Feeding Flocks Research Articles

Foraging Area Fidelity of Individual Seabirds in Relation to Tidal Cycles and Flock Feeding

I studied the foraging behavior of Black-legged Kittiwakes (Rissa tridactyla) using radio transmitters, remote receiving stations, and a boat to track individuals. I tested the hypothesis that individual birds exhibit foraging area fidelity. I also investigated evidence for information transfer among birds and the role of tidal cycles in foraging behavior. Flight speed, feeding methods, chick growth rates, and fledging success were similar for radio-tagged birds and birds without radio-tags, suggesting that radio-tags had no significant effect. Birds made an average of two foraging trips per day for 13 d, during a spring and a neap tide series. Twenty-four of 26 birds returned repeatedly (X = 19.2 trips) to particular areas to feed; these birds were considered faithful to a foraging area. During spring and neap tides, 17 and 14 birds, respectively, timed their foraging trips to coincide with daily tidal cycles. Foraging area fidelity indicates that kittiwakes learn and remember when and where to forage and therefore may gain local knowledge of their foraging area. Individuals possessing such knowledge may have an advantage over individuals without local knowledge. Foraging area fidelity in individuals indicates that information transfer among colonial seabirds may be less important than previously expected. Birds were selective in joining feeding flocks (13 of 35 flocks were joined). Those that were joined most often were large feeding flocks that occurred in the birds’ preferred feeding areas. Birds often fed alone (n = 26 observations) rather than with feeding flocks (n = 9 observations). Individuals of a mated pair did not forage in the same area, suggesting that information transfer regarding foraging did not occur between mates. Some mates were unable to coordinate their tidally timed foraging with nesting activities, and chicks were left unattended, which may have reduced the number of chicks fledged. Such gaps in attendance may reflect why birds such as kittiwakes have delayed maturation and poor reproductive success during the first few years of breeding.

Hydrological Constraints on Tricolored Heron and Snowy Egret Resource Use

We investigated resource use by breeding Tricolored Herons (Egretta tricolor) and Snowy Egrets (E. thula) at a regional scale (1,312 km2) during three years (1987-1989) with dissimilar water levels. We documented the direction and distance flown from the colony to feeding sites, habitat selection, daily movement patterns, feeding flock size, and diet at the Rodgers River Bay colony in Everglades National Park, Florida. The general pattern of flight direction was similar between the two species; both species flew northwest in 1987, a wet year, and northeast in 1988, a year with moderate surface water drying. During drought conditions in 1989, only Tricolored Herons nested successfully. Despite similarities in flight direction, habitat selection differed between the two species. Tricolored Herons were more consistent in habitat use among years, whereas Snowy Egret habitat selection was more variable. However, shifts in habitat selection in response to varying hydrologic conditions among years were greater than differences in habitat selection between the two species within a year. Prey selection was similar for the two species in 1987, indicating that they procured similar prey from different habitat types. Regardless of water conditions, Snowy Egrets were significantly more gregarious than Tricolored Herons, and feeding sites on consecutive days by radio-tagged Snowy Egrets were farther apart than those used by Tricolored Herons. Our results suggest that habitat selection occurs at a coarser scale than previously recognized and that studies conducted at single sites over short temporal scales may be ignoring important aspects of habitat selection.

Does father know best? Social learning from kin and non-kin in juvenile ringdoves

Parent-offspring transmission is usually thought to be the major route for cultural learning. We tested this assumption in ringdoves ( Streptopelia risoria), a flock feeding Columbid that is easily raised in captivity. An aviary study first determined the foraging associations of juveniles placed with their sibling, their parents and a pair of non-kin adults. Juveniles foraged more often with kin than with non-kin and joined food discoveries in proportion to these foraging associations; aggression was relatively rare and came more often from unrelated adults than from parents. Two cage experiments showed that parents were not copied more often than unrelated adults when the two tutor types provided different, but equally productive, solutions to a feeding problem. Neither the ingestion of unfamiliar food (two seed types the juveniles had never encountered) nor the learning of a new food searching skill (opening a box containing seed) showed a differential effect of father versus non-kin tutors. Animals that scramble compete for food may thus acquire social information from whatever knowledgeable individuals are present, whether these are kin, unrelated conspecifics or heterospecifics.

