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.     

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.     

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.     

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.     

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.     

The effects of water on patch use by two Simpson Desert granivores (Corvus coronoides and Pseudomys hermannsburgensis)

Abstract Water loss while foraging may affect the overall value of food to desert animals. When water is scarce, foragers may alter activity and shun certain types of food due to elevated water loss. When water is abundant, foragers can exploit food patches more thoroughly and remain active over a broader range of ambient conditions. In short, food and water may be complementary resources. The presence of water raises the marginal value of food, particularly those foods low in water content. We tested for the complementarity of food and water to foragers at a sand dune site in the Simpson Desert of arid Australia. To do so, we quantified patch exploitation of foragers in the presence or absence of bowls filled with water. In order to quantify patch use, we provisioned feeding trays with granulated peanuts mixed into a sand substrate. In these trays we measured giving-up densities (GUD; the amount of food left in a tray after a foraging bout) of diurnal (mostly Australian ravens, Corvus coronoides) and nocturnal foragers (mostly sandy inland mouse, Pseudomys hermannsburgensis). The presence of water affected the GUD of ravens but not of rodents. For the ravens, GUD dropped about 50% in response to added water. For ravens, water and food are strongly complementary. In addition, ravens had lower GUD in the open than the bush microhabitat, and lower GUD at the bottom than the tops of sand dunes.     

Behavioural and Hormonal Stress Responses to Social Separation in Ravens,Corvus corax

Social life is profitable, but it facilitates conflicts over resources and creates interdependence between individuals. Separating highly social animals triggers intense reactions aimed at re‐establishing lost connections. Less is known, however, about behavioural and physiological responses to separation in socially facultative species, where individuals temporarily form groups and may subsequently leave them. Non‐breeding common ravens (Corvus corax) gather in large numbers at feeding and roosting sites, but otherwise spend time seemingly solitary or in small subgroups. We here studied how ravens cope with being socially isolated, and investigated the life characteristics that might explain potential individual differences. For this, we individually separated captive subadult ravens (n = 25) and housed them in physical and visual isolation from their group members across 4 d. During the separation period, we collected behavioural data and measured the amount of immunoreactive corticosterone metabolites from bird droppings to assess the ravens’ physiological stress response. We found behavioural indicators of stress at the start of the separation, when ravens showed higher levels of tension than of comfort – a pattern that reversed at the end of the separation. Furthermore, we found that the upbringing of ravens affected their behaviour during separation. Hand‐raised birds produced more vocalisations in the beginning of the separation, and were less active at the end, while the reverse pattern occurred with parent‐raised ravens. Contrary to our predictions, we did not find differences in hormonal responses between the beginning and end of the separation period or any link between hormonal responses and behaviours. Ravens’ behavioural responses to social separation stress seem to be dependent on their arousal states, although possible links with hormonal reactions remain unclear. Our results show that behavioural reactions are not always linked with hormonal responses to stress, and further emphasise the importance of investigating effects of early‐life experiences.     

Evidence that tufted puffins Fratercula cirrhata use colony overflights to reduce kleptoparasitism risk

Predation, foraging and mating costs are critical factors shaping life histories. Among colonial seabirds, colony overflights may enhance foraging or mating success, or diminish the risk of predation and kleptoparasitism. The latter possibility is difficult to test because low predation or kleptoparasitism rates could be due either to low danger or to effective counter-tactics by prey. Tufted puffins Fratercula cirrhata breeding at a large colony in British Columbia, Canada, deliver several loads of fish each day to their nestlings and are targets for kleptoparasitism by glaucous-winged gulls Larus glaucescens . In the present study, we documented the ecological conditions under which overflights occurred in order to assess when overflights were made and to statistically isolate the effect of overflights on kleptoparasitism risk at this site. Load-carrying puffins engaged in overflights under ecological conditions associated with relatively high rates of kleptoparasitism. Further, when ecological factors determining risk were statistically controlled, overflights were correlated with marginally lower chances of kleptoparasitism than when the risk factors were ignored. The results suggest that breeding puffins at this site use overflights for kleptoparasite avoidance. This tactic is used sparingly, suggesting it is costly. Costs of overflight behaviour might contribute to the impact of kleptoparasitism on the breeding success of tufted puffins.     

