bluuebunny/crossref_metadata_2025_split huggingface dataset:
Determinants of geographical range sizes: a test using introduced New Zealand birds
Authors: Duncan Richard P. , Blackburn Tim M. , Veltman Clare J. 
Published: March 2003

Summary1. The 34 species of birds that have been successfully introduced to New Zealand offer a unique opportunity to study patterns of variation in geographical range sizes, and to test mechanisms that may be responsible for that variation, because the New Zealand range sizes have established within the last 160 years and because there are data on the starting conditions, including the year of first recorded release and the subsequent effort put into the introduction of each species.2. We collated data on geographical range sizes, life history traits, dates of introduction and initial introduction effort for the birds successfully introduced to New Zealand. To test whether range size–life history correlations show a consistent pattern between regions, we collated further data on geographical range sizes and life history traits for British breeding birds.3. The geographical range sizes of birds introduced to New Zealand did not depend on the length of time since they were introduced. Instead, large geographical ranges were exhibited by species whose preferred habitat is widespread in New Zealand, species with life history traits associated with higher rates of population growth (high fecundity, fast development and small body size), species that are partial migrants in part of their natural range, and species that were initially introduced more often and in greater numbers to New Zealand.4. The strength and direction of geographical range size–life history correlations in introduced New Zealand and British breeding birds were very similar. There was also a strong positive correlation between the geographical range sizes of the species introduced to New Zealand from Britain and their geographical range sizes in Britain. However, the similarity of the correlations between life history traits and geographical range sizes in both regions was not a simple consequence of this; the similarity persisted when species introduced from Britain were excluded from the New Zealand data.5. We discuss the implications of these results for understanding variation in geographical range sizes in the introduced New Zealand avifauna.
Food regulates the Serengeti wildebeest: a 40‐year record
Authors: Mduma Simon A. R. , Sinclair A. R. E. , Hilborn Ray 
Published: March 2003

Summary 1. The migratory wildebeest (Connochaetes taurinus (Burchell)) population of Serengeti has been monitored for 40 years (1958–1998). The population increased from 1963 to 1977, stabilized from 1977 to 1993 and declined during drought in 1993–94. These changes provided a significant contrast in densities and the opportunity to determine the life history stages where survival was density dependent and their causes. 2. There was a delay in age of maturity and a decline of both yearling and adult pregnancy rates at higher densities. Changes in yearling pregnancy had minimal effect on number of calves born. Adult pregnancy rate dropped from 95% to 88% 3. The greatest absolute mortality occurred in newborn calves (0–4 months old). The most variable survival was in dry‐season calf mortality (5–11 months old). 4. Density dependence was clearly present in adult mortality and this appeared to be the regulating life stage. Density dependence was also found in dry‐season calf mortality. The drop in pregnancy rate was only weakly density dependent. 5. The main cause of mortality (75% of cases) was undernutrition. Bone marrow condition showed that more animals died in extremely poor condition as the population increased and predators did not cause these deaths. 6. Predation played only a minor role in limiting the wildebeest population. Predators caught animals in moderate condition during the increase phase and increasingly healthy condition during years of high population density. The main predators were lion [Panthera leo (Linnaeus)] and hyaena (Crocuta crocuta Erxleben). 7. Adult mortality was significantly negatively related to food per capita. This could be viewed either as curvilinear density dependence, or as two phases of mortality. The first phase was during population increase when mortality was inversely density dependent and may have been caused by a constant predation rate. The second was during stability and decline and comprised the additive effects of undernutrition and predation. 8. Rainfall in the dry season was the most important extrinsic determinant of food supply for the wildebeest population but food was related to mortality only when population density was incorporated. 9. At present the wildebeest population appears regulated by natural causes through food supply. Human‐caused mortality appears to be a minor factor. However, if the number of wildebeest killed by humans increases as human population increases, then there could be a permanent decline in wildebeest, and a major change in the whole ecosystem, because wildebeest are the keystone species. Continued monitoring is essential for conservation of the Serengeti ecosystem.
Hatching asynchrony in the Eurasian kestrel Falco tinnunculus: an experimental test of the brood reduction hypothesis
Authors: Wiehn Jürgen , Ilmonen Petteri , Korpimäki Erkki , Pahkala Maarit , Wiebe Karen L. 
Published: March 2003

