The effect of kleptoparasite and host numbers on the risk of food‐stealing in an avian assemblage

Kleptoparasitism involves the theft of resources such as food items from one individual by another. Such food‐stealing behaviour can have important consequences for birds, in terms of individual fitness and population sizes. In order to understand avian host–kleptoparasite interactions, studies are needed which identify the factors which modulate the risk of kleptoparasitism. In temperate European intertidal areas, Eurasian oystercatchers Haematopus ostralegus feed primarily on bivalve molluscs, which may be stolen by kleptoparasitic species such as carrion crows Corvus corone and herring gulls Larus argentatus. In this study we combined overwinter foraging observations of oystercatchers and their kleptoparasites on the Exe Estuary, UK, with statistical modelling to identify the factors that influence the likelihood of successful food stealing behaviour occurring. Across the winter, 16.4% of oystercatcher foraging attempts ended in successful kleptoparasitism; the risk of theft was lowest in February (10.8%) and highest in December (36.3%). Using an information theoretic approach to compare multiple logistic regression models we present evidence that the outcome of host foraging attempts varied with the number of kleptoparasites per host within the foraging patch for two out of five individual months, and for all months grouped. Successful, kleptoparasitism was more likely to occur when the total number of all kleptoparasites per host was greater. Across the entire winter study period, oystercatcher foraging attempts that resulted in kleptoparasitism were associated with a mean number of kleptoparasites per host that was more than double that for foraging attempts that ended in the oystercatcher successfully consuming the mussel. Conversely, the stage of the tidal cycle within the estuary did not affect the outcome of oystercatcher foraging attempts. Our study provides evidence that bird numbers influence the risk of kleptoparasitism within avian assemblages.     

Movement coordination and signalling in ravens (Corvus corax): an experimental field study

Vagrant non-breeding ravens frequently attract conspecifics to rich ephemeral food sources. There, grouping may allow them to overcome the defence of territorial breeders. Here, we focus on ravens making use of regular food supplies in a game park, where they divert food from the provision of park animals. We investigated if ravens foraging in the Cumberland game park (Grünau, Austria) are attentive towards one another when they experience some unpredictability in food provisioning. We confronted a group of 30–50 ravens with two different treatments. Ten minutes ahead of the feeding of either wolves or wild boars we showed buckets containing pieces of meat to the ravens flying overhead. In the reliable cue treatment (RCT), the meat was placed next to one of the two enclosures, whereas in the unreliable cue treatment (UCT), the buckets were placed simultaneously in front of both enclosures though only in one of the enclosures were the animals fed 10 min later. Thus, during RCT but not during UCT, ravens could predict where food would become available. Only during UCT, ravens moved in large groups between the two feeding sites. Many ravens moving at the same time in the same direction may indicate some co-ordination in space and time, which is most likely achieved by social attraction among individuals. Furthermore, the number of ravens approaching and leaving, respectively, a feeding site cross-correlated with a temporary increase in the rate of a food-associated call, the yell. This suggests that in addition to watching each other, calling may have contributed to group formation. Possible benefits of group formation during food inspection are discussed. Received: 9 February 2000 / Received in revised form: 30 May 2000 / Accepted: 1 June 2000     

Do kleptoparasites reduce their own foraging effort in order to detect kleptoparasitic opportunities?: An empirical test of a key assumption of kleptoparasitic models

Determining if, or when, individuals trade off time spent personally feeding against time spent monitoring others for kleptoparasitism opportunities is essential to an understanding of the evolution of scrounging and usurpation behaviours. We provide a first field test of whether kleptoparasites reduce their personal foraging effort in situations where the frequency and rewards of kleptoparasitism increase. We provided experimental food patches for wild European blackbirds that varied in the distribution of prey and that had a potentially high rate of kleptoparasitism within pairs of blackbirds feeding in them. Although individuals differed in their rate of kleptoparasitism, they did not vary in the size of the reward that they gained from kleptoparasitism. As prey became more clumped, kleptoparasitism rate and its reward per incident increased on average. There was, however, no evidence that individuals that were kleptoparasitising more quickly and/or at a higher frequency had lower personal foraging effort. In contrast, foraging effort increased in both birds compared to when they were foraging alone, independent of dominance, kleptoparasitic opportunity or reward. Our evidence suggests that in some circumstances a kleptoparasite can detect kleptoparasitic opportunities without compromising its own personal foraging rate.     

