Great Bustard Population Research Articles

Abundance and demographic structure of the great bustard (<i>Otis tarda</i>) (Otididae, Aves) population at lek areas in the Saratov and Volgograd Trans-Volga region

The results of our studies of the great bustard population in the Saratov and Volgograd Trans-Volga region are presented. For the first time in this territory, censuses of the numbers of bustards were carried out in their lek areas in the spring for 3 years. This made it possible to obtain more reliable data on the numbers of individuals of this species, as well as on the demographic structure in the model area and compare them with those obtained during autumn population counts. In our opinion, as a result of agriculture intensification, the nesting habitats of the bustard have been sharply reduced, which was caused by changes in the structure of crop areas and the massive use of pesticides. In the absence of necessary protection measures, this has led to a reduction in the numbers of birds in the Saratov Trans-Volga region by almost 10 times over 20 years. This trend is observed throughout almost the entire world range of the great bustard, where there are no long-term effective measures aimed at preserving the natural habitats of the species. In the Trans-Volga region, the number of lek areas has decreased by approx. half and the numbers of individuals of the species in the remaining leks have decreased. The demographic structure of the bustard population when studied in lek areas shows a significant predominance of males over females, which contradicts the stability of the population. Due to the reduction in the number of habitats suitable for bustards to nest, the success of their reproduction has significantly decreased. Spring treatment of crop areas with pesticides by ground and aerial methods coincides with the nesting period of birds, which is a powerful factor of disturbance and leads to the loss of clutches. Females whose clutches have been preserved, have the problem of acute food shortage after their chicks hatched, which also leads to elimination of some part of the broods.

A túzok (Otis tarda L.) ízeltlábú táplálékforrás kínálatának vizsgálata alföldi előfordulási területein

We have carried out the tests in the four lowland regions where the Great Bustards are found: (1) Apajpuszta – Csepeli-plain (1985), (2) Mezőnagymihály – Borsodi Mezőség (1991), (3) Nagyiván – Hortobágy (1988) and Dévaványa – Dévaványai-plain (1985, 1987) (Map 1). We have collected the Arthropods in the study areas with BARBER pitfalls. Each of the annually selected habitats had 5 pitfalls in operation, which we have placed along a line at five meters from each other. The traps were in operation from May to July, sometimes until August. We emptied them in two-week (14-14 day) periods. During laboratory processing, we have determined the number of individuals and measured the total mass of individuals of each taxon per period. The individual and mass data were recorded in tables for each measurement (=every two-week period) and the dominance relationships were calculated, then the basic data were used to calculate the various community-ecological indicators (diversity, evenness, etc.). In addition to the calculated individual and mass ratios, we have also calculated the energy value and dominance of the samples. We have applied cluster analysis to group the animal food sources of each habitat. We have used the BRAY-CURTIS index as a similarity index. Based on the results obtained – regardless of the place and time of the examination – if we want to generalize, then we can say the following facts: (1) The mass and energy ratios that most characterize the food source prove that agricultural habitats are more favourable. (2) The food supply of the habitats changes during the reproduction period, partly for biological (seasonal dynamics of the taxa), partly for reasons of cultivation (mowing, harvesting) and with the maturation of the plants (disappearance of green phytomass), shows fluctuations, decreases or increases. The decrease is typical for cereals and natural grasslands, and the cyclical change is typical for mowed grasslands and alfalfa. We have experienced an increase in spring-sown (e.g. maize) habitats. (3) We cannot generally talk about the goodness of food sources within the same year, we must determine and prioritize the food supply in each period. (4) The extent and quality of the supply of Arthropod food sources is also different in the topographically different places of the Great Hungarian Plain. (5) It can be concluded that, based on the trappings, the decisive role of those taxa in the food supply of Arthropods can be demonstrated, which also play a decisive role in the animal food spectrum of the Great Bustards, so above all the Coleoptera and the Orthoptera. (6) Taking all of this into consideration, we should establish that regardless of the year, place, or period, the more favourable Arthropod food supply of agricultural habitats was one of the motivating factors for the habitat change of Great Bustard, and even today it is one of the explanations for the settlement and nesting of Great Bustards on arable land. (7) The examination of the Arthropod food sources of the Great Bustard showed that the Arthropod resource sets of the agricultural habitats are still suitable for the maintenance of Great Bustard populations. (8) Environmentfriendly farming, the restrained use of chemicals, and the increase in the diversity of food weeds increase the diversity and mass of phytophagous Arthropods and their predators, thus increasing the resources of the habitat.

