bluuebunny/crossref_metadata_2025 huggingface dataset:
A field study of size–fitness relationships in the parasitoid Asobara tabida
Authors: Ellers Jacintha , Van Alphen Jacques J.M. , Sevenster Jan G.
Published: March 2003

1.  Many evolutionary models of parasitoid behaviour assume a positive correlation between size and fitness. In this paper we study the size–fitness relationship in the laboratory and in the field using females of the solitary parasitoid Asobara tabida (Hymenoptera: Braconidae).

2.  In the laboratory, fecundity, fat reserves and longevity without food were positively correlated with size.

3.  Release–recapture experiments in the field showed that dispersal diminishes fat reserves. Dispersal ability is size‐dependent: larger females, with larger fat reserves, disperse over larger distances than smaller females.

4.  The form of the relationship between size and fitness in the field was estimated in two ways: one based on a comparison of the size distribution of released and recaptured females; the other based on the egg load and fat reserves of wild‐caught females. Both showed an accelerating increase of fitness with size.

5.  The majority of females appeared to be time‐limited. Therefore, the increase in fitness with size is predominantly due to a larger dispersal ability and not to a higher egg load.
Plumage dichromatism of birds predicts introduction success in New Zealand
Authors: Sorci Gabriele , Møller Anders Pape , Clobert Jean
Published: March 2003

1.  Reintroduction of plants and animals has become a common practice in the attempt to establish, re‐establish or augment endangered populations. The fate of these translocation programmes is likely to depend, on the one hand, on several ecological attributes of the introduced species, on the other hand on introduction effort.

2.  In a recent paper, Veltman, Nee & Crawley (1996) reported the correlates of introduction success of exotic birds released by humans in New Zealand before 1907: introduction effort (minimum number of release events and minimum number of released propagules) accounted for most of the variance in introduction success.

3.  One of the factors potentially affecting the establishment of new populations concerns the intensity of sexual selection. Sexually selected species may be more vulnerable to extinction risks for several reasons (they may be more sensitive to both environmental and demographic stochasticity) and therefore have lower introduction success when compared to nonsexually selected species.

4.  In the present study the data set reported by Veltman et al. (1996) was used to address the particular issue concerning sexual selection and introduction success. Each bird species was scored as monochromatic or dichromatic; plumage dichromatism is supposed to have evolved under sexual selection pressures. These scores were then correlated to the fate of introduction.

5.  Plumage dichromatism was found to be, as expected, a significant predictor of introduction success, after removing the potential confounding effect of introduction effort. Dichromatic species had lower introduction success than monochromatic species. This pattern holds true for nonpasserines and passerines, although the correlation between plumage dichromatism and introduction success was not significant for the latter.

6.  These results indicate that dichromatic species may experience reduced chances of establishing new populations when compared to monochromatic species. The presumed mechanisms involved in this phenomenon range from a reduced ability to adapt to a novel environment, to an increase in the risks of extinction through environmental and demographic stochasticity.
Costs of ant attendance for aphids
Authors: STADLER  , DIXON
Published: March 2003

1.  Interactions between aphids and ants are considered to be mutualistic, with both partners benefiting. Costs associated with such interactions are likely to be less obvious, although they can be expected, especially if these associations are facultative.

2.  It is demonstrated here that there are costs in several life‐history parameters to individual aphids resulting from ant attendance. Over several generations Aphis fabae cirsiiacanthoides feeding on Cirsium arvense, at a range of developmental stages, suffered significant costs when tended by Lasius niger, e.g. in terms of a prolonged developmental time, delayed offspring production, proportionally smaller gonads, fewer well developed embryos and a reduced mean relative growth rate. These effects are similar to those observed when aphids feed on poor quality plants.

3.  This is the first indication that there is a cost for aphids associated with ant attendance. The significance of this for the evolution of ant attendance in aphids is discussed.
Evolutionary consequences of habitat fragmentation in a localized butterfly
Authors: THOMAS CHRIS D. , HILL JANE K. , LEWIS OWEN T.
Published: March 2003

1.  Theoretical studies of the costs and benefits of migration predict that evolutionary changes in dispersal traits may take place in response to habitat fragmentation.

2.  For the butterfly Plebejus argus, we investigated five morphological characters potentially associated with flight ability, in locations that varied in level of habitat fragmentation. The traits were: total mass, relative thorax mass (containing flight muscle), relative abdomen mass (containing reproductive organs), relative wing area and wing aspect ratio. All characters were measured on individuals reared in a common environment.

3.  Morphology was related to the level of habitat fragmentation in both limestone and heathland habitats. Total mass increased with decreasing heathland habitat area (over the range 13 000–10 ha). Relative allocation to the thorax increased, while allocation to the abdomen decreased, with declining limestone habitat area (over the range 3·5–0·2 ha). Morphological characters were not significantly correlated with habitat isolation.

