What genetics tell us about the conservation of the critically endangered Balearic Shearwater?

The Balearic shearwater Puffinus mauretanicus is one of the most critically endangered seabirds in the world. The species is endemic to the Balearic archipelago, and conservation concerns are the low number of breeding pairs, the low adult survival, and the possible hybridization with a sibling species, the morphologically smaller Yelkouan shearwater (P. yelkouan). We sampled almost the entire breeding range of the species and analyzed the genetic variation at two mitochondrial DNA regions. No genetic evidence of population decline was found. Despite the observed philopatry, we detected a weak population structure mainly due to connectivity among colonies higher than expected, but also to a Pleistocene demographic expansion. Some colonies showed a high imbalance between immigration and emigration rates, suggesting spatial heterogeneity in patch quality. Genetic evidence of maternal introgression from the sibling species was reinforced, but almost only in a peripheral colony and not followed, at least to date, by the spread of the introgressed mtDNA lineages. Morphometric differences were not correlated with mtDNA haplotypes and introgression is probably due to a secondary contact between the two species several generations ago. Overall, results suggested that the very recent demographic decline in this critically endangered species has not yet decreased its genetic variability, and connectivity found among most colonies should help to reduce species extinction risk. Spreading of introgression should be monitored, but the species is not jeopardized at the moment by genetic factors and the major conservation actions should concentrate at enhancing adult survival.     

Viability of mountain caribou in British Columbia, Canada: Effects of habitat change and population density

Population viability analyses (PVA) are frequently employed to develop recovery plans and inform management of endangered species. Translating results from PVA into meaningful management recommendations often depends on an understanding of how population parameters change with environmental conditions as well as population density. The decline of mountain caribou (Rangifer tarandus caribou) in British Columbia, Canada, is believed to be caused by apparent competition with alternative prey species following changes to the forest age structure from timber harvest and wildfire. In addition, populations have been shown to decline at faster rates at low population density. To evaluate the potential effects of habitat change and population density on population persistence, we used stochastic projection models for 10 distinct populations varying in initial size from <10 to approximately 150 females. In an initial model, we used estimates of vital rates based on information sampled from >350 radiocollared caribou between 1984 and 2004. We then compared the results of the initial model to a set of models that evaluated the effects of habitat conditions and population density via their expected relationships to female adult survival. Assuming that vital rates remain constant over a 200-year time frame, only three populations have high probabilities (>0.95) of extinction. When models incorporate the declines in adult female survival know to occur with increasing proportions of young forest and declining population densities, all 10 populations are predicted to decline to extinction within <200 years. Based on our results, we suggest that PVA models that fail to incorporate the effects of changes in vital rates with habitat and population density may lead to overly optimistic assessments of the probability of population persistence in endangered species.     

A risk analysis for the common hamster (Cricetus cricetus)

The common hamster (Cricetus cricetus) has suffered a dramatic decline in Western Europe though the species is characterized by an extremely high reproductive potential. An individual-based model was developed to determine the risk factors which lead to population decline and to investigate management actions for viable populations. Demographic stochasticity was considered by (a) describing reproduction on an individual basis and (b) distinguishing behavioural patterns according to age and sex. Unknown model parameters were determined by comparing field data and simulated patterns in a specific multi-criteria fashion. We conclude that disturbances such as agricultural management and highway construction are most dangerous to the common hamster in autumn. Complex land management is required which should aim at reducing habitat-specific mortality and favouring the safeguarding of adult and subadult females to protect the most sensitive component of the population. According to the model results, large habitat size is not sufficient for survival, and habitat connectivity may be even more important. Late timing of the harvest and following cultivations was most favourable for population survival.     

Responses of cycads with different life histories to the impact of plant collecting: simulation models to determine important life history stages and population recovery times

Collection of plants and seeds from wild populations threatens a large number of cycad species. We investigated to what extent individual life history stages contribute to population growth (λ) and compared two species with major differences in life histories in the African genus Encephalartos: Encephalartos cycadifolius, a highly persistent grassland species that resprouts after fire, and Encephalartos villosus, a relatively fast growing, non-sprouting forest species. Several harvesting scenarios impacting different sized individuals were simulated to determine the sensitivity of the two functional types to harvesting. In both species λ was most sensitive to changes in abundance of adult plants. The harvesting of seeds had minimal impact on population growth rates, whereas harvesting of adult plants led to rapid population decline. This response from two very different functional types suggests that the conservation of adult plants is critical for all cycad species. Despite similar responses to adult mortality, the two species had substantially different population growth rates. This determined recovery time after harvesting of adult individuals. Encephalartos cycadifolius is typical of highly persistent plant species associated with low levels of recruitment and unable to recover from even small losses of adults within a reasonable conservation time frame (<100 years). Our results suggest that the ability to recover from loss of individuals is an important factor that should be considered when assessing the vulnerability of wild populations to threats.     

