bluuebunny/crossref_metadata_2025_split huggingface dataset:
Climatic effects on breeding and morphology: evidence for phenotypic plasticity
Authors: Przybylo Robert , Sheldon BeN C. , Merilä JuhA 
Published: March 2003

Summary 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

Summary 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

Summary 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

Summary 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

Summary1. 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 model2. 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

Summary1. 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

Summary1. 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

Summary

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

Summary

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

SummaryThe 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.
The response of spotted hyaenas to long‐term changes in prey populations: functional response and interspecific kleptoparasitism
Authors: Höner Oliver P. , Wachter Bettina , East Marion L. , Hofer Heribert 
Published: March 2003

Summary

Over the last three decades the main prey species (wildebeest Connochaetes taurinus, zebra Equus burchelli, Thomson’s gazelle Gazella thomsoni, and Grant’s gazelle Gazella granti) of spotted hyaenas Crocuta crocuta in the Ngorongoro Crater, Tanzania, substantially declined in numbers, whereas buffalo Syncerus caffer numbers increased strongly. This provided a ‘natural experiment’ to investigate how a generalist predator such as the spotted hyaena responds to long‐term changes in prey populations. Here we compare data on the feeding ecology of Crater hyaenas from the late 1990s (period II) with equivalent data from the late 1960s (period I).
Hyaenas showed strong hunting preferences for wildebeest calves and gazelle fawns during both periods I and II. Adult buffaloes, prey difficult to hunt for hyaenas, were rarely hunted in either period despite a large increase in buffalo numbers from period I to II.
Hyaenas exhibited a functional response to the changes in prey populations by killing more buffalo calves and adult wildebeest during period II, relatively easy prey categories for hyaenas to hunt.
The proportion of carcasses scavenged and acquired by kleptoparasitizing other predators increased from period I to II. This was facilitated by an increase in the relative number of lion kills available to hyaenas during the same period.
Hyaenas defended a similar proportion of their kills against kleptoparasitic attempts by lions during both periods. Since the ratio of kills hyaenas kleptoparasitized from lions in relation to kills lost to lions increased from period I to period II, hyaenas obtained more food resources from lions during period II than I.
The success of hyaenas kleptoparasitizing lions was influenced by the presence of male lions and the ability of hyaenas to recruit sufficient clan members to appropriate carcasses from lions.
This study is the first to demonstrate both a functional response of a predator to changes in the size of prey populations and an associated change in foraging behaviour of the predator.