Are trellis vineyards avoided? Examining how vineyard types affect the distribution of great bustards

A significant restructuring of vineyards is currently taking place in the European Union (EU) as a result of the implementation of a restructuring and conversion of vineyards regulation (CE 1493/1999) in southwest Europe, so that trellis vineyards are rapidly replacing traditional vineyards (e.g., surface area from 18.1 % in 2010 to 34 % in 2015 in Castilla-La Mancha, Central Spain). These changes may influence patterns of space use in birds, which may avoid modified habitats. We assess how the location of traditional and trellis vineyards might influence the distribution of great bustard (Otis tarda), a globally threatened species. We estimate Resource Selection Functions (RSFs) to quantify the relative probability of use of different areas by the great bustards, and use the RSF’s to simulated scenarios of conversion from traditional to trellis vineyards (low - 10 %, medium - 30 %, and high rate - 60 %) to quantify the potential impact of such modifications on the availability of suitable great bustard habitat. Our results revealed that great bustards significantly avoid trellis vineyards, especially at closer distances. Transition scenarios show how an increase in the proportion of traditional vineyards converted to trellis vineyards greatly decrease the proportion of suitable habitat for great bustard. Compared to current conditions, the percentage loss of suitable habitat increased steadily with higher rates of converted vineyards, up to 60 % loss of suitable habitat at the highest rate of conversion. Because the effect of transforming traditional vineyards to trellis vineyards depends both on the amount of habitat available for bustards before the transformation occurs and on the overall area covered by vineyards, a correct estimation of transformable vineyard area will require a case-by-case assessment to assure a low impact on bustard populations. We identified alternative vineyard management options that would mitigate impacts on the great bustard populations.

The Probable Strong Decline of the Great Bustard Otis tarda tarda Population in North-Western China

The Great Bustard population in Xinjiang Province, the Chinese stronghold of Otis tarda tarda, is suspect to be declining, but little is known about its trends and numbers. Using the same methods, we compared two former published counts from the province (1988 and 1992) with ours (2013–2016), exhaustively covering the two known bustard areas of Tacheng (summer) and Qapqal (winter). A strong decline (92%) is reported, with a minimum 19 birds found in Tacheng and none in Qapqal. Cooperation between scientists is needed to study the causes of decline in O. T. tarda in Xinjiang and elsewhere.—Wang, M., Gonzalez, M.A., Yang, W., Neuhaus, P., Blanco-Fontao, B. & Ruckstuhl, E.K. (2018). The probable strong decline of the Great Bustard Otis tarda tarda population in north-western China. Ardeola, 65: 291–297.

Abundance dynamics of the Trans-Volga great bustard (Otididae, Aves) population

The results of our study of the abundance dynamics of the great bustard population in the southern part of the Saratov Trans-Volga region for the period from 1998 to 2015 are presented on the basis of censuses carried out before the migration for wintering in the fall and on leks in the spring. The abundance dynamics was analyzed, and the factors causing it were identified. Data on the distribution of the great bustard population density over an account area of 12000 km2 are presented.

Assessing the extinction risk of the great bustard Otis tarda in Africa

We studied the dynamics and trend of the last extant population of great bustards Otis tarda in Africa. Moroccan great bustards are the southernmost population of this species, and thus show the characteristics of a peripheral population: small size, isolation and low gene flow. Available counts indicate a severe population decline (62% in the last 15 yr), as well as a contrac- tion of the species' distribution. We used a population viability analysis (PVA) to evaluate the quasi-extinction risk and to identify the most important threats. The estimated geometric growth rate of the more realistic of a set of possible scenarios was 0.87 (95% CI: 0.85, 0.89). This implies a 13% annual decline over 50 yr. However, projections derived from these results should be inter- preted with caution, because models have a great deal of uncertainty and vital rates from Iberian populations may be different from those of the Moroccan population. PVA showed the negative consequence of human-induced mortality. According to the model that best fits our census data and if present threats remain in the coming years, this peripheral population could go extinct in ca. 20 yr. Agricultural intensification, infrastructure developments and new power lines in rural areas where the species occurs are causing habitat destruction and fragmentation and increasing artifi- cial mortality. Urgent conservation measures, especially to reduce human-induced mortality, are needed to save African great bustards from extinction. We suggest that these findings can be gen- eralized to other peripheral great bustard populations living in highly humanized landscapes.