Survival rates and causes of mortality in Eurasian lynx (Lynx lynx) in multi-use landscapes

Causes of mortality were described for 245 radio-marked Eurasian lynx (Lynx lynx) in five different Scandinavian study areas. Furthermore, the survival rates and the population growth rates were estimated for three of the study areas where 202 lynx were followed for a total of 314 radio-years. The main causes of mortality in adult Eurasian lynx in all our study areas were overwhelmingly anthropogenic, with starvation, vehicle collisions, intra- and interspecific killing and disease only having a minor role. The mean mortality rates for adults increased from 2% to 17% when hunting and poaching were included, i.e., an increase by a factor of eight. This in turn had a large impact on population growth rates, which changed from more than a 20% annual increase to only a 2-4% when hunting and poaching were included. Poaching accounted for 46% of the mortality in adult lynx. Poaching and legal harvest appear to be primarily motivated by conflicts; lynx depredation on semi-domestic reindeer in northern Scandinavia, competition with hunters for roe deer in southern Scandinavia, and depredation on free-ranging domestic sheep in all Norway. The lowest poaching rate was found in the Hedmark study area in Norway, which also had a high legal harvest. The poaching rate was higher in one of the Swedish study areas (Sarek) where legal hunting was lower than in other areas. On the other hand, both the poaching rate and the legal harvest were high in the Akershus/Østfold study area in Norway. Thus, there does not seem to be a simple relationship between an increased legal harvest and decreased poaching as is commonly expected. The most important conservation actions are to combat poaching through both law enforcement and measures designed to increase tolerance.     

Anthropogenic and natural disturbance lead to differing patterns of gene flow in the Rocky Mountain tailed frog, Ascaphus montanus

Understanding the influence of habitat alteration on population structure and persistence is critical for effective conservation strategies. Timber harvest and wildfire are two of the most prevalent disturbances across temperate forests, yet the long-term effects of these two forces on population connectivity have rarely been studied. We studied populations of the Rocky Mountain tailed frog (Ascaphus montanus) across landscapes that have experienced either timber harvest or broad scale fires. Rocky Mountain tailed frogs generally require forested habitat, and are a species of concern in managed forests. We used landscape genetic techniques to test the explanatory power of alternative paths of connectivity across both burned and harvested forests and identified topographical and climatic variables that significantly influenced gene flow. We found that timber harvest and fire led to different patterns of genetic connectivity. Widespread terrestrial gene flow was maintained across previously burned areas and connectivity was primarily limited by solar radiation. In contrast, gene flow among populations in managed forests followed riparian corridors, presumably to avoid the loss of cover due to timber harvest. Gene flow along riparian corridors in Rocky Mountain tailed frogs differs from a population of the closely related coastal tailed frog (Ascaphus truei), for which gene flow appears to occur primarily over land. These results suggest management of Rocky Mountain tailed frogs should focus on maintaining riparian buffer zones, which may be less effective for the coastal species. Thus, consideration of landscape differences is essential even for conservation of closely related, morphologically similar species.     

Conservation biology for suites of species: Demographic modeling for Pacific island kingfishers

Conservation practitioners frequently extrapolate data from single-species investigations when managing critically endangered populations. However, few researchers initiate work with the intent of making findings useful to conservation efforts for other species. We presented and explored the concept of conducting conservation-oriented research for suites of geographically separated populations with similar natural histories, resource needs, and extinction threats. An example was provided in the form of an investigation into the population demography of endangered Micronesian kingfishers (Todiramphus cinnamominus). We provided the first demographic parameter estimates for any of the 12 endangered Pacific Todiramphus species, and used results to develop a population projection matrix model for management throughout the insular Pacific. Further, we used the model for elasticity and simulation analyses with demographic values that randomly varied across ranges that might characterize congener populations. Results from elasticity and simulation analyses indicated that changes in breeding adult survival exerted the greatest magnitude of influence on population dynamics. However, changes in nestling survival were more consistently correlated with population dynamics as demographic rates were randomly altered. We concluded that conservation practitioners working with endangered Pacific kingfishers should primarily focus efforts on factors affecting nestling and breeder survival, and secondarily address fledgling juveniles and helpers. Further, we described how the generalized base model might be changed to focus on individual populations and discussed the potential application of multi-species models to other conservation situations.     

