Ontogenetic niche shifts in two populations of juvenile threespine stickleback, Gasterosteus aculeatus, that differ in pelvic spine morphology

Most research on ontogenetic niche shifts has focused on changes in habitat or resource use related to food resource distribution and heterospecific size-limited predation. Cannibalism, an intraspecific interaction, can also affect habitat selection or resource use by vulnerable size classes. Morphological defenses, such as spines, increase the effective size of an individual, making it more difficult to consume. The importance of such defense structures in affecting niche shifts in early life history stages is unclear. Using a combination of field observations and experiments in aquaria and wading pools, we examined the relative roles of cannibalism and morphology in determining juvenile habitat use in two populations of threespine stickleback that differ in pelvic spine morphology. Juveniles were categorized into three size classes: small (5–10 mm), medium (11–15 mm), and large (15–25 mm). In experiments assessing the relative vulnerability of juveniles to cannibalism by adults, we documented a significant difference among size classes in the number of juveniles eaten such that more large juveniles were eaten from the population lacking pelvic spines. The natural distribution of small and large juveniles in two distinct littoral microhabitats, open water and vegetation, was determined in each lake. In both populations, small juveniles were more abundant in vegetation. In the population with pelvic spines, a greater proportion of large juveniles was observed in open water than in vegetation. In the population without pelvic spines, the proportion of large juveniles did not differ between the two habitats. Experiments comparing juvenile habitat use in the presence or absence of adult conspecifics suggest that differences in habitat use may not only depend on the size of the individual, or the size of the individual relative to the size of the adult predator, but also on the degree of development or expression of defensive structures.     

Subordination in mixed-age bird flocks–a removal study

During the winters of 1979 to 1986, flocks (7–17 per season, 78 total) of Willow Tits Parus montanus (generally consisting of an adult pair and four juveniles) were studied in a subalpine mixed forest in Central Norway. The mean survival rate between December and early April for juveniles (0.32) was significantly lower than that found for adults (0.74). As it has previously been found that adult Willow Tits, being dominant, prevent the juveniles in a flock from using preferred foraging sites, a removal experiment was made to see if the survival of juveniles improved when living in flocks with no adults present. After removal of the adult pair from 13 flocks in early January, the juveniles from these flocks survived significantly less well, up to early April, than either the juveniles in 11 flocks in which the adults were present (controlled for flock size) or those in 14 flocks consisting of an adult pair and four juveniles. Despite a skewed access to the resources in favour of dominant adults, the cost of being a subordinate juvenile is probably outweighed by the improved predator evasion and improved food finding efficiency due to the greater experience of the adult pair possessing the territory.     

Competition in size-structured populations: mechanisms inducing cohort formation and population cycles

In this paper we investigate the consequences of size-dependent competition among the individuals of a consumer population by analyzing the dynamic properties of a physiologically structured population model. Only 2 size-classes of individuals are distinguished: juveniles and adults. Juveniles and adults both feed on one and the same resource and hence interact by means of exploitative competition. Juvenile individuals allocate all assimilated energy into development and mature on reaching a fixed developmental threshold. The combination of this fixed threshold and the resource-dependent developmental rate, implies that the juvenile delay between birth and the onset of reproduction may vary in time. Adult individuals allocate all assimilated energy to reproduction. Mortality of both juveniles and adults is assumed to be inversely proportional to the amount of energy assimilated. In this setting we study how the dynamics of the population are influenced by the relative foraging capabilities of juveniles and adults.In line with results that we previously obtained in size-structured consumer-resource models with pulsed reproduction, population cycles primarily occur when either juveniles or adults have a distinct competitive advantage. When adults have a larger per capita feeding rate and are hence competitively superior to juveniles, population oscillations occur that are primarily induced by the fact that the duration of the juvenile period changes with changing food conditions. These cycles do not occur when the juvenile delay is a fixed parameter. When juveniles are competitively superior, two different types of population fluctuations can occur: (1) rapid, low-amplitude fluctuations having a period of half the juvenile delay and (2) slow, large-amplitude fluctuations characterized by a period, which is roughly equal to the juvenile delay. The analysis of simplified versions of the structured model indicates that these two types of oscillations also occur if mortality and/or development is independent of food density, i.e. in a situation with a constant juvenile developmental delay and a constant, food-independent background mortality. Thus, the oscillations that occur when juveniles are more competitive are induced by the juvenile delay per se. When juveniles exert a larger foraging pressure on the shared resource, maturation implies an increase not only in adult density, but also in food density and consequently fecundity. Our analysis suggests that this correlation in time between adult density and fecundity is crucial for the occurrence of population cycles when juveniles are competitively superior.     