SEABIRD COMMUNITY STRUCTURE ALONG A PRODUCTIVITY GRADIENT: IMPORTANCE OF COMPETITION AND ENERGETIC CONSTRAINT

The oceanic eastern tropical Pacific supports a speciose seabird community that feeds in flocks and depends for food on schools of tunas and dolphins, which force prey to the surface. We analyzed data collected throughout an area of 40 3 10 6 km 2 of open ocean over 10 years (1979-1988) during 1136 at-sea days, from 1750 feeding flocks comprising 51 688 birds of 49 species. A PCA identified three distinct flock types based on species composition: ''Sooty Tern Flocks'' ( n 5 941 flocks) with a large number of Sooty Terns (Sterna fuscata), ''Juan-Wedge Flocks'' ( n 5 402 flocks) with a large number of Juan Fernandez Petrels (Pterodroma externa) and Wedge-tailed Shearwaters (Puffinus pacificus ), and ''Booby Flocks'' ( n 5 407 flocks) with a large number of Red-footed and Masked Boobies (Sula sula and S. dactylatra, respectively). Flock types exhibited largely disjunct distributions that mirrored patterns in thermocline depth and surface water chlo- rophyll content. Sooty Tern Flocks occurred in areas with deepest thermocline (77.6 m) and lowest chlorophyll (0.14 mg/m 3 ), Juan-Wedge Flocks in areas of intermediate ther- mocline (68.8 m) and chlorophyll (0.16 mg/m 3 ), and Booby Flocks in areas of shallowest thermocline (62.5 m) and highest chlorophyll (0.17 mg/m 3 ). These differences were sta- tistically significant. As thermocline depth and surface water chlorophyll are reliable in- dicators of surface water productivity, we conclude that Sooty Tern Flocks foraged in waters of low productivity, Juan-Wedge Flocks in waters of intermediate productivity, and Booby Flocks in waters of highest productivity. Differences in published cost-of-flight values support the hypothesis that energetic con- straint determines flock structure at the low-productivity end of the system. Sooty Terns have the lowest flight costs (4.8 W) and feed in waters of lowest productivity. Flight costs are progressively higher for those species feeding in more productive waters (9.9 W for Wedge-tailed Shearwaters, 19.0 W for Red-footed Boobies). Differences in body size sup- port the hypothesis that interference competition determines flock structure at the high- productivity end of the system. The largest species, Masked Boobies (1987 g) and Red- footed Boobies (1003 g), feed in areas of highest productivity; progressively smaller species, Juan Fernandez Petrels (430 g) and Wedge-tailed Shearwaters (388 g), and Sooty Terns (180 g), feed in areas of decreasing productivity. We hypothesize that this community is largely structured by two factors, competition and energetic constraint, that operate at opposite ends of a productivity gradient. As productivity decreases (low prey abundance), flight proficiency is increasingly important because only species with relatively low flight costs will be able to transit between prey patches. As productivity increases (high prey abundance), competitive ability is increasingly important because competitive dominants will exclude other species from feeding flocks. Our hypothesis is in accordance with em- pirical data and theoretical models designed to explain factors structuring communities for a wide variety of habitats and taxa.

Where and When To Feed: Sex and Experience Affect Access To Food in Wintering Snow Buntings

AbstractIn group-foraging species an individual's ability to feed in the most beneficial situations could influence its overall foraging success more than its ability to find and handle food. Here we examined whether sex, age or site experience of individual snow buntings Plectrophenax nivalis in winter flocks could help explain how frequently they were found in advantageous foraging situations. We found that inexperienced females were scarcer than expected in large flocks, that females fed less often in the early parts of flock feeding bouts, and that older and more experienced birds were more likely to feed in central flock positions. In addition, the likelihood of moving around the feeding station increased more with flock-size in females, while older birds were better at retaining access to the feeding patch. Previously reported differences in peck-rate between sex and age/experience categories of this species are therefore likely to be magnified by differential access to sources of food. We suggest that experience may help individuals to integrate within the feeding flock, but aggression from larger males may cause females to delay feeding or to feed in smaller flocks.

