Foraging ecology of the Great Crested Grebe (Podiceps cristatus L.) at a mesotrophic-eutrophic reservoir

The foraging ecology of the Great Crested Grebe wasstudied at Dobczyce Reservoir in southernPoland. The mean diving frequency of grebes withoutyoung was 19.3 h^–1, while grebes with young dived onaverage 32.5 h^–1. The mean number of apparent prey caughtby grebes without young was 0.5 fish h^–1. Birds with young caught a greater number, on average4.5 fish h^–1. Probably, birds fishing forthemselves ate much prey underwater. The total lengthof the fish consumed by adult grebes was on average10.0 cm. Adults ate larger fish in the breeding seasonthan in the autumn migration period. Young birds werefed fish about 8.3 cm long. More than 50% of preywere bleak. The mean success of dives of birds withoutyoung was 3.3% and for individuals with young 14.8%.A comparison of the weight of the daily food demandwith the weight of fish brought to the surface showed that it was 70% lower than the dailyfood demand. The poor relation between prey size anddiving success shows that grebes can eat both small andlarge fish below the water surface.     

Feeding associations between Red-necked Grebes Podiceps griseigena and Velvet Scoters Melanitta fusca in winter

Red-necked Grebes Podiceps griseigena wintering along the coast of Jseren, southwest Norway, were found to feed either solitarily or in close association with Velvet Scoters Melanitta fusca feeding on bottom prey (mainly echinoderms) dug out from the sandy substrate. By scuba diving at spots where both species were feeding, we found that stirring the sand caused polychaete worms to emerge from the substrate. Bottom samples showed that these worms (on average >1000 per m²) vastly outnumbered other animal taxa living in the substrate and that polychaetes were probably the most likely prey of grebes associated with scoters. Food competition between the two species and/or kleptoparasitism by the grebes were probably not involved (no food-stealing attempts by the grebes toward scoters handling prey brought to the surface, no aggression by scoters toward grebes and no effect of the grebes on the time budget of the scoters). Red-necked Grebes associating with scoters were mainly first-year birds, and the frequency of association decreased from a maximum of nearly 60% of the grebes in November to c. 5% before spring departure in April. Grebes feeding solitarily hunted in a manner resembling divers (Gavia) , and they brought fish to the surface significantly more often than grebes feeding with scoters. Solitary grebes spent less time feeding and more time preening and swimming than did grebes associating with scoters. The association appears to be a way naive birds could easily obtain prey. Skills needed to dive for more nutritious but agile fish are probably gradually acquired through the winter as more of the grebes adopt solitary feeding.     

A novel strategy to escape a poor habitat: red-necked grebes transfer flightless young to other ponds

Animals confronted with the threat of the death of their offspring may exhibit unusual and risk-prone behaviours. Grebes (Podicipediformes) are water birds which cannot effectively walk, thus unfledged young are assumed to be unable to depart from their natal ponds by land. We provide evidence that red-necked grebes Podiceps grisegena, breeding on ponds with scarce food resources, transferred their flightless young (2–4 weeks old) to other, unconnected ponds by land or air. Although a large proportion of breeding grebes in the study area nested on food-poor fish ponds acting as ecological traps, where they suffered significant brood losses, brood movements to new ponds accounted for only 3.3% of such breeding attempts. The infrequency of this strategy may be explained by the lack of suitable territories in close proximity and the high risk of predation or fatal injury. The means of chick transfer remains unclear; the chicks may have followed or been carried by parents shuffling across the pond levees; alternatively, parents may have carried the young on their backs in flight. Our findings indicate that red-necked grebes assess the current level of resources available for chicks and may adopt novel and risky strategies to escape total brood failure.     

Foraging rhythm of great crested grebes Podiceps cristatus adjusted to diel variations in the vertical distribution of their prey Osmerus eperlanus in a shallow eutrophic lake in The Netherlands

Summary Great crested grebes Podiceps cristatus only foraged for an hour or two during dawn and again during dusk on Lake IJsselmeer in August-September. During this time of the year the adult birds are in wing moult and temporarily unable to fly. The food of grebes consisted almost exclusively of smelt Osmerus eperlanus, the most numberous pelagic fish. Simultaneous sonar registrations and trawl net fishing showed that smelt moved to the water surface during the twilight periods. During day and night they were concentrated near the bottom. We argue that grebes have the best foraging opportunities during twilight when much of their prey is near the surface, where light intensities allow the fish to be detected and captured. When the smelt are in the upper water layers the distance to the covered to get the prey (i.e. diving time and cost) is also least.     

