Dominance behaviour,body weight and fat variations,and partial migration in European blackbirds Turdus merula

Summary A partially migratory population of European blackbirds Turdus merula was studied in central Europe during the non-breeding season. Body weight and the amount of subcutaneous fat deposits showed seasonal variations typical for each age and sex category. Juveniles were fattest in autumn and spring and both juveniles and females lost fat and body weight in mid-winter. Adults and males reached their maximum body weight in mid-winter. These findings together with data from Schwabl (1983) suggest that juveniles and females constitute the main fraction of those birds migrating from the area in autumn. In winter mainly adults and males have access to food resources. These patterns may be regulated by dominance behaviour, since adults and males are usually dominant over juveniles and females throughout the non-breeding season. Subordinate birds that stay in the area probably suffer from a higher winter mortality but have the advantage of early breeding.     

Risk-sensitive foraging in common shrews (Sorex araneus L.)

Summary Risk-sensitive foraging theory predicts that predators which face starvation if there is a temporary shortfall in their food supply should choose feeding sites on the basis of variation in as well as mean expected reward rate. For a given mean reward rate they should choose high variance feeding sites (be risk-prone) if they are running below energy requirement, but low variance sites (be risk-averse) if they are running above.Common shrews presented with a choice between constant and variable feeding stations were more likely to visit the variable station when they were running below energy requirement and more likely to visit the constant station when they were running above. However, the tendency towards risk-aversion above requirement was greater than that towards risk-proneness below.When all shrews were considered together, the probability of visiting the variable station correlated negatively and continuously with intake relative to requirement.     

Antipredation role of clumped nesting by marsh-nesting red-winged blackbirds

Summary Red-winged blackbirds, Agelaius phoeniceus, breed in marshes in high densities and their nests are frequently clumped. Because predation is consistently the most important cause of redwing nesting mortality, high densities of breeding individuals could be an anti-predation adaptation. In our study site predation by marsh wrens, Cistothorus palustris, was the main cause of redwing nesting losses. In situations when marsh wrens were near, predation rates on redwing nests decreased with increasing female density. Group life could reduce predation because of improved nest defense, selfish herd effects, or predator dilution effects. We differentiated between these possibilities by introducing experimental colonies consisting of 3, 6, and 9 artificial nests near and away from active redwing nests. The experimental colonies near active nests suffered less predation, but predation rates were not correlated with colony size or a nest's location within the colony. Therefore, the advantage of group life in this population is probably mutual nest protection.     

Experimental manipulation of the dawn and dusk chorus in the blackbird Turdus merula

Summary We present experimental and correlational data that demonstrate that both the nutritional state of the male and his mate's fertility affect the timing and intensity of dawn and dusk song in the blackbird (Turdus merula). Food supplementation leads to paired males singing earlier, for longer, and at higher peak rate at both dusk and dawn succeeding the treatment. We argue that the timing, not just the intensity, of song conveys information about the male's phenotypic condition. Both dawn and dusk song start earlier and have longer duration as a male's mate reaches peak fertility indicating that, in agreement with literature on the great tit (Parus major), song at this time has particular significance in mate guarding for paired males of monogamous species. We discuss the relative importance of these factors as functional explanations for the dawn chorus. Offprint requests to: I.C. Cuthill     

Central place foraging in wheatears (Oenanthe oenanthe L.): foraging itineraries when feeding nestlings

Summary Parent wheatears (Oenanthe oenanthe L.), foraging to meet their own needs and to provide food for a central place (CP, i.e. the nest), have to make decisions with respect to the configuration of foraging itineraries during round trips in the territory and to the directionality of their movements. These problems were studied in two pairs breeding in an agricultural area in central Swecen. All birds started a round trip by hunting from perches close to the CP and then moving to perches progressively further away in a roughly straight line until the first prey item was loaded. Loaded prey were either delivered singly (single prey loading: SPL) or with other prey (multiple prey loading: MPL). When the bird decided to return with several items to the CP, it abruptly changed the direction of its movements by making a left, right or backward turn and started to visit perches progressively nearer the CP, again following a roughly straight line. The decision to return continue loading was affected by the size of the prey as shown by the fact that prey carried singly was significantly larger than the first item in an MPL for all individuals. The distance to the CP also seemed to affect this decision as prey provisioned singly on average were collected significantly closer to the nest than the first item in an MPL by one pair. Both the size of prey loaded singly and load size of MPL increased with distance from the CP in one pair. The concentrated use of the territory in the other pair made any effect of distance difficult to detect. Great flexibility in foraging decisions was observed in cases when an individual, because of the size of the last prey captured, altered its decision to deliver an MPL and transported an SPL to the nest instead. On the basis of these results we propose a set of rules followed by predators such as wheatears when making decisions about delivering prey to a CP (Fig. 4).     

