Probabilistic movement model with emigration simulates movements of deer in Nebraska, 1990–2006

Movements of deer can affect population dynamics, spatial redistribution, and transmission and spread of diseases. Our goal was to model the movement of deer in Nebraska in an attempt to predict the potential for spread of chronic wasting disease (CWD) into eastern Nebraska. We collared and radio-tracked >600 white-tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus) in Nebraska during 1990–2006. We observed large displacements (>10 km) for both species and sexes of deer, including migrations up to 100 km and dispersals up to 50 km. Average distance traveled between successive daily locations was 166 m for male and 173 for female deer in eastern Nebraska, and 427 m for male and 459 for female deer in western Nebraska. Average daily displacement from initial capture point was 10 m for male and 14 m for female deer in eastern Nebraska, and 27 m for male and 28 m for female deer in western Nebraska. We used these data on naturally occurring movements to create and test 6 individual-based models of movement for white-tailed deer and mule deer in Nebraska, including models that incorporated sampling from empirical distributions of movement lengths and turn angles (DIST), correlated random walks (CRW), home point fidelity (FOCUS), shifting home point (SHIFT), probabilistic movement acceptance (MOVE), and probabilistic movement with emigration (MOVEwEMI). We created models in sequence in an attempt to account for the shortcomings of the previous model(s). We used the Kolmogrov–Smirnov goodness-of-fit test to verify improvement of simulated annual displacement distributions to empirical displacement distributions. The best-fit model (D = 0.07 and 0.08 for eastern and western Nebraska, respectively) included a probabilistic movement chance with emigration (MOVEwEMI) and resulted in an optimal daily movement length of 350 m (maximum daily movement length of 2800 m for emigrators) for eastern Nebraska and 370 m (maximum of 2960 m) for western Nebraska. The proportion of deer that moved as emigrators was 0.10 and 0.13 for eastern and western Nebraska, respectively. We propose that the observed spread of CWD may be driven by large movements of a small proportion of deer that help to establish a low prevalence of the disease in areas east of the current endemic area. Our movement models will be used in a larger individual-based simulation of movement, survival, and transmission of CWD to help determine future surveillance and management actions.     

Towards an ecological solution to the folivore paradox: patch depletion as an indicator of within-group scramble competition in red colobus monkeys (Piliocolobus tephrosceles)

A number of socioecological models assume that within-group food competition is either weak or absent among folivorous primates. This assumption is made because their food resources are presumed to be superabundant and evenly dispersed. However, recent evidence increasingly suggests that folivore group size is food-limited, that the primates prefer patchily distributed high-quality foods, and display some of the expected responses to within-group scramble competition. To investigate this apparent contradiction between theoretical models and recent empirical data, we examined the foraging behaviour of red colobus monkeys (Piliocolobus tephrosceles) in Kibale National Park, Uganda. We found that red colobus monkeys foraged in a manner that suggests they deplete patches of preferred foods: intake rate slowed significantly during patch occupancy while movement rate, an index of foraging effort, increased. Furthermore, patch occupancy was related to the size of the feeding group and the size of the patch. These results suggest that within-group scramble competition occurs, may limit folivore group size, and should be considered in models of folivore behavioural ecology.     

All clear? Meerkats attend to contextual information in close calls to coordinate vigilance

Socio-demographic factors, such as group size and their effect on predation vulnerability, have, in addition to intrinsic factors, dominated as explanations when attempting to understand animal vigilance behaviour. It is generally assumed that animals evaluate these external factors visually; however, many socially foraging species adopt a foraging technique that directly compromises the visual system. In these instances, such species may instead rely more on the acoustical medium to assess their relative risk and guide their subsequent anti-predator behaviour. We addressed this question in the socially foraging meerkat (Suricata suricatta). Meerkats forage with their head down, but at the same time frequently produce close calls (‘Foraging’ close calls). Close calls are also produced just after an individual has briefly scanned the surrounding environment for predators (‘Guarding’ close calls). Here, we firstly show that these Guarding and Foraging close call variants are in fact acoustically distinct and secondly subjects are less vigilant (in terms of frequency and time) when exposed to Guarding close call playbacks than when they hear Foraging close calls. We argue that this is the first evidence for socially foraging animals using the information encoded within calls, the main adaptive function of which is unrelated to immediate predator encounters, to coordinate their vigilance behaviour. In addition, these results provide new insights into the potential cognitive mechanisms underlying anti-predator behaviour and suggest meerkats may be capable of signalling to group members the ‘absence’ of predatory threat. If we are to fully understand the complexities underlying the coordination of animal anti-predator behaviour, we encourage future studies to take these additional auditory and cognitive dimensions into account.     