Food competition and niche separation between fish and the Red-necked Grebe Podiceps grisegena (Boddaert, 1783)

During the breeding season, the aquatic bird Red-necked Grebe Podiceps grisegena feeds on small aquatic animals, which are also an important food source for many fish species. Because grebes have to provide substantial amounts of invertebrates to their brood, competitive interactions with fish may be expected during the breeding season of grebes. Based on this hypothesis, the distribution of Red-necked Grebes, the abundances of macroinvertebrates, fish, and vegetation, as well as the water depth were determined in a shallow lake during two consecutive breeding seasons. The Red-necked Grebes only utilised the reed bed as breeding site, whereas the adjacent Lagoon, although potentially suitable as breeding site, was not used. Macroinvertebrate biomass was 5–10 times higher in the reed beds than in the Lagoon. The abundance of fish was low in the reed bed but considerably higher in the Lagoon. The results indicate habitat separation between breeding Red-necked Grebe and fish in Lake Tåkern, and that the separation is related to distribution of food resources birds and fish have in common.     

Effects of small-bodied fish on invertebrate prey and foraging patterns of waterbirds in Aspen Parkland wetlands

Competition between large-bodied fish and waterbirds for aquatic invertebrates is well documented in oligotrophic lakes. Recent evidence suggests that small-bodied fish that colonize eutrophic, hypoxia-prone wetlands such as prairie potholes can also reduce aquatic invertebrates, but the effects of these reductions on breeding waterbirds have so far not been directly documented. We added brook stickleback (Culaea inconstans) and fathead minnow (Pimephales promelas) to a fishless wetland in Aspen Parkland potholes in central Alberta, Canada. We monitored invertebrate biomasses and the foraging effort of blue-winged teal (Anas discors) and red-necked grebe (Podiceps grisegena) before and after the addition, relative to reference wetlands with and without fish. Fish reduced the biomass of gastropod prey of blue-winged teal, and teals increased foraging effort when fish were added. When the fish failed to overwinter due to hypoxic conditions, gastropod biomass increased, but teal foraging effort did not return to pre-treatment levels. Amphipods and chironomids increased following fish addition, possibly due to indirect positive effects of fish. Red-necked grebes did not exhibit any changes in foraging effort as a result of the fish addition or the subsequent fish extirpation. Grebes in Aspen Parkland appear to treat fish and invertebrates as equivalent prey. This study suggests that small-bodied fish in eutrophic systems can reduce some important invertebrate prey and change foraging behaviour of blue-winged teal and other waterbirds that rely on those invertebrates. Land-use practices that encourage survival of colonizing fish through drought years in Aspen Parkland wetlands, such as wetland consolidation, should not be encouraged.     

Effect of trophic status of a deep-water lake on breeding Great Crested Grebes Podiceps cristatus during a phase of recovery from eutrophication: a long-term study

Capsule: Trophic status of a deep-water lake was the main driver of changes in breeding population size of Great Crested Grebes Podiceps cristatus while reproductive success was also strongly affected by weather parameters.

Aims: To determine the effects of changes in nutrient status of a formerly highly-eutrophicated deep-water lake and other environmental parameters on a Great Crested Grebe population during a phase of re-oligotrophication.

Methods: Annual surveys were carried out on a natural lake in Switzerland over a period of 25 years to determine breeding population size and reproductive success. The effects of phosphorus content, other limnological parameters and weather variables were analysed with quasi-Poisson models.

Results: The breeding population increased from 80 pairs in 1992 to 417 pairs in 2001, after which numbers showed strong fluctuations. Total phosphorus content in the lake had a strong negative effect on breeding population size. A significant positive correlation was found with the national population index. Reproductive success fluctuated strongly but showed an overall decline. The model indicated positive effects on reproductive success of phosphorus and negative effects of the number of days with strong wind. Rapid water-level increases in early summer and water transparency in June led to higher proportions of late broods.

Conclusion: Phosphorus concentration was identified as the main driver affecting the breeding population of Great Crested Grebes during the phase of recovery of the lake from a highly-eutrophic state. Results indicate that mesotrophic to eutrophic conditions enabled a large population and high breeding success. Reproductive output was further negatively affected by strong wind during a critical breeding phase.     