Body condition influences ontogeny of foraging behavior in juvenile southern elephant seals

Ontogeny of diving and foraging behavior in marine top predators is poorly understood despite its importance in population recruitment. This lack of knowledge is partly due to the difficulties of monitoring juveniles in the wild, which is linked to high mortality early in life. Pinnipeds are good models for studying the development of foraging behaviors because juveniles are large enough to robustly carry tracking devices for many months. Moreover, parental assistance is absent after a juvenile departs for its first foraging trip, minimizing confounding effects of parental input on the development of foraging skills. In this study, we tracked 20 newly weaned juvenile southern elephant seals from Kerguelen Islands for up to 338 days during their first trip at sea following weaning. We used a new generation of satellite relay tags, which allow for the transmission of dive, accelerometer, and location data. We also monitored, at the same time, nine adult females from the colony during their post‐breeding trips, in order to compare diving and foraging behaviors. Juveniles showed a gradual improvement through time in their foraging skills. Like adults females, they remarkably adjusted their swimming effort according to temporal changes in buoyancy (i.e., a proxy of their body condition). They also did not appear to exceed their aerobic physiological diving limits, although dives were constrained by their smaller size compared to adults. Changes in buoyancy appeared to also influence their decision to either keep foraging or return to land, alongside the duration of their haul outs and choice of foraging habitat (oceanic vs. plateau). Further studies are thus needed to better understand how patterns in juveniles survival, and therefore elephant seal populations, might be affected by their changes in foraging skills and changes in their environmental conditions.     

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.     

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.     

Avoidance of Filial Cannibalism in the Amphipod Gammarus pulex

Adult animals that cannibalise juvenile conspecifics may gain energy but also risk filial cannibalism, that is, consumption of their own offspring. However, individuals vary in the magnitude of the costs and benefits of cannibalism depending on factors such as their current energy reserves or the probability that they have offspring in the vicinity. They may therefore also vary in the extent to which they participate in cannibalism. This study investigated whether the sex or brooding status of adult amphipods (Gammarus pulex) influenced whether they participated in cannibalism of juveniles. For females carrying embryos within their brood pouch, we also investigated two hypotheses to explain the presence or absence of cannibalistic behaviour by determining whether cannibalism was correlated with factors that might reflect energy demands (body length, brood size), or that might reflect a temporal change in cannibalistic behaviour (corresponding to stage of brood development). All reproductive classes of adults participated in some level of juvenile cannibalism, but females carrying offspring at an advanced stage of development (close to emergence from the brood pouch) consumed significantly fewer juveniles than other groups. Females thus appear to significantly reduce cannibalism of juveniles concurrent with the time when their own eggs are hatching within the brood pouch, prior to the release of their offspring. Because the experiment tested female responses to unfamiliar juveniles, this reflects a temporal change in behaviour rather than a response to phenotypic recognition cues, although additional direct recognition cannot be ruled out. Brooding females with large brood sizes or large body lengths, which might have disproportionately greater energetic demands, were not more likely to cannibalise juveniles. We also noted that juveniles that survived in trials where cannibalism occurred were significantly more likely to be found at the water surface, suggesting a possible adaptation to escape cannibalistic adults. Overall, our results provide evidence that amphipods use indirect temporal cues to avoid filial cannibalism.     