Metapopulation dynamics and conservation of the marsh fritillary butterfly: Population viability analysis and management options for a critically endangered species in Western Europe

This study investigates the dynamics and viability of a marsh fritillary butterfly Euphydryas aurinia metapopulation in a Belgian successional landscape. Based on capture-mark-recapture and winter nest census data, we first estimated demography (survival and recruitment rates, population size, density dependence) and dispersal parameters (emigration rate, effect of patch connectivity on dispersal, mortality during dispersal). Then using RAMAS/GIS platform, we parameterised a population viability analysis (PVA) model with these parameters to simulate the future of this metapopulation under different scenarios.The metapopulation does not seem viable even if natural reforestation is controlled by adequate management. In its present state, the patch system is not able to sustain enough individuals: due to the large temporal fluctuations in demographic parameters, a carrying capacity far higher than currently would be necessary to limit extinction risk to 1%, suggesting the existence of an extinction debt for the species in Belgium. The situation of E. aurinia appears much worse compared to two other fritillary species threatened in Belgium, for which similar PVA are available. It is therefore urgent to increase the carrying capacity of the patch system. How and where it is achieved are of secondary importance for the gain in viability: improvement of habitat quality through restoration, or increase of habitat quantity via enlargement of existing patches and/or creation of new habitat in the matrix. A regime of management based on regular re-opening and maintenance of habitat patches may be the only guarantee of long-term persistence for this critically endangered species in Belgium.     

Estimating impacts of poison operations on non-target species using mark-recapture analysis and simulation modelling: an example with saddlebacks

Poison operations to control or eradicate exotic mammals are a key component of conservation management in the South Pacific. They also result in by-kill of native species. It is therefore important to develop reliable methods for estimating by-kill and assessing its impact. The North Island saddleback (Philesturnus carunculatus rufusater), a rare New Zealand forest bird, was reintroduced to Mokoia Island in 1992, and 4 years later there was an aerial drop of cereal pellets containing Brodifacoum aimed at eradicating mice. We used mark-recapture analysis on resighting data collected from 1992 to 1997 to estimate the by-kill of saddlebacks attributable to this poison drop. We nominated a set of candidate models to explain the data, and compared these using Akaike's Information Criterion. The analysis showed that saddleback survival was substantially lower than expected in the 6-week interval after the poison drop, taking age, density dependence, season and random variation into account. Taking expected survival rates into account, the probability of an adult being killed was estimated to be 0.45 (95% CI=0.34-0.56), and the probability of a juvenile being killed was estimated to be 0.35 (95% CI=−0.05-0.75). We then used a simulation model developed for the population to assess the longer-term impact of this mortality. While the mortality set back the expansion of the population by 1-2 years, we predicted that the population would have recovered fairly quickly and ultimately reach the same carrying capacity. Mark-recapture analysis permits precise estimates of background survival rates unconfounded by seasonal or random variation.     

Dispersal of breeding, adult male Phocarctos hookeri: Implications for disease transmission, population management and species recovery

Dispersal impacts on a range of population parameters making it a key piece of information in species conservation. Despite its importance, dispersal is poorly characterized for many species: pinnipeds are no exception. Understanding dispersal patterns of the New Zealand sea lion Phocarctos hookeri is crucial in the conservation management of the species as its recovery to a non-threatened status hinges on range recolonisation. In this study, we examined the movements of breeding adult male New Zealand sea lions within and following the breeding season of the 2002/03 austral summer using a novel multi colony approach. Based on resightings of 202 individually identifiable adult males, we found (1) a previously unappreciated, high level of dispersal by adult territorial males between breeding colonies during the pupping period and (2) that breeding males disperse to the extremes of the species’ range at the end of female oestrous. Our findings are contrary to the current paradigm of otariid breeding behaviour, which is believed to consist of prolonged, uninterrupted male territoriality based on intense male-male competition and sustained fasting. Adult male dispersal between colonies and across the species range has important implications for adult males as vectors of disease in three recent epizootics, species management and species recovery via recolonisation as males are apparently remaining part of a localized, vulnerable breeding population.     