The loading effect in central place foraging wheatears (Oenanthe oenanthe L.)

Summary The behaviour of five adult wheatears (Oe. oenanthe) delivering prey (maggots and mealworms) from artificial patches to the nestlings was studied. The existence of a so-called loading effect was confirmed. Handling times for subsequent prey items increased with the number held in the beak, which led to a decrease in collecting rates with patch time and, in most cases, to positively accelerating loading functions when collecting times for prey items are plotted as a function of load size. All birds seemed to become more efficient at loading prey in the experimental patches during the course of the nestling period. Loading functions for maggots and mealworms were slightly different. When forcing the birds to visit several cups (only one item in each cup) and remove a layer of moss before reaching the prey (low-density patches), all birds took fewer prey and two of them stayed longer in them than in highdensity patches (one cup filled with prey items). This was due to an increase in search times with the number of prey held in the beak.From knowledge of the loading functions and travel times to the nest, it is possible to predict the optimal load sizes according to a mathematical solution of the delivery rate model of Orians and Pearson (1979). By transforming collecting and travel times to energy expenditures, it is also possible to derive predictions from an energy efficiency model (maximizing energy delivery per unit energy expended in a round-trip). The observed average load sizes did not differ significantly from those predicted by the delivery rate model, but they were significantly smaller in all cases than those predicted by the energy efficiency model. For birds feeding nestlings, it may be more important to sacrifice efficiency in energy expenditure in favour of greater delivery rates, thereby maximizing the growth rate of the young.     

Seasonal variation in ejaculate traits of male red-winged blackbirds (Agelaius phoeniceus)

Many reproductive traits, including ejaculate characteristics, usually show remarkable seasonal variation, but the potential for such dynamics in sperm morphology has been overlooked. Several studies have revealed high within-male repeatability in sperm morphology, but samples have typically been collected within a few hours or days, and the consistency of sperm morphology over longer periods remains unexplored. Here, we tested whether ejaculate traits, including sperm morphology, sperm number, and sperm velocity, exhibit seasonal phenotypic plasticity in a long-lived seasonal breeder, the red-winged blackbird (Agelaius phoeniceus). We found absolute and/or relative flagellum length and sperm velocity to increase across the season, whereas sperm numbers within ejaculates declined. Sperm morphological traits were further positively associated with harem size or the total number of offspring that fledged in each male’s territory, suggesting that sperm morphology is likely to be linked to male reproductive quality. The underlying mechanisms of these patterns of seasonal variation remain unresolved, but we discuss our results in the context of dynamics of reproductive hormones, testicular structures and function, and reproductive behavior.     

The influence of pollen quality on foraging behavior in honeybees (Apis mellifera L.)

Individual and colony-level foraging behaviors were evaluated in response to changes in the quantity or nutritional quality of pollen stored within honeybee (Apis mellifera L.) colonies. Colonies were housed in vertical, three-frame observation hives situated inside a building, with entrances leading to the exterior. Before receiving treatments, all colonies were deprived of pollen for 5 days and pollen foragers were marked. In one treatment group, colony pollen reserves were quantitatively manipulated to a low or high level, either by starving colonies of pollen or by providing them with a fully provisioned frame of pollen composed of mixed species. In another treatment group, pollen reserves were qualitatively manipulated by removing pollen stores from colonies and replacing them with low- or high-protein pollen supplements. After applying treatments, foraging rates were measured four times per day and pollen pellets were collected from experienced and inexperienced foragers to determine their weight, species composition, and protein content. Honeybee colonies responded to decreases in the quantity or quality of pollen reserves by increasing the proportion of pollen foragers in their foraging populations, without increasing the overall foraging rate. Manipulation of pollen stores had no effect on the breadth of floral species collected by colonies, or their preferences for the size or protein content of pollen grains. In addition, treatments had no effect on the weight of pollen loads collected by individual foragers or the number of floral species collected per foraging trip. However, significant changes in foraging behavior were detected in relation to the experience level of foragers. Irrespective of treatment group, inexperienced foragers exerted greater effort by collecting heavier pollen loads and also sampled their floral environment more extensively than experienced foragers. Overall, our results indicate that honeybees respond to deficiencies in the quantity or quality of their pollen reserves by increasing the gross amount of pollen returned to the colony, rather than by specializing in collecting pollen with a greater protein content. Individual pollen foragers appear to be insensitive to the quality of pollen they collect, indicating that colony-level feedback is necessary to regulate the flow of protein to and within the colony. Colonies may respond to changes in the quality of their pollen stores by adjusting the numbers of inexperienced to experienced foragers within their foraging populations.     