Behavioral signature of intraspecific competition and density dependence in colony‐breeding marine predators

In populations of colony‐breeding marine animals, foraging around colonies can lead to intraspecific competition. This competition affects individual foraging behavior and can cause density‐dependent population growth. Where behavioral data are available, it may be possible to infer the mechanism of intraspecific competition. If these mechanics are understood, they can be used to predict the population‐level functional response resulting from the competition. Using satellite relocation and dive data, we studied the use of space and foraging behavior of juvenile and adult gray seals (Halichoerus grypus) from a large (over 200,000) and growing population breeding at Sable Island, Nova Scotia (44.0 ^oN 60.0 ^oW). These data were first analyzed using a behaviorally switching state‐space model to infer foraging areas followed by randomization analysis of foraging region overlap of competing age classes. Patterns of habitat use and behavioral time budgets indicate that young‐of‐year juveniles (YOY) were likely displaced from foraging areas near (<10 km) the breeding colony by adult females. This displacement was most pronounced in the summer. Additionally, our data suggest that YOY are less capable divers than adults and this limits the habitat available to them. However, other segregating mechanisms cannot be ruled out, and we discuss several alternate hypotheses. Mark–resight data indicate juveniles born between 1998 and 2002 have much reduced survivorship compared with cohorts born in the late 1980s, while adult survivorship has remained steady. Combined with behavioral observations, our data suggest YOY are losing an intraspecific competition between adults and juveniles, resulting in the currently observed decelerating logistic population growth. Competition theory predicts that intraspecific competition resulting in a clear losing competitor should cause compensatory population regulation. This functional response produces a smooth logistic growth curve as carrying capacity is approached, and is consistent with census data collected from this population over the past 50 years. The competitive mechanism causing compensatory regulation likely stems from the capital‐breeding life‐history strategy employed by gray seals. This strategy decouples reproductive success from resources available around breeding colonies and prevents females from competing with each other while young are dependent.     

The evolution of juvenile-adult interactions in populations structured in age and space

We study the evolution of a spatially structured population with two age classes using spatial moment equations. In the model, adults can either help juveniles by increasing their survival, or adopt a cannibalistic behaviour and consume juveniles. While cannibalism is the sole evolutionary outcome when the population is well-mixed, both cannibalism and parental care can be evolutionarily stable if the population is viscous. Our analysis allows us to make two main technical points. First, we present a method to define invasion fitness in class-structured viscous populations, which allows us to apply adaptive dynamics methodology. Second, we show that ordinary pair approximation introduces an important quantitative bias in the evolutionary model, even on random networks. We propose a correction to the ordinary pair approximation that yields quantitative accuracy, and discuss how the bias associated with this approach is precisely what allows us to identify subtle aspects associated with the evolutionary dynamics of spatially structured populations.     

Sex,Age, and Population Density Affect Aggressive Behaviors in Island Lizards Promoting Cannibalism

Island populations may evolve distinct behavioral repertoires as a response to the conditions of insular life. Strong intraspecific competition is typical in insular lizards and may include cannibalism. In this study, we investigated sexual and age patterns of aggression in two populations of the Skyros wall lizard (Podarcis gaigeae), one from the main island of Skyros (Aegean Sea, Greece) and another from the satellite islet Diavates. The latter is terrestrial predator‐free biotope, hosting a dense population of large‐bodied lizards that have been reported to exert cannibalism. In staged encounters, we examined the aggressive propensities of adult male and female lizards against their age‐peers and juveniles. Males from both populations were much more aggressive than females toward juveniles and other adults. Males from Diavates were more frequently aggressive to juveniles and other male lizards than males from Skyros. Diavates cannibals also captured their targets at shorter latency. We ascribe this distinct behavioral pattern to the high population density. Infanticide and intramale aggressiveness confer two great advantages to cannibals: food and elimination of future rivals.     