Social foraging and dominance relationships: the effects of socially mediated interference

In socially foraging animals, it is widely acknowledged that the position of an individual within the dominance hierarchy of the group has a large effect upon its foraging behaviour and energetic intake, where the intake of subordinates can be reduced through socially mediated interference. In this paper, we explore the effects of interference upon group dynamics and individual behaviour, using a spatially explicit individual-based model. Each individual follows a simple behavioural rule based upon its energetic reserves and the actions of its neighbours (where the rule is derived from game theory models). We show that dominant individuals should have larger energetic reserves than their subordinates, and the size of this difference increases when either food is scarce, the intensity of interference suffered by the subordinates increases, or the distance over which dominant individuals affect subordinates increases. Unlike previous models, the results presented in this paper about differences in reserves are not based upon prior assumptions of the effects of social hierarchy and energetic reserves upon predation risk, and emerge through nothing more than a reduction in energetic intake by the subordinates when dominants are present. Furthermore, we show that increasing interference intensity, food availability or the distance over which dominants have an effect also causes the difference in movement between ranks to increase (where subordinates move more than dominants), and the distance over which dominants have an effect changes the size of the groups that the different ranks are found in. These results are discussed in relation to previous studies of intra- and interspecific dominance hierarchies.     

Correlates of boldness in three-spined sticklebacks (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>)

Behavioural variation is known to occur between individuals of the same population competing for resources. Individuals also vary with respect to their boldness or shyness. An individuals position along the shy-bold axis may be defined as the extent to which it is willing to trade off potentially increased predation risks for possible gains in resources. Similarly, group living may be interpreted as a trade-off between anti-predatory tactics and foraging efficiency. The responses of three-spined sticklebacks (Gasterosteus aculeatus) were tested across four social contexts to assess relative boldness or shyness and to further examine whether their behaviour would be consistent within and between contexts. Individuals displayed consistent responses within and between the first two contexts: those individuals which resumed foraging rapidly after a simulated aerial predator attack also displayed low shoaling tendencies. Such fish were deemed to be bold, whilst those which displayed the converse behaviour, slow resumption of foraging and a high shoaling tendency, were deemed to be shy. In a third context, bold individuals out-competed shy conspecifics for food. Boldness was also positively correlated with growth over a 6-week period. The position adopted by an individual within a group is usually interpreted as a trade-off between predation risk and foraging efficiency—both are greater at the front of a mobile group. Bold individuals showed significantly stronger tendencies towards front positions than shy conspecifics. The results suggest that, contrary to some previous studies on other animals, bold or shy behaviour in sticklebacks is consistent between contexts.Communicated by T. Czeschlik     

Client preferences by Caribbean cleaning gobies: food,safety or something else?

Predation risk is amongst the most pervasive selective pressures influencing behaviour and animals have been repeatedly shown to trade-off foraging success for safety. We examined the nature of this trade-off in cleaning symbioses amongst Caribbean coral reef fishes. We predicted that cleaning gobies (Elacatinus evelynae and Elacatinus prochilos) should prefer fish clients that pose a low risk of predation (e.g. herbivores) over clients that may have more ectoparasites but pose a higher risk (e.g. piscivores). Our field observations revealed that cleaners did clean preferentially client species with more parasites but predatory and non-predatory clients had similar ectoparasite loads. Despite the lack of a foraging advantage for inspecting predators, cleaners did not avoid risky clients. On the contrary, a larger proportion of visiting predators than non-predators was inspected, gobies initiated more interactions with predatory clients, and predators were attended to immediately upon arrival at cleaning stations. This preferential treatment of dangerous clients may allow the rapid identification of cleaners as non-prey item or may be due to the effect of predators on the rest of the cleaners’ clientele, which avoided cleaning stations whilst predators were present. Dealing with potentially risky clients may allow gobies to regain access to their main food source: non-predatory clients.     

Behavioral and life-history components of division of labor in honey bees (Apis mellifera L.)

