Disparity between Adult and Juvenile American Robins Turdus migratorius Foraging for Ground Invertebrates and Cherry Fruits

Although it has long been known that juveniles often have foraging skills inferior to those of adults, it has generally been assumed that animal prey are more difficult to capture than fruit, and thus that juveniles foraging on fruit should be similar to adults in their efficiency. To examine these ideas, we investigated the abilities of juvenile and adult American robins Turdus migratorius to forage for ground invertebrates and fruits of the black cherry tree Prunus serotina. We hypothesized that juveniles, lacking the experience of adults, would not have the skills of adults and therefore would be less proficient invertebrate and fruit foragers. Juveniles captured 69% of invertebrates at which they struck compared with 80% of adults’ strikes that ended in capture. However, juveniles made more strikes than adults, so mean prey capture per minute was the same. Juveniles were also less skilled fruit foragers. Juveniles were twice as likely as adults to fail to pick a cherry (55% of adults’ vs. 28% of juveniles’ attempts ended in success). However, there was no significant difference in the proportion of juveniles and adults that dropped a cherry once it was picked. As a result of their low levels of success, juveniles consumed about half the number of cherries per minute as did adults. Contrary to prior assumptions, skills involved in fruit foraging may not be so easily acquired and many omnivorous species, like the American robin, must learn both invertebrate and fruit foraging skills.     

Microhabitat use and prey selection of the coral-feeding snail <Emphasis Type="Italic">Drupella cornus</Emphasis> in the northern Red Sea

Corallivorous gastropods of the genus Drupella are known for population outbreaks throughout the Indo-Pacific region. Despite their potential to destroy wide areas of coral reef, prey preferences have never been analyzed with respect to prey availability, and juvenile ecology and food selectivity remain largely unknown. Here, the influence of water depth, coral abundance, colony shape, prey species, and intraspecific attraction among snails on distribution patterns, prey selection, and microhabitat use of D. cornus was studied in the northern Red Sea. Special emphasis was put on ontogenetic differences. The snails were most abundant in the shallowest reef zone (1 m depth). Adults were associated with several substrates and coral growth forms, whereas juveniles were highly cryptic and restricted to live branching corals. The genus Acropora was significantly preferred over other acroporid and pocilloporid corals. As revealed by resource selection ratios, Acropora acuminata was preferred by juveniles, A. selago by adults. In aquarium experiments, intraspecific attraction was high among both life stages. Overall, significant differences in juvenile and adult microhabitat and prey use suggest that juveniles have more specific habitat requirements, and indicate ecological impacts on coral communities different from that of adults. Prey preferences seem to depend on both coral genus and colony shape. Acropora corals provide the best combination of food and shelter and therefore determine distribution patterns of D. cornus.     

Prey selection and dietary response by wolves in a high-density multi-species ungulate community

Studies on predation by the wolf (Canis lupus) have often reported contradictory results about the role of prey density and vulnerability on wolf prey use. We investigated dietary response and prey selection by wolves in a high-density and multi-species ungulate community, analysing scats collected over a period of 11 years in the Casentinesi Forests, Italy. The second most abundant species, wild boar (Sus scrofa), was found to be the main wolf prey, and we did not observe any dietary response of wolves to variations in the density of either primary or secondary prey species. Selection patterns were uniform throughout the study period. Wolves strongly selected for wild boar piglets, while roe deer (Capreolus capreolus) fawns and adults, red deer (Cervus elaphus) adults and fallow deer (Dama dama) adults were avoided. Wolf preference for wild boar was inversely density dependent. Within each species, juveniles were preferred to adults. Medium-sized, young individuals of both wild boar and roe deer were optimal prey, although with different selection patterns related to the different anti-predator strategies adopted by each prey species. The results of this study suggest that in productive ecosystems with high density and high renewal rates of prey, selection patterns by wolves are determined by prey vulnerability, which is connected to prey age and body size. The different patterns of wild boar versus cervids use by wolf across Europe seems to be related to their relative abundances, while the strong selection of wild boar in Italian Apennines with respect to the more frequent avoidance in central-eastern Europe is better explained by higher piglet productivity and smaller body size of adults boar in Mediterranean temperate forests.     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Ontogenetic variation in the autecology of the greater earless lizard Cophosaurus texanus