Impacts of survival and reproductive success on the long-term population viability of reintroduced great bustards Otis tarda in the UK

AbstractReintroductions aim to re-establish species within their historical ranges through the release of wild- or captive-bred individuals following extirpation (or extinction) in the wild. There is no general agreement on what constitutes a successful reintroduction but the probability of the population achieving long-term persistence should be addressed. Here we review a 10-year trial reintroduction of the great bustard Otis tarda, a globally threatened bird species, to the UK and assess the long-term population viability. Despite changes in rearing and release strategy, initial post-release survival probability remained consistently low, with only 11.3% of bustards (n = 167) surviving from release to 1 year post-release. Nineteen breeding attempts were made by eight females; however, only one chick survived > 100 days after hatching, and no wild juveniles have recruited into the population. Using demographic rates from the UK population and wild populations elsewhere, and stochastic population modelling, we investigate the viability of this reintroduced population by predicting population size over the next 10 years. Under current demographic rates the population was predicted to decline rapidly. Self-sufficiency was predicted only using the highest estimates from the UK population for first-year and adult survival, and recruitment rates from wild populations elsewhere. Although changes have been made in rearing, release strategies, habitat management and release sites used, these changes appear to have a modest effect on long-term viability. Substantial improvements in survival rates and productivity are necessary to establish a viable great bustard population in the UK, and we consider this unlikely.

Conservation of Great Bustard (Otis Tarda) Population of the Mosoni-Plain - A Success Story

Abstract At the turn of the 19-20th centuries, the Great Bustard population of the Kisalföld (Little Hungarian Plain) numbered 4000 specimens. By 1990, only about 100 individuals remained in the Hungarian and Austrian territories. Of the many possible negative factors of current times, the greatest pressure on the Great Bustard population stems from unfavorable crop structures, extensive use of intensive agricultural technologies and predation. During the past decades, we have seen a shift in nesting place locations from natural-like habitats to agrar-type habitats. This change may be explained by the more favorable structure and microclimate of this latter habitat type paralleled with greater food source availability. In order to escape this ecological trap, we have to engage in active conflict resolution that provides protection the region's Bustard population. For this very reason, the MOSON Project was founded in 1992 at the northern part of the Mosoni-Plain in the territory of Lajta-Hanság Co. Later, several Austrian regions joined the project. On these territories, out of the above mentioned 100 specimens, only 20 birds lived at the time. As the result of active habitat management of Great Bustards and coexisting small game species (mostly due to the influence of set-aside areas) as well as effective predator control (especially the Red Fox) resulted in an increase of the Great Bustard population. By the end of the 1990's, the population grew to 120-130 individuals which number was limited by the carrying capacity of this territory. Consequently, the species continued to reoccupy new regions in the Hungarian and Austrian territories. These days, the number of Great Bustards in these protected regions is estimated to be 400 individuals. In 1998, the Mosoni-plain was given IBA (HU-001) status, and in 2004, the region was protected under the Natura 2000 EU nature conservation network.

A link between historical population decline in the threatened great bustard and human expansion in Iberia: evidence from genetic and demographic data

Early anthropogenic impacts on the abundance and distribution of wild species are difficult to document, but can help us to understand the causes and relative importance of current declines. Genetic data can be of use in inferring historical demographic events, but the accuracy of these inferences depends on the availability and precision of demographic parameters that are difficult to obtain in the field. Here, we use demographic data on Iberian populations of the threatened great bustard (Aves: Otis tarda), obtained from an intensive population monitoring programme over the last 20 years, to estimate critical population parameters (population size and generation time), which are then used in a Bayesian Skyline Plot (BSP) analysis of mitochondrial DNA sequence data to assess changes in population size over the last several thousand years. BSP showed a sudden and sharp great bustard population decline coinciding with human expansion in Iberia, and the associated agricultural and urban development and increased hunting pressure. These results illustrate the importance of human population size as a possible ultimate cause of an environmental impact that occurred in the historical past, a fact that has often been neglected. Our results also suggest the role of human activities in driving historical population declines in great bustards, and underscore the importance of precise, long-term field data to infer past demographic trends from parameters of extant populations. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 518–527.