Variability exists among worker honey bees for components of division of labor. These components are of two types, those that affect foraging behavior and those that affect life-history characteristics of workers. Variable foraging behavior components are: the probability that foraging workers collect (1) pollen only; (2) nectar only; and (3) pollen and nectar on the same trip. Life history components are: (1) the age the workers initiate foraging behavior; (2) the length of the foraging life of a worker; and (3) worker length of life. We show how these components may interact to change the social organization of honey bee colonies and the lifetime foraging productivity of individual workers. Selection acting on foraging behavior components may result in changes in the proportion of workers collecting pollen and nectar. Selection acting on life-history components may affect the size of the foraging population and the distribution of workers between within nest and foraging activities. We suggest that these components define possible sociogenic pathways through which colony-level natural selection can change social organization. These pathways may be analogous to developmental pathways in the morphogenesis of individual organisms because small changes in behavioral or life history components of individual workers may lead to major changes in the organizational structure of colonies. Correspondence to: R.E. Page, Jr.     

Influences on dusky dolphin (<Emphasis Type="Italic">Lagenorhynchus obscurus</Emphasis>) fission-fusion dynamics in Admiralty Bay,New Zealand

In societies characterized by a high degree of fission-fusion dynamics, individuals adjust their grouping patterns according to the shifting balance of costs and benefits associated with grouping. This study examines influences on fission-fusion dynamics for dusky dolphins (Lagenorhynchus obscurus) in Admiralty Bay, New Zealand. This area is an important foraging habitat for dusky dolphins during the winter and spring. Admiralty Bay has little predation risk, but nearshore mussel farms may infringe on available habitat. I used generalized estimating equations to determine the influences of coordinated foraging, predation risk, and presence of mussel farms on party size, rate of fission-fusion, and behavioral state. I conducted 168 boat-based group focal follows totaling 168 h. The proportion of individuals observed foraging was positively related to party size and rate of party fusion. Resting had no effect on party size and did not vary according to location. Near mussel farms, traveling decreased, and rate of party fission decreased. I conclude that (1) coordinated foraging strategies are a primary influence on fission-fusion dynamics within this population, (2) dolphins may respond to decreased predation risk by not adjusting party size or location during resting, and (3) areas near mussel farms are not used for traveling.     

A general random walk model for the leptokurtic distribution of organism movement: Theory and application

The movement of organisms is usually leptokurtic in which some individuals move long distances while the majority remains at or near the area they are released. There has been extensive research into the origin of such leptokurtic movement, but one important aspect that has been overlooked is that the foraging behaviour of most organisms is not Brownian as assumed in most existing models. In this paper we show that such non-Brownian foraging indeed gives rise to leptokurtic distribution. We first present a general random walk model to describe the organism movement by breaking the foraging of each individual into events of active movement and inactive stationary period; its foraging behaviour is therefore fully characterized by a joint probability of how far the individual can move in each active movement and the duration it remains stationary between two consecutive movements. The spatio-temporal distribution of the organism can be described by a generalized partial differential equation, and the leptokurtic distribution is a special case when the stationary period is not exponentially distributed. Empirical observations of some organisms living in different habitats indicated that their rest time shows a power-law distribution, and we speculate that this is general for other organisms. This leads to a fractional diffusion equation with three parameters to characterize the distributions of stationary period and movement distance. A method to estimate the parameters from empirical data is given, and we apply the model to simulate the movement of two organisms living in different habitats: a stream fish (Cyprinidae: Nocomis leptocephalus) in water, and a root-feeding weevil, Sitona lepidus in the soil. Comparison of the simulations with the measured data shows close agreement. This has an important implication in ecology that the leptokurtic distribution observed at population level does not necessarily mean population heterogeneity as most existing models suggested, in which the population consists of different phenotypes; instead, a homogeneous population moving in homogeneous habitat can also lead to leptokurtic distribution.     

Foraging behaviour and brain morphology in recently emerged brook charr, Salvelinus fontinalis

Recently emerged brook charr (Salvelinus fontinalis) foraging in still-water pools along the sides of streams are either active, feeding on insects from the upper portion of the water column away from the stream bank, or sedentary, feeding on crustaceans emerging from the hyporheic zone near the stream bank. We tested whether the frequency of movement displayed by individual brook charr searching for prey in the field was related to the relative volume of the telencephalon, a brain region involved with movement and space use in fishes. Movement of individuals searching for prey was quantified in the field, individuals were captured and volumes of the telencephalon and of the olfactory bulbs, a brain region neighbouring the telencephalon but not implicated in space use, were measured. Individuals with larger telencephalon volumes moved more frequently on average while searching for prey in the field than did individuals with smaller telencephalon volumes. The frequency of movement was unrelated to differences in the volume of the olfactory bulbs, suggesting that the relationship between telencephalon volume and movement was not a consequence of differences in overall brain size. Demonstrating a correlation between foraging behaviour and brain morphology for brook charr exhibiting different foraging tactics suggests that diversification in brain structure and function could be important aspects of the foraging specialization believed to occur during early stages in the evolution and development of resource polymorphisms.     

