Sexual,Seasonal and Group Size Differences in the Allocation of Time between Vigilance and Feeding in the Greater Rhea,Rhea americana

We studied the effect of sex and group size on the proportion of time a greater rhea, Rhea americana, allocates to vigilance and feeding during the breeding and the non-breeding seasons. We analysed 175 records of focal animals that were feeding alone or in groups of 2 to 26 birds. In both seasons, males spent more time in vigilance and less time in feeding than females. Both sexes spent more time in vigilance and less time in feeding during the breeding season. Sexual and seasonal differences in vigilance were the result of different mechanisms. Males had shorter feeding bouts than females but there were no sexual differences in the length of the vigilance bouts. On the contrary, seasonal differences were the result of males and females having longer vigilance bouts during the breeding season but there were no seasonal differences in the length of the feeding bouts. During the non-breeding season, individual vigilance was higher in rheas foraging alone than in groups. In this case, solitary birds had longer vigilance and shorter feeding bouts than birds foraging in groups. We discuss the possible effect of intragroup competition and food availability on the allocation of time between feeding and vigilance in this species.     

Diving pattern and performance in relation to foraging ecology in the gentoo penguin, Pygoscelis papua

We present data on diving pattern and performance (dive depth, duration, frequency and organization during the foraging trip) in gentoo penguins Pygoscelis papua , obtained using time-depth recorders ( n = 9 birds, 99 foraging trips). These data are used to estimate various parameters of foraging activity, e.g. foraging range, prey capture rates, and are compared in relation to breeding chronology. Foraging trip duration was 6 h and 10 h, and trip frequency 1.0/day and 0.96/day, during the brooding and creche periods, respectively. Birds spent on average 52%of each foraging trip diving. Dive depth and duration were highly bimodal: shallow dives (< 21 m) averaged 4 m and 0.23 min, and deep dives (> 30 m) 80 m and 2.5 min, respectively. Birds spent on average 71%and 25%of total diving time in deep and shallow dives, respectively. For deep dives, dive duration exceeded the subsequent surface interval, but shallow dives were followed by surface intervals 2–3 times dive duration. We suggest that most shallow dives are searching/exploratory dives and most deep dives are feeding dives. Deep dives showed clear diel patterns averaging 40 m at dawn and dusk and 80–90 m at midday. Estimated foraging ranges were 2.3 km and 4.1 km during the brood and creche period, respectively. Foraging trip duration increased by 4 h between the brood and creche periods but total time spent in deep dives (i.e. time spent feeding) was the same (3 h). Of 99 foraging trips, 56%consisted of only one dive bout and 44%of 2–4 bouts delimited by extended surface intervals > 10 min. We suggest that this pattern of diving activity reflects variation in spatial distribution of prey rather than the effect of physiological constraints on diving ability.     

Good foragers can also be good at detecting predators

The degree to which foraging and vigilance are mutually exclusive is crucial to understanding the management of the predation and starvation risk trade-off in animals. We tested whether wild-caught captive chaffinches that feed at a higher rate do so at the expense of their speed in responding to a model sparrowhawk flying nearby, and whether consistently good foragers will therefore tend to respond more slowly on average. First, we confirmed that the time taken to respond to the approaching predator depended on the rate of scanning: as head-up rate increased so chaffinches responded more quickly. However, against predictions, as peck rate increased so head-up rate increased and mean length of head-up and head-down periods decreased. Head-up rate was probably dependent on peck rate because almost every time a seed was found, a bird raised its head to handle it. Therefore chaffinches with higher peck rates responded more quickly. Individual chaffinches showed consistent durations of both their head-down and head-up periods and, therefore, individuals that were good foragers were also good detectors of predators. In relation to the broad range of species that have a similar foraging mode to chaffinches, our results have two major implications for predation/starvation risk trade-offs: (i) feeding rate can determine vigilance scanning patterns; and (ii) the best foragers can also be the best at detecting predators. We discuss how our results can be explained in mechanistic terms relating to fundamental differences in how the probabilities of detecting food rather than a predator are affected by time. In addition, our results offer a plausible explanation for the widely observed effect that vigilance continues to decline with group size even when there is no further benefit to reducing vigilance.     

