to find the daily foraging routine that minimizes mortality. The principal results are (1) Variability in food finding leads to routines with feeding concentrated early in the day, (2) digestive constraints cause feeding to be spread more evenly through the day, (3) even under fairly severe digestive constraints, the stomach is generally not full and (4) optimal fat reserve levels are higher in more variable environments and under digestive constraints. This model suggests that the characteristic daily feeding pattern of small birds is not due to digestive constraints but is greatly influenced by environmental variability.     

Origin, age, sex and breeding status of wandering albatrosses (Diomedea exulans), northern (Macronectes halli) and southern giant petrels (Macronectes giganteus) attending demersal longliners in Falkland Islands and Scotia Ridge waters, 2001–2005

A total of 547 sightings of 291 banded wandering albatrosses Diomedea exulans and 21 sightings of 14 banded giant petrels Macronectes spp. were made from toothfish longliners operating on the southern Patagonian Shelf during 2001–2005. This included 25% of the wandering albatrosses with Darvic bands that bred at Bird Island (South Georgia) during this period. Thirteen of the northern Macronectes halli and southern giant petrels Macronectes giganteus had been banded at South Georgia, and there was one sighting of a southern giant petrel from Argentina. Male and female wandering albatrosses of all age classes except young birds (<15 years old) were equally likely to attend longline vessels. Most sightings of all age classes were made during the incubation period and fewest during the brood period. Eighty-six percent of birds sighted had bred at least once before, with half currently breeding and half on sabbatical (i.e. between breeding attempts). Almost half of the wandering albatrosses were sighted on more than one occasion. The data confirms that the southern Patagonian shelf is an important foraging area for wandering albatrosses and northern and southern giant petrels, and that some individuals show consistent associations in multiple years with longline vessels fishing in the region.     

Environmental heterogeneity and the evolution of foraging behaviour in long ranging greater albatrosses

Habitat selection in heterogeneous environments is assumed to allow diversification. Wide‐ranging species like pelagic seabirds present a paradox, in that their diversity appears difficult to reconcile with a frequent lack of geographical isolation between populations. We studied the foraging strategies of three closely related species of greater albatrosses, wandering albatross, Diomedea exulans, Amsterdam albatrosses D. amsterdamensis and royal albatross, D. epomophora, in relation to environmental heterogeneity at coarse‐grained and fine‐grained scales. During the incubation period the three species foraged at long distances from their colonies. We observed significant differences between the species in the duration of foraging trips and the distance travelled per day. There were significant differences in preference for habitat types in relation to bathymetric features, and in chlorophyll a concentrations in the waters traversed. Royal albatross preferred shallower waters (<1500 m depth), which were rich in chlorophyll (>0.5 mg/m³), while the other species spent on average 80% of their time in waters deeper than this, where chlorophyll levels were lower. Wandering albatrosses foraged in colder waters than Amsterdam albatrosses. Patterns of activity divided the species into two groups: those exploiting oceanic habitats (wandering and Amsterdam albatrosses) spent high proportions of time on the water (49%), and had on average 1.35 takeoffs and landings per hour, while royal albatross, which foraged mainly over neritic waters spent only 35% of their time sitting on the water, and made on average 2.6 takeoff per hour. Further, royal albatross showed a similar pattern of activity during all periods of the day, while wandering and Amsterdam albatrosses were mostly inactive during the night. We link these differences in activity to prey patch availability in two contrasting habitats – continental shelf areas compared to open ocean habitats. The divergent styles of foraging observed in this study suggest that these closely‐related and wide‐ranging species could effectively co‐exist by dividing the resources available to them by different modes of exploitation.     

A new look at optimal foraging behaviour; rule of thumb in the rainbow trout

A realistic model of foraging behaviour in stream-living rainbow trout was developed to address some shortcomings of previous tests of optimal foraging theory. The model predicts prey selection based upon both energy-maximization and number-maximization strategies. Trout diet selection on the basis of prey size and prey calorific content was examined under laboratory conditions.
Diet composition was strongly influenced by relative abundance of prey types under all feeding regimes, as predicted by the model. Additional important factors include fish stomach capacity and daily digestive capacity. Experimental results strongly suggest that prey selection in rainbow trout is not based on calorific content. Examination of energy (kJ) obtained from observed diets showed no energy penalties associated with 'suboptimal' food choices, since all fish (regardless of their choices) consumed far more energy than required for their basic metabolic needs.     