4.  Significant family effects for total mass, relative thorax mass, relative abdomen mass and (for females only) relative wing area indicate that these traits may have a heritable component, and therefore have the potential to respond to selection acting on flight ability.

5.  We suggest that evolutionary changes in life history traits are taking place in response to changes in landscape structure: in P. argus, these traits may be influenced by the effects of mate‐location strategy on emigration rates. Specific changes in traits can be complex, and may vary among species and populations.

6.  
P. argus had a 50% chance of occurring in heathland fragments of 33 ha, and was present on all heathlands above 50 ha, but population systems may not yet have achieved equilibrium. P. argus is unlikely to undertake major evolutionary changes in response to reduced habitat area in heathlands > 50 ha. Extinctions from smaller heathlands can be explained most plausibly by population/vegetation dynamics. Therefore, evolutionary changes in morphology may be more likely to be symptomatic of populations with altered costs and benefits of migration, rather than to be a direct cause of extinction.
Experimental evidence for density dependence of reproduction in great tits
Authors: BOTH CHRISTIAAN
Published: March 2003

1.  Density dependence of avian reproduction has often been analysed using correlations between annual mean reproductive output and population density. Experiments are necessary to prove that density is the cause of the observed patterns, but so far, three out of four experimental studies do not support a direct causal effect of density on reproduction.

2.  This paper presents experimental evidence that reproductive decisions in great tits, Parus major L., are causally affected by breeding density. The breeding density of great tits was manipulated by providing nest‐boxes at different densities in an ecologically homogeneous area.

3.  Within years the densities in the high and low density plots differed approximately 8‐fold. During the 11 years of the experiment, clutch size, nestling mass and the proportion of birds starting a second brood were all lower in the high density plot. In 5 years with equal breeding densities in both parts, clutch size did not differ between the plots. The patterns found were consistent with the density effects as predicted from the non‐experimental data.
Examination of the nitrogen limitation hypothesis in non‐cyclic populations of cotton rats (Sigmodon hispidus)
Authors: Schetter Timothy A. , Lochmiller Robert L. , Leslie David M. , Engle David M. , Payton Mark E.
Published: March 2003

1. Nitrogen‐containing nutrients have long been considered a frequently limiting resource to the growth of herbivore populations (nitrogen limitation hypothesis). To explore this hypothesis, we examined the relationships between availability of essential amino acids and concentrations of phenolics in the diets of hispid cotton rats (Sigmodon hispidus) in central Oklahoma and the intrinsic characteristics of their non‐cyclic populations. We posited that lower quality proteins (i.e. essential amino acid composition) and elevated phenolic levels (protein digestion inhibitors) in diets of cotton rats from low‐ compared to high‐density populations, especially during the breeding season, would be supportive of the nitrogen limitation hypothesis. Replicated low‐ and high‐density populations were censused by live‐trapping at 3‐month intervals. Samples of stomach digesta were collected from cotton rats in similar habitats adjacent to trapping grids to determine the botanical and nutrient composition of their diets.

2. During the breeding season, concentrations of essential amino acids were as much as 43% greater in diets of cotton rats from high‐density populations. Dicots, typically higher in protein than monocots, were an important component of diets and were preferred forage in all seasonal collections. Seeds and arthropods were frequently utilized by cotton rats as additional high‐quality sources of essential amino acids. Concentrations of total phenolics in the diet (greater in diets from low‐density populations) were consistent with the nitrogen limitation hypothesis.

3. Density was consistently higher in the high‐density populations throughout the study. Other demographic and body condition parameters were similar between low‐ and high‐density populations in the non‐breeding season, but reproductive activity was greater in high‐density populations during the breeding season. Total number of juveniles recruited into the trappable population over the entire study was about five times greater in high‐ compared to low‐density populations.

4. Our data did not refute the nitrogen limitation hypothesis where levels of essential amino acids and phenolic compounds in the diet during the breeding season may determine annual peak densities of cotton rats that can be supported in their habitat. However, we could not rule out the involvement of other environmental variables such as overhead cover (as well as other unmeasured variables) as contributing factors to determining annual peak densities.
The interactive effects of age and food supply on the breeding ecology of great skuas
Authors: RATCLIFFE N. , FURNESS R.W. , HAMER K.C.
Published: March 2003

Age‐specific variation in the reproductive performance of great skuas in Shetland, U.K., was investigated during a period of fluctuating sandeel availability between 1988 and 1993.

Increased sandeel abundance was associated with earlier laying dates, increased clutch volumes and improved fledging success. Sandeel abundance had no effects on clutch size and hatching success.

Parental age improved haying date and clutch size in a nonlinear manner. Laying date became earlier in the youngest age classes, with the effect being negligible in older birds. Clutch size increased with age up to 18 and then declined in older birds. Clutch volume increased with age and the probability of nest predation declined with age.

There were no significant interactive effects of age and year on laying date nor clutch volume, indicating that birds of all ages benefited equally from increases in sandeel abundance. This suggests that lack of breeding experience rather than deficient foraging skills are responsible for young birds laying later.