Colonial and single breeding in fieldfares,Turdus pilaris L.: a comparison of nesting success in early and late broods

Summary Pairs of fieldfares breed singly or in colonies. Their breeding season was subdivided into an early part, when the trees were still leafless, and a late part, when the trees had leaves. Early colonial pairs has a significantly higher nesting success than single pairs. However, amongst late broods the nesting success of both categories of pairs was the same. Colonies of only two pairs show a weak trend suggesting that it could be advantageous for early broods to have a neighbour nearby, while the nesting success of late broods seems to be highest if the neighbour is farther away. The number of neighbours is more important than the distance to the nearest neighbour when colonies of different sizes are examined. The nesting success of colonial broods is also influenced by the time of breeding. Success increases with colony size, in early colonies, but the opposite applies for the late breeding season.A little owl was placed near single and colonial nests to show how communal defence contributed to the greater success of early colonial nests. Predators entering a colony risk being contaminated by significantly more faeces due to the higher rate of attacks by fieldfares using aimed defecation near colonial nests and these could affect the predators' flying ability.The possibility of fieldfare colonies serving as information centres for food finding is also discussed.     

Genotype and colony environment affect honeybee (Apis mellifera L.) development and foraging behavior

We examined the interaction of genotype and environment on foraging-behavior development and forage choice in honeybees. High- and low-pollen-hoarding strains and unselected wild-type bees were co-fostered in pairs of colonies manipulated to differentially stimulate high and low pollen foraging. The high-pollen-foraging stimulus consisted of high amounts of larvae, a known stimulus for pollen foraging, plus low amounts of pollen, known to induce pollen foraging. The low-pollen-foraging stimulus consisted of low amounts of larvae plus high amounts of pollen. We estimated the median age at which bees initiated foraging, determined forage choice, and the quality and quantity of resources collected. High-strain bees consistently foraged at younger ages than workers from the other sources. High-strain bees appeared to be more sensitive to the pollen-foraging-stimulus treatments, showing greater differences in foraging age and behavior. Three-way interactions of genotype, pollen foraging stimulus, and colony pair (replicate) were statistically significant for most foraging variables measured suggesting that additional, unknown environmental factors also affect foraging behavior. Our results suggest there is a functional relationship between age of first foraging and forage choice with a strong genetic component that is modulated by colony environment.     

Migratory activity by hatchling loggerhead sea turtles (Caretta caretta L.): evidence for divergence between nesting groups

The South Florida subpopulation of loggerhead sea turtles (Caretta caretta L.) nests with great fidelity on either the southeast or the southwest coast of Florida, USA. The hatchlings that emerge from those nests must swim in opposite directions and search for different surface currents to migrate away from continental shelf waters. In this laboratory study, we compared the pattern of swimming activity shown by the hatchlings from each coast over the first 6 days of migration. Turtles from both coasts were equally active during their “frenzy” period (the first 24 h of swimming) and during the daylight hours of the 5 days that followed (the “postfrenzy” period). However, the west coast turtles were significantly more active than the east coast turtles during the nocturnal portion of the postfrenzy period. This difference may be related to the greater distance southwest coast turtles must negotiate to locate surface currents for transport out of the Gulf of Mexico and into the Atlantic Ocean basin. These differing behavioral strategies may be genetically determined, as similar correspondence between activity and distance is well known among migratory populations of birds and fish and is often based upon inherited programs of endogenously driven activity. Alternatively, behavioral differences between the two nesting groups could be a manifestation of phenotypic plasticity that arises as the hatchlings respond to unique environmental cues on each coast.     

Zinc and copper induced changes in physiological characteristics of Vigna mungo (L.)