Individual diet differences in a molluscivore shorebird are associated with the size of body instruments for internal processing rather than for feeding

Especially in birds, it is widely found that the size of individual prey items follows the size of the instruments of prey capture, handling and processing, i.e. bill size. In fact, this is the natural history basis of major discoveries on adaptive evolution in the face of changing food resources. In some birds, e.g. the molluscivore shorebirds ingesting hard‐shelled prey, most of the prey processing takes place within the digestive tract. This study of a salvaged sample of actively feeding great knots Calidris tenuirostris accidentally drowned in fishing nets in northern China, is the first documentation of diet selection at the level of the individual in previously well‐studied molluscivore shorebirds. Diet composition was not associated with the length of the bill, but with the mass of the muscular gizzard. Gizzard mass, which unlike bill length is a phenotypically flexible trait, enables great knots to adjust to changing food resources as an individual, i.e. instantly responding to the food on offer. For migratory species like great knots which rely on seasonal sequences of interdistant feeding areas offering prey with a variety of characteristics, the capacity to individually adjust appears a key adaptation.     

6. SHORT NOTES

Feare, Christopher J. 1981. Breeding schedules and feeding strategies of Seychelles sea-birds. Ostrich 52:179–185.

A comparison of the breeding schedules of Seychelles seabirds with their feeding strategies showed no relation between synchronous breeding and the ranges over which the birds forage, but there was a relation with flock feeding. Flock feeding birds are largely dependent upon schools of predatory fishes that drive prey species to the surface, and food is thus localized and transient. Synchronous colonial breeding probably helps individuals to locate these localized food sources, but species that are dependent mainly on dispersed prey would derive no benefit from synchrony, and they do, in fact, breed throughout the year.

Within shoals of prey, food is probably superabundant, but feeding space may be limiting. The dark backs of flock feeding birds render them inconspicuous to foraging birds (conspecifics and other species), thereby tending to reduce competition for space within the feeding flocks. The white plumage of species that feed on dispersed prey appears to act as a spacing out mechanism, reducing interference between feeding birds.     

Foraging and provisioning strategies of black-browed albatrosses in relation to the requirements of the chick: natural variation and experimental study

We studied regulation of the food supply to black-browed albatrosschicks at Kerguelen by simultaneously recording the provisioningrates achieved by individual parents and satellite trackingforaging birds during two seasons, by studying changes in adultmass, and by experimentally manipulating the food requirementof chicks. In 1994 black-browed albatrosses had a higher breedingsuccess and produced heavier chicks that grew faster than in1995. They spent a similar time foraging but brought heaviermeals to their chick in 1994. Satellite tracking indicated thatin both seasons birds foraged in the same oceanographic area,250 km from the colony. Travel times to and from this area remainedunchanged, and similar times were spent foraging there. In ourstudy area, black-browed albatrosses appear to rely on a foodresource that is predictable in location, but whose availabilityvaries from one year to the next. The principal difference betweenyears of differing food availability was that birds broughtlarger meals when food was more abundant Costs of commutingto nearby feeding areas are probably low and allow the deliveryof energy to the chick at a high rate. A study carried out in1991 indicated that there was no relationship between the changesin adult mass from one trip to the next and the duration offoraging trips or feed mass, suggesting that adult body conditionhad little influence on the provisioning strategy of this species.An experiment whereby some chicks were deprived of food andothers received supplementary food showed that parents of underfedchicks spent the same time foraging and brought slightly largeramounts of food to their chicks as control parents. We suggestthat parents are searching for food to the maximum limits oftheir ability and thus cannot reduce further foraging time,but underfed chicks can swallow more food. Parents of overfedchicks delivered less food and increased the time between feeds.The reduction in provisioning frequency is interpreted as thecapacity of parents to modify their foraging behavior accordingto the nutritional status of the chick, but the reduction offeed mass is probably the result of chicks being close to theirmaximum assimilatory capacity. Comparison between Procellariiformspecies indicates extensive differences in the degree to whichparents can regulate the supply of food to their chicks. Neriticspecies like black-browed albatrosses appear to have a reducedability to regulate, and especially to increase provisioningrates, whereas more pelagic species may have a greater regulationability     