An Experimental Analysis of Spatial Position Effects on Foraging and Vigilance in Brown-Headed Cowbird flocks

Several observational studies have found that the costs and benefits of social foraging vary as a function of spatial position in the group. However, it is difficult to make mechanistic inferences because several confounding factors, such as food deprivation levels, food availability, neighbor distance, and group size can mask or amplify spatial position effects. We attempted to address experimentally the effect of spatial position on foraging and vigilance in a group, controlling for many confounding factors. We used enclosures that restricted physical but not visual interactions between brown-headed cowbirds and manipulated spatial position, flock size, and neighbor distance. Pecking rate (number of pecks per trial duration) was not related with position, but instantaneous pecking rate (number of pecks per foraging bout duration) was higher at the edge. The proportion of time spent head-up (scanning and food-handling) was also higher at the edge. For pecking rate and proportion of time spent scanning, changes in neighbor distance influenced the behavior of edge birds to a lesser extent than central birds. These results suggest that cowbirds at the edge perceived greater predation risk and that during the limited foraging time available, edge birds tried to compensate by foraging at a faster rate.     

Effect of Group Size on Individual and Collective Vigilance in Greater Rheas

We studied the effect of group size on the proportion of time that greater rheas, Rhea americana, allocated to vigilance and feeding during the non‐breeding season. We tested whether: (1) the proportion of time that one bird allocates to vigilance (individual vigilance) decreases with group size, and (2) the proportion of time that at least one bird of the group is vigilant (collective vigilance) increases with group size. We analyzed video‐recordings of birds that were foraging alone or in groups from two to 12 birds. The proportion of time allocated to individual vigilance decreased and the proportion of time spent feeding increased with group size. In both cases the main significant difference was between birds foraging alone and in groups. Collective vigilance did not vary with group size and it was lower than expected if vigilance bouts were random or sequential. Our results indicate that rheas foraging in large groups would not receive the benefit of an increase in collective vigilance, although they could still benefit from a reduction of predation risk by the dilution effect.     

Foraging methods can affect patch choice: an experimental study in Mallard (Anas platyrhynchos)

Animals can adapt to changes in feeding conditions by switching between foraging methods. Dabbling ducks use different foraging methods, including dabbling in deep water with the head and neck submerged, and grubbing in the mud (or shallow water) where the eyes are above the surface, so the bird can visually monitor its environment while foraging. Deep foraging is considered to provide lower intake rates and to have high associated costs, such as predation risk, compared to shallow foraging. Ducks should thus prefer shallow foraging and switch to deeper methods when feeding conditions deteriorate. We conducted a set of experiments with Mallard to assess the importance of intake rate as a cue to choose between patches associated with different foraging methods, and evaluate the influence of food depletion on the decision to switch between methods. When 50 g of wheat were presented in two patches, one at a depth of 5 cm and one at 35 cm, most of the foraging was in the shallow area. Reducing food abundance to 10 g in the shallow area led to an increase in deep foraging, although the birds still preferred the shallow area at the beginning of the tests despite the fact that it did not provide a higher intake rate. This area was used until complete depletion, and birds did not turn to deep foraging before ensuring that the shallow patch was empty. These results show that food depletion affects the choice between feeding patches hence foraging method. However the value of intake rate is not the main cue for decision, rather the birds appear to choose between patches with different methods on account of their respective costs.     

The effect of habitat complexity on the functional response of a seed-eating passerine

Recent population declines of seed-eating farmland birds have been associated with reduced overwinter survival due to reductions in food supply. An important component of predicting how food shortages will affect animal populations is to measure the functional response, i.e. the relationship between food density and feeding rate, over the range of environmental conditions experienced by foraging animals. Crop stubble fields are an important foraging habitat for many species of seed-eating farmland bird. However, some important questions remain regarding farmland bird foraging behaviour in this habitat, and in particular the effect of stubble on farmland bird functional responses is unknown. We measured the functional responses of a seed-eating passerine, the Chaffinch Fringilla coelebs , consuming seeds placed on the substrate surface in three different treatments: bare soil, low density stubble and high density stubble. Stubble presence significantly reduced feeding rates, but there was no significant difference between the two stubble treatments. Stubble reduced feeding rates by reducing the maximum attack distance, i.e. the distance over which an individual food item is targeted and consumed. The searching speed, handling time per seed, proportion of time spent vigilant, duration of vigilance bouts and duration of head-down search periods were unaffected by the presence of stubble. The frequency of vigilance bouts was higher in the bare soil treatment, but this is likely to be a consequence of the increased feeding rate. We show the influence of a key habitat type on the functional response of a seed-eating passerine, and discuss the consequences of this for farmland bird conservation.     