We investigated the potential for ontogenetic resource partitioning within a population of the Texas earless lizard Cophosaurus texanus Results from focal samples, line transects, haphazard observations, and stomach flushes compiled in summer (July 1993) and autumn (September 1995) revealed differences in microhabitat use and diet between adults and juveniles Juveniles use rock perches more frequently than adult males, and consume smaller prey than adults No ontogenetic differences in thermal ecology were observed Diet differences between juveniles and adults may be attributed to the inability of juveniles to handle large prey items Differences between juveniles and adults in prey size and microhabitat use suggest ontogenetic variation in foraging, predator avoidance, and territory defense     

Determinants of dietary specialization: a comparison of two sympatric species of sea snakes

Why do some predator species specialize on only a single type of prey whereas others take a broad range? One critical determinant may be the ontogenetic range of body sizes of the predator compared to that of its prey. If any single prey taxon spans only part of the range of prey sizes ingestible by the predator, then the predator will be more likely to take multiple prey taxa. We exploit a model system that provides a robust opportunity to test this hypothesis. We studied two sympatric species of predatory sea snakes, similar in size and general ecology that feed on anguilliform fishes from different habitats in the Great Lagoon of New Caledonia. Eel species from soft‐bottom habitats must construct their own burrows, and thus tend to be more slender‐bodied and less variable in body size than eel species that inhabit variable‐sized crevices among hard coral. As a result, a laticaudine sea snake species (Laticauda saintgironsi) that feeds on hard‐coral‐dwelling eels relies primarily on a single prey species: juveniles take young eels whereas adults consume adult eels of the same species. In contrast, a laticaudine species (L. laticaudata) that forages on soft‐bottom eels switches its prey ontogenetically: juveniles take small eel species whereas adults consume large eel species. Thus, habitat‐imposed constraints on the range of body sizes within each prey taxon generate a striking difference in the degree of dietary specialization of two closely related, sympatric predator species.     

Extended Mother–Offspring Relationships in Crayfish: The Return Behaviour of Juvenile Procambarus Clarkii

Crayfish shows a relatively complex parental behaviour compared with other invertebrates, but the literature provides only anecdotal accounts of this phenomenon. In Procambarus clarkii, we described the ‘return’ behaviour of third‐stage juveniles when offered four types of adults: biological mothers, foster mothers, non‐brooding females and males. Then, we analysed the posture and behaviour of the adults to understand the role played by the putative mother in attracting the juveniles. Contrary to non‐brooding individuals, both biological and foster mothers displayed a relatively rare locomotion, executed few cleaning and feeding acts, and never attempted to prey on juveniles. They often assumed a ‘spoon‐like telson posture’ that seemed to facilitate offspring’s approaches. Juveniles increased the frequency of tail‐flips away in the presence of non‐brooding adults; conversely, they accepted foster mothers, along with biological mothers, but not as fast as the latter. Taken together, these results suggest that mother–offspring relationships in P. clarkii are more refined than previously thought, being possibly a key factor enabling this species to thrive in harsh environmental conditions.     

Age and Sex-Based Differences in the Use of Prey Sensory Cues in Wolf Spiders (Araneae: Lycosidae)

Differences in foraging patterns mediated by sensory cues were examined between adult and juvenile male and female wolf spiders (Schizocosa rovneri; Lycosidae). Patch residence time for thirty-one spiders were tested among juveniles and adults in artificial foraging patches. Patches varied in sensory information provided by live prey (crickets) as follows: visual stimuli alone; vibratory stimuli alone; visual and vibratory stimuli together; and control (no stimuli). Spiders moved between patches for one hour, but could not feed. Adult Schizocosa rovneri use primarily visual information to determine patch residence time, but juveniles use vibratory cues as well. Significant age and sex-based differences in the use of sensory cues suggest that observed divergent foraging strategies are partly due to the use of different perceptual cues in prey detection.     