Undirected Song in Wild Zebra Finch Flocks: Contexts and Effects of Mate Removal

AbstractWild zebra finches, Taeniopygia guttata, sing frequently throughout the year but the function of undirected song, the most prevalent type, is unknown. Paired males commonly sang undirected song in feeding flocks where it occurred more frequently in the non‐breeding season than in the breeding season. Song rate varied greatly among males. The context of the singer and the absence of any overt responses from flock members excludes direct aggressive and mate‐attracting functions. Performance of undirected song may have energetic costs and predatory risks. The finding that singers are active individuals with ‘spare time’, that are neither hungry, nor tired, nor sexually motivated, suggests that singers are advertising their ‘quality’ as potential mates and/or participants for extra‐pair copulations. When the female partner in non‐breeding pairs was experimentally removed, the rate of undirected song of the ‘widower’ increased significantly. Males quickly re‐paired. This experiment supports the hypothesis that singing in the flock has a mate‐attracting role.

Roost Departure by European Starlings Sturnus vulgaris: Effects of Competition and Choice of Feeding Site

Starlings leave their winter roosts each morning in a series of exoduses so that the process of departure is prolonged, taking up to 50 min. Assuming that Starlings attempt to maximise the time available for feeding on short winter days, we predicted that: (1) birds that feed far from the roost would leave earlier than birds that feed nearby, and (2) birds that are less efficient at foraging or are competitively inferior should either leave before, or follow, better birds, and should feed closer to the roost. We tested these predictions by radio-tracking birds as they left roosts and while feeding at farms to determine when these individuals left and where they fed. We also determined the ages and sexes of Starlings from different exoduses, and assessed the social structure of feeding flocks at farms close to and far from the roost. Starlings that fed further from a roost did not leave earlier than birds that fed nearby, and first-year females left later than adult males but did not follow them. Farms differed significantly in the age structure of their feeding Starlings; some had proportionately more adults whilst others had proportionately more first-year birds. These data failed to support our predictions. In order to reconcile the observed departure pattern with the heterogeneous population structure at feeding sites we propose that dominant, usually adult birds, depart the roost first and attain the best feeding sites, and that when the subordinate younger birds overfly these sites later they can gauge the potential level of competition and decide whether to stop or fly on.

Dispersal and Association among Common Ravens

We examined the dispersal and gregariousness of 10 ravens from one feeding flock through daily and nocturnal monitoring for 70 days and nights over an area of approximately 5,000 km2. One of the birds took a mate and established residency within this area. Another passed through the area on infrequent intervals having an apparently much larger range. Two stayed only a day or less. All but one bird wandered widely, taking up residency lasting for one to several weeks in ranges from 190-3, 100 km2. The 10 birds from the feeding flock did not stay together, although up to two occasionally overlapped at nocturnal roosts. The birds roosted and dispersed independently of one another with no indication of any "flock" cohesiveness. There was a great variation of response from "vagrants" to "residents", with one grading into the other through temporary residences and periods of wandering.

Time budget variation in relation to flock size in carrion crows, Corvus corone corone

Abstract. Time budgets of carrion crows feeding in flocks of variable size were recorded in a field experiment. Variation in surveillance, successful and unsuccessful pecking, walking and aggression was analysed in relation to flock size while controlling for the effects of confounding variables (time of day, food quality and density, temperature, observer). Time devoted to surveillance and surveillance rate decreased non-linearly with increasing flock size, while time elapsing between two successive surveillance episodes increased linearly with flock size. Surveillance behaviour did not vary with food density. Surveillance was significantly higher when a predator was flying over the flock or had just disappeared than for apparently undisturbed crows. Feeding rates reached a maximum in flocks of 10 crows, decreased in larger flocks and increased monotonically with food density. The number of aggressive encounters/min was positively correlated with flock size but was independent of food density. In the range of flock sizes considered (1-17 birds), crows benefited from being in a flock in terms of both increased feeding efficiency and, presumably, increased surveillance in the flock. It is also suggested that the increase in bird density with flock size, rather than the increase in flock size per se, was responsible for higher levels of aggression in large flocks.