Costs and benefits of pocket gopher foraging: linking behavior and physiology

Animals can attain fitness benefits by maintaining a positive net energy balance, including costs of movement during resource acquisition and the profits from foraging. Subterranean rodent burrowing provides an excellent system in which to examine the effects of movement costs on foraging behavior because it is energetically expensive to excavate burrows. We used an individual-based modeling approach to study pocket gopher foraging and its relationship to digging cost, food abundance, and food distribution. We used a unique combination of an individual-based foraging-behavior model and an energetic model to assess survival, body mass dynamics, and burrow configurations. Our model revealed that even the extreme cost of digging is not as costly as it appears when compared to metabolic costs. Concentrating digging in the area where food was found, or area-restricted search (ARS), was the most energetically efficient digging strategy compared to a random strategy. Field data show that natural burrow configurations were more closely approximated by the animals we modeled using ARS compared to random diggers. By using behavior and simple physiological principles in our model, we were able to observe realistic body mass dynamics and recreate natural movement patterns.     

Behavioural interference between ungulate species: roe are not on velvet with fallow deer

Interference is expected to occur at feeding areas between species with a similar diet, but few studies have tested this idea for wild ungulates. We analysed interactions between fallow deer, European roe deer and wild boar, in three sites, in a Mediterranean area. We expected that interference should be greater between deer than between them and wild boar. We documented the negative effects of behavioural interference by fallow on foraging behaviour of roe deer, under field conditions. Deer species built up 90% interference interactions, with fallow always dominant on roe, also through direct aggression. Although roe deer decreased feeding and increased vigilance levels in proximity (<50 m) of either fallow deer or wild boar, they were displaced significantly more often by the former than by the latter. Fallow deer were neither displaced nor alarmed by roe and rarely by wild boar. No deer species displaced wild boar. Interference was significantly greater on solitary roe deer, especially females, in spring and roe left the feeding ground most often in the smallest site (13 ha). Roe deer avoided areas where the local density of fallow deer was the highest. During our 4-year-study, roe deer density decreased whereas fallow deer numbers increased. Behavioural interference may explain how fallow deer outcompete roe deer through spatial exclusion from feeding sites and avoidance of areas with high densities of the former. Fallow deer evolved in semi-arid, relatively poor habitats of Asia Minor: interspecific defence of crucial resources could have developed as a beneficial tactic for its survival.     

Spatio-temporal ranging behaviour and its relevance to foraging strategies in wide-ranging wolverines

Conservation of carnivores in an increasingly changing environment is greatly helped by understanding the decision-making processes underlying habitat patch choice. Foraging theory may give us insight into spatio-temporal search patterns and consequent foraging decisions that carnivores make in heterogeneous and fluctuating environments. Constraints placed on central-place foragers in particular are likely to influence both foraging decisions and related spatio-temporal movement patterns. We used discrete choice models to investigate the spatio-temporal ranging behaviour of GPS collared female wolverines (Gulo gulo) with dependent cubs in south-central Norway. Activity patterns, home range use and selection for elevation were analyzed in relation to spatial and temporal covariates (daily and seasonal) and related to different foraging behaviours. In spring, wolverines showed restricted movement patterns around rendezvous sites at high elevations by day, whereas during the night animals were active at lower elevations. Over the summer, this daily pattern in intensity of use diminished and their overall selectiveness for elevation decreased as cubs grow more mobile and independent. At the onset of autumn, wolverines showed intensive use of the profitable forest-alpine tundra ecotone. We argue that reproducing wolverines deployed a foraging strategy attuned to altering their movement patterns throughout the summer to address a continuous, but diminishing, trade-off between providing both food and shelter for their offspring. Incorporating spatially and temporally explicit activity patterns and home range use in discrete choice resource selection models thus enhances the understanding of the motives behind wolverine resource utilization in space and time. Such knowledge may provide guidance to managers designing regional-scale zoning, in order to facilitate carnivore recovery and to minimize conflicts with human activities.     