There was a significant interactive effect of year and age on fledging success that was associated with changes in sandeel availability. Success was uniformly low for birds of all ages in 1988–90 when sandeel abundance was low. Success increased with age in 1991 and 1992 during which time food supply improved. During 1993 food supply was abundant and fledging success was high in all age classes.

Age‐specific improvements in laying date, clutch size and nest predation were best explained by learning of skills associated with experience of breeding. Improvements in fledging success with age were probably best explained by the learning of skills associated with foraging.
Can grazing sheep compensate for a daily foraging time constraint?
Authors: Iason G. R. , Mantecon A. R. , Sim D. A. , Gonzalez J. , Foreman E. , Bermudez F. F. , Elston D. A.
Published: March 2003

1. Theoretical studies of large herbivore foraging assume that total daily grazing time is a key constraint on daily intake and diet choice. We experimentally tested this assumption and investigated the effects of food availability on the ability of grazing sheep to compensate for restriction of available daily grazing time.

2. Foraging behaviour, intake and diet digestibility by sheep, were measured on grass pastures in a replicated 2 × 2 factorial experiment, in which overnight access to pasture was varied (restricted overnight and continuous access) on two sward heights (5·5 and 3·0 cm), representing high and low food availability.

3. Regardless of food availability, the overnight‐restricted sheep fed for almost all of the available grazing time by grazing for fewer, longer foraging bouts, but still had much shorter total daily grazing time than the continuous access sheep.

4. In response to overnight penning, the sheep had a significantly higher instantaneous rate of intake achieved mainly via larger bites. The continuous access sheep were hence not maximizing their short‐term rate of intake, whilst grazing according to the daily schedule considered normal for sheep.

5. The behavioural responses to overnight food restriction were able to counteract the reduction in daily grazing time only where food availability was high. In contrast on short swards overnight grazing restriction led to a reduction in total daily intake. We suggest that the interactions between the factors considered as constraints on foraging behaviour of herbivores are, as yet, only poorly quantified.
Eliminating autocorrelation reduces biological relevance of home range estimates
Authors: DE Solla Shane R. , Bonduriansky Russell , Brooks Ronald J.
Published: March 2003

1. Destructive subsampling or restrictive sampling are often standard procedures to obtain independence of spatial observations in home range analyses. We examined whether home range estimators based upon kernel densities require serial independence of observations, by using a Monte Carlo simulation, antler flies and snapping turtles as models.

2. Home range size, time partitioning and total straight line distances travelled were tested to determine if subsampling improved kernel performance and estimation of home range parameters.

3. The accuracy and precision of home range estimates from the simulated data set improved at shorter time intervals despite the increase in autocorrelation among the observations.

4. Subsampling did not reduce autocorrelation among locational observations of snapping turtles or antler flies, and home range size, time partitioning and total distance travelled were better represented by autocorrelated observations.

5. We found that kernel densities do not require serial independence of observations when estimating home range, and we recommend that researchers maximize the number of observations using constant time intervals to increase the accuracy and precision of their estimates.
Different impact of pupal predation on populations of Epirrita autumnata (Lepidoptera; Geometridae) within and outside the outbreak range
Authors: Tanhuanpää Miia , Ruohomäki Kai , Kaitaniemi Pekka , Klemola Tero
Published: March 2003

1. We studied the effect of pupal predation on Epirrita autumnata populations within and outside the outbreak range, in northern and southern Finland, respectively. The purpose of the study was to evaluate the potential of pupal predation to contribute to the contrasting dynamics of northern and southern populations. In addition to overall pupal survival, we separately estimated the influence of small mammal and invertebrate predation.

2. Each summer for a period of 5 years, pupae inside their cocoons were buried in the ground. The pupae were re‐collected before the flight season and checked for damage. Direct and delayed density dependence of pupal survival was tested using the current and the previous year larval density indices, respectively.

3. Annual survival of pupae from both small mammal and invertebrate predation was lower in the south. Daily survival rates did not differ between the south and the north indicating that the difference in annual pupal survival is most likely due to the longer pupal period in the south.

4. In the south, pupal survival was negatively density dependent and resulted from small mammal predation. The results indicate that small mammals may prevent E. autumnata populations from reaching high densities in the south.

5. In the northern study area with a current outbreak, total pupal survival was density independent. Also small mammal predation was density independent, most likely due to predator satiation. Moreover, there was no difference in pupal survival between northern areas with and without a current outbreak. The results indicate that pupal predation was not a significant factor in termination of the outbreak, nor is it likely to contribute to the maintenance of the cyclic density fluctuations.