Summary 1. We tested the brood reduction hypothesis by manipulating hatching spans of Eurasian kestrel Falco tinnunculus (Linnaeus) broods during two low vole years (1996–97) in western Finland. In addition, half the broods in 1997 were food supplemented during the entire nestling period. 2. Nestling mortality was high, occurring in 55% of nests, and the effect of manipulated hatching span on fledging success was more pronounced than either the effects of year or supplementary food. In contrast to the predictions tested, fledging success was higher in synchronous than asynchronous nests, both when food was limiting and when food conditions were substantially improved by food supplements. 3. We found no evidence that brood reduction resulted in higher quality (body mass) of surviving young in low vole years. On average, fledglings were heavier at synchronous than asynchronous nests. This was only the case, however, when food was limiting. 4. Most nestling mortality occurred when the chicks were 2 weeks old, and age of death did not differ between the brood types. Parents at synchronous nests fed their broods at a higher rate than at asynchronous broods, this divergence arising only in the latter part of nestling period. 5. This is the first hatching span experiment where synchronous hatching seemed more profitable than asynchronous during conditions of both food scarcity and food abundance. The higher parental workload at synchronous nests could, however, counterbalance the benefits of synchronous hatching. 6. Hatching in natural broods of Eurasian kestrel is more synchronous in low vole years compared to high vole years. The fact that synchronous broods out‐performed asynchronous broods in both ‘low’ years of the study is consistent with the facultative manipulations. The benefit of asynchrony in high vole years is still unclear.
Eggload dynamics and oviposition rate in a wild population of a parasitic wasp
Authors: Casas Jérôme , Nisbet Roger M. , Swarbrick Sue , Murdoch William W. 
Published: March 2003

Summary 1. This paper develops and tests in the field a model that predicts the oviposition rate and the rate of egg maturation of a synovigenic parasitic wasp during a foraging period. The parasitoid is Aphytis melinus, the highly successful biological control agent of California redscale (Aondiniella aurantii), a pest of citrus worldwide. 2. Females were sampled in the interior canopy of grapefruit trees either just before and at the end of the foraging period over 2 days and were dissected to determine the starting and ending eggload distributions. A group of females was caught before the onset of activity and kept in vials in the field in trees with honey but without access to hosts during the foraging period. Their eggload at the end of the day was used to estimate the egg maturation rate during the foraging period. 3. Two stochastic models are used to predict the eggload distribution at the end of the day. Both use the observed starting eggload distribution, the observed length of the foraging period and the estimated rate of egg maturation. The model providing the better fit uses an oviposition rate which is an increasing function of the eggload. 4. The eggload does not attain a steady state distribution during a foraging period. One‐third to one‐half of the population is predicted to experience egg‐limitation at some time during the foraging period. Five percent of the population will experience egg‐limitation a second time within a single day. 5. The common occurrence of egg‐limitation over a single day and the relatively high rate of change of states between egg‐ and time‐limitation imply that the rate of nutrient acquisition and use are likely to be subject to strong evolutionary pressures.
Geometric scaling of microhabitat patches and their efficacy as refugia during disturbance
Authors: Lancaster Jill 
Published: March 2003