A nine-year study of successful breeding in a Bonelli’s eagle population in southeast Spain: a basis for conservation

We analyse the factors influencing breeding success in a healthy population of Bonelli’s eagle (Hieraaetus fasciatus) from south-east Spain, a raptor that has suffered a serious decline in Europe. Between 18 and 33 pairs were annually monitored during the period 1994-2002. Several factors that may affect four breeding parameters were studied, namely human presence, vegetation, relief, climatic factors, intra- and inter-specific relationships, diet, prey abundance, nest building, adult mortality and age of reproduction. A consistently high breeding success was registered during the study period (productivity=1.43, SD=0.11), which was probably the result of high adult survival, adequate prey availability and mild weather conditions. However, a certain vulnerability to the presence of golden eagles (Aquila chrysaetos) and to human disturbance was observed. One interesting result was the reduced survival of young chicks on north-facing cliffs owing to colder conditions, which may partially explain the decline of the Bonelli’s eagle populations along its European distribution limits. We discuss the role of this healthy subpopulation in an Iberian metapopulation context, and propose that the potential interference of golden eagles should be taken into account when designing management strategies for Bonelli’s eagles. Finally, we pinpoint some conservation priorities and the importance of reducing the main causes of adult mortality (i.e. shooting and electrocution) to assure successful reproduction and survival of this species.     

Human caused mortality in the endangered Scandinavian wolverine population

The wolverine is a relatively long lived mustelid species with low average annual reproduction which in theory makes them sensitive to changes in survival rates. In a multi-year study we monitored 211 radio-marked wolverines to estimate age-specific survival rates and mortality causes in an endangered population. Our data suggests that poaching forms a substantial part of wolverine population dynamics in northern Scandinavia causing up to 60% of adult mortality. Average annual adult survival rate was 0.91. Male and female survival rates did not differ. Twenty-five adult wolverines were confirmed to have died during the study. Fifty-two percent of confirmed adult mortality was human caused. The most important cause of adult mortality was poaching (9). Annual adult survival was lower (0.86) when 15 adult resident wolverines that were assumed to have died due to poaching were included in the analyses. Natural mortality among subadults and adults was less frequent in our study population compared to North American wolverine populations. Adult survival was significantly lower during the snow season (December-May) than during the snow-free season (June-November), while natural mortality was more evenly distributed between seasons. We explain this by the frequency of poaching which is higher during snow season. Our results underline the need to frame the underlying human dimension factors behind poaching to facilitate conservation and management of endangered populations.     

Demography and conservation of the Tristan albatross Diomedea [exulans] dabbenena

The Tristan albatross Diomedea [exulans] dabbenena is the third rarest albatross species, with a breeding population of around 1500 pairs almost totally restricted to Gough Island in the Tristan da Cunha group, central South Atlantic. During January 2000, the entire breeding population of Gough was surveyed for the first time since 1956, and 2400 incubating pairs were counted. An analysis of the areas that are likely to have been surveyed most accurately in the past suggests that the population has decreased by around 28% over 46 years. The number of large chicks counted over three successive seasons (1999-2001) was highly variable (range 318-1129). The average count over this period (705 chicks) is less than counts made in 1979 (792) and 1982 (798). A total of 656 chicks were counted in September 2001, giving an island breeding success of just 27.3%. However, breeding success varied considerably in different areas of the island, ranging from 17.6 to 68.0%. During the 2001 season most breeding failures were of large chicks, and over 4 years where data were available, 75% of breeding failures occurred during the chick period. Predation by introduced house mice Mus musculus is the most likely cause of chick mortality. In a small study population, birds began breeding at an average age of 9.7 years and annual adult survival from 1985 to 2001 was 92.6% (SE=1.6%). Both breeding success and adult survival estimates are low in comparison with other Diomedea species and population modelling predicts a population decreasing at an annual rate of 2.9-5.3%. Further research is needed urgently to assess whether breeding success is typical, and to confirm that mouse predation is the cause of chick mortality. The low productivity of this species will compound the negative impacts of longline fishing mortality, which are likely to be reducing adult and juvenile survival.     