Decision time modulates social foraging success in wild common ravens,Corvus corax

Social foraging provides several benefits for individuals but also bears the potential costs of higher competition. In some species, such competition arises through kleptoparasitism, that is when an animal takes food which was caught or collected by a member of its social group. Except in the context of caching, few studies have investigated how individuals avoid kleptoparasitism, which could be based on physical strength/dominance but also cognitive skills. Here, we investigated the foraging success of wild common ravens, Corvus corax, experiencing high levels of kleptoparasitism from conspecifics when snatching food from the daily feedings of captive wild boars in a game park in the Austrian Alps. Success in keeping the food depended mainly on the individuals’ age class and was positively correlated with the time to make a decision in whether to fly off with food or consume it on site. While the effect of age class suggests that dominant and/or experienced individuals are better in avoiding kleptoparasitism, the effect of decision time indicates that individuals benefit from applying cognition to such decision-making, independently of age class. We discuss our findings in the context of the ecological and social intelligence hypotheses referring to the development of cognitive abilities. We conclude that investigating which factors underline kleptoparasitism avoidance is a promising scenario to test specific predictions derived from these hypotheses.     

Scrounging Tactics in Free-Ranging Ravens, Corvus corax

Social foraging allows individuals to scrounge, i.e. to exploit the food others have made available. The conditions promoting scrounging as an alternative foraging tactic have yet received limited attention. We presently examine whether ravens, as opportunistic scavengers, adjust their foraging tactics according to the potential costs involved in accessing a particular food source. We observed wild ravens foraging in a game park, at the enclosures of wolves, Canis lupus, and wild boars, Sus scrofa. Wolves may aggressively defend their food and even kill ravens, whereas wild boars do not. When co‐feeding with wolves, the ravens showed higher scrounging rates than with wild boars. Only at the wolves, they tended to specialize either on scrounging or on getting food directly from the site. However, scrounging techniques differed in relation to the state of food depletion. Early on, after food became available, the ravens most frequently displaced others from food, whereas towards the end, stealing, solicited sharing, and cache raiding became prevalent. These techniques differed in their profitability and their use was related to the scroungers’ age, social status and affiliative relationships. This suggests that ecological conditions, such as co‐feeding with potential predators, may influence the individuals’ decision whether or not to scrounge in competition for food. Social conditions, on the other hand, may affect the way how to get at food possessed by others and may thus, to a large extent, determine the profitability of scrounging.     

Food calling in wild ravens (Corvus corax) revisited: Who is addressed?

Numerous birds and mammals use vocal signals to advertise feeding opportunities but often such signals vary with individual and contextual factors. Non-breeding ravens call at food that is difficult to access, resulting in the attraction of nearby conspecifics. Although callers may benefit from group formation in various ways, we recently found substantial individual variation in food calling. We here explored whether this variation can be partly explained by the social dynamics in raven foraging groups, together with already known effects of age class and sex. Specifically, we expected ravens to respond to the presence or absence of affiliates that could act as cooperative partners in the forthcoming feeding event, that is they should call when other ravens were present but they themselves were alone rather than when they were also in company of an affiliation partner. We observed the vocal behaviour of individually marked wild ravens and, simultaneously, categorized their affiliative behaviour with other ravens in the minutes before experimentally controlled feedings. In line with our prediction, individuals were less likely to produce food-associated calls when they were in close contact with an affiliation partner prior to feeding as compared to when they were alone. Furthermore, sex and age class influenced food calling as females called more often than males and younger birds called more often than adult ravens. In conclusion, these results suggest that ravens attempt to find support from a particular cooperative partner by broadly advertise feeding opportunities via food-associated calls, especially when they have low chances in contest competition due to their age and sex. These findings lend further support to the assumption of raven flocks being structured by social relationships and individual birds flexibly controlling their vocal signalling according to the current flock composition.     