Summary1. A field experiment was carried out to determine whether the accumulation of stream invertebrates in refugium patches during high flow disturbances was influenced by the geometric scaling of the patches, i.e. edge to size relationships.2. Flow was reduced experimentally by creating artificial flow refugia and comparing invertebrate colonization of refugium and control patches. There were eight sizes of colonization cage (1·5 cm mesh) with surface area/volume quotients (SA/V) which ranged from 1·2 to 0·5. Control cages were open to the flow; artificial refugia were baffled upstream by 0·3 cm mesh to reduce flow. Four, week‐long experimental trials were carried out during the winter months in an upland Scottish stream. On two occasions, discharge was high, on two it was low.3. The results are consistent with the hypothesis that stream invertebrates accumulate in refugia during high flow disturbances. The density of colonists was higher in refugia than control cages after high discharge for some taxa. There was no difference between treatments for any taxon at low discharge.4. The shape of soft‐edged microhabitat patches appears to be important in mediating the impact of disturbance in streams. Small refugium patches (high edge to size ratio) appeared to be more efficacious than large patches for taxa that accumulate in refugia during disturbances. Density increased with SA/V in all trials and for all taxa, but the slopes of the lines relating density to SA/V differed between control and refugium patches after disturbance for some taxa.5. According to trait‐based analysis, taxa that have high mobility or that are vulnerable to high hydraulic forces may be especially dependent upon small flow refugia to maintain population size. Whether movement is by active or passive means is less important.
Individual variation in intake rate: the relative importance of foraging efficiency and dominance
Authors: Stillman R. A. , Caldow R. W. G. , Goss‐Custard J. D. , Alexander M. J. 
Published: March 2003

Summary1.Individual variation in the intake rate of foraging animals arises largely from variation in two characteristics: their intrinsic ability to forage in the absence of competitors (foraging efficiency) and the detrimental effect of competitors on their intake rate (susceptibility to interference). We use a model to explore the relative importance of foraging efficiency and dominance, which influences susceptibility to interference, in determining the intake rate of foraging animals at a range of competitor densities.2.The model is parameterized and tested for a shorebird–prey system, oystercatchersHaematopus ostralegusL. feeding on musselsMytilus edulisL., in which interference occurs because dominant individuals steal prey from subdominant ones. In both oystercatchers which open mussels by stabbing their bill between the shell valves (stabbers) and those which hammer a hole through the shell (hammerers), foraging efficiency is predicted to be the major determinant of intake rate at competitor densities below 100–150 birds ha‐1and dominance the major determinant at higher densities. In reality, a very similar relationship is found in stabbers, but in hammerers foraging efficiency remains the most important determinant of intake rate across the full range of observed competitor densities, in contrast to the model's prediction. Oystercatchers typically forage at densities in the range 100–250 birds ha‐1, in the region in which foraging efficiency and dominance are of approximately equal importance to stabbers, while foraging efficiency is most important to hammerers. We suggest reasons for the difference in the model's predictive power for stabbers and hammerers.3.We use the model to make the general predictions that the relative importance of foraging efficiency is higher when (i) it varies more between individuals, (ii) prey encounter rate is high, (iii) handling time is short, (iv) prey stealing (kleptoparasitic) attacks occur over short distances, (v) the probability of stealing prey is low, and (vi) movement speed while foraging is low. In the oystercatcher–mussel system, prey encounter rate is lower and handling time longer than in most other shorebird–prey systems. This suggests that the importance of foraging efficiency in determining intake rate is likely to be greater in many other shorebirds.4.Most studies of between individual variation in intake rate have focused on the importance of dominance rather than variation in foraging efficiency. Both the model predictions and field data suggest that variation in foraging efficiency may be a more important source of variation in intake rate than its previous depth of study would suggest.
Aphid suitability and its relationship to oviposition preference in predatory hoverflies
Authors: Sadeghi Hussein , Gilbert Francis 
Published: March 2003