Schooling tendency and foraging benefit in the ocean surgeonfish

Summary Field observations of individuallytagged ocean surgeonfish (Acanthurus bahianus) indicated that schooling provided a foraging benefit: an increase in relative forging time. The magnitude of this increase differed among individuals, primarily due to time budgets of the fish when they were not in schools. The proportion of nonschooling time devoted to foraging was positively correlated with fish size, while the proportion of schooling time that was spent forging was similar for all individuals. Schooling tendency (percentage of time in schools) was positively related to magnitude of foraging benefit and negatively related to fish size. When fish size was held constant by partial correlation, schooling tendency remained positively related to foraging benefit. These results provide evidence that fish which derive a greater foraging benefit from school membership spend more time in schools.     

Marine aquaculture and bottlenose dolphins’ (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>) social structure

In this study, we investigate association patterns of 249 bottlenose dolphin feeding groups off Sardinia Island (Italy) from January 2000–May 2007 and describe how their association behaviour is related to their response to food patches created by a marine fin fish farm. We also tested the hypothesis that dolphins have different social structures with different feeding activities: Associations should decrease during opportunistic feeding behaviours as it is easier to capture prey, and cooperation is not as necessary. Sixteen individually identified bottlenose dolphins were observed participating in both opportunistic and not opportunistic feeding activities, with a mean of 30 ± 8 times and 9.6 ± 1 times, respectively. Bottlenose dolphins show non-random social behaviour during feeding and this behaviour differs depending on their specific foraging activity. Dolphin associations during feeding can be divided into three categories: acquaintances, affiliates, and feeding associates. Association behaviour during fish farm feeding is consistent with our hypothesis that during opportunistic behaviours, benefits from cooperation decrease, as it is easier to capture prey. Group size homogeneity in both feeding activities demonstrates that the number of dolphins engaging in foraging is not necessarily related with cooperation levels. Moreover, an adult dolphin may prefer to associate with a specific individual, independent of the sex, who shares the same foraging priorities. This study is the first to show how aquaculture is not only directly affecting marine predators but could also indirectly affect their social structure and behaviour.     

Learning,retention and coding of nest-associated visual cues by the Australian desert ant, <Emphasis Type="Italic">Melophorus bagoti</Emphasis>

A variety of social insects use visual cues for homing. In this study, we examine the possible factors affecting the learning and retention of nest-associated visual cues by the Australian desert ant Melophorus bagoti and the manner in which such cues are encoded by foraging ants. We placed four prominent cylindrical landmarks around a nest and trained foragers from that nest to a food source. Ants were tested with the landmark array in a distant testing field after (1) a known number of exposures to the landmarks (1, 3, 7 or 15 trials, spread over a period of 1 day, 2 days or ≥3 days) and (2) after a known period of delay (0, 24, 48, 96 or 192 h). The results show that a combination of an increase in training trials and an increase in number of training days affected the acquisition of landmark memory. Moreover, once the landmarks were learnt, they became a part of long-term memory and lasted throughout the ants’ foraging lifetime. To examine visual cue encoding behaviour, ants trained under similar conditions for 4 days were tested with (1) an identical landmark array, (2) landmarks of the same size used in training, but placed at twice the distance from each other, and (3) landmarks whose dimensions were doubled and placed at twice the distance from each other. In conditions (1) and (3), the ants searched extensively at the centre of the four landmarks, suggesting that, similar to the Saharan ant (genus Cataglyphis) and the honeybee, M. bagoti too uses a snapshot to match the view of the landmarks around the nest. But contrary to the snapshot model, in condition (2), the ants did not search extensively at the centre of the landmarks, but searched primarily 0.5 m from the landmark, the distance from each landmark to the nest during training. We discuss how various search models fare in accounting for these findings.     

Determinants of multiple central-place territory use in wild young-of-the-year Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis>)

Patterns of space use provide key insights into how animals exploit local resources and are linked to both the fitness and distribution of individuals. We studied territory size, mobility, and foraging behavior of young-of-the-year Atlantic salmon Salmo salar in relation to several key environmental factors in Catamaran Brook, New Brunswick, Canada. The 50 study fish were all multiple central-place foragers (i.e., alternated among several sit-and-wait foraging stations) and showed great variability in territory size and the total distance traveled within the territories. Territory size increased with the mean distance traveled between consecutive foraging stations, the number of stations visited, and the mean foraging radius. Fish also varied greatly in how much of the total travel distance was associated with foraging at a station (14.8–91.8%) versus switching among stations (4.6–84.3%). As predicted, fish in slow-flowing waters, where drifting prey were scarce, used larger multiple central-place territories than individuals in faster, more productive waters. Interestingly, however, the most mobile fish did not inhabit slow-running waters as predicted but were found at intermediate (optimal) water current velocities. Hence, our study suggests that among some multiple central-place foragers, increased mobility may not only serve to increase prey encounter rate but may reflect an attempt to patrol territories in favorable habitats. Further studies are needed to determine the generality and the ultimate benefits of multiple central-place space use among stream-dwelling fish and other animals.     