6. We conclude that low pupal survival and its density dependence are likely to contribute to stability of E. autumnata populations in southern Fennoscandia. On the contrary, shorter pupal period and the lack of density dependent pupal predation may make northern populations more prone to outbreaks. The potential regulating effect of pupal predation in southern populations is due to small mammals being able to respond to changes in E. autumnata densities.
Dietary protein constraint on age at maturity: an experimental test with wild deer mice
Authors: McAdam Andrew G. , Millar JohN. S.
Published: March 2003

1.Five populations ofPeromyscus maniculatus borealis(Mearns) were live‐trapped in the Kananaskis Valley, Alberta, throughout the breeding seasons of 1996 and 1997, to test the hypothesis that growth and maturation of young‐of‐the‐year (YY) females are limited by dietary protein.

2.In 1996, two populations received a high protein (30%) food supplement, one received a high energy, but low protein (14%) food supplement and two others were unsupplemented. In 1997, three populations received the protein (30%) supplement and two served as controls.

3.In 1996, control populations had high nestling growth rates and many (43%) YY females bred. In 1997, nestling growth rates were lower in control populations and no YY females bred.

4.Supplementation of high protein food resulted in consistently high nestling growth rates in both 1996 and 1997, and a significantly higher proportion of YY females bred than controls in both years.

5.Supplementation of high energy, but low protein food had no effect on YY growth or maturation.

6.Neither protein nor energy supplementation had any effects on the number of litters conceived per season, litter success, litter size or sex ratio of litters born to over‐wintered females.

7.We conclude that growth and maturation of YY femalePeromyscus maniculatusin this area are limited by the availability of dietary protein.
Life‐history consequences of predation for a subantarctic beetle: evaluating the contribution of direct and indirect effects
Authors: Ernsting G. , Brandjes G. J. , Block W. , Isaaks J. A.
Published: March 2003

1.Recently, a small predatory beetle,Trechisibus antarcticus(Carabidae), was accidentally introduced onto the island of South Georgia, sub‐Antarctic.

2.From the presumed site of introduction the beetle is invading the coastal lowland area, building up high densities locally in the tussock‐forming grassParodiochloa flabellata.

3.In the coastal area the endemic detritivorous/herbivorous beetleHydromedion sparsutum(Perimylopidae) is common, especially in and beneath the tussocks.

4.The first three, out of six, larval instars ofH. sparsutumare easily taken prey by the carabid.

5.In sites colonized by the carabid, total abundance and the ratio between larval and adult numbers ofH. sparsutumare far lower, and its adult body size clearly larger, than in comparable sites where the carabid is absent.

6.Two hypotheses are proposed for explaining the increase in adult body size ofH. sparsutum: (i) the increase is a direct effect of predation: selection by the predator favours large hatchlings and/or larvae with a high growth rate; and (ii) the increase is an indirect effect of predation: by lowering the density ofH. sparsutum, predation has increased its per capita food supply, enabling a higher growth rate and a larger adult body size.

7.A food addition experiment in a carabid‐free site showed availability of high quality food to be insufficient for sustaining the initial larval population.

8.In the laboratory, females from the predator‐infested sites produced larger eggs and hatchlings than females from the carabid‐free sites, but mass specific growth rates of the larvae were not higher.

9.Field and laboratory data give stronger support to the food hypothesis than to the size selectivity hypothesis.
Competition for early arrival in migratory birds
Authors: Kokko Hanna
Published: March 2003

1. It is widely accepted that the arrival order of migratory birds is correlated with the condition of the birds, which leads to high quality individuals occupying prime sites. However, the theoretical backgrounds for this argument have been lacking. A simple game‐theoretic model of arrival timing is provided which investigates the evolutionary stability of condition‐dependent arrival order in territorial migrant birds.

2. Competition for territories or other priority‐dependent benefits can lead to arrival dates far preceding the cost‐minimizing date (the optimum date in the absence of competition) for all but the weakest individuals. Increasing the number of competitors can generate a ‘cascading’ competition for early arrival, which advances arrival dates further apart from the individual optimum dates for the onset of breeding.

3. At equilibrium, arrival order corresponds strictly to condition order only if marginal costs of advancing arrival are always larger for individuals in lower condition. If spring mortality vacates territories for later‐arriving birds, the criterion for ‘honest’ arrival order becomes still stricter: differential survival costs should exist, but survival differences among individuals (or, alternatively, territory quality differences) should not be very large.

4. If the habitat is saturated so that there is a risk of not obtaining a territory at all, or if worst territories are of much lower value than the rest, competition may lead to the majority of the population arriving within a fairly short interval, followed by a much later floating fraction. This synchrony in the arrival of breeders imposes an increasing cost for the lesser fit breeding birds. Thus, arrival costs paid are not necessarily highest for earliest arriving individuals, but for those who have the most to lose if they drop a few steps in the arrival order.

5. Competition for high quality territories can also lead to partial migration, in which case birds in good condition are expected to be most likely to remain resident.
Does large size make daughters of the blue‐footed booby more expensive than sons?
Authors: Torres Roxana , Drummond Hugh
Published: March 2003

1. In species where size dimorphism emerges during parental care, individuals of the larger sex are presumably more expensive to produce than individuals of the smaller sex.