Assessment of the diets of cod Gadus morhua and whiting Merlangius merlangus juveniles in a frontal region close to the Norwegian Trench: co‐existence or competition?

Over 200 pelagic juvenile cod Gadus morhua and whiting Merlangius merlangus were sampled in spring 2001 at stations inside and outside a frontal region close to the Norwegian Trench, and their stomach contents analysed in the context of zooplankton samples taken at the same time and same locations. Merlangius merlangus juveniles appeared to feed on a diverse zooplankton community, whereas G. morhua juveniles appeared to preferentially select Pseudocalanus elongatus, even though these prey items were not the dominant component of the prey community. The results suggest that during the pelagic juvenile stage, M. merlangus and G. morhua are direct competitors for zooplanktonic prey. In addition, because M. merlangus are more opportunistic, they may be more tolerant to changes in the species composition of the prey community, and therefore out‐compete G. morhua juveniles at times of low preferred prey abundance or under changing environmental conditions.     

Foraging Ontogeny is not Linked to Delayed Maturation in Squirrel Monkeys (Saimiri sciureus)

The acquisition of complex foraging behaviors by young is a proposed cause of a prolonged juvenile phase in many vertebrates, including primates. I compared the foraging behaviors of infant, juvenile and adult squirrel monkeys to determine if significant age‐related differences in foraging behavior and efficiency were present. Infants and juveniles differed from each other in patterns of prey and fruit foraging, but few differences existed between juveniles and adults. Despite differing in the use of foraging substrates, young juveniles (8–12 mo) were as efficient as older juveniles (1–4 yr) and adults at capturing and processing large prey. Young juveniles (<1 yr) were limited in their ability to consume husked palm fruits due to an inability to peel them to obtain pulp. By 1 yr of age, however, foraging behaviors of adults and juveniles were nearly indistinguishable. The absence of meaningful differences between adults and juvenile foraging is not consistent with the hypothesis that the need to develop foraging skills accounts for the pattern of extended juvenility in squirrel monkeys.     

Feeding ecology and seasonal diet overlap between Stellifer brasiliensis and Stellifer stellifer in a tropical estuarine ecocline

Seasonal freshwater discharge was important for defining habitat utilization by different ontogenetic phases of Stellifer brasiliensis and Stellifer stellifer along the estuarine ecocline. The middle estuary was important as a nursery and feeding ground for young‐of‐the‐year, and a feeding ground for sub‐adults and adults of both species. These species are zoobenthivorous, but during their life cycle and between different habitats and seasons, their trophic guild can change to opportunist and zooplanktivore. During the late rainy season in the lower estuary, all phases of both species, except juveniles of S. brasiliensis and adults of S. stellifer, showed a niche overlap indicating similarity in prey utilization. The diet composition was qualitatively similar, showing an evident niche overlap of intra and interspecific competition among the Stellifer spp. Although the niches of these species appeared to significantly overlap, some resource partitioning patterns were apparent. The niche overlap was significantly reduced due to the seasonal difference in habitat use and prey consumption along the ecocline of the estuary by different ontogenetic phases. The ingestion of blue nylon fragments by both species was observed and quantified.     

Play behaviour and active training of Montagu’s harrier (Circus pygargus) offspring in the post-fledging period

Play bouts and active training of juveniles by Montagus harrier (Circus pygargus) adults in the post-fledging period were observed. Fledglings often played with prey and with a variety of inanimate objects such as bits of moss, regurgitated pellets, sticks and a wad of hay. Inanimate objects selected for play were, in length, very similar to the common vole (Microtus arvalis), which is the most common prey of the species during their breeding period. Some recently fledged individuals were trained to capture invertebrate prey by adults demonstrating techniques for the fledglings and thus develop their hunting skills. Training sessions took place only in the foraging areas of the adult birds.     