Illegal killing slows population recovery of a re-introduced raptor of high conservation concern - The red kite Milvus milvus

The re-introduction of extirpated species is a valuable conservation tool. Red kites Milvus milvus are declining over much of their European range and have been re-introduced to England and Scotland, following their extinction due to widespread human persecution during the 19th century. Considerable regional variation in population growth exists. This study aims to identify the proximate demographic and ultimate environmental constraints on red kites in north Scotland, a region with low population growth. Productivity in north Scotland was high compared to other Scottish and Welsh populations and equal to English populations with high population growth rates. In north Scotland, annual survival of wild-fledged birds was low for first-year birds compared to other Scottish populations and second-year survival declined over time. In north Scotland, 40% of 103 red kites found dead were killed illegally, mainly by direct poisoning. In the absence of illegal killing, we estimate that annual survival rates in wild red kites might increase from 0.37 to 0.54, 0.72 to 0.78 and 0.87 to 0.92 for first, second-year and adult birds respectively. Demographic rates from this study produce population trends that recapitulate observed trends for the north Scotland population (leading to a population of c40 pairs by 2006). Models in which the additive illegal killing mortality is excluded, predict a population trajectory and size (c300 pairs by 2006) very similar to that found in the Chilterns, a rapidly growing population (320 pairs in 2006) in south-east England re-introduced at the same time, but where rates of illegal killing are much lower. We conclude that illegal killing of red kites is the cause of poor population growth in north Scotland and the key challenge facing government is to find a way to eliminate this killing.     

Dispersal and establishment of floodplain grassland species as limiting factors in restoration

The restoration of grassland and its dependence on the dispersal of characteristic plant species was analysed in the Saale River floodplain near Halle (Germany). Species composition and soil nutrient content were investigated in grassland converted to extensive management in 1989 (“restoration grassland”) and in adjacent grassland that had never been managed intensively (“old grassland”). In two experiments dispersal of Silaum silaus and Serratula tinctoria was studied following introduction of these species into “restoration grassland”. Seedling establishment was recorded and compared with the “old grassland”. Ten years after conversion to extensive management, characteristic floodplain grassland species had only reappeared in locations very close to “old grassland”. There were still differences in soil nutrient content of both grassland sites but a comparison of seedling survival provided evidence that conditions of establishment were similar. However, establishment rates appeared to be low in both grassland types indicating that a large initial input of seeds is required for re-establishment. Seeds of Silaum silaus and Serratula tinctoria were dispersed very short distances. About 75% of the seedlings were found within 1.5 m of parent plants. Management and flooding did not increase dispersal distances. The results strongly suggest that poor dispersal was the main limiting factor in determining the success of restoration. The implications of this result for nature conservation are discussed.     

Microsatellite analysis of genetic population structure of the endangered cyprinid Anaecypris hispanica in Portugal: implications for conservation

The endangered fish species Anaecypris hispanica is restricted to eight disjunct populations in the Portuguese Guadiana drainage. The genetic structure of these populations was studied in order to determine levels of genetic variation within and among populations and suggest implications for conservation of the species. Based on five microsatellite loci, the null hypothesis of population homogeneity was tested. Tests for genetic differentiation revealed highly significant differences for pairwise comparisons between all populations, and substantial overall population subdivision (F_ST=0.112). All sampled populations contained unique alleles. Our findings indicate marked genetic structuring and emphasise limited dispersal ability. The high levels of genetic diversity detected within and among A. hispanica populations suggest, however, that the observed fragmentation and reduction in population size of some populations during the last two decades, has impacted little on levels of genetic variability. Data imply that most A. hispanica populations should be managed as distinct units and that each has a high conservation value containing unique genetic variation. It is argued that geographic patterns of genetic structuring indicate the existence of eight management units.     

Female biased mortality caused by anthropogenic nest loss contributes to population decline and adult sex ratio of a meadow bird

Meadow breeding birds such as the whinchat Saxicola rubetra have been declining due to increased farming intensity. In modern grassland management, the first mowing and the bird’s breeding cycle coincide, causing high nest destruction rates and low productivity of grassland bird populations. However, it is virtually unknown whether the mowing process directly affects adult survival by accidentally killing incubating females. We studied adult survival of an Alpine whinchat population during two breeding seasons using either colour-ringing or radio-tracking of 71 adults. Assessing territories, mowing phenology and nest destruction from 1988 to 2007 allowed changes in the factors associated with female mowing mortality to be estimated. Adult survival over 5-day-periods was Φ = 0.986, but during the period of mowing female survival was strongly reduced (Φ = 0.946). As a result, 80.6% of the males, but only 68.4% of the females survived the breeding season. Mowing undoubtedly killed two of 20 radio-tagged females when they were laying or incubating. In the 20-year period, an increasing proportion of nests were destroyed before the chicks hatched and this change was associated with an increased distortion of the adult sex ratio. Modelling the population growth rate showed that including the additional effect of mowing on female mortality resulted in a 1.7 times faster local population decline. These results are consistent with the hypothesis that the extinction of whinchat populations in the lowlands of central Europe was caused not only by habitat degradation and low productivity, but also by increased man-made female mortality.     