Satellite telemetry reveals long‐distance migration in the Asian great bustard Otis tarda dybowskii

The range of the great bustard stretches 10 000 km across Eurasia, one of the largest ranges of any threatened species. While movement patterns of the western subspecies of great bustard are relatively well‐understood, this is the first research to monitor the movements of the more endangered Asian subspecies of great bustard through telemetry and to link a breeding population of Asian great bustards to their wintering grounds. Using Argos/GPS platform transmitter terminals, we identified the annual movement patterns of three female great bustards captured at their breeding sites in northern Mongolia. The 4000 km round‐trip migration we have recorded terminated at wintering grounds in Shaanxi, China. This route is twice as long as has previously been reported for great bustards, which are among the heaviest flying birds. The journey was accomplished in approximately two months each way, at ground velocities of 48–98 km h−1, and incorporated multiple and variable stopover sites. On their wintering grounds these birds moved itinerantly across relatively large home ranges. Our findings confirm that migratory behavior in this species varies longitudinally. This variation may be attributable to longitudinal gradients in seasonality and severity of winter across Eurasia. The distance and duration of the migratory route taken by great bustards breeding in Mongolia, the crossing of an international border, the incorporation of many stopovers, and the use of a large wintering territory present challenges to the conservation of the Asian subspecies of great bustard in this rapidly changing part of the world.

Nest site habitat selection and nesting performance of the Great Bustard Otis tarda in southern Portugal: implications for conservation

SummaryWe present results on breeding parameters, nesting performance and nest habitat selection of the globally threatened Great Bustard Otis tarda, gathered during a 3-year study in Castro Verde (southern Portugal). A total of 107 nests were located. The estimated average onset of laying was 22 April (SD = 12 days, n = 8). The incubation period ranged from 25 March to 15 June. Nearly 95% of females incubated between 1 April and 31 May. Mean clutch size was 2.12 (SD = 0.69, n = 86), showing no significant inter-annual differences. There was no significant relationship between clutch size and average nest finding date, although, on average, nests were found later each year. Hatching occurred between 21 April and 26 June. The estimated proportion of eggs hatched per nest (hatching percentage) was 87.5% in 2002 (n = 8), 64.0% in 2003 (n = 33) and 75.8% in 2004 (n = 33). Overall nesting success for the three years was 71% (n = 107). The main causes of nesting failure were agricultural activities, mainly ploughing and harvesting, followed by corvid predation. Nests were spatially aggregated up to 800–1,500 m. At larger scales, the spatial distribution of nests can be considered random or uniform. Females showed a clear preference for nesting in cereal fields, followed by young fallows, old fallows, ploughed fields and then other habitats, although the intensity of selection of particular habitats varied between years. Nests were associated with both cereals and fallows, thus aggregating where those habitat types occurred within the study area. These results provide the first complete description of the nesting process in a Great Bustard population, as well as important cues to the assessment of conservation measures implemented in Castro Verde, an area holding around 80% of the Portuguese population of this species (application of EU agri-environmental schemes, power line signalling, water-point provision in dry years).

Population Increase of the Great BustardOtis tardain Its Main Distribution Area in Relation to Changes in Farming Practices

We update and present relevant information regarding the abundance and distribution of the great bustard Otis tarda in Castilla y León (Spain) in 2008, compare it with previous census results, and analyse the effects of agricultural changes on the provincial abundance and distribution of the species. The study area was surveyed from four-wheel drive vehicles driven at low speed (20–30 km/h) along predetermined transects, stopping frequently at prominent spots. The intention was to detect all the great bustards present in the study area. The great bustard population in Castilla y León during the breeding season of 2008 was 14,025 birds: 5,637 males, 7,760 females and 628 individuals whose sex could not be determined. The population was 34% greater in 2008 than in 1998 when the area surveyed in both censuses is compared. Population increases were recorded in all provinces except for Soria, where no birds were observed. Not all local populations within provinces increased: we detected population declines of over 10% in 43 (27%) of the 160 polygons surveyed both in 1998 and 2008, population increases of over 10% in 93 (58%) polygons, and largely stable populations (changes less than 10%) in 24 (15%) polygons. Provincial increases in bustard numbers were positively correlated with increases in the extent of unirrigated legume crops. The great bustard population in Castilla y León in 2008 was the largest in the world, accounting for 27–32% of the global population and about 47% of the estimated Spanish population. The spread of unirrigated legumes, crops that are promoted by agrienvironment schemes, may have fostered the observed population increase between 1998 and 2008. A survey of the region every ten years is proposed to monitor future population trends.