Experimental Manipulation of Food Accessibility Affects Conflict Management Behaviour in Ravens

Conflict management strategies such as reconciliation and bystander affiliation have been described for a variety of species. A common determinant seems to be a ‘complex’ social life, with individuals relying on affiliate relationships or social bonds. Little is known, however, about the strategic and flexible use of conflict management skills in experimental settings in species other than primates. We here investigated conflict and post‐conflict behaviour of ravens by manipulating the accessibility of food and, thus, the likelihood of aggressive interactions while foraging. Specifically, we presented birds with a certain amount of highly preferred food that varied in the number of pieces (one piece, two pieces or, as a control, small pieces matching the number of participating birds) and observed their agonistic behaviour during feeding and their affiliative behaviour afterwards. The results showed that high levels of conflicts during feeding in the 1‐piece and 2‐piece conditions led to high levels of affiliation after feeding. Depending on the experimental condition, this effect is best explained (a) by the affiliative behaviour of former aggressors (1‐piece condition) and (b) by the affiliation directed to the receivers of aggression after feeding (2‐piece condition). Those dyads that engaged in allo‐preening after feeding also engaged in allo‐preening outside the experimental setting, suggesting that socially bonded individuals provided third‐party affiliation to victims of aggression. Moreover, socially bonded ravens fed close to each other in the experiment when food was clumped, indicating that they actively coordinated their behaviour when there was a high conflict potential. Taken together, these findings support the assumption that ravens use their social bonds to avoid conflicts by choosing with whom to feed, and to buffer effects of conflicts by engaging in third‐party affiliation as post‐conflict behaviour.     

Red-gartered Coot Fulica armillata feeding on the grapsid crab Cyrtograpsus angulatus: advantages and disadvantages of an unusual food resource

The behaviour of Red-gartered Coots feeding on an unusual food source was examined at Mar Chiquita Coastal Lagoon, Argentina. The grapsid crab Cyrtograpsus angulatus made up all observed prey items, and 61% were small. Both handling and foraging duration increased with the size of captured crabs, but foraging efficiency decreased. Crab availability affected both the dive duration of the Coots and their foraging decisions with regard to prey-size selection. Two species of gull were observed kleptoparasitizing Coots, especially when the Coot was handling medium or large crabs. Feeding by Coots on Cyrtograpsus angulatus has not been previously documented and may be a feeding innovation. Our estimations suggest that Coots were foraging optimally, since smaller crabs were more energetically profitable.     

Flight performance, echolocation and foraging behaviour in noctule bats Nyctalus noctula

The noctule Nyctulus noctula (Schreber, 1774) is a relatively large (c. 25 g) insectivorous bat which catches insects on the wing (by aerial hawking). Emergence at a maternity roost was earliest relative to sunset when females were lactating, and bats may then have risked predation by flying at higher light levels during a period of high energy demand. Flight performance was quantified by using stereophotogrammetry. At feeding sites bats flew at 6.0 ± 2.1 m/s. This was faster than predicted minimum power speed (V _mp), and either between V _mp and maximum range speed (V _mr), or close to their predicted V _mr, depending on which aerodynamic model of flight power requirements was used. The echolocation behaviour of noctules is flexible. Long duration, low frequency calls (c. 20 kHz) with little frequency modulation were emitted while cruising, but at foraging sites the calls became more frequency-modulated. As the noctule is traditionally thought of as using low frequency echolocation, it was expected to receive weak echoes from small targets and therefore to specialize in eating large insect prey. Although the bats ate mainly beetles, large numbers of small dipterans were also eaten. The noctule is probably able to detect such small items because, when foraging, its calls become broadband and sweep from high frequencies. Higher harmonics are also present, and these may assist in the detection of small prey. In noctules, as in many bats, there appears to be a 1:1 link between wingbeat and call production during the search phase of foraging.     

Cooperative hunting in Brown-Necked Raven (Corvus rufficollis) on Egyptian Mastigure (Uromastyx aegyptius)

We describe cooperative hunting by Brown-necked Raven (Corvus ruficollis) on Egyptian Mastigure (Uromastyx aegyptius) in the Arava Valley, Israel. At first, in all nine observed hunts, the ravens were observed to be in the vicinity and were seen simultaneously. The Mastigure was almost always at a distance from the burrow entrance and either foraging on the bushes or lying flattened on a bush sunning itself. The attack started when a circling pair of ravens flew in at high speed and landed on the entrance of the burrow, cutting off the escape route of the lizard. Following this maneuver, the other ravens attacked the lizard. The ravens pecked randomly at the most exposed part of the lizard, eventually causing its death. Only when the lizard was evidently dead did the two individuals that blocked the escape route join in the feeding ravens.