Horned grebe habitat use of constructed wetlands in Alberta,Canada

Wetland degradation has resulted in declines in populations of aquatic birds throughout North America. Horned grebe (Podiceps auritus), a migratory diving bird experiencing population decline, may benefit from wetland construction. We examined horned grebe use of borrow pits (ponds created during highway construction) in Alberta, Canada. Our goals were to document patterns of occurrence and breeding success of grebes on borrow pits and to determine if occupied and unoccupied sites differed in local habitat or landscape characteristics. In May 2003 and 2007, 330 constructed ponds were surveyed for horned grebes. We chose 100 occupied and 100 unoccupied ponds for additional surveys in 2007 and 2008, and collected habitat and landscape data for these sites. We used generalized linear mixed model and generalized linear model regression, coupled with Akaike's Information Criterion, to determine which environmental variables were most effective in explaining occurrence of horned grebes. The best model included all measured local and landscape habitat features. Horned grebes occurred on 36% of ponds in May 2003 and 2007, and chicks were produced on 74.5–81.3% of occupied ponds in 2007 and 2008. Grebes occupied larger ponds with more emergent and riparian vegetation and avoided ponds that supported beavers, contained human structures, and were primarily surrounded by forest within 1 km. On ponds with grebe pairs, chicks were produced more often on sites surrounded by more riparian vegetation. We conclude that the construction of small wetlands offers a promising means of increasing breeding habitat for horned grebes and present specific recommendations for breeding pond construction. © 2012 The Wildlife Society.     

Late‐moulting Black‐necked Grebes Podiceps nigricollis show greater body mass in the face of failing food supply

From August to December, thousands of Black‐necked Grebes Podiceps nigricollis concentrate during the flightless moult period in salt ponds in the Odiel Marshes, southern Spain, where they feed on the brine shrimp Artemia parthenogenetica. We predicted that because Black‐necked Grebes moulted in a food‐rich, predator‐free environment, there would be no net loss of body mass caused by the use of fat stored to meet energy needs during remigial feather replacement (as is the case for some other diving waterbirds). However, because the food resource disappears in winter, we predicted that grebes moulting later in the season would put on more body mass prior to moult because of the increasing risk of an Artemia population crash before the moult period is completed. Body mass determinations of thousands of birds captured during 2000–2010 showed that grebes in active wing‐moult showed greater mass with date of capture. Early‐moulting grebes were significantly lighter at all stages than late‐moulting birds. Grebes captured with new feathers post‐moult were significantly lighter than those in moult. This is the first study to support the hypothesis that individual waterbirds adopt different strategies in body mass accumulation according to timing of moult: early‐season grebes were able to acquire an excess of energy over expenditure and accumulate fat stores while moulting. Delayed moulters acquired greater fat stores in advance of moult to contribute to energy expenditure for feather replacement and retained extra stores later, most likely as a bet hedge against the increasing probability of failing food supply and higher thermoregulatory demands late in the season. An alternative hypothesis, that mass change is affected by a trophically transmitted cestode using brine shrimps as an intermediate host and Black‐necked Grebes as final host, was not supported by the data.     

Feather eating in Great Crested Grebes Podiceps cristatus: a unique solution to the problems of debris and gastric parasites in fish-eating birds

The occurrence of feathers in stomachs of 407 Great Crested Grebes Podiceps cristatus from Lake IJsselmeer, The Netherlands, is described. Only four of the 8718 identified stomach feathers originated from a species other than Great Crested Grebes. In all, 68% were breast and belly feathers and 19% were flank feathers. Flank feathers were positively selected from those becoming available during moult, and also occurred more in the ejected feather pellets than expected from the intake. This suggests that flank feathers, which are long and curved and have a black tip, may be especially suited for the formation of pellets. Feather ingestion was more than balanced by feather production. Feathers were most abundant in stomachs in autumn and were least numerous in winter. Most of this variation can be explained by seasonal differences in the intensity of body moult. In addition, the type of prey was related to the number of ingested feathers. When the diet consisted of smelt, which leave little indigestible matter, more feathers were eaten than with a diet of perch and pikeperch, which leave more fish debris. We suggest the hypothesis that ingested feathers, in the absence of other indigestible matter, contribute substance to the stomach content, enabling the formation of pellets that can be ejected. The habit of regularly ejecting the stomach contents minimizes the chance that any serious population of gastric parasites will build up in the upper part of the alimentary tract.     