2. We evaluated whether female offspring of the blue‐footed booby, because of their larger size, require greater parental feeding expenditure than males.

3. A field experiment compared growth rates of sons and daughters when hand‐fed the same amount of food, and a descriptive field study compared the parental feeding rates to fledgling sons and daughters.

4. Females maintained greater rates of mass and ulna increase than males when fed the same amount of food.

5. First‐hatched chicks had higher probability to be fed than second‐hatched chicks. However, male and female fledglings were fed at similar rates by their parents.

6. No evidence that daughters of the blue‐footed booby receive a greater feeding expenditure than sons was found. Faster motor development of males in this facultatively siblicidal species might explain why they grow more slowly than females with the same food budget.
Comparative patterns of philopatry and dispersal in two common mole‐rat populations: implications for the evolution of mole‐rat sociality
Authors: Spinks Andrew C. , Jarvis Jennifer U. M. , Bennett Nigel C.
Published: March 2003

1. The aridity food‐distribution hypothesis (AFDH) maintains that ecological constraints and foraging risks in arid environments curtail dispersal and promote the evolution of cooperatively foraging social groups within African mole‐rat populations. To assess the influence of habitat constraints upon mole‐rat social behaviour and hence the validity of the AFDH, we investigated the differential patterns of dispersal and philopatry in two populations of the common mole‐rat, Cryptomys hottentotus hottentotus Lesson, one from a more ‘optimal’ habitat (mesic) and another from a ‘suboptimal’ habitat (arid).

2. Rates of immigration and emigration were markedly lower in the arid relative to the mesic site, reflecting divergence in the ecological constraints upon dispersal.

3. The rates of dispersal at both sites increased as a function of group size. However, for any given group size, colony attrition was significantly greater at the mesic site than at the arid site, probably reflecting the relaxation of constraints upon dispersal in mesic areas.

4. There was no sex bias in dispersal at either study site.

5. At the mesic sites dispersing and nondispersing individuals were of comparable mass, whilst at the arid site there was a trend for dispersing animals to be heavier than their nondispersing counterparts.

6. This investigation revealed marked differences between the study populations in the nature of dispersal. This reflects adaptive variation in social behaviour between the regions, and the results suggest that delayed dispersal and cooperation may be more crucial to individual survival in arid than in mesic areas. As such, these findings provide support for the underlying contention of the AFDH that ecological constraints on successful dispersal and colony formation in arid areas have promoted a greater degree of social cohesion in mole‐rats occurring in these regions, ultimately advancing the evolution of group‐living and cooperative foraging in the Bathyergidae.
Climatic effects on breeding and morphology: evidence for phenotypic plasticity
Authors: Przybylo Robert , Sheldon BeN C. , Merilä JuhA
Published: March 2003

1. A trend towards increasing average spring temperatures has been observed in Europe over the past 25 years. This climatic variation has been attributed to a natural, large‐scale atmospheric phenomenon, the North Atlantic Oscillation (NAO). A number of studies have reported associations between the winter NAO‐index and both breeding phenology and cohort‐specific morphology. These studies have been cross‐sectional rather than longitudinal and therefore have not been able to determine whether the changes result from phenotypic plasticity or microevolutionary processes.

2. We analysed (i) cross‐sectional correlations between the winter NAO‐index and breeding performance (laying date, clutch size, fledging success and number of recruits produced) and morphological traits (tarsus and wing length), and (ii) within‐individual variation in the same traits for individuals experiencing different values of the NAO‐index in a population of collared flycatchers (Ficedula albicollis) breeding on Gotland, Sweden, over a period of 16 years (1980–95).

3. None of the four measures of breeding performance changed consistently over the study period, while tarsus length of males (and marginally females) decreased over time. Of the six traits investigated using cross‐sectional data, only laying date was related to variation in the NAO‐index.

4. All characters investigated showed significant repeatability within individuals among years, revealing the importance of factors specific to individuals in determining their value. However, within individuals the NAO‐index significantly affected laying date and clutch size such that females laid earlier and produced larger clutches after warmer, moister winters.

5. Our data show a high degree of concordance between cross‐sectional and longitudinal analyses of the effect of NAO on laying date and clutch size. The similarity of responses across and within individuals suggests that the population‐level response to the NAO‐index can be explained entirely as the result of phenotypic plasticity.
Fragment size and the demography of an area‐sensitive songbird
Authors: Zanette Liana
Published: March 2003

1. Many songbird species are absent from small forest fragments. One possible explanation for this finding is that populations in small fragments decline to extinction because reproductive success is too low to compensate for adult mortality (λ < 1). I tested this hypothesis from 1995 to 1997 by measuring reproductive success, adult survival, and population viability for four banded populations of the Eastern yellow robin (Eopsaltria australis, White), an area‐sensitive songbird. The populations inhabited two small (55 ha, termed S1, S2) and two large (500–1000 ha, termed L1, L2) forest fragments set within an agricultural landscape.