Visual perception and social foraging in birds

Birds gather information about their environment mainly through vision by scanning their surroundings. Many prevalent models of social foraging assume that foraging and scanning are mutually exclusive. Although this assumption is valid for birds with narrow visual fields, these models have also been applied to species with wide fields. In fact, available models do not make precise predictions for birds with large visual fields, in which the head-up, head-down dichotomy is not accurate and, moreover, do not consider the effects of detection distance and limited attention. Studies of how different types of visual information are acquired as a function of body posture and of how information flows within flocks offer new insights into the costs and benefits of living in groups.     

Time budgets of Nuthatches Sitta europaea with supplementary food

Individual time budgets during winter and spring were studied in a population of Nuthatches Sitta europaea in south-central Sweden. In late winter birds spent about 90% of daytime foraging, males feeding somewhat less than females, possibly due to male dominance. In early spring, foraging time decreased to about 65% of daytime activity, and sex-related differences became more pronounced. Males then increased time spent resting and singing. Females were nestbuilding, spent less time resting and also devoted more time to foraging than did males. Prior to the observation periods, food density for part of the population was experimentally increased by supplying sunflower seed. In spite of this, time budgets in the experimental group were similar to those of the control group. It is possible that the intensity of foraging, rather than actual time devoted to it, was influenced by food density and that, as a result, birds in the experimental group were able to maintain a higher level of vigilance. Also, an increase in food supply may be less important to a hoarding bird such as the Nuthatch. However, the result remains that Nuthatch time budgets in winter and spring are not readily influenced by an increase in food density.     

Satellite tracking of male rockhopper penguins Eudyptes chrysocome during the incubation period at the Falkland Islands

The foraging patterns of ten male rockhopper penguins Eudyptes chrysocome from the Falkland Islands were recorded during the incubation period by using satellite telemetry. Irrespective of study site and year, two different foraging areas could be identified. Three foraging trips were directed towards the slope of the Patagonian Shelf c. 140 km to the northeast of the breeding colony and these trips had a duration of 11–15 days. The seven other rockhopper penguins travelled c. 400 km towards the edge of the Falkland Islands' waters; all these foraging trips followed an anti-clockwise direction and lasted 16–27 days. The calculated mean daily travelling speed, based on the time spent underwater and the distance covered between two positions, was significantly higher in birds travelling to the edge of the Falkland Islands' waters compared with those foraging at the shelf slope (4.4±1.6 km/h vs. 3.4±2.0 km/h, respectively). The consistent foraging patterns exhibited during the long trips may be linked to the Falklands current, allowing the penguins to reach remote areas while reducing their energy expenditure. Potential interactions between commercial fisheries and hydrocarbon exploration are discussed.     

鄱阳湖六种野鸭越冬行为及觅食策略

2019年10月-2020年1月和2020年10月-2021年1月,采用瞬时扫描法和焦点动物法对鄱阳湖区6种野鸭越冬行为和觅食策略进行了观察.结果 表明,6种野鸭最多的4种行为均为取食、休息、修整和运动.绿头鸭(Anas platyrhynchos)、赤麻鸭(Tadorna ferruginea)和针尾鸭(Anas a...     

Mallards Feed Longer to Maintain Intake Rate under Competition on a Natural Food Distribution

Animals foraging in groups may benefit from a faster detection of food and predators, but competition by conspecifics may reduce intake rate. Competition may also alter the foraging behaviour of individuals, which can be influenced by dominance status and the way food is distributed over the environment. Many studies measuring the effects of competition and dominance status have been conducted on a uniform or highly clumped food distribution, while in reality prey distributions are often in‐between these two extremes. The few studies that used a more natural food distribution only detected subtle effects of interference and dominance. We therefore conducted an experiment on a natural food distribution with focal mallards Anas platyrhynchos foraging alone and in a group of three, having a dominant, intermediate or subordinate dominance status. In this way, the foraging behaviour of the same individual in different treatments could be compared, and the effect of dominance was tested independently of individual identity. The experiment was balanced using a 4 × 4 Latin square design, with four focal and six non‐focal birds. Individuals in a group achieved a similar intake rate (i.e. number of consumed seeds divided by trial length) as when foraging alone, because of an increase in the proportion of time feeding (albeit not significant for subordinate birds). Patch residence time and the number of different patches visited did not differ when birds were foraging alone or in a group. Besides some agonistic interactions, no differences in foraging behaviour between dominant, intermediate and subordinate birds were measured in group trials. Possibly group‐foraging birds increased their feeding time because there was less need for vigilance or because they increased foraging intensity to compensate for competition. This study underlines that a higher competitor density does not necessarily lead to a lower intake rate, irrespective of dominance status.     