Effects of tail autotomy on survival,growth and territory occupation in free‐ranging juvenile geckos (Oedura lesueurii)

Abstract Many animals autotomize their tails to facilitate escape from predators. Although tail autotomy can increase the likelihood of surviving a predatory encounter, it may entail subsequent costs, including reduced growth, loss of energy stores, a reduction in reproductive output, loss of social status and a decreased probability of survival during subsequent encounters with predators. To date, few studies have investigated the potential fitness costs of tail autotomy in natural populations. I investigated whether tail loss influenced survival, growth and territory occupation of juvenile velvet geckos Oedura lesueurii in a population where predatory snakes were common. During the 3‐year mark–recapture study, 32% of juveniles voluntarily autotomized their tails when first captured. Analysis of survival using the program mark showed that voluntary tail autotomy did not influence the subsequent survival of juvenile geckos. Survival was age‐dependent and was higher in 1‐year‐old animals (0.98) than in hatchlings (0.76), whereas recapture probabilities were time‐dependent. Growth rates of tailed and tailless juveniles were very similar, but tailless geckos had slow rates of tail regeneration (0.14 mm day⁻¹). Tail autotomy did not influence rock usage by geckos, and both tailed and tailless juveniles used few rocks as diurnal retreat sites (means of 1.64 and 1.47 rocks, respectively) and spent long time periods (85 and 82 days) under the same rocks. Site fidelity may confer survival advantages to juveniles in populations sympatric with ambush foraging snakes. My results show that two potential fitness costs of tail autotomy – decreased growth rates and a lower probability of survival – did not occur in juveniles from this population. However, compared with juveniles, significantly fewer adult geckos (17%) voluntarily autotomized their tails during capture. Because adults possess large tails that are used for lipid storage, the energetic costs of tail autotomy are likely to be much higher in adult than in juvenile O. lesueurii.     

Guppy populations differ in cannibalistic degree and adaptation to structural environments

There is considerable variation in cannibalism between different species and also between individuals of different species, although relatively little is known about what creates this variation. We investigated the degree of cannibalism in guppy (Poecilia reticulata) populations originating from high and low predation environments in Trinidad, and also how cannibalism was affected by the presence of refuges. Females from two populations were allowed to feed on juveniles from two populations in aquaria trials. The cannibalism was size-dependent and varied depending on both juvenile and female origin. Low predation females were more efficient cannibals and low predation juveniles were better at avoiding cannibalism compared to high predation guppies when no refuges were present. The high predation females were superior cannibals and the high predation juveniles were better at escaping cannibalism than the low predation guppies when refuges were present. We discuss whether the differences in cannibalism and response to refuge addition relate to predation-induced habitat shifts and differences in the guppies’ natural environment.     

Ontogenetic development underlies population response to mortality

Understanding demographic responses to mortality is crucial to predictive ecology. While classic ecological theory posits reductions in population biomass in response to extrinsic mortality, models containing realistic developmental change predict the potential for counterintuitive increase in stage-specific biomass, i.e. biomass overcompensation. Patterns of biomass overcompensation should be predictable based on differences in the relative energetic efficiencies of juvenile maturation and adult reproduction. Specifically, in populations where reproduction is the limiting process, adult-specific mortality should enhance total reproduction and thus juvenile biomass. We tested this prediction by inducing an array of stage-specific harvesting treatments across replicate populations of Daphnia pulex. In accordance with reproductive regulation, the greatest biomass response occurred in the juvenile Daphnia stage and this response occurred most strongly in response to adult mortality. Nevertheless, we failed to detect significant biomass overcompensation and instead report largely compensatory effects. In total, our work demonstrates that knowledge of population structure is necessary to accurately predict population dynamics, but cautions that further research is needed to illuminate the factors generating over-compensatory versus compensatory responses across natural populations.     

