Assessing the applicability of stable isotope analysis to determine the contribution of landfills to vultures' diet

A review of risk factors at the human-animal-environmental interface of garbage dumps that are driving current and emerging zoonotic diseases

An increasing trend in zoonotic and emerging infectious diseases (EIDs) has been observed worldwide. Most EID outbreaks originate from wildlife, and these outbreaks often involve pathogen-host-environment interaction. Garbage dumps act as an interface between humans, animals, and the environment, from which EIDs could arise. Therefore, this review considers the presence of important pathogens associated with animals and vectors at garbage dumps from a One Health perspective, looking at animal, human, and environmental factors that play a role. A narrative review was performed focusing on four key points, including garbage dumps, animals, waste pickers, zoonoses and EIDs. Articles addressing the presence of terrestrial animals, insects in garbage dumps, and infectious diseases among waste pickers were included in this study. There were 345 relevant articles covering 395 species of terrestrial animals and insects, consisting of 4 species of amphibians, 180 species of birds, 84 species of insects, 114 species of mammals, and 13 species of reptiles. Furthermore, 97 articles (28.12 %) addressed pathogens found in those populations. About half of the articles were interested in bacterial diseases (52.58 %), followed by parasitic diseases (30.93 %) and viral diseases (30.93 %). Zoonotic pathogens were described in 53.6 % of all articles, while 19.59 % focused on drug-resistant microbes, 13.40 % on rodent-borne diseases, and 7.21 % on vector-borne diseases. Garbage dumps would play a role in the emergence of diseases. The relevant factors at garbage dumps that may increase the risk of disease emergence include increased animal populations and density, increased vector population, newly evolved strains of pathogens, increased interaction between humans, domestic animals, wildlife, and vectors, and socio-economic factors. Therefore, sustainable waste management will reduce waste generation, and improve waste collection, and disposal which helps reduce the emergence of new diseases.

Levels of persistent organic pollutants (POPs) and the role of anthropic subsidies in the diet of avian scavengers tracked by stable isotopes

Persistent Organic Pollutants (POPs) have been identified as a significant factor driving declines in wildlife populations. These contaminants exhibit a dual tendency to biomagnify up the food chains and persist within tissues, rendering long-lived vertebrates, such as raptors, highly vulnerable to their adverse effects. We assessed the concentrations of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) in fledglings of two vulture species, the Egyptian vulture (Neophron percnopterus) and the griffon vulture (Gyps fulvus), coexisting in northern Spain. Vultures, currently facing a severe threat with a population decline exceeding 90%, represent one of the most critically endangered avian groups in the Old World. Despite this critical situation, there remains a scarcity of research examining the intricate relationship between contaminant levels and individual foraging behaviors. In parallel, we analyzed stable isotope levels (δ15N and δ13C) in fledgling's feathers and prey hair to determine the association between individual dietary and contaminant burdens. Our findings revealed higher levels of PCBs in Egyptian vultures, while pesticide concentrations remained very similar between focal species. Furthermore, higher individual values of δ13C, indicating a diet based on intensive farming carcasses and landfills, were associated with higher levels of PCBs. While the levels of POPs found do not raise immediate alarm, the presence of individuals with unusually high values reveals the existence of accessible contamination sources in the environment for avian scavengers. The increasing reliance of these birds on intensive livestock farming and landfills, due to the decline of extensive livestock farming, necessitates long-term monitoring of potential contaminant effects on their populations.

Ecosystem productivity drives the breeding success of an endangered top avian scavenger in a changing grazing pressure context

Environmental conditions and resource availability shape population dynamics through direct and indirect effects of climate, biological interactions and the human modification of landscape. Even when a species seems dependent on predictable anthropogenic food resources or subsidies, ecosystem-level factors can still determine population dynamics across taxa. However, there is still a knowledge gap about the cascade effects driven by climate, vegetation functioning, resource availability and governmental policies on key aspects of species reproduction for top scavengers. Here we put to good use 22 years (2000-2021) of extensive population monitoring from the endemic Canary Egyptian vulture (Neophron percnopterus majorensis) on the Fuerteventura Island (Canary Islands, Spain) to study the relative importance of demographic factors, ecosystem conditions and availability of anthropogenic food sources on breeding success. Our results suggest that ecosystem-level primary productivity, the number of livestock animals present on the island and Density-dependent processes determine the temporal changes in the breeding success of this species. We firstly accounted for a top-down effect of livestock on island vegetation, where overgrazing directly reduces landscape-level vegetation biomass. We, consequently, found a bottom-up effect between vegetation and the Egyptian vulture's breeding success. In this context, minimal changes in ecological conditions can impact the species inhabiting these ecosystems, with direct consequences on a key population stage, such as breeding season, when energy requirements are higher. These results are especially relevant because cascading and indirect effects of ecosystem processes and governmental policies are often overlooked when pursuing conservation goals of endangered species.

Resource predictability modulates spatial-use networks in an endangered scavenger species

BACKGROUND

Changes in human-induced resource availability can alter the behaviour of free-living species and affect their foraging strategies. The future European Landfill Waste Directive and Circular Economy Action Plan will reduce the number of predictable anthropogenic food subsidies (PAFS), above all, by closing landfills to preclude negative effects on human health. Obligate avian scavengers, the most threatened group of birds worldwide, are the most likely group of species that will be forced to change their behaviour and use of space in response to landfill site closures. Here, we examine the possible consequences of these management decisions on the foraging patterns of Egyptian vultures (Neophron percnopterus) in an expanding population in the Iberian Peninsula.

METHODS

We tracked 16 individuals in 2018-2021, including breeders and non-breeders, and, using a combination of spatial-use and spatial-network modelling, assessed landscape connectivity between key resources based on movement