The return of the white-tailed eagle (Haliaeetus albicilla) to northern Germany: Modelling the past to predict the future

Linking age-specific vital rates to population growth through demographic matrix models can enhance our understanding of crucial population processes, vital in a conservation context. The white-tailed eagle (Haliaeetus albicilla) population in the Federal State of Schleswig-Holstein, Germany, has been monitored since re-colonisation in 1947 and provides a well-documented example of a recovery. We test how demographic models capture growth trajectories of a recovering population and how applicable they are in guiding population management of endangered species. From 1947 to 1974, the population was stable but the growth rate predicted by an age-structured matrix model was −6.1% per annum. The small but stable population must have been maintained by immigration. From 1975 to 2008, observed and predicted population growths were very similar (6.7% and 4% per annum respectively). Elasticity and life-stage simulation analyses identified adult and pre-breeding survival as key vital rate elements. While the prospective analyses identified survival as the key vital rate influencing population growth, the increasing reproduction rate allowed the recovery to take place; thus caution is needed when prospective modelling makes management recommendations. Nevertheless, conservation efforts should address key mortality factors such as lead poisoning and collision with wind turbines. A logistic model predicted a maximum carrying capacity of 255 pairs for the Federal State, but using the highest currently observed density (1.4 pairs per 100 km²) and differences in habitat suitability, a more likely carrying capacity was estimated at 122 pairs. Under both scenarios, current population growth should slow soon.     

Habitat associations of the endangered damselfly, Coenagrion mercuriale, in a water meadow ditch system in southern England

The damselfly Coenagrion mercuriale (Charpentier) (Odonata: Coenagrionidae) is threatened throughout Europe. It is listed on the EC Habitats and Species Directive and is the only odonate currently given priority status in the UK Biodiversity Action Plan. Information about the habitat preferences of this species is required to guide conservation and management programmes. We obtained estimates of density of mature adult C. mercuriale during an intensive mark-release-recapture study over 7.65 km of a water meadow ditch network in the Itchen Valley, Hampshire. Detailed habitat information was also collected, including a variety of physical variables, and data about the in-channel and bankside vegetation. C. mercuriale density and movement were analysed in relation to habitat variables and local population size using Generalized Linear Models. Mean adjacent population density was the single most important factor determining density. However the species was also associated with a number of habitat features, the most important of which were: a channel substrate consisting primarily of silt, wide underwater ledges (berms), in-channel emergent dicots, and bankside monocots. The presence of trees was negatively associated with damselfly density. Mean net lifetime movement was greatest from sections with low density, with smaller than average berms, and with deeper water. The causes and consequences of these findings are discussed in relation to the conservation and management of this rare species.     

Challenging conservation of migratory species: Sahelian rainfalls drive first-year survival of the vulnerable Lesser Kestrel Falco naumanni

In the context of attested global changes, accurate estimation of whether climatic fluctuations impact on population demographic parameters is needed for adequate management, especially for migratory species. We present a capture-recapture analysis linking survival rates of the vulnerable Lesser Kestrel (Falco naumanni) to annual rainfalls in the Sahel region, considered as a proxy of climatic conditions on wintering grounds. Recapture data were obtained from field observations of individuals ringed and sexed as juveniles over a 14-year monitoring period (1994-2007). We addressed a common but problematic situation in birds where: (i) sex is known with certainty for first-summer or older birds but only suspected for juveniles, and (ii) a large proportion of individuals never return to the study population (e.g. transient behavior). Transient behavior and unknown sexes were explicitly integrated considering a two age class in a multistate capture-recapture model. Survival was time-varying for juveniles (geometric mean: 0.499 ± 0.021) but constant - and higher - for adults (0.718 ± 0.013). Yearling survival probabilities were strongly correlated with rainfalls in the Sahel, suggesting a high dependence of juvenile upon the wintering conditions. While taking sex uncertainty into account, we detected no sex-dependence in survival. Incorporating the sensitivity of survival of wintering migratory birds to climatic variables such as precipitations in arid Sahelian ecosystem may allow to model conservation scenarios with a greater realism. Finally, we encourage the development of international management strategies for migratory species on wintering areas in addition to the existing conservation actions on summering Mediterranean grounds.