安徽沿江浅水湖泊越冬水鸟群落的集团结构

长江中下游湖泊是越冬水鸟的重要栖息地,随着湖泊渔业养殖强度的不断加大,湖泊湿地严重退化,水鸟的越冬生态受到影响。为揭示长江中下游浅水湖泊越冬水鸟对湿地资源的利用特征,2008年12月至2009年3月,通过扫描取样法采集安徽省长江沿江升金湖、菜子湖和武昌湖3个浅水湖泊30种越冬水鸟的取食行为百分比数据,利用聚类分析法对越冬水鸟进行集团划分,并采用无倾向对应法(DCA)分析越冬水鸟的取食特征。聚类分析结果表明,安庆沿江湖泊越冬水鸟群落可分为4个集团,即深水取食集团G1、挖掘和啄取集团G2、浅水取食集团G3和泥滩拾取集团G4。G2集团的鸟种最多,共有13种,优势种为鸿雁(Anser cygnoides)、豆雁(Anser fabalis);G3集团次之,共6种,优势种为小天鹅(Cygnus columbianus)、白琵鹭(Platalea leucorodia);G4集团共5种,优势种为黑腹滨鹬(Calidris alpina)、鹤鹬(Tringa erythropus)和红脚鹬(Tringa totanus);G1集团水鸟种类有6种。这些水鸟的觅食生境主要在湖泊滩涂和浅水区域,其食物资源的可利用性和觅食对策共同决定群落组成结构。DCA分析表明,取食方式及取食时运动方式组成的觅食对策决定了集团食物资源的分割,草滩中取食鸟类主要采用静止取食和啄取方式,泥滩取食集团主要采取拾取及奔-停取食,深水区取食集团则主要采用潜水方式取食,因此,维持湖泊不同区域的丰富食物资源对于保护湖泊丰富的水鸟资源具有重要意义。     

Diving depths of Shags Phalacrocorax aristotelis breeding on the Isle of May

Information on maximum dive depth and the time spent at various depths was obtained from 49 Shags Phalacrocorax aristotelis. On average, the maximum dive depth was 33–35 m; the overall maximum was 43 m. Shags dived repeatedly to the same depth and spent c. 55% of the time between 25 and 34 m which indicated that they were foraging close to the seabed. About 46% of the time underwater was spent foraging and 54% travelling. Average underwater swimming speed was 1.7-1.9 m per second.     

The effects of predation risk on the use of social foraging tactics

The effects of predation on the use of social foraging tactics, such as producing and scrounging, are poorly known in animals. On the one hand, recent theoretical models predict increased use of scrounging with increasing predation risk, when scroungers seeking feeding opportunities also have a higher chance of detecting predators. On the other hand, there may be no relation between tactic use and predation when antipredator vigilance is not compatible with scanning flockmates. We investigated experimentally the effects of predation risk on social foraging tactic use in tree sparrows, Passer montanus. We manipulated predation risk in the field by changing the distance between shelter and a feeder. Birds visited the feeder in smaller flocks, spent less time on it and were somewhat more vigilant far from shelter than close to it. Increased predation risk strongly affected the social foraging tactic used: birds used the scrounger tactic 30% more often far from cover than close to it. Between-flock variability in scrounging frequency was not related to the average vigilance level of the flock members, and within-flock variability in the use of scrounging was negatively related to the vigilance of birds. Our results suggest that in tree sparrows, the increased frequency of scrounging during high predation risk cannot simply be explained by an additional advantage of increasing antipredator vigilance. We propose alternative mechanisms (e.g. increased stochasticity in food supply, and that riskier places are used by individuals with lower reserves) that may explain increased scrounging when animals forage under high predation risk.     