Progressive parenting behavior in wild golden lion tamarins

Young primates in the family Callitrichidae (the marmosets and tamarins) receive extensive and relatively prolonged care from adults. Of particular note, callitrichid young are routinely provisioned until well after weaning by parents and helpers, which is in stark contrast to typical juvenile primates, who must acquire most of their food independently once they are weaned. Adults of some callitrichid species produce a specialized vocalization that encourages immature group members to take proffered food from the caller. Here, I report that wild adult golden lion tamarins (Leontopithecus rosalia) not only used this food-offering call to encourage young, mobile offspring to approach and take captured prey from them, but as the young began to spend significant time foraging for themselves and to acquire prey by independent means, the frequency of these vocalizations in the context of food transfer declined. Adults then began to use food-offering calls in a novel context: to direct juveniles to foraging sites that contained hidden prey that the adults had found but not captured. During the period of these most frequent adult-directed prey captures, the independent prey-capture success rates of juveniles improved. Thus, adults modified their provisioning behavior in a progressive developmentally sensitive manner that may have facilitated learning how to find food. I hypothesize that as a result of these demonstrations by adults, juveniles either may be encouraged to continue foraging despite low return rates or to learn the properties of productive prey-foraging substrates in a complex environment.     

Post-migratory care of young by Crab Plovers Dromas ardeola

The Crab Plover Dromas ardeola is the only waterbird species known to provision offspring well after the post-reproductive migration and through overwintering. A few previous quantitative studies have reported inconclusively that juveniles begged rarely, and also indiscriminately at other juveniles. Here, we describe the feeding behaviour of adult and juvenile Crab Plovers during the first part of the wintering period. Juveniles begged frequently for food, always toward adults, and obtained 0.1 prey items/min from them. Begging birds obtained the largest prey items of those captured by the adults. When foraging alone, juveniles captured prey at the same rate as adults, but captured smaller crabs. The feeding success of adults was not altered by the presence of the begging juvenile. Juveniles depended partly on adults during the first part of the overwintering period, but were almost independent towards the end. Crab Plovers may adopt such unusually prolonged care because they need to abandon their breeding areas quickly, when environmental conditions are extreme. Large numbers of Crab Plovers overwinter in a few areas, now subject to human alteration. Isolated adults may forage on other areas such as narrow and disturbed shores, but juveniles may require wide beaches, suitable for group foraging, that should be considered as key areas for the recruitment of juveniles into the population.     

Social learning of hunting skills in juvenile marsh harriers <Emphasis Type="Italic">Circus aeruginosus</Emphasis>

The impact of social factors on the improvement of hunting skills of juvenile marsh harriers during their first autumnal migration were studied in SE Poland. While foraging with adult birds, juveniles performed more dives on prey both in terms of number of trials and rates. Hunting sessions of juveniles were more efficient in the presence of adults than in the absence of adults. Juveniles hunting with adults and other juveniles could select adequate habitat patches in which access to prey is easier. The role of vertical and horizontal transmission of information in the development of hunting skills in juvenile marsh harrier were confirmed because faster development of hunting ability was achieved in the social hunting after the end of their postfledging dependency period.     

Why do Juvenile Chinese Pit‐Vipers (Gloydius shedaoensis) Select Arboreal Ambush Sites?

Ontogenetic shifts in habitat use are widespread, especially in ectothermic taxa in which juveniles may be an order of magnitude smaller than large adult conspecifics. The factors that generate such habitat shifts are generally obscure, but we studied an unusual system that allowed us to compare consequences of habitat selection between adults and juveniles. Pit‐vipers (Gloydius shedaoensis) on a small island in north‐eastern China feed almost entirely on seasonally migrating birds. During the spring bird‐migration period, individual snakes consistently re‐used either arboreal or terrestrial ambush sites. Snakes in trees were smaller (and more philopatric) than snakes on the ground. This ontogenetic shift in habitat use may reflect the difficulty of capturing birds on the ground, especially by small snakes. In laboratory trials, large (adult) pit‐vipers struck faster, further and more accurately than did small (juvenile) snakes. In experiments with free‐ranging snakes, the proportion of strikes hitting the bird was lower for juveniles than for adults, and lower for terrestrial snakes than for arboreal snakes. Additionally, adult snakes generally seized the bird by the head whereas juveniles frequently struck the body or wings (and thus, obtained a less secure grip). Arboreal ambush sites may facilitate prey capture not only because they give access to smaller birds but also because they render the bird's location more predictable and, hence, enable the snake to position itself optimally prior to the prey's arrival. Because juvenile pit‐vipers are less capable strikers, and are small relative to available prey items, they may benefit from the greater ease of prey capture from branches. Thus, the ontogenetic shift in habitat selection within this species may be because of ontogenetic shifts in the vipers’ ability to capture and ingest large, mobile prey.     