Role of kin recognition in oviposition preference and cannibalism by the predatory mite <Emphasis Type="Italic">Gynaeseius liturivorus</Emphasis>

Animals often select oviposition sites to minimize the predation risk for eggs and juveniles, which are more vulnerable to predation than adults. When females produce eggs in clusters, the eggs and juveniles are likely to suffer from cannibalism. Although cannibalism among siblings is known to be lower than among non-siblings, there have been few investigations into the possibility that females select oviposition sites that reduce the risk of cannibalism for the offspring. To test this possibility, we examined oviposition preference by adult females of the predatory mite Gynaeseius liturivorus in response to the presence of her own eggs and to eggs of other females, offering plastic discs as oviposition substrates. Although females did not clearly show a preference for plastic discs on which they had oviposited, they avoided plastic discs on which other females had oviposited. When eggs of other females were artificially placed on clean plastic discs, adult female mites avoided these discs, suggesting that the eggs were used as cues for oviposition preference. Cannibalism among juvenile siblings was lower than among non-siblings. These observations show that adult females and juveniles of G. liturivorus discriminate kin relationships among conspecific individuals. Therefore, oviposition preference by adult female G. liturivorus may lead to the reduced risk of cannibalism among offspring.     

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.     

The cannibalistic behaviour of two Gammarus species (Crustacea: Amphipoda)

This study examines the cannibalistic behaviour of the freshwater amphipods Gammarus duebeni celticus Stock & Pinkster, 1970 and G. pulex (L., 1758). In the first experiment, interactions were staged among all combinations of single adult males, single adult females, adults in the precopulatory mate-guarding phase and juveniles. Cannibalism by inter-moult individuals on newly moulted conspecifics occurred in all interaction categories in both species. Gammarus d. celticus , however, were significantly more cannibalistic than G. pulex. Cannibalism between and within sex and size categories (males > females > juveniles) was facilitated by the vulnerability of individuals at moult. Individuals of smaller size categories, however, did not cannibalize newly moulted conspecifics of larger size categories. Males were less cannibalistic on newly moulted females than on newly moulted males and juveniles and, when in the precopulatory condition, appeared to defend females from cannibalistic attacks. In a second experiment, stream conditions were simulated in the laboratory and replicated populations monitored for nine weeks. High levels of cannibalism, and the species and sex differences in cannibalism identified in the first experiment, were confirmed under these heterogeneous conditions. Cannibalism by males on their newly moulted female mating partners, termed 'reversed' sexual cannibalism, was further investigated. When males were deprived of foraging opportunities, cannibalism of precopulatory partners was significantly more frequent. The occurrence of 'reversed' sexual cannibalism is thus interpreted as a conflict between motivation to feed and motivation to mate.     

Size-dependent effects of an invasive herbivorous snail (Pomacea canaliculata) on macrophytes and periphyton in Asian wetlands

1. The invasive golden apple snail (Pomacea canaliculata), native to South America, is a serious pest on rice seedlings in south‐east Asia and has also been shown to consume large amounts of macrophytes in natural wetlands, with large effects on ecosystem functioning. Earlier studies suggest that the snail undergoes an ontogenetic diet shift, feeding on algae and detritus as juveniles and shifting to aquatic macrophytes as adults. 2. Here, we study the effects of snail populations with a size‐structure typical of either populations at an invasive front or the size‐structure of established populations. In an enclosure experiment performed in a wetland in Laos, we compared treatments with small snails only (3 mm; invasive treatment) to treatments with small, medium sized (10 mm) and adult (>25 mm) snails (established treatment). The effects of snail grazing on three aquatic macrophyte species and periphytic algae were quantified. 3. We found that snails of all sizes had a strong negative effect on the biomass of all macrophyte species and periphytic algae. There was no evidence of an ontogenetic diet change, i.e. snails in both the invasive and established treatments affected macrophyte biomass. Foraging was size‐dependent in that small snails had higher relative foraging capacity (g plant consumed per g of snail) compared with medium and adult snails. Small snails, therefore, depressed growth of medium snails at increasing densities through exploitative competition for preferred resources, while adult snails did not grow at all in the presence of small snails. 4. Density dependence is common in freshwater invertebrates, including gastropod populations, but differences in size dependent foraging‐ and competitive‐ability have rarely been demonstrated in this group of organisms. Knowledge about intra‐specific differences in ecological performance may, however, both deepen our understanding of the processes that underlie population dynamics in invertebrates such as gastropods, and help develop control strategies for invasive golden apple snails.     