The group size effect and synchronization of vigilance in the Tibetan wild ass

Vigilance behavior is considered as an effective strategy for prey species to detect predators.An individual benefits from living in a group by reducing the time spent being vigilant without affecting the probability of detecting a predator.However,the mechanism producing a decrease in vigilance with increasing group size is unclear.Many models of vigilance assume that group members scan independently of one another.Yet in recent studies,the other 2 patterns of vigilance,coordination and synchronization,were reported in some species.In 2 summers(2018 and 2019),we studied the group-size effect on vigilance and foraging of Tibetan wild ass in Chang Tang Nature Reserve of Tibet.We also tested whether individuals scan the environment independently,tend to coordinate their scans,or tend to synchronize their vigilance.The results showed that individuals decreased the time spent on vigilance with increasing group size,while increased the time spent foraging.Group members scanned the environment at the same time more frequently and there was a positive correlation between group members'behaviors,indicating that Tibetan wild asses tend to synchronize their vigilance.     

Daily routines and predator encounters in Yellowhammers Emberiza citrinella in the field during winter

Diurnal and seasonal variation in foraging behaviour of Yellowhammers Emberiza citrinella and activity of their avian predators were studied in an unmanipulated winter field situation around Uppsala, Sweden. In December, when time available for foraging was lowest, Yellowhammers seemed to be time-stressed. In order to meet their energetic needs, they reduced the time allocated to vigilance and increased the time allocated to foraging. Yellowhammers did not systematically change the time spent foraging during the day in December, which indicates time-stress, while they decreased the time spent foraging during the day in both November and February. Predator activity was highest in the afternoon, when Yellowhammers spent the least time foraging. Yellowhammers may have adopted a routine with decreasing foraging rates over the day in November and February, when time available for foraging was longer, in order to avoid foraging during periods of high predator activity. The diurnal activity pattern of predators together with daylength and energetic needs are factors that might be important in shaping daily foraging routines in small birds.     

Detecting predators and locating competitors while foraging: an experimental study of a medium-sized herbivore in an African savanna

Vigilance allows individuals to escape from predators, but it also reduces time for other activities which determine fitness, in particular resource acquisition. The principles determining how prey trade time between the detection of predators and food acquisition are not fully understood, particularly in herbivores because of many potential confounding factors (such as group size), and the ability of these animals to be vigilant while handling food. We designed a fertilization experiment to manipulate the quality of resources, and compared awareness (distinguishing apprehensive foraging and vigilance) of wild impalas (Aepyceros melampus) foraging on patches of different grass height and quality in a wilderness area with a full community of predators. While handling food, these animals can allocate time to other functions. The impalas were aware of their environment less often when on good food patches and when the grass was short. The animals spent more time in apprehensive foraging when grass was tall, and no other variable affected apprehensive behavior. The probability of exhibiting a vigilance posture decreased with group size. The interaction between grass height and patch enrichment also affected the time spent in vigilance, suggesting that resource quality was the main driver when visibility is good, and the risk of predation the main driver when the risk is high. We discuss various possible mechanisms underlying the perception of predation risk: foraging strategy, opportunities for scrounging, and inter-individual interference. Overall, this experiment shows that improving patch quality modifies the trade-off between vigilance and foraging in favor of feeding, but vigilance remains ultimately driven by the visibility of predators by foragers within their feeding patches.     

Antipredator vigilance in birds: modelling the 'edge' effect

Many animals spend a large proportion of their time either foraging for food or watching out for predators (antipredator vigilance). There have been many theoretical and empirical studies investigating the trade-off between these two activities, especially in birds. Previous models of antipredator vigilance assume that all birds within the group spend the same amount of time feeding. However, many empirical studies have shown that individuals on the edge of flocks are more vigilant. Here we describe a vigilance model which investigates the effect of position on the birds' strategies by dividing the feeding area into an inner and outer region. The model examines how various parameters such as food availability and predation risk affect the strategies of individual birds according to whether they are in the inner or outer region. Our model predicts that birds in the outer group are always more vigilant than those in the inner region. Birds in the centre receive a higher payoff in each of the wide range of scenarios that we have considered, and so our model also indicates why dominant birds would choose to feed in the centre of the group; a prediction in accord with several empirical studies.     

The advantages of social foraging in downy woodpeckers

I investigated the advantages gained by downy woodpeckers (Picoides pubescens) which join mixed-species winter flocks. Woodpeckers foraging alone showed high levels of vigilance as measured by head-cocking rates, and low feeding rates. Woodpeckers foraging with one or two flock members showed intermediate rates of head-cocking and feeding, while woodpeckers foraging with flocks of three or more birds showed low head-cocking rates and high feeding rates. Although local enhancement and copying may contribute to the woodpeckers' increased foraging efficiency in a flock, these do not appear to be the main factors. As downy woodpeckers spend less time on vigilance, they devote more time to foraging, thereby increasing their foraging efficiency