Summary 1. The fitness consequences of feeding on different aphids (apple, blackberry, dock, elder, nettle, pea, rose and sycamore aphids) for two species of generalist insect predator (the aphidophagous larvae of Episyrphus balteatus and Syrphus ribesii— Diptera:Syrphidae) were measured in the laboratory. The relevant literature studies on prey specialization in the Syrphidae were summarized for the first time. 2. Both the literature evidence reviewed and the data from the experiment indicated that fitness components and overall individual fitnesses are broadly similar among all aphid species, with some exceptions. 3. We correlated individual fitness to two estimates of oviposition preference, an indirect (field distribution of larvae) and a direct measure (egg distribution under laboratory conditions). In the non‐resident, migratory E. balteatus significant correlations were absent, but there were indications of a weak preference‐performance correlation in the largely resident S. ribesii.
Factors affecting breeding dispersal in the facultatively colonial lesser kestrel: individual experience vs. conspecific cues
Authors: Serrano David , Tella José L. , Forero Manuela G. , Donázar José A. 
Published: March 2003

Summary

The role of individual experience vs. the use of conspecific cues on breeding dispersal decisions have seldom been determined in colonial birds. We studied causes of breeding dispersal in the lesser kestrel (Falco naumanni), a species that breeds in colonies of variable size as well as solitarily. During a 6‐year study in Spain, we gathered information on 486 subsequent breeding attempts and on 26 explanatory variables which evaluated individual experience, conspecific cues in terms of breeding performance and colony size, and different ecological and populational characteristics.
Two decisions were separately analysed: whether or not to disperse, and how far to move. Generalized Linear Mixed Models (GLMMs) allowed us to identify the relative contribution of each explanatory variable while controlling for the non‐independence of individual dispersal decisions across years.
Females seemed to disperse more often than males (34% vs. 19%), and both sexes apparently dispersed less with age. However, a GLMM showed that experience (i.e. the number of years a bird bred in a particular colony) was the only factor influencing breeding dispersal. Birds showed higher site fidelity the greater their experience in a colony, which could be related to benefits derived of increased local familiarity. A second GLMM showed that, before birds acquired experience in a particular colony, individual nest failure due to predation and proximity to other colonies increased the probability of dispersal, dispersal being also higher in colonies with poor conspecific breeding success. Furthermore, solitary nesting birds were more prone to disperse and dispersal probability decreased the larger the colony of origin, according to fitness expectations associated with colony size.
A GLMM explaining dispersal distances retained two variables – birds dispersed farther the lower the breeding density in the surroundings, and the larger the distance to the nearest colony. Dispersing birds tended to settle within their previous foraging areas (median dispersal distance = 1·6 km), being constrained by the availability of nearby colonies.
Lesser kestrels mainly cue on their own breeding performance and experience in a particular colony at the time of taking a dispersal decision. However, inexperienced birds also partially cue on the size and breeding success of their own colonies (but not on the size or breeding performance of other colonies), and birds moved larger distances when dispersing from areas of low populational density. These results support some degree of conspecific attraction.
Spatio‐temporal patterns of mink and muskrat in Canada during a quarter century
Authors: Viljugrein Hildegunn , Lingjærde Ole CHR. , Stenseth Nils CHR. , Boyce Mark S. 
Published: March 2003

Summary Fur‐return data on muskrat (Ondatra zibethicus) and mink (Mustela vison) from 80 posts of the Hudson’s Bay Company were analysed. From each post we obtained the number of furs bought from trappers during 25 years (1925–49). The ecological information content of the data was assessed using questionnaires sent to each post. We conclude that fur‐return data reflect relative abundance of muskrat and mink, and not primarily information about the trappers. The most‐common temporal pattern of the two species are cycles with a period of 8–9 years, with muskrat cycling 1–2 years ahead of the mink. This pattern, as would be expected in a typical predator–prey relationship, is found throughout the boreal forest region, with the exception of several posts in eastern Canada. In the eastern region, the two species were found to fluctuate in synchrony. The data show broad‐scale synchrony. Peaks (and troughs) are roughly aligned in phase over the entire Canadian boreal forest. In some areas the phase of the cycle is either ahead or behind the average cycle. Peaks (and troughs) of mink and muskrat cycles generally appear first in the Athabasca Basin region (for mink in the Hudson’s Bay region of Manitoba and Ontario) and from this epicentre spread throughout Canada within 2–3 years. Synchrony (i.e. mean correlation) declines with distance; for distances less than 450 km, similarity among populations is higher than the average.
Migratory and sedentary blackcaps in sympatric non‐breeding grounds: implications for the evolution of avian migration
Authors: Pérez‐Tris Javier , Tellería José Luis 
Published: March 2003