The effect of deleterious concentration of zinc and copper provided either individually or in combination in the nutrient media was investigated in order to assess the effect of metal interaction in Vigna mungo (L.). Both metals showed negative effect and led to a marked decrease in seed germination (20%), seedling growth (91.7%) and nitrate reductase activity (85.7%) with the increase in metal concentrations. The present study also emphasizes on the response of catalase and peroxidase enzyme under zinc and copper stress. Both antioxidant enzymes exhibited an increasing trend under different treatment conditions but it was reverse at highly toxic metal concentration. The results showed active involvement of peroxidase enzyme in regulating oxidative stress rather than catalase enzyme, as the specific activity of peroxidase enzyme got increased by 8.94% under the combined metals stress whereas catalase activity got declined by 60.97% in comparison to control due to excessive stress. The combined effect of copper and zinc metal was more pronounced in comparison to their individual effects.     

Memory dynamics and foraging strategies of honeybees

Summary The foraging behavior of a single bee in a patch of four electronic flower dummies (feeders) was studied with the aim of analyzing the informational components in the choice process. In different experimental combinations of reward rates, color marks, odors and distances of the feeders, the behavior of the test bee was monitored by a computer in real time by several devices installed in each feeder. The test bee optimizes by partially matching its choice behavior to the reward rates of the feeders. The matching behavior differs strongly between stay flights (the bee chooses the feeder just visited) and shift flights (the bee chooses one of the three alternative feeders). The probability of stay and shift flights depends on the reward sequence and on the time interval between successive visits. Since functions describing the rising probability of stay flights with rising amounts of sucrose solution just experienced differ for the four feeders, it is concluded that bees develop feeder-specific memories. The choice profiles of shift flights between the three alternative feeders depend on the mean reward rate of the feeder last visited. Good matching is found after visits to the low-reward feeders and poor matching following departure from the high-reward feeders. These results indicate that bees use two different kinds of memories to guide their choice behavior: a transient short-term working memory that is not feeder-specific, and a feeder-specific long-term reference memory. Model calculations were carried out to test this hypothesis. The model was based on a learning rule (the difference rule) developed by Rescorla and Wagner (1972), which was extended to the two forms of memories to predict this operant behavior. The experiments show that a foraging honeybee learns the properties of a food source (its signals and rewards) so effectively that specific expectations guide the choice behavior. Correspondence to: R. Menzel     

Migratory activity by hatchling loggerhead sea turtles (Caretta caretta L.): evidence for divergence between nesting groups

The South Florida subpopulation of loggerhead sea turtles (Caretta caretta L.) nests with great fidelity on either the southeast or the southwest coast of Florida, USA. The hatchlings that emerge from those nests must swim in opposite directions and search for different surface currents to migrate away from continental shelf waters. In this laboratory study, we compared the pattern of swimming activity shown by the hatchlings from each coast over the first 6 days of migration. Turtles from both coasts were equally active during their “frenzy” period (the first 24 h of swimming) and during the daylight hours of the 5 days that followed (the “postfrenzy” period). However, the west coast turtles were significantly more active than the east coast turtles during the nocturnal portion of the postfrenzy period. This difference may be related to the greater distance southwest coast turtles must negotiate to locate surface currents for transport out of the Gulf of Mexico and into the Atlantic Ocean basin. These differing behavioral strategies may be genetically determined, as similar correspondence between activity and distance is well known among migratory populations of birds and fish and is often based upon inherited programs of endogenously driven activity. Alternatively, behavioral differences between the two nesting groups could be a manifestation of phenotypic plasticity that arises as the hatchlings respond to unique environmental cues on each coast.     