2. Females produced more fledglings but a similar number of independent young per capita in the small compared to the large fragments. Local female survival did not vary with fragment size.

3. Population models were used to calculate the finite rate of increase (λ) from the number of young produced (female yearlings per female per year) and adult female survival rates. Combining data from all four populations, λ = 0·96, indicating that robins produced 4% fewer female young per year than were needed to compensate for adult losses. Combining data from the two small fragments, λ = 1·02, suggesting that populations in small fragments were viable on average. In contrast, λ = 0·85 for the two large fragments combined, indicating that, on average, 15% fewer young were produced than were needed to replace missing adults each year. Averaging across fragments within the same size class masked the variability in population dynamics that existed between the replicates. Considering each of the four fragments separately, there was one potential demographic source (S2, λ = 1·12; L2, λ = 1·23) and one demographic sink (S1, λ = 0·90; L1, λ = 0·46) within each of the fragment size categories.

4. Observed changes in population sizes were similar to those predicted by the population models. Combining all populations, recruitment replaced most but not all missing females, and the observed population size declined by 8% per year. In both the small and large fragments, observed recruitment was insufficient to replace missing females and population sizes declined 5% and 8% per year, respectively. Within each of the fragment size categories, there was one stable population (S2 and L2) and one population that declined in size (S1 and L1, respectively, declined by 14% and 20% per year).

5. Reproductive success, adult survival, and population viability did not vary with fragment size. To explain area‐sensitivity, I suggest that a threshold in fragment size exists below which populations are never viable, but above which populations may or may not be viable depending on factors unrelated to fragment size, such as nest predation rates.
Raptor predation and population limitation in red grouse
Authors: Thirgood Simon J. , Redpath Stephen M. , Rothery Peter , Aebischer Nicholas J.
Published: March 2003

1. We assessed the impact of predation by hen harriers and peregrine falcons on a red grouse population in southern Scotland during 1992–98. Grouse density in April, July and October declined during this time, coincident with an increase in the numbers of breeding harriers and peregrines.

2. Winter losses of grouse between October and April averaged 33% and were density‐dependent. Raptors were the cause of about 70% of winter mortality and they killed about 30% of the grouse present in October. We were unable to determine whether winter mortality in raptors was additive to other losses.

3. Summer losses of adult grouse between April and July averaged 30% and were density‐dependent. Raptors were the cause of more than 90% of the early summer mortality of adult grouse. Summer losses of grouse chicks between May and July averaged 45% and were not density‐dependent. Harriers killed about 28% of grouse chicks by late July and about 37% by the end of August. Summer raptor predation on adult grouse and chicks appeared to be largely additive to other losses and we estimated that it reduced autumn grouse densities by about 50%.

4. A model combining the estimated reduction in autumn grouse density caused by raptors with the observed density dependence in winter loss predicted that, in the absence of raptors for 2 years, grouse density in spring would be 1·9 times greater, and grouse density in autumn 3·9 times greater, than in the presence of raptors. The model suggested that raptor predation prevented the grouse population from increasing and was thus a limiting factor.
What omnivores eat: direct effects of induced plant resistance on herbivores and indirect consequences for diet selection by omnivores
Authors: Agrawal Anurag A. , Klein Corinne N.
Published: March 2003

1. Omnivory, where an animal crosses trophic boundaries, is thought to be common in natural and managed communities. Foraging theory predicts that omnivores will balance their diet as a result of nutritional needs, food quality and availability of alternate foods. We investigated diet selection of the western flower thrips [Frankliniella occidentalis (Pergande)], a herbivore under some circumstances, and a predator under other circumstances.

2. We demonstrate that induced plant resistance can indirectly affect diet selection by thrips. The density of herbivorous spider mites is reduced on induced plants compared to controls and is also positively correlated with the number of mite eggs that the thrips consume.

3. In choice tests, mite eggs from induced plants were less preferred by thrips than eggs from control plants. Mite egg size was also smaller on induced plants compared to controls. However, this alone did not explain the diet selection by thrips. In a field experiment, thrips avoided colonizing induced plants, but they were attracted to induced plants that we inoculated with mites. Thus, the presence of herbivorous prey on induced plants attenuated the negative effects of induced resistance on thrips colonization.

4. Feeding decisions of thrips can be influenced by several factors. We showed previously (Agrawal, Kobayashi & Thaler 1999a) Influence of prey availability and induced host plant resistance on omnivory by western flower thrips. (Ecology, 80, 518–523.) that reducing plant quality can cause omnivores to shift towards relatively more predation than herbivory. We show now that on induced plants, reduced prey density and quality may antagonize this shift towards increased predation.