Regulation of foraging trips and incubation routine in male and female wandering albatrosses

The resolution of the conflict between eggcare and foraging was studied in male and female wandering albatrosses. The foraging zone and range, duration of incubation shifts and foraging trips, and associated changes in body mass were studied. Costs during incubation, expressed as the time spent incubating and the proportional loss of body mass, were similar for both sexes. The mass gained at sea was related to the duration of foraging trips, but the relationship was much less significant in males, where foraging ranges, though similar on average to those of females, were very variable. Males foraged in more southerly waters than females, and gained mass more rapidly. Only females appeared to regulate the duration of foraging trips, and this compensated for the mass lost during the incubation fast. Previous breeding experience had no influence on foraging efficiency. Egg desertion because of depletion of body reserves was very rare because birds have a wide safety margin, i.e. the difference between the average body mass when relieved and that at nest desertion. This safety margin enables the birds to compensate for the high variability in the duration of foraging trips, and is probably a reason for the high breeding success of wandering albatrosses. Decisions to return from the sea to the nest or to desert the nest are probably related to the status of body reserves, and have been selected in the large wandering albatross so that both present and future reproductive success are maximised.     

Postnatal dispersal of wandering albatrosses Diomedea exulans: implications for the conservation of the species

Many large marine vertebrates are today threatened by human activities and it is therefore crucial to obtain information on their distribution and behaviour at sea. In particular little is known about the time necessary for juveniles to acquire the foraging skills of adults. We tracked 13 juvenile wandering albatrosses Diomedea exulans by satellite telemetry during their first year at sea. They covered an average distance of 184,000 km during the first year and restricted their dispersal to the unproductive waters of the subtropical Indian Ocean and Tasman Sea. This region of low wind velocities does not overlap with the foraging areas used by adults. After an innate phase of rapid dispersal with a fixed flight direction, young birds progressively increased their daily flight distances and attained adult flight efficiency within their first six months at sea. The complete overlap of the juveniles’ foraging ranges with major long‐line fisheries in the subtropical waters constitutes a major threat that could jeopardize the long term recovery ability of populations of the endangered wandering albatross in the Indian Ocean.     

Postnatal dispersal of wandering albatrosses Diomedea exulans: implications for the conservation of the species

Many large marine vertebrates are today threatened by human activities and it is therefore crucial to obtain information on their distribution and behaviour at sea. In particular little is known about the time necessary for juveniles to acquire the foraging skills of adults. We tracked 13 juvenile wandering albatrosses Diomedea exulans by satellite telemetry during their first year at sea. They covered an average distance of 184,000 km during the first year and restricted their dispersal to the unproductive waters of the subtropical Indian Ocean and Tasman Sea. This region of low wind velocities does not overlap with the foraging areas used by adults. After an innate phase of rapid dispersal with a fixed flight direction, young birds progressively increased their daily flight distances and attained adult flight efficiency within their first six months at sea. The complete overlap of the juveniles' foraging ranges with major long-line fisheries in the subtropical waters constitutes a major threat that could jeopardize the long term recovery ability of populations of the endangered wandering albatross in the Indian Ocean.     