Intriguing compensation by adult female spiders for food limitation experienced as juveniles

We conducted a field experiment to test for food limitation in immature stages, and its consequences for mature females, in the territorial, cannibalistic spider Lycosa tarentula (L.). Randomly selected antepenultimate juveniles were provided supplemental prey until they matured, at which time supplemental feeding ceased. Immature stages of L. tarentula are food-limited. Supplemented juvenile spiders decreased foraging activity, disappeared at a lower rate and grew faster than the control spiders, which had been exposed only to ambient prey levels. Fed juvenile females were less hungry at maturity, as judged by an index of body condition, and showed higher mating success as adults, as judged by cohabitation rates with mature males. Foraging theory predicts that in order to compensate for residual effects of food limitation, adult female spiders that had experienced a shortage of prey as juveniles – the controls – would have to exhibit a greater increase in foraging activity upon maturing than the prey-supplemented group. Contrary to expectation, the control females did not increase their foraging activity, but the previously fed females did forage more actively as adults. Furthermore, the difference in mass gain during the mating period between the two groups was opposite from what the difference in change in foraging activity would predict. Control females, the spiders that had not changed their foraging activity, gained mass more rapidly than the previously fed females, with the result that the two groups were similar in mass by the end of the mating period. We hypothesize that an increased rate of sexual cannibalism may have been one mechanism by which control females compensated for the food limitation that they had experienced as immatures.     

Do naive juvenile seabirds forage differently from adults?

Foraging skills of young individuals are assumed to be inferior to those of adults. The reduced efficiency of naive individuals may be the primary cause of the high juvenile mortality and explain the deferment of maturity in long-lived species. However, the study of juvenile and immature foraging behaviour has been limited so far. We used satellite telemetry to compare the foraging movements of juveniles, immatures and breeding adult wandering albatrosses Diomedea exulans, a species where foraging success is positively influenced by the distance covered daily. We showed that juveniles are able to use favourable winds as soon as the first month of independence, but cover shorter distances daily and spend more time sitting on water than adults during the first two months after fledging. These reduced movement capacities do not seem to be the cause of higher juvenile mortality. Moreover, juveniles almost never restrict their movement to specific areas, as adults and immatures frequently do over shelf edges or oceanic zones, which suggest that the location of appropriate areas is learned through experience. Immatures and adults have equivalent movement capacities, but when they are central place foragers, i.e. when adults breed or immatures come to the colony to display and pair, immatures make shorter trips than adults. The long duration of immaturity in this species seems to be related to a long period of learning to integrate the foraging constraints associated with reproduction and central place foraging. Our results indicate that foraging behaviour of young albatrosses is partly innate and partly learned progressively over immaturity. The first months of learning appear critical in terms of survival, whereas the long period of immaturity is necessary for young birds to attain the skills necessary for efficient breeding without fitness costs.     

From cradle to early grave: juvenile mortality in European shags Phalacrocorax aristotelis results from inadequate development of foraging proficiency

In most long-lived animal species, juveniles survive less well than adults. A potential mechanism is inferior foraging skills but longitudinal studies that follow the development of juvenile foraging are needed to test this. We used miniaturized activity loggers to record daily foraging times of juvenile and adult European shags Phalacrocorax aristotelis from fledging to the following spring. Juveniles became independent from their parents 40 days post-fledging. They compensated for poor foraging proficiency by foraging for approximately 3 h d(-1) longer than adults until constrained by day length in early November. Thereafter, juvenile foraging time tracked shortening day length up to the winter solstice, when foraging time of the two age classes converged and continued to track day length until early February. Few individuals died until midwinter and mortality peaked in January-February, with juvenile mortality (including some of the study birds) five times that of adults. In their last two weeks of life, juveniles showed a marked decline in foraging time consistent with individuals becoming moribund. Our results provide compelling evidence that juveniles compensate for poor foraging proficiency by increasing foraging time, a strategy that is limited by day length resulting in high winter mortality.