Summary

Several hypotheses have attempted to explain why sedentary populations persist in the wintering grounds of many migratory bird species. For instance, residents may overcome the flooding of their range if they are better competitors than migrants. Alternatively, each population fraction may use different resources or even different habitat types, for example because residents benefit from site tenacity while migrants benefit from resource tracking.
To evaluate these hypotheses, we studied the distribution of sympatric migratory and sedentary blackcaps (Sylvia atricapilla) in southern Spain during two winters. We distinguished two habitat types: forests, which are used by residents for breeding, and shrublands, to which blackcaps spread in winter with the arrival of migrants. Shrublands are unsheltered habitats that show more abundant but less diverse fruits than forests. We investigated (i) if blackcaps show habitat segregation with respect to migratory behaviour, sex, age and body size, (ii) what resources are used by each population fraction within each habitat type and (iii) how habitat occupancy affects the body condition of individuals.
Residents were almost completely restricted to forests, while migrants occupied both habitat types. Among migrants, adults predominated in forests and juveniles in shrublands, but no sexual segregation was found. Body size was larger in residents than in migrants, and these were larger in forests than in shrublands (especially juveniles). If larger birds are dominant, these results support the idea that residents may endure in forests and exclude the most subordinate migrants (juveniles and small birds) towards shrublands.
Within forests, migrants and residents tracked fruit abundance, but residents were also associated with the most suitable breeding sites. Migrants tracked fruits less closely in shrublands, due probably to a higher fruit abundance and a lower availability of shelter.
Habitat segregation did not affect muscular development of migrants. However, migrants accumulated more fat in shrublands. Therefore, differences in nutritional quality cannot explain why adult and larger blackcaps predominate in forests. Instead, other factors such as food diversity or exposure to predators might account for this distribution.
Our results provide a mechanism to explain the persistence of sedentary populations despite migrants overflowing their range and using the same resources. Residents may challenge the arrival of conspecifics because they are better competitors. From an evolutionary perspective, this shows that non‐breeding processes may be as important as breeding benefits accrued to migrants for explaining the dynamics of migratory and sedentary populations.
Movement parameters of ungulates and scale‐specific responses to the environment
Authors: Johnson Chris J. , Parker Katherine L. , Heard Douglas C. , Gillingham Michael P. 
Published: March 2003

Summary

Most studies of animal movements and habitat selection do not recognize empirically that different components of the environment are important to animals at different scales. Often, availability of habitats is defined at one or more arbitrary spatio‐temporal scales, but use of those habitats is constrained to one scale. Identification of scalar movement is the first step in developing models to explain why animals select or move to certain parts of their range. We used a non‐linear curve‐fitting model of movement rates to identify discontinuities in the scales of movement by woodland caribou Rangifer tarandus caribou collared with global positioning system (GPS) collars.
We differentiated intrapatch from interpatch movements, but were unable to distinguish interpatch from migratory‐type movements for most combinations of individual caribou by season. Model fit was stronger for winter than summer movements. We suggest that increased patch heterogeneity during the winter resulted in interseason variation in movements and corresponding model fit.
Responses by caribou to the environment were scale‐dependent. When we applied logistic regressions, land‐cover type, energetic costs of movement, and predation risk differentiated the two scales of movement. Intrapatch movements had a lower cost of movement, were associated with cover types where foraging behaviours probably occurred, and were closer to areas of higher predator risk than interpatch movements.
Application of the non‐linear model will aid in developing mechanism‐based approaches to studying resource selection and animal behaviour.