Divergent foraging strategies of three co-occurring north Pacific flatfishes

Despite facing similar constraints imposed by the environment, significant variation in life history traits frequently exists among species generally considered to comprise a single ecological guild. For juvenile flatfishes, constraints on foraging activity include variation in light and prey availability, as well as predation risk. This paper describes the visual constraints on, and divergent foraging strategies of three co-occurring north Pacific flatfish species, northern rock sole (Lepidopsetta polyxystra), Pacific halibut (Hippoglossus stenolepis), and English sole (Pleuronectes vetulus). Visual foraging abilities measured in the laboratory decreased rapidly below 10⁻⁴ μmol photons·m⁻² s⁻¹, and were similar among species. Despite similar sensory constraints, field sampling in August 2004 at a Kodiak Island nursery site (Holiday Beach, 57^o41.2′ N, 152^o27.7′ W) identified species differences in diets, diel foraging patterns, and within-nursery depth distributions. Northern rock sole and English sole fed primarily on bivalve siphons and polychaetes, whereas mysids dominated the diets of Pacific halibut. Northern rock sole were geographically the most widespread but feeding activity was temporally restricted to the dusk period. Pacific halibut were rare in shallow depths (<5 m) and fed most intensively prior to dusk. English sole fed throughout the daylight hours and were abundant only in the shallowest (<5 m) habitats. These differences in diets, foraging times, and habitat use appear related to previously documented species-specific behavioral characteristics as well as general spatial (increasing with depth) and temporal (increasing during foraging activity) variations in predation risk. At one extreme, the conservative behavioral strategy of northern rock sole may permit use of a broader range of foraging habitats, whereas English sole may be restricted to shallow water by limited behavioral responses to predation threat. These observations demonstrate that the appearance of habitat partitioning is not due to differences in sensory ability, but reflects multi-faceted, species-specific responses to the ecological tradeoffs between foraging and predation risks.     

The hunting handicap: costly signaling in human foraging strategies

Humans sometimes forage or distribute the products of foraging in ways that do not maximize individual energetic return rates. As an alternative to hypotheses that rely on reciprocal altruism to counter the costs of inefficiency, we suggest that the cost itself could be recouped through signal benefit. Costly signaling theory predicts that signals can provide fitness benefits when costs are honestly linked to signaler quality, and this information is broadcast to potential mates and competitors. Here, we test some predictions of costly signaling theory against empirical data on human food acquisition and sharing patterns. We show that at least two types of marine foraging, turtle hunting and spearfishing, as practiced among the Meriam (a Melanesian people of Torres Strait, Australia) meet key criteria for costly signaling: signal traits are (1) differentially costly or beneficial in ways that are (2) honestly linked to signaler quality, and (3) designed to effectively broadcast the signal. We conclude that relatively inefficient hunting or sharing choices may be maintained in a population if they serve as costly and reliable signals designed to reveal the signaler's qualities to observers.     

Shifting foraging strategies in colonies of the social wasp Polybia occidentalis (Hymenoptera, Vespidae)

Social insect colonies can be expected to forage at rates that maximize colony fitness. Foraging at higher rates would increase the rate of worker production, but decrease adult survival. This trade-off has particular significance during the founding stage, when adults lost are not replaced. Prior work has shown that independent-founding wasps rear the first workers rapidly by foraging at high rates. Foraging rates decrease after those individuals pupate, presumably reducing the risk of foundress death. In the swarm-founding wasps, colony-founding units have many workers, making colony death by forager attrition less likely. Do swarm-founding wasps show similar shifts in foraging rates during the founding stage? We measured foraging rates of the swarm-founding wasp, Polybia occidentalis at four stages of colony development. At each stage, foraging rates correlated with the number of larvae present, which, in the founding stages, correlated with the number of cells in the new nest. Thus, foraging rates appear to be demand-driven, with the level of demand in the founding stage set by the size of nest that is constructed. During the founding stage, foraging rates per larva were high initially, suggesting that colonies minimize the development times of larvae early in the founding stage. Later in the stage, foraging rates decreased, which would reduce worker mortality until new workers eclose. This pattern is similar to that shown for independent-founding wasps and likely results from conflicting pressures to maximize colony growth and minimize the risk of colony death by forager attrition.     

Strong response of an invasive plant species (Centaurea solstitialis L.) to global environmental changes

Global environmental changes are altering interactions among plant species, sometimes favoring invasive species. Here, we examine how a suite of five environmental factors, singly and in combination, can affect the success of a highly invasive plant. We introduced Centaurea solstitialis L. (yellow starthistle), which is considered by many to be California's most troublesome wildland weed, to grassland plots in the San Francisco Bay Area. These plots experienced ambient or elevated levels of warming, atmospheric CO2, precipitation, and nitrate deposition, and an accidental fire in the previous year created an additional treatment. Centaurea grew more than six times larger in response to elevated CO2, and, outside of the burned area, grew more than three times larger in response to nitrate deposition. In contrast, resident plants in the community responded less strongly (or did not respond) to these treatments. Interactive effects among treatments were rarely significant. Results from a parallel mesocosm experiment, while less dramatic, supported the pattern of results observed in the field. Taken together, our results suggest that ongoing environmental changes may dramatically increase Centaurea's prevalence in western North America.