Feeding behaviour of free-ranging penguins determined by oesophageal temperature

Sea birds play a major role in marine food webs, and it is important to determine when and how much they feed at sea. A major advance has been made by using the drop in stomach temperature after ingestion of ectothermic prey. This method is less sensitive when birds eat small prey or when the stomach is full. Moreover, in diving birds, independently of food ingestion, there are fluctuations in the lower abdominal temperature during the dives. Using oesophageal temperature, we present here a new method for detecting the timing of prey ingestion in free-ranging sea birds, and, to our knowledge, report the first data obtained on king penguins (Aptenodytes patagonicus). In birds ashore, which were hand-fed 2-15 g pieces of fish, all meal ingestions were detected with a sensor in the upper oesophagus. Detection was poorer with sensors at increasing distances from the beak. At sea, slow temperature drops in the upper oesophagus and stomach characterized a diving effect per se. For the upper oesophagus only, abrupt temperature variations were superimposed, therefore indicating prey ingestions. We determined the depths at which these occurred. Combining the changes in oesophageal temperatures of marine predators with their diving pattern opens new perspectives for understanding their foraging strategy, and, after validation with concurrent applications of classical techniques of prey survey, for assessing the distribution of their prey.     

Experimental study of cognitive aspects of ambivalent foraging as exemplified by the great tit

A hypothesis of ambivalent foraging is proposed based on ideas about dual treating of the prey by a consumer: the food value attracts while the danger repulses. The foraging strategy of the great tit was investigated experimentally with the use of artificial “food patches” with variable amounts of dangerous prey (live red wood ants) and non-dangerous prey (fly larvae). With non-dangerous prey, the behavior of the birds corresponded to the known marginal value theorem: they proceeded with foraging until the resources were exhausted. We found the threshold amount of dangerous prey that prevents tits from hunting.     

Stomach fullness modulates prey size choice in the frillfin goby,Bathygobius soporator

Behaviours related to foraging and feeding in predator–prey systems are fundamental to our understanding of food webs. From the perspective of a predator, the selection of prey size depends upon a number of factors including prey vulnerability, prey size, and the predator's motivation to eat. Thus, feeding motivation and prey visual cues are supposed to influence predator decisions and it is predicted that prey selection by visual cues is modulated by the predator's stomach fullness prior to attacking a prey. This study was conducted using an animal model from the rocky shores ecosystem, a predatory fish, the frillfin goby Bathygobius soporator, and a benthic prey, the mottled shore crab Pachygrapsus transversus. Our results demonstrate that frillfin gobies are capable of visually evaluating prey size and that the size evaluation process is modulated by the level of stomach fullness. Predators with an empty stomach (0% fullness) attacked prey that was larger than the predicted optimal size. Partially satiated predators (50% stomach fullness) selected prey close to the optimal size, while fully satiated predators (100% stomach fullness) showed no preference for size. This finding indicates an integrative response of the predator that depends on the input of both internal and external sensory information when choosing prey. Predator perceptions of visual cues (prey size) and stomach fullness modulate foraging decisions. As a result, a flexible feeding behaviour emerges, evidencing a clearly adaptive response in line with optimal foraging theory predictions.     

Foraging patterns of breeding birds in eucalypt forest and woodland of southeastern Australia

The foraging ecology of eucalypt forest and woodland birds was studied on three 10 ha plots in southeastern Australia. Quantitative data were obtained for 41 species of which 31 were insectivorous, eight were nectar-feeders, and two were parrots that fed primarily on eucalypt seeds. Birds-of-prey, large omnivores and frugivores were uncommon. Insectivorous birds differed in foraging behaviour, the substrates on which they found prey, and foraging height. Nectar- feeders exploited a variety of carbohydrates including nectar, honeydew, lerp, manna and sap. Nectarivorous birds were separated by foraging behaviour, substrate, height and by the extent to which they used the different types of carbohydrates. Carbohydrates were also an important food resource for some insectivores. By understanding how birds exploit food resources within forest and woodland environments, the features of the environment which need to be conserved or manipulated to manage forest avifaunas can be identified. For example, in addition to the substrates such as foliage and bark, usually associated with the foraging of forest birds, carbohydrates and loose bark were identified as important resources for birds in eucalypt forests and woodlands. The broad importance of these two resources to the avifauna had not been previously appreciated, yet both may be sensitive to environmental changes associated with logging and other forest management practices which alter the composition or age-class structure of forests.