Ecological and social determinants of spacing behaviour in desert baboon groups

This paper investigates the determinants of individual spacing behaviour in a desert baboon population (Papio cynocephalus ursinus). Patterns of neighbour proximity and neighbour density were examined among adults in four groups under different ecological and social conditions (through instantaneous sampling during focal follows). Initial analysis of these data shows that (1) the use of vertical substrates (refuges such as tall trees and cliff faces) can confound patterns of spacing, and (2) individual differences in spacing can depend on the spatial scale over which it is measured. To minimise these substrate and scale effects, this analysis focuses on animals which are off refuges and examines spacing behaviour through its underlying statistical `dimensions' (identified through factor analysis). Analysis of these dimensions indicates that sex, group size, activity-habitat and female reproductive state can all have independent effects on spacing: (1) males are more dispersed than females in smaller groups, (2) male and female dispersion increases with time spent in foraging habitats, and (3) female dispersion is reduced during lactation. According to the hypotheses tested, these results indicate that feeding competition only affects spacing behaviour during foraging while predation risk plays little or no role in spacing. Most aspects of spacing behaviour are best explained by male reproductive strategies and their social repercussions. Received: 25 May 1998 / Accepted after revision: 18 July 1998     

Group size and foraging efficiency in yellow baboons

Summary I studied the foraging behaviour of adults in three different-sized groups of yellow baboons (Papio cynocephalus) at Amboseli National Park in Kenya to assess the relationship between group size and foraging efficiency in this species. Study groups ranged in size from 8 to 44 members; within each group, I collected feeding data for the dominant adult male, the highest ranking pregnant female, and the highest ranking female with a young infant. There were no significant differences between groups during the study in either the mean estimated energy value of the food ingested per day for each individual (385±27 kJ kg^-1 day^-1) or in the estimated energy expended to obtain that food (114±3 kJ kg^-1 day^-1). Mean foraging efficiency ratios, which reflect net energy gain per unit of foraging time, also did not vary as a function of the size of the group in which the baboons were living. There was substantial variation between days in the efficiency ratios of all animals; this was the result of large differences in energy intake rather than in the energy expended during foraging itself. The members of the smallest group spent on the average only one-half as much time feeding each day as did individuals in the two larger groups. However, they obtained almost as much energy while foraging, primarily because their rate of food intake while actually eating tended to be higher than the rate in the other groups. The baboons in the small group were observed closer to trees that they could climb to escape ground predators, and they also were more likely to sit in locations elevated above the ground while resting. Such differences would be expected if the members of the small group were less able to detect approaching predators than individuals that lived in the larger groups. The results of this study suggest that predator detection or avoidance, rather than increased foraging efficiency, may be the primary benefit of living in larger groups in this population.     

Social dominance in migrant and resident turkey vultures at carcasses: evidence for a despotic distribution?

We placed carcasses in three different vegetation types in the heterogeneous savannas of central Venezuela to investigate the role of social dominance in habitat use by flocking migrant and resident turkey vultures (Cathartes aura meridionalis and C. a. ruficollis). Migrants foraged primarily in savanna habitats while residents foraged almost exclusively in gallery forest. In the gallery forest residents discovered carrion first significantly more often than migrants, despite there being equal densities of residents and migrants foraging over this habitat. Because residents fed in smaller groups than migrants at carcasses they had higher feeding rates. There was also a negative relationship between group sizes of residents and migrants. The feeding rate of residents declined in response to increased group size of migrants, but group size of residents had no effect. Migrant group size also had a greater effect on resident feeding rates than king vulture presence or absence. When the effect of migrant and resident group size on feeding rates in migrants was compared, the most significant factor was migrant group size. A second analysis showed that both resident group size and presence or absence of king vultures had a significant effect on feeding rates in migrants. Rates of agonistic encounters in migrant and resident turkey vultures increased weakly in relation to group size. However, there was an increase in residents' encounter rate with migrants in relation to increased migrant group size; there was no difference in resident encounter rates with other residents in relation to resident group size. Migrants dominated residents in almost all agonistic interactions over carcasses. We suggest that savanna habitats were less attractive to residents for foraging because they held larger groups of migrants.