The diet ofEnoplus brevis (Nematoda) in a supralittoral salt marsh of the North Sea

The gut content of nearly 2000 specimens ofEnoplus brevis was quantitatively analysed.E. brevis is an opportunistic omnivore. Main food categories found were cyanobacteria, diatoms, oligochaetes, nematodes, and rotifers. With a few exceptions, the diet of females and males was identical. Predominantly, juveniles fed on cyanobacteria and adults on animal prey. Experiments revealed that living oligochaetes are attacked and preyed upon. The quantities of the food categories eaten byEnoplus varied according to their ambient abundance, both temporarily and spatially on a small scale. At least the nematode prey was strongly selected: some species were significantly preferred and others avoided.     

Distribution and feeding behaviour of field populations of the water cricket Velio, caprai (Hemiptera)

1. The distribution of individuals, prey capture success and prey choice were studied in a water cricket (Velia caprai) population in southern Ireland. Groups of different sizes were observed in the field during the summer and all potential prey animals entering the groups were counted. Surface drift samples were collected to assess the proportion of profitable prey items. 2. Two types of Velia groups were observed: mixed groups (adults and juvenile instars 4 and 5) and juvenile groups (instars 1–3), A greater proportion of the available prey was captured by the mixed groups. 3. Individuals in large groups had a lower capture rate than individuals in small groups. 4. No differences in capture rates could be found between males, females and juveniles, but differences in prey choice were apparent. Females captured more large prey than expected. 5. Water cricket distribution in the stream did not follow the ‘ideal free’ model; too few individuals were found at the most profitable sites. Food might not be the limiting factor during summer, and other factors such as intraspecific interference and predator avoidance could be more important.     

Size related variability in the summer diet of the blackfin tuna (Thunnus atlanticus Lesson, 1831) from Tobago,the Lesser Antilles

Blackfin tuna (Thunnus atlanticus) is a small epipelagic oceanic species known only from the western Atlantic. In Tobago, the Lesser Antilles, blackfin tuna is caught by the artisanal fishery. The diet of this species was examined during the summer of 2004 for fish landed at the Charlotteville fish market in Tobago. T. atlanticus ranged from 32 to 91 cm FL (0.7–12.4 kg). Overall numerical abundance of prey items comprised fish (48%), crustaceans (46%) and cephalopods (6%). Prey species included small pelagics such as anchovies (ranked as most important prey overall), juveniles of larger pelagics such as jacks, juveniles of fish found in coral reef communities as adults, e.g. squirrelfishes, and some mesopelagic species. The importance of major diet categories differed significantly with predator size, with fishes becoming more important and crustaceans less important with increasing size of the blackfin tuna. This study has improved our scant knowledge of the blackfin tuna diet in the Lesser Antilles.     

Influence of diet on the predation rate of <Emphasis Type="Italic">Orius laevigatus</Emphasis> on <Emphasis Type="Italic">Frankliniella occidentalis</Emphasis>

The predation rate of fifth instars and female adults of Orius laevigatus (Fieber) fed honeybee pollen, Ephestia kuehniella Zeller eggs or an egg yolk based artificial diet on second instars of the western flower thrips Frankliniella occidentalis (Pergande) was examined in the laboratory. Predation rate of both fifth instars and female adults was not influenced by their diet. Despite a lower body weight, O. laevigatus reared on artificial diet or pollen killed as many prey as their peers reared on E. kuehniella eggs, suggesting that body weight is not a reliable predictor of predation rate. The use of non-prey foods for O. laevigatus for mass production or as a supplementary food to sustain its populations in the field when prey are scarce is discussed.