A test of the producer-scrounger foraging game in captive flocks of spice finches, Loncbura punctulata

Group foraging allows the co-existence of a strategy (producer) that involves searching for food, and its alternative (scrounger) exploiting the food of the producer. The use of producer and scrounger strategies has been modelled as an alternative-option scramble which assumes strong negative frequency-dependence of the scrounger's pay-offs. We tested this assumption in a flock feeding situation by manipulating the proportion of scroungers in flocks of spice finches, Lonchura punctulata. In a first experiment we found that: (1) the food intake of scroungers, and to a lesser extent producers, was negatively affected by an increase in the proportion of scroungers; (2) the food intake of producers and scroungers was equal when the proportion of scroungers was small, suggesting that producers, who exploited 35.4% of their patches by scrounging were opportunistically adjusting their use of the strategies until the pay-offs equalized. In a second experiment we tested whether finches could vary their use of the two strategies in response to changes in foraging conditions brought about by an increase in the cost of producing. As predicted by the game, finches reduced their use of the producer strategy and increased their use of the scrounger strategy when the cost of producing increased. These results suggest that spice finches can alter their allocation to each foraging alternative by experience and that the producer-scrounger game is a realistic model for predicting group foraging decisions.

Population Variabilities of Bird Guilds in Kansas during Fall and Winter: Weekly Censuses versus Christmas Bird Counts

Fall and winter (September through March) weekly census data from 1978 through 1992 were examined for 18 species of birds in forests and cultivated fields near Manhattan, Kansas. The 18 species were grouped into five feeding guilds: frugivores; granivores on the ground; acorn cachers; leaf, bud, and twig gleaners; and trunk and branch gleaners. Frugivores exhibited the most variable populations, both within years and between consecutive years. Granivores on the ground exhibited the second most within-year population variability. Among the remaining guilds, there were no statistically significant differences among either the within- or between-year variabilities. Although data were limited, there were significant correlations between the mean numbers in some species of frugivores, granivores, and acorn cachers and relative annual fruit or seed crop size in the area. The size and mobility of feeding flocks, the annual variability of food crops, and the manner of food exploitation all influenced the between-year variability of winter bird species population density. The rank order of between-year variability for species populations censused on Christmas Bird Counts in the same area correlated significantly with between-year variability of our weekly censuses. The within-year nomadic tendencies of some of the species with the greatest between-year variability decreased the correlation between Christmas Bird Counts and mean weekly censuses for these species.

Age, Habitat and Tide Effects on Feeding Activity of Emperor Geese during Autumn Migration

I studied feeding activity of Emperor Geese (Chen canagica) on the Alaska Peninsula during autumn migration, 1991. Scan samples were used to estimate the proportion of birds feeding in flocks as a measure of feeding intensity. Most geese fed during low tides and roosted during high tides. However, flocks with disproportionately more juveniles continued to feed during high tides in either blue mussel (Mytilus edulis) beds (during relatively low high tides) or in vegetated habitats. Feeding intensity was higher in mussel habitats than in mud/sand or vegetated habitats, and juveniles fed more than adults. Juvenile geese probably have greater nutritional needs than adults, and feeding during high tide may represent their attempt to satisfy these disproportionate demands. Vegetated habitats may be used when high value bivalve prey are unavailable due to tidal inundation.