Underground cabling and marking of power lines: conservation measures rapidly reduced mortality of West-Pannonian Great Bustards Otis tarda

SummaryCollisions with power lines represent an important mortality factor for Great Bustards Otis tarda throughout the distribution range of the species. This study evaluates the success of two conservation measures implemented in the West-Pannonian distribution range to reduce the number of power line collision casualties: (1) extensive underground cabling of 43.1 km power lines, and (2) marking of 89.7 km power lines starting in 2005 and 2006, respectively. The mortality rate of Great Bustards in our study area (covering 686.5 km2) decreased significantly between 2002 and 2011, predominantly caused by reduced mortality due to power line collisions. Univariate tests indicate that underground cabling and power line marking significantly decreased power line collision casualties. Generalised linear models (GLMs) highlighted the prominent effect of underground cabling. Our results indicate that five years after underground cabling and marking of power lines within core areas of the West-Pannonian distribution range of the Great Bustard, the population already benefited through a significantly decreased mortality rate. Both conservation measures most likely contributed strongly to the rapid recovery of the West-Pannonian Great Bustard population observed within the last decade.

Assessing the effects of a highway on a threatened species using Before–During–After and Before–During–After-Control–Impact designs

Due to the growing awareness of potential impacts of roads, managers demand well-designed studies about the implications of linear infrastructures on ecosystems. We illustrate the application of Before–During–After and Before–During–After-Control–Impact designs (BDA and BDACI) to assess effects of highway construction and operation using a population of great bustards ( Otis tarda) as a model. Based on a time series of demographic and distribution data (1997–2009), we developed generalized additive models and classification trees to test the effect of road distance on bustard distribution, identify road-effect distances and explore the seasonality of these effects. Two control zones were selected to test the changes between construction phases on productivity, and population trends using TRIM models. From the start of the road construction, great bustards tended to avoid close proximity to the highway (ca. 560–750 m threshold distance). The exclusion band was narrower during the breeding season. In addition, family groups were less tolerant to highway operation disturbances, as shown by their higher distance effect (ca. 1300 m). Population trends did not differ between impact and control zones during the construction. However, once the highway was in operation, bustard numbers declined gradually up to 50% in the impact zone, remained stable in the closest control zone, and increased in the zone located at the greatest distance from the highway. The effects on density of family groups were less evident. Our approach provides information relevant to great bustard conservation and suggests methods for obtaining information of interest to road managers, that could be applied to linear infrastructures with others species.

Nest‐site selection by Great Bustards Otis tarda suggests a trade‐off between concealment and visibility

Great Bustards Otis tarda have expanded their habitat range from historical occupancy of natural steppes to arable farmland, where the species initially benefited from favourable feeding conditions. More recently, the species has suffered severe declines due partly to agricultural intensification. Nest losses and juvenile mortality are amongst the factors most seriously affecting survival probabilities of many populations of this endangered species, suggesting that management of nesting habitats would bring conservation benefits. We studied nest‐site selection in a Great Bustard population of central Spain by radiotracking 42 females for periods of between 1 and 4 years. Females selected nest‐sites in fallows or cereal fields, in areas of low patch‐type diversity, far from human infrastructure, and with good horizontal visibility. These results suggest that females look for shelter, but also need to have good visibility while incubating, and they support the hypothesis that nest selection is a trade‐off between concealment and visibility. We interpret both preferences as adaptations to reduce predation pressure, one of the main causes of nest failure in this species. Nests were placed on slopes significantly orientated to the southeast, which suggests that females also seek sites protected from the cold north‐westerly winds that are prevalent in the study area. To reduce nest destruction, harvesting should be delayed as long as possible and habitat conservation measures should not be restricted to lek sites but also include nesting areas, which are frequently located far from leks.

Recent status and distribution of the Great Bustard, Otis tarda, in Turkey

The records of the Great Bustard, Otis tarda (Linnaeus, 1758), in Turkey from the period 1989–2008 were compiled and analysed to update a previous assessment published in 1989. Despite a sharply increased birdwatching activity in Turkey, only 106 records could be found from published and unpublished sources for this 20-year period. 870 of the total of 1066 individuals were recorded during the breeding season and 196 in the non-breeding season. The population is split into two subpopulations; one focused in Inner Anatolia and the other in East and South-east Anatolia, with the latter exceeding the Inner Anatolian population in numbers. Both subpopulations have declined significantly since the 1989 assessment, although, in the absence of comprehensive field surveys, it is not possible to quantify the degree. As several breeding sites have been abandoned, the size of the distribution area has shrunk in recent decades. In particular, the species is no longer present at several sites situated at the edge of the distribution area. The species winters in south-eastern Anatolia and in small numbers also in the large river deltas on the Black Sea and Mediterranean coasts. However, the winter population is also sharply decreasing. The Turkish Great Bustard population is much smaller than predicted, and apparently comprises 200–300 breeding pairs at the most.