5. Induced plant resistance has negative effects on both the plant‐based food resource (direct effect) and the animal‐based food resource (indirect effect) of omnivorous thrips. Thus, variation in the quantity and quality of food items interact to determine the diet selection of omnivores.
Evidence for density‐dependent survival in adult cormorants from a combined analysis of recoveries and resightings
Authors: Frederiksen Morten , Bregnballe Thomas
Published: March 2003

1. The increasing population of cormorants (Phalacrocorax carbo sinensis) in Europe since 1970 has led to conflicts with fishery interests. Control of cormorant populations is a management issue in many countries and a predictive population model is needed. However, reliable estimates of survival are lacking as input for such a model

2. Capture–recapture estimates of survival of dispersive species like cormorants suffer from an unknown bias due to permanent emigration from the study area. However, a combined analysis of resightings and recovery of dead birds allows unbiased estimates of survival and emigration.

3. We use data on 11 000 cormorants colour‐ringed as chicks in the Danish colony Vorsø 1977–97 to estimate adult survival and colony fidelity. Recent statistical models allowing simultaneous use of recovery and resighting data are employed. We compensate for variation in colour‐ring quality, and study the effect of population size and winter severity on survival, as well as of breeding success on fidelity by including these factors as covariates in statistical models.

4. Annual adult survival fluctuated from year to year (0·74–0·95), with a mean of 0·88. A combination of population size in Europe and winter temperatures explained 52–64% of the year‐to‐year variation in survival. Differences in survival between sexes was less than 1%. Cormorants older than ≈ 12 years experienced lower survival, whereas second‐year birds had survival similar to adults. Colony fidelity declined after 1990 from nearly 1 to ≈ 0·90, implying 10% permanent emigration per year. This change coincided with a decline in food availability.

5. Apparently, survival was more severely affected by winter severity when population size was high. This could be caused by saturation of high‐quality wintering habitat, forcing some birds to winter in less good habitat where they would be more vulnerable to cold winters. There was thus evidence for density dependence in adult survival, at least in cold winters.

6. The high population growth rate sustained by EuropeanPh. c. sinensisin the 1970s and 1980s can partly be accounted for by unusually high survival of immature and adult birds, probably caused by absence of hunting, low population density and high food availability.
Diagnosing a decline in return rate of 1‐year‐old cormorants: mortality, emigration or delayed return?
Authors: Frederiksen Morten , Bregnballe Thomas
Published: March 2003

1. In long‐lived birds with delayed recruitment, variation in prebreeding population parameters is difficult to study, although potentially important. Cormorants (Phalacrocorax carbo sinensis) often return to their natal colony when 1 year old, enabling direct study of first‐year survival and emigration. Density‐dependent regulation mechanisms may be strong at this stage of the life cycle.

2. In the Danish cormorant colony Vorsø, 11 000 chicks have been colour‐ringed between 1977 and 1997. Returning birds have been resighted in the colony and dead recoveries have been collected. Since 1990, the proportion of a cohort observed in the colony as 1‐year‐olds (return rate) has declined from 0·40 to 0·10. Possible explanations include increased mortality, increased emigration and a later age of first return to the colony. Breeding success has also declined strongly as a consequence of low food availability.

3. We used capture–recapture analysis of recovery and resighting data to investigate variation in first‐year survival, emigration and resighting probabilities. Survival fluctuated widely (0·42–0·75, mean 0·58); emigration increased in the 1990s from 0·05 to 0·15; resighting probability declined from 0·75 to 0·20 after 1990.

4. First‐year survival was particularly low in 1993 and 1996. The causes of the year‐to‐year variation were not clear. Survival may have been affected by food availability during the post‐fledging period.

5. The declining return rate was caused mainly by decreasing resighting probability of 1‐year‐olds, although increasing emigration also contributed. The biological mechanism was that increasing numbers of cormorants did not return to the natal colony as 1‐year‐olds. We suggest that this was a consequence of low physiological condition among newly fledged young, caused by low food availability in the 1990s.

6. We conclude that declining food availability has had several consequences for colony dynamics. First‐year emigration and the age of first return to the colony have increased and, in the worst years, survival has decreased. If the decline in food availability was due to cormorant predation, this would constitute an example of density‐dependent regulation of immature colony attendance.
Habitat‐specific effects of food abundance on the condition of ovenbirds wintering in Jamaica
Authors: Strong* Allan M. , Sherry Thomas W.
Published: March 2003

1. Food availability has been considered one of the most important factors limiting bird populations, yet the effects of food abundance on non‐breeding insectivores has rarely been investigated. We studied the effects of food abundance on the body condition of ovenbirds (Seiurus aurocapillusL.), a sexually monomorphic, ground‐foraging, Neotropical–Nearctic migrant warbler during two winters in three habitats on their wintering grounds in Jamaica.