Competitive food exploitation of smelt Osmerus eperlanus by great crested grebes Podiceps cristatus and perch Perca fluviatilis at Lake IJsselmeer,The Netherlands

The impact of predation by an avian predator (great crested grebe Podiceps cristatus) and a predatory fish (perch Perca fluviatilis) on a common resource, the small planktivorous fish smelt (Osmerus eperlanus) was studied in Lake IJsselmeer, The Netherlands, in July–October 1985–1988. At this time of the year the grebes are in simultaneous wing-moult and therefore temporarily flightless. Four factors limit the predation pressure by grebes: (1) an underwater visibility threshold of c. 40 cm Secchi depth is the lower limit at which large numbers attend the moulting site, (2) a lower density threshold of exploitable smelt biomass of c. 30 kg/ha determines the actual fishing areas, (3) a fish size threshold of 6.5 cm total length further limits prey availability, and (4) the vertical movements of the prey impose a strong (diel) time constraint on the birds, allowing only crepuscular foraging. Perch, the other main predator in this system, is less restricted in its foraging. Perch were found to be in direct competition with the grebes, preying heavily on the juvenile smelt. Over the 4 years of study 90% of all predation from 15 July to 15 October (72–233 kg/ha) was due to perch. The high level of predation by birds and fish was only possible due to a continuous immigration of smelt into the area (1.45 kg/ha per day). In three of the four years, however, the combined predation by fish and birds surpassed the immigration rate of the prey, which led to a strong reduction in smelt stocks in the study area. A conceptual model is developed to describe the different sets of constraints on the grebes' foraging. First, the state of eutrophication in relation to the weather condition determines the degree of algal blooms, and thus underwater visibility, in late summer. This is the major factor governing the numbers of grebes on the moulting area. Second, the size of the population of predatory fish determines the overall food availability (biomass and size distribution of smelt). These factors are partly interconnected and related to human action (pollution, fisheries). It is suggested that, despite deteriorating visibility conditions, the largest moulting site for grebes in Europe at Lake IJsselmeer exists because the stocks of predatory fish are kept low by overfishing.     

Ecology of migrant Black-necked Grebes Podiceps nigricollis at Mono Lake, California

Autumnal migrant Black-necked or Eared Grebes Podiceps nigricollis begin arriving in large numbers at Mono Lake, California, in August. Juveniles appear to arrive later than adults, and the number of grebes at Mono Lake peaks in September and October. The grebes leave by November or December. Stomachs of 73 grebes collected in the Autumns of 1980 and 1981 reveal that brine shrimp Artemia monica comprise over 90% of the diet. The remainder of the diet is composed of the larvae, pupariaand adults of a brine fly Ephydra hians and small numbers of shore bugs Saldula arenicola and 5. opiparia and other terrestrial arthropods. The grebes do not feed at night but rest instead in large nocturnal aggregations over deep water. Grebe fat stores and total body-weight increase from August to October, and adults moult their regimes in August and September. In late autumn brine shrimp densities decline dramatically and the grebes leave the lake.     

Energy Budgets for Eared Grebes on the Great Salt Lake and Implications for Harvest of Brine Shrimp

ABSTRACT About 1.5-million eared grebes (Podiceps nigricollis), representing half of the North American population, stage on Utah's Great Salt Lake, USA (GSL) during autumn migration to forage on brine shrimp (Artemia franciscana). Indirectly competing with birds for brine shrimp are commercial harvesters who annually collect >1 million kg (dry wt) of shrimp cysts (i.e., hardened eggs), an amount that during some years equals up to half of all brine shrimp cysts produced annually on the GSL. No information was available regarding what impact this commercial harvest was having on eared grebes. We determined daily energy requirements of eared grebes so that regulations governing brine shrimp cyst harvest would better reflect foraging needs of grebes. We measured basal metabolic rate (BMR) of eared grebes from June 2000 to October 2000. Mean BMR of 106 adult and subadult eared grebes was 0.023 kJ/g/hour (SD = 0.004), and mean BMR of 37 juveniles was 0.024 kJ/g/hour (SD = 0.003). Resting and preening metabolic rates were 1.2 times higher than BMR, whereas diving-bout metabolic rate was 1.7 times higher than BMR. Daily energy needs of an average-sized grebe (550 g) during November were 391 kJ. Meeting this energy need requires daily consumption of 24,400 adult brine shrimp. In addition, grebes must consume 2,100–5,200 adult shrimp daily to obtain enough energy reserves to continue their migration to California, USA, and Mexico. Hence, grebes need to consume 26,500–29,600 adult brine shrimp daily while staging on GSL. To achieve this high harvest rate, grebes need adult brine shrimp densities at >380 shrimp/m³during autumn. Commercial harvest of brine shrimp cysts from GSL should be curtailed when cyst densities fall below 20,000 cysts/m³to ensure enough adult brine shrimp for grebes during the subsequent year.     