Flock feeding and food intake in Little Egrets Egretta garzetta and their effects on food provisioning and reproductive success

Food provisioning to chicks of Little Egret Egretta garzetta in a colony at Ligagneau in the Camargue, southern France, was measured using electronic nest balances. Both pair members supplied food to the chicks, and each performed three to five alternating foraging trips.The loading rate during the first trip of the day (1.09 g/min) was more than three times as high as that during later trips (0.32 g/min) and accounted for 20% of the daily amount of food delivered. This food was collected while the birds were feeding in dense aggregations on mosquitofish which had become temporarily concentrated due to hypoxic conditions overnight in the surrounding marsh. Later in the day, when this concentrated food source was no longer available, the birds fed solitarily.The total amount of food delivered per day remained constant at 385 g as chick age increased from 10 to 20 days. Given the observed loading rates, 385 g is near the maximum amount of food the pair can deliver during a 16.5‐h daylight period when they forage continuously throughout the day but only one parent at a time. This indicates that a time constraint limits the amount of food delivered to a brood during this stage of the breeding cycle. Given this time constraint, the total amount of food delivered per day would have been reduced by 25% if the birds had not been able to benefit from the concentrations of mosquitofish during the early morning.Breeding success was significantly higher at Ligagneau (3.25 chicks/nest) than in other Camargue colonies (2.69 chicks/nest). In addition, post‐fledging survival of chicks with a low rank in the brood hierarchy was probably better at Ligagneau than elsewhere in the Camargue. We attribute this elevated reproductive output at Ligagneau to the exceptionally large amount of food collected by egrets during the early morning aggregations, which were probably formed in response to predictable concentrations of mosquitofish in the permanent marshes surrounding this colony. Such aggregations were rarely observed in the temporary marshes surrounding the other colonies, probably because the occurrence and location of fish concentrations are difficult to predict in this habitat due to rapidly falling water levels.

Foraging dynamics in goose flocks: the cost of living on the edge

The effects of flock position on the foraging performance of individual barnacle geese, Branta leucopsis, were determined by comparing foraging behaviour, vegetation quality and diet of geese at the edge and centre of feeding flocks. Birds on the edge of flocks fed in a hurried fashion compared with centre positions (shorter bouts of foraging) probably because of the increased amount of vigilant behaviour and aggressive encounters. In energetic terms, however, the costs of grazing and vigilance balanced out, so that it was as ‘expensive’ to feed in either flock position. Geese appeared to obtain more food when in edge positions because they pecked faster and obtained more food per peck as a result of the higher biomass of grass. A series of calculations, which included measures of energy expenditure, gross intake and energetic quality of the food, showed that the net energetic payoff of birds in edge positions was substantially higher than that of those in the centre. The relative merits of taking positions in the flock edge for single geese, pairs and families are discussed. The finding that dominant families were most often in edge positions and avoided the centre, and the fact that families attacked neighbours more often than did any other geese, suggests that they actively attempt to monopolize the best food resource available to flock members which is usually on the edges of the flock.

Great tits benefit from feeding in mixed-species flocks: a field experiment

Patch depletion, food intake and aggressive interactions were studied in flocks of great tits, Parus major, and in mixed-species flocks of great tits and marsh tits, P. palustris, exploiting one or two patches. Both male and female great tits had a higher feeding rate in mixed-species flocks than in single-species flocks. Food was depleted more quickly and aggression was lower in mixed flocks. The difference in intake rates between males and females was lower when they were feeding over two patches than on one patch because of the dispersion and consequent reduction in aggressive interactions in the group. Marsh tits and female great tits were more likely than male great tits to be attracted to a neighbouring food patch by another individual. Marsh tits outside the flock were a greater attraction to individuals from the group than were great tits. Individuals appeared to feed less efficiently in two patches as they spent longer there than in one patch of the same size and food density.

A Novel Context for Bird Song: Predator Calls Prompt Male Singing in the Kleptogamous Superb Fairy‐wren, Malurus cyaneus

AbstractThe cooperatively breeding superb fairy‐wren, Malurus cyaneus, sings two types of song; a variable, complex song sung by both sexes, and a relatively invariant, stereotyped song sung only by males. Field observations and playback experiments revealed that the male song could be triggered by a variety of loud avian and non‐avian vocalizations, but was triggered most frequently by the calls of predators or potential predators. The male song is sung both within the territory and in winter feeding flocks outside the territory. Acoustic properties of the song suggest that it is a long‐range signal. A possible explanation for this unusual phenomenon is that the male song has evolved through sexual selection as an honest signal to females of male quality, for the purpose of obtaining extra‐pair copulations.