2. Prey biomass decreased from early to late winter in all habitats. Concurrently, ovenbird body mass, corrected for differences in structural size, decreased significantly in five of six habitat–year combinations. Only in second growth scrub habitat in the 1995–96 winter did ovenbirds show no decrease in corrected body mass, and during this period there was no significant decrease in ant biomass, the dominant prey item of ovenbirds in all habitats.

3. Significant positive correlations were found between ovenbirds' rate of feather regrowth and the biomass of ants on their home ranges in early winter, and between overwinter change in ovenbird body mass and the biomass of ants on their home ranges in late winter.

4. The results of both the habitat‐ and home‐range‐based analyses suggest a similar threshold of ant biomass (2·5–3·0 mg m−2), below which ovenbirds did not maintain their body mass.

5. The results suggest that late winter rainfall mediates the biomass of prey abundance, which in turn affects the ovenbirds' overwinter body condition.
Associations between energetics and over‐winter survival in the short‐tailed field vole Microtus agrestis
Authors: Jackson D. M. , Trayhurn P. , Speakman J. R.
Published: March 2003

Over‐winter mortality in small mammals is influenced strongly by low ambient temperatures. Individuals with greater thermogenic capacities might then be expected to survive better than those with lower thermogenic capacities.

 To test this hypothesis, short‐tailed field voles Microtus agrestis (Linnaeus) were captured during the winters of 1995/96 and 1996/97 at two field sites near Aberdeen (57°N). The captured animals were tagged and taken back to the laboratory, where their resting metabolism (RMR), thermogenic capacity (NA induced metabolism excluding RMR) and body mass were measured.

 Body mass, RMR and thermogenic capacity did not differ significantly between the start and end of winter in voles that were captured at both times.

 Body mass varied significantly over the winter months, being lowest in January and highest in March. Thermogenic capacity also varied over the winter and the variation was linked significantly with changes in ambient temperature, suggesting that either voles with greater thermogenic capacity were more likely to be active on cold nights, or voles were flexible in their thermogenic capacities. RMR did not vary significantly over the winter.

 Animals that survived the winter had a significantly higher residual RMR than those that died (or permanently emigrated) but the survivors did not have significantly greater body masses, RMRs, thermogenic capacities or residual thermogenic capacities.
Optimal patch‐leaving behaviour: a case study using the parasitoid Cotesia rubecula
Authors: Tenhumberg Brigitte , Keller Mike A. , Possingham Hugh P. , Tyre Andrew J.
Published: March 2003

Parasitoids are predicted to spend longer in patches with more hosts, but previous work on Cotesia rubecula (Marshall) has not upheld this prediction. Tests of theoretical predictions may be affected by the definition of patch leaving behaviour, which is often ambiguous.

 In this study whole plants were considered as patches and assumed that wasps move within patches by means of walking or flying. Within‐patch and between‐patch flights were distinguished based on flight distance. The quality of this classification was tested statistically by examination of log‐survivor curves of flight times.

 Wasps remained longer in patches with higher host densities, which is consistent with predictions of the marginal value theorem (Charnov 1976). Under the assumption that each flight indicates a patch departure, there is no relationship between host density and leaving tendency.

 Oviposition influences the patch leaving behaviour of wasps in a count down fashion (Driessen et al. 1995), as predicted by an optimal foraging model (Tenhumberg, Keller & Possingham 2001).

 Wasps spend significantly longer in the first patch encountered following release, resulting in an increased rate of superparasitism.
Parasite‐induced maternal response in a natural bird population
Authors: Buechler K. , Fitze P.S. , Gottstein B. , Jacot A. , Richner H.
Published: March 2003

The timing and mechanism of the maternal response to ectoparasites is investigated in a host–parasite system consisting of great tits and a haematophageous ectoparasite, the hen flea. It has been demonstrated previously that a maternal response to this parasite enhances survival and fertility of the offspring. This may have arisen by either a maternally transferred protection via the egg, or by a parental response affecting the common rearing environment after hatching. Two experiments aimed to differentiate between the two possibilities are reported here.

First, mothers were either exposed to or kept free of ectoparasite during egg production, and subsequently the newborn nestlings were cross‐fostered between the two treatments. The experimental design discriminates as to whether the maternal effect arises before or after hatching. Within the same nest, the nestlings originating from previously exposed mothers grew faster than nestlings of unexposed mothers.

Secondly, we tested for the transfer of parasite‐induced immunoglobulins (IgG) via the egg. Mothers were kept free of ectoparasites until they had laid the first egg and were then either exposed to or kept free of ectoparasites to the end of laying. The IgG‐concentration significantly increased from the first to the eighth egg of exposed mothers, but not in eggs of unexposed ones.

In summary, the first experiment shows that ectoparasites can induce a beneficial maternal response at egg laying, and the second experiment suggests that the maternal effect is due to immunoglobulins transferred via the egg. Maternal responses to other parasites, e.g. blood parasites, are known for chicken in captivity. In natural populations of birds both the timing and mechanism of the response are poorly understood, despite their relevance for behavioural and population ecology.