Book Reviews

Evans, M.S. (ed.) (1988) Toxic Contamination and Ecosystem Health: A Great Lakes Focus.
Kelly, M. (1988) Mining and the Freshwater Environment.
Chart, T.E. and Bergerson, E.P. (1988) Methods for long-term identification of Salmonids: a review.
Gilderhus, P.A., Dawson, V.K. and Allen, J.L. (1988) Deposition and persistence of rotenone in shallow ponds during cold and warm seasons.
Hunn, J.B. (1988) Field assessment of the effects of contaminants on fish.
Jennings, D.P. (1988) Bighead carp (Hypothalmichthys nobilis): a biological synopsis.     

Energy requirements of a Great white pelican (Pelecanus onocrotalus) population and its impact on fish stocks

The Great white pelican Pelecanus onocrotalus is a major predator of some freshwater fish stocks in much of Africa. Detailed population time-activity budgets allowed us to estimate that the south-western Cape Province population consumed 184 tonnes offish annually. Of this, 69% was required for adult existence, 17% for chicks and 14% for flight. Per capita consumption averaged 840 g per day, or 8–8% of body weight; very much less than the figure of 17–6% used in some energy budget calculations. Our integrative bioenergetics approach lends some support for the use of 10% of body weight as the amount of food consumed per day by large piscivorous birds for initial estimates of their impact on stocks.
Predation by Great white pelicans may account for some 10–25% offish production, a shortage of reliable data on fish production in African freshwater ecosystems makes a more precise estimate impossible. Food supplies and shoreline length may both be important factors for pelican populations.     

Nocturnal courtship,copulation, and egg-laying in the western grebe (<Emphasis Type="Italic">Aechmophorus occidentalis</Emphasis>) and Clark’s grebe (<Emphasis Type="Italic">Aechmophorus clarkii</Emphasis>)

Grebes (Podicipedidae) are primarily diurnal although migration occurs at night and some species forage opportunistically at night. We used motion-activated cameras to study nocturnal reproductive activities of the western grebe, Aechmophorus occidentalis, and Clark’s grebe, Aechmophorus clarkii, at Clear Lake, California, USA, during 2014–2016. During 9283 h of monitoring, 19.4% of rushing displays (n = 75) and 8.4% of copulations (n = 435), plus two weed dancing displays and four instances of egg-laying, occurred at night. Nocturnal copulation and egg-laying often occurred on empty, recently abandoned nests, but some incubating pairs of grebes also copulated at night. Nocturnal reproductive activities occurred more frequently at nests attached to submergent vegetation in open water far from shore when water levels were low than on nests attached to emergent vegetation close to shore when water levels were high, probably due to a reduced risk of predation by nocturnal mammals farther from shore. Nocturnal reproductive activities were not in response to elevated light levels at night, daylight disturbances, diurnal predation, high temperatures on hot days, or sexual interference during daylight. Instead, grebes attempted to increase mating investment and minimize parental investment by exploiting recently abandoned nests at night, and perhaps by undertaking extrapair copulations under the cover of darkness, but some pairs of grebes may simply extended routine reproductive activities into the night. Reproductive activities are probably limited at night by the need to feed during daylight and sleep at night, and by the increased risk of predation by mammals at night. Given the high costs of construction and maintenance of floating nests and the obvious benefit of quickly exploiting abandoned nests, nocturnal reproductive activities may occur more frequently in grebes and other waterbirds using floating nests than in other groups of birds whose reproductive activities occur mostly during daylight.     

Between-lake differences in the diet and provisioning behaviour of Black-throated Divers Gavia arctica breeding in Scotland

Surveys at 23 Black-throated Diver breeding lakes in Scotland showed large between-lake differences in the species and size range of potential prey. The study lakes were classified into four types according to the main size-taxa prey classes present. Type 1 lakes lacked small fish, Type 4 lacked salmonids < 300 mm and Types 2 and 3 both had all four of the main prey classes. Diver diet for 30 families at the study lakes was quantified from 7943 prey items seen fed to chicks, and 153 items seen eaten by adults, during 662 h of observations. At all lake types adults mostly ate salmonids of 120–240 mm in length, especially Brown Trout Salmo trutta , and European Eels Anguilla anguilla up to c.  350 mm. The prey eaten by chicks ranged widely, from c.  0.03 g to 300 g, with important implications for feeding behaviour. Young chicks (days 1–8) rejected items greater than 70 mm long. The diet of young chicks consisted mostly of small fish, either Three-spined Sticklebacks Gasterosteus aculeatus (Type 2 lakes) or Minnow Phoxinus phoxinus (Type 3 and Type 4 lakes). At other lakes (Type 1) it consisted mostly of mayfly larvae (Ephemeroptera); invertebrates were the only abundant potential small prey in these lakes. At all lakes the chick diet became more similar to that of adults as chicks grew. Adults provisioning chicks used two foraging strategies. In 'excursion foraging', adults hunted away from the chicks and carried prey back for them; in 'attendance foraging', the chicks accompanied the foraging adult(s). Excursion foraging was used mostly before day 4 and was strongly and positively correlated with the abundance of fish < 80 mm in length. This behaviour is probably adaptive, but is probably tenable only where small fish are relatively abundant. Young chicks fed mainly on small fish had higher survival rates than those fed on invertebrates.     

Diet and foraging effort of Adélie penguins in relation to pack-ice conditions in the southern Ross Sea

We investigated the diet and aspects of foraging effort among Adélie penguins (Pygoscelis adeliae) breeding at three colonies on Ross Island, in the southwestern Ross Sea – Capes Royds, Bird and Crozier – during the chick-provisioning period of three austral summers, 1994–1995, 1995–1996 and 1996–1997. During the study period, pack-ice cover differed in waters offshore of these colonies, by colony, seasons and year. Diet differed among colonies only slightly. The fish Pleuragramma antarcticum was the most important prey, especially during years or periods within years when little pack ice was present. With respect to krill, which composed the remainder of diet, juvenile Euphausia crystallorophias were consumed predominantly in a year of heavy pack-ice cover; more adult krill were consumed in 2 years when pack ice was sparse. Foraging trip duration differed by colony, season and year and was related directly to distance from the colony to the nearest pack ice. The amount of food brought to chicks increased as trip duration increased, to a point (2 days), but then decreased as duration increased further (up to 4 days). On the basis of data on mass of parents and of meal sizes to chicks, it appeared that on the longest trips more of the food gathered by parents was used for self maintenance; on the longest trips, parents lost body mass. Successful foraging during chick rearing, the period when adult foraging is most intense, appears to depend on the proximity of pack ice to nesting colonies for this penguin species. Received: 1 October 1997 / Accepted: 25 April 1998     

How many can you catch? Factors influencing the occurrence of multi-prey loading in provisioning Greater Crested Terns§

Seabirds use several methods to transport food to their chicks; most species carry food in their stomachs or crops, but some terns and auks carry prey in their bills. Terns usually only carry one prey item at a time, limiting the rate at which they can provision their chicks, and restricting their effective foraging range. However, some terns do occasionally carry multiple prey, which should offer a selective advantage, but there are very few studies investigating the factors influencing the occurrence of multi-prey loading. We investigated the occurrence of multi-prey loads in provisioning Greater Crested Terns (Swift Tern) Thalasseus bergii bergii breeding on Robben Island, South Africa. Of 24 173 loads photographed, 1.3% comprised multiple prey items. Up to 11 fish were carried at once, but most multi-prey loads contained two Anchovies Engraulis encrasicolus, the most common prey item for this population of terns. Mixed species prey were recorded for the first time in a tern. Multi-prey loads occurred more frequently during mid- and late-provisioning, presumably because large chicks can cope with multiple prey, and have higher energetic requirements than small chicks. Mean standard length of Anchovies in multi-prey prey loads was less than Anchovies in single loads, possibly suggesting terns compensate for smaller prey sizes by bringing multiple prey back to their chick. The orientation of multiple Anchovies in a tern’s bill tended to be the same, suggesting that they were captured from polarised fish schools. At least some multi-prey loads were caught in a single dive.