Observations of Foraging Northern Fulmars (<i>Fulmarus Glacialis</i>) in the Canadian High Arctic

We summarize observations of foraging northern fulmars (Fulmarus glacialis) in the Barrow Strait-Lancaster Sound region of the Northwest Territories from June to September 1984-90 and in Admiralty Inlet, N.W.T., in July 1989. In each year, fulmars scavenged hunter-killed marine mammal remains in the vicinity of Resolute Bay. Large feeding flocks, aggregated primarily along tide lines and at upwelling sites, exploited primarily calanoid copepods by surface seizing and diving. Late-season onshore movement of Arctic cod (Boreogadus saida) resulted in flocks of several thousand fulmars capturing cod by surface and pursuit diving. We determined experimentally that northern fulmars are capable of diving to 3 m to retrieve cod.Key words: northern fulmar, Fulrmarus glacialis, feeding behaviour, diving, Barrow Strait, Lancaster Sound

Flock Feeding and Food Intake in Little Egrets: The Effects of Prey Distribution and Behaviour

SUMMARY (1) Little egrets, Egretta garzetta, from a breeding colony in the Camargue, southern France, tended to feed in dense aggregations during early morning. Later in the day the birds dispersed over the feeding area and foraged alone. (2) Of prey captured in these aggregations, 83% were mosquitofish, as revealed by regurgitates collected from chicks. (3) Capture rate of individuals within an aggregation decreased rapidly with time (-0.22 fish min-2) but the birds were able to improve their food intake by visiting a new site as maximum capture rate, measured directly after the initiation of an aggregation, decreased by only 006 fish min2. So the egrets visited a series of aggregations and, as a result, the average capture rate decreased from 6-6 fish min

Vigilance and feeding behaviour in large feeding flocks of laughing gulls, Larus atricilla, on Delaware Bay

Laughing gulls ( Larus atricilla) forage on horseshoe crab ( Limulus polyphemus) eggs during May in Delaware Bay each year. They feed in dense flocks, and foraging rates vary with vigilance, bird density, number of steps and location in the flock, whereas time devoted to vigilance is explained by number of steps, density, location and feeding rates. The time devoted to vigilance decreases with increasing density, increasing foraging rates and decreasing aggression. Birds foraging on the edge of flocks take fewer pecks and more steps, and devote more time to vigilance than those in the intermediate or central parts of a flock.

The Effect on Winter Wheat of Grazing by Brent Geese Branta bernicla

(1) Brent geese (Branta b. bernicla) fed on fields of winter wheat from November to March, and particularly during December to February. On undisturbed fields the feeding flock increased in size during mid-morning and remained stable until dusk when all the geese departed to the roost site. (2) Grazing of winter wheat led to a loss of c. 750% of the biomass of leaves and shoots of winter wheat. Ungrazed leaves increased in ash and fibre, and decreased in water-soluble carbohydrate content between early and midto late winter. Grazed wheat had a lower ash content and more water-soluble carbohydrates in March than ungrazed wheat. (3) Grazed wheat ripened later than ungrazed wheat. (4) Average grain yield losses were 6, 9 and 100%, respectively, on three fields where traditional scaring devices were deployed, and 7%/o when there was no scaring. Losses were due to either reduced ear density (one field), fewer grains per ear (two fields), smaller grain sizes (three fields), or a combination of these factors. (5) Straw yield was reduced by grazing on one field and the harvest index reduced on two fields. (6) Grazing did not lead to a higher biomass of weeds in the grazed plots. (7) Treading by brent geese caused slight soil compaction (shear strength) on two fields but no reduction in infiltration capacity. An experiment showed that treading, which exceeded that caused by the geese, did not significantly reduce grain yields; defoliation was responsible for the loss.