Sex‐biased juvenile survival in a bird with extreme size dimorphism, the great bustard Otis tarda

We explored sex‐biased mortality patterns in a species showing the most extreme sexual dimorphism among birds, the great bustard Otis tarda. Between 1991 and 2005 we studied juvenile and immature survival in a sample of 361 great bustards radio‐tagged at two different populations in Spain, Villafáfila and Madrid. Mortality decreased with age, from high rates during the first year (0.70), to 0.10 in the second year. Using the known‐fate model in program MARK we found that monthly survival increased throughout the first year. Offspring showing higher body mass at marking, i.e. those hatched earlier in the season and those with better body condition, survived in higher proportion. This was probably related to the earlier breeding dates of more experienced mothers, as well as to the observed decrease in food availability as the season progresses. Monthly survival estimates were higher in females than in males, which suggests that juvenile males are more vulnerable to reduced food availability and other factors due to their much faster growth rates. The proportion of non‐natural deaths increased with age, and was higher in the Madrid population, where illegal hunting and collision with powerlines showed a high incidence. The male‐biased mortality found in young birds in this study explains the female‐biased population sex ratios observed in great bustard populations. The different degrees of incidence of human‐induced causes of mortality found between both populations studied suggest that such differences may contribute to the variation observed in the adult sex ratio among populations.

What triggers facultative winter migration of Great Bustard (Otis tarda) in Central Europe?

The Great Bustard is on the brink of extinction in Central Europe. Its population is known to suffer high mortality during hard winters, particularly when severe weather conditions cause migration. Long-term winter food management in two populations in Germany did not prevent migration events. To identify migration-triggering factors we tested the potential influence of snow, temperature, phase of winter and development of a tradition of migration. Comparing migratory behaviour with long-term local weather records, we found that snow cover is a much stronger trigger for migration than frost and low temperatures. We conclude that snow heavily affects the Great Bustard’s energy balance mediated not only by limited food access but also by the particular properties of its plumage. This could explain migration events despite food availability and is consistent with our results concerning a tendency for females to undertake facultative winter migration more than males. Available data are currently insufficient to confirm or reject the idea that Great Bustard populations develop a tradition of migratory behaviour following a previous winter migration, and we found no evidence for a decrease in the disposition of the Great Bustard to migrate during the course of winter.

Long-term trends in great bustard ( Otis tarda) populations in Portugal suggest concentration in single high quality area

Areas of occurrence, population trends and extinction patterns for great bustard Otis tarda populations in Portugal are described for a 22-year period (1980–2002). The major population trends were a large decline in the 1984–1995 period, followed by a fast post-1995 increase. Most of this variation was explained by the trend observed in a single site, Castro Verde, where population has been increasing, in contrast with all other areas where populations have been declining. Eight local extinctions were documented, and probability of extinction increased fast below a threshold of 30 individuals in the initial (1980) population. Agricultural intensification, illegal hunting, road and power line building, and afforestations were the main causes of population decline and extinctions. The exceptional character of the Castro Verde population is explained by the maintenance of good habitat quality, the existence of a bustard conservation project and of an agri-environmental scheme promoting farm management compatible with bustard conservation. Stochastic computer simulations, using VORTEX, suggested that migration of individuals from other sites into Castro Verde was likely, as local productivity alone could not explain the observed population increase. The Portuguese population was estimated at 1150 birds in 2002, of which 912 were concentrated in Castro Verde. The present trend for the increasing concentration of the Portuguese population of great bustards in a single site might lead to increased probability of extinction, particularly due to environmental stochasticity.

Ecological Segregation of Bustards (Otididae) in the North of the Lower Volga Region: Evolutionary and Adaptive Aspects

Based on the concept of an ecological niche, the processes of segregation in the Lower Volga populations of great and little bustards (Otis tarda L. and Tetrax tetrax L). have been studied in the evolutionary and adaptive aspects. The results have shown that segregation of the ecological niches of the two species occurs mainly on a trophic basis, when bird specialization in the use of food resources provides the possibility of changing the nesting biotopes. Some trophic adaptations are temporary and do not modify a species-specific stereotype, whereas other adaptations lead to the development of new forms of behavior and their evolutionary stabilization. Spatial characteristics (biotopic preference, nest location, etc.) complement the identified hierarchical series of parameters characterizing the ecological niches of the species studied. In the Transvolga region of Saratov oblast, the total sizes of great and little bustard populations in the breeding season reach approximately 4100 and 5900 birds, respectively.