Common ravens, Corvus corax, preferentially associate with grey wolves, Canis lupus, as a foraging strategy in winter

One foraging strategy that scavengers can employ to discover unpredictable food sources is to associate directly with predators who inadvertently provide food. The common raven, a well known feeding generalist, is also a prominent scavenger of wolves' kills and is found to be in close association with this predator. We tested the hypothesis that ravens preferentially associate with wolves in winter as a kleptoparasitic foraging strategy. The presence, absence and behaviour of ravens was documented during winter observations of wolves, coyotes, Canis latrans, and elk, Cervus elaphus, as well as the landscape in the absence of these three species. Ravens were found to be in close association with wolves when they were travelling, resting and hunting prey. In comparison, ravens showed no significant association with coyotes, elk or areas on the landscape in the absence of wolves. We also compared ravens' discovery success of wolf-killed and nonwolf-killed carcasses and their behavioural response upon discovery. Ravens found all wolf kills almost immediately and remained at the carcass to feed alongside wolves after the death of the prey. In contrast, ravens were less successful discovering experimentally placed carcasses in the same study region, and did not land or feed despite the availability of fresh, exposed meat. Our results show that ravens' association with wolves is not just an incidental and proximate by-product of the presence of fresh meat. Instead, we show that ravens preferentially associate with wolves in both the presence and absence of food, resulting in the discovery of carcasses and suppression of ravens' innate fear of novel food sources. Through this mode of social foraging, ravens may experience increased foraging efficiency in the use of an otherwise spatially and temporally unpredictable food source.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .     

Foraging Decision Rules and Prey Species Preferences of Northwestern Crows (Corvus caurinus)

Categorization of similar prey types and the application of decision rules by dietary generalists can enhance the efficiency of foraging decisions and facilitate the inclusion of novel prey types in the diet. While considerable research attention has been directed toward investigation of these concepts in invertebrates, few have assessed categorization and decision rules used by generalist vertebrate predators. In this study, we experimentally investigated decision rules and prey preferences of northwestern crows (Corvus caurinus) feeding on littleneck clams (Tapes philippinarum) and whelks (Nucella lamellosa). We presented crows with three species‐size combinations: small clams (2.0–2.9 cm length) paired with large whelks (4.0–4.9 cm), small clams paired with medium whelks (3.0–3.9 cm), and large clams (4.0–4.9 cm) with large whelks. Profitability estimates based on observations of crows feeding on these prey species indicated that clams were always the more energetically profitable option; however, in prey choice trials crows consistently selected the heavier prey species, regardless of differences in profitability. These results show that crows apply a general decision rule according to which they select heavier prey items when feeding on hard‐shelled prey requiring similar handling techniques, and that while such decision rules may approximate optimal choices they may not always follow predictions based solely on prey profitability. We discuss these results in the context of behavioural flexibility of generalist predators, and predicting impacts of intertidal avian predators on prey populations.     

Intraspecific kleptoparasitism improves chick growth and reproductive output in Common Terns Sterna hirundo

Kleptoparasitism is a well‐known foraging strategy used opportunistically by many seabirds. Here, we investigated the effect of intraspecific kleptoparasitism on chick growth and reproductive output in Common Terns Sterna hirundo. Effects were compared between two groups comprising (1) individuals using kleptoparasitism during the chick‐rearing period (kleptoparasitic group, n = 18), and (2) individuals in pairs that never performed kleptoparasitism throughout the season (‘honest’ group, n = 21). The null models best described variation in mass at day 3 and the pre‐fledging mass, indicating no significant effect of the explanatory variables. However, the best models describing the linear growth rate (days 3–13) and peak mass included the parents' foraging strategy (kleptoparasitic vs. honest parents) as an explanatory variable. These two growth parameters were higher in chicks of kleptoparasitic parents. Kleptoparasitic foraging strategy was also associated with higher pre‐fledging survival, as the reproductive performance (i.e. number of fledglings) was significantly higher in the kleptoparasitic than in the honest group. We suggest that by stealing food (and consequently feeding offspring more frequently with high‐quality prey), kleptoparasitic parents are able to produce higher quality chicks with enhanced survival.     

Temporary intermissions in capturing prey (<Emphasis Type="Italic">Daphnia</Emphasis>) by planktivorous fish (<Emphasis Type="Italic">Rutilus</Emphasis><Emphasis Type="Italic">rutilus</Emphasis>): Are they due to scramble competition or the need for antipredation vigilance?

Capture rates in planktivorous fish may differ in individuals foraging alone or in a group, and this may result either from the altered risk of predation due to vigilance sharing in the group, or from a difference in the intensity of scramble competition for encountered prey items. Changes in capture frequency and the feeding pattern observed in young roach (Rutilus rutilus) feeding alone and in a group of three on a high density prey (Daphnia), in the presence and in the absence of predator odor, were used to determine which of these two alternate explanations is more likely. Earlier studies revealed that a foraging roach captures Daphnia prey in uninterrupted sequences of captures occurring every 1–3 s. Such multiple captures are separated by intermissions of 10–20 s, with their duration being likely to determine the overall capture rate. An experiment was performed to examine whether feeding in a group of three permits higher capture rates (hypothesis 1), and whether the intermittent foraging pattern is due to the need to invest more time for vigilance when foraging alone (hypothesis 2). Video recordings were made of many series of subsequent prey captures by roach feeding on high Daphnia densities, alone or in a group, and in the presence or absence of predator odor. Analysis of these data revealed that the mean duration of intermissions between bursts of feeding activity was significantly greater in the presence of predator odor, which resulted in a significant decrease in the capture rate. Furthermore, when the roach were feeding in a group, these intermissions were reduced to a greater extent in the presence of predator odor than in its absence, implying that the intermission intervals represent an investment for vigilance as an effective antipredation defense that permits increased food intake regardless of whether or not it is enhanced by the resource or the interference competition.     

Habitat utilisation, feeding tactics and age related feeding efficiency in the Caspian Gull Larus cachinnans

The feeding behaviour of the Caspian Gull Larus cachinnans was analysed in southern Poland in 2001. During the pre-breeding period, most birds foraged on a refuse dump and some foraged in a river valley. During incubation, similar numbers of birds foraged on fishponds, gravel pits and the refuse dump. During the chick-rearing period, fishponds were the most important foraging grounds. The foraging success of three main foraging tactics was analysed: digging on refuse, fishing and kleptoparasitism. We found that digging success was higher in juveniles than in immature or adult birds. However, older birds moved and ate more items per unit of time than juveniles, which indicates that older birds improved their energy gain simply by a higher speed of searching. The opposite was found for fishing success. As juvenile birds made fewer attempts than immature or adult birds, fishing success was higher in adults. Adults and immature birds interrupted more attacks than juveniles, which indicates that older birds were better able to assess the probability of fish catching than juveniles. Kleptoparasitism was observed almost exclusively on the refuse dump during the pre-breeding period. Young birds kleptoparasitised more frequently than adults, but they had a lower rate of success. However, the lower success in young birds was due to victim choice, rather than differences in flight skills. Young birds kleptoparasitised Black-headed Gulls Larus ridibundus and Jackdaws Corvus monedula more frequently than adults, but none of the attacks towards these species was successful. Generally, Caspian Gulls kleptoparasitised conspecifics more often than expected from species frequency. Only attacks towards conspecifics yielded any success.     

Foraging behavior of redheads (Aythya americana) wintering in Texas and Louisiana

Redheads, Aythya americana, concentrate in large numbers annually in traditional wintering areas along the western and northern rim of the Gulf of Mexico. Two of these areas are the Laguna Madre of Texas and Chandeleur Sound of Louisiana. We collected data on 54,340 activities from 103 redhead flocks in Texas and 51,650 activities from 57 redhead flocks in Louisiana. Males and females fed similarly, differing neither in levels of feeding (percent of all birds in flock that were feeding) (p>0.90) nor in percentages of birds feeding by diving, tipping, dipping, or gleaning from the surface (p>0.10). The foraging level of redheads in the upper Laguna Madre region was relatively constant throughout two winters. Foraging of redheads in early winter in Louisiana was significantly greater than redhead foraging in the upper Laguna Madre, but by late winter, foraging by redheads in Louisiana had declined to the same level as that shown by redheads foraging in the upper Laguna Madre. The overall foraging level of redheads from Chandeleur Sound was greater (41%) than that of redheads in the upper Laguna Madre (26%), yet it was quite similar to the 46% foraging level reported for redheads from the lower Laguna Madre. Redheads in the upper Laguna Madre region of Texas fed more by diving than did those in the Chandeleur Sound and the lower Laguna Madre. Diving increased in frequency in late winter. Greater reliance by redheads on diving in January and February indicates that the birds altered their foraging to feed in deeper water, suggesting that the large concentrations of redheads staging at this time for spring migration may have displaced some birds to alternative foraging sites. Our results imply that the most likely period for food resources to become limiting for wintering redheads is when they are staging in late winter.     

Central place foraging in a human-dominated landscape: how do common cranes select feeding sites?

Human infrastructure and disturbance play an important role when animals select resources in human-modified landscapes. Theory predicts that animals trade food intake against costs of movement or disturbance to optimize net energy gain and fitness, but other necessary resources may also constrain the decisions, e.g. when animals repeatedly need to return to a central location, such as a nest, waterhole or night roost. Central place foraging theory states that the probability of occurrence of an animal decreases with the distance to the central location while selectivity for food items or foraging sites providing high net energy gain should increase with distance. We studied foraging patterns of common cranes Grus grus feeding in an agricultural landscape adjacent to a wetland to which they return for night roost. We used availability of spilled grains on harvested fields and distance to human settlement as proxy for site quality (i.e. increased likelihood of increased net energy gain with increased food availability and less disturbance). As predicted by theory, our results clearly show that cranes were more likely (more than twice as high resource selection function scores) to select foraging sites close to roosts. However, contrary to predictions, the selection of high quality sites in terms of high food availability decreased with distance to roost sites. Nevertheless, our results indicate that cranes were more likely to select sites with low risk of human disturbance far from roost sites, and were more tolerant to disturbance close to roost sites. How different species respond to the local and environmental conditions will increase the understanding of the species’ resource requirement, and also where in the landscape to prioritize conservation or management actions (e.g. mitigation of human disturbance and crop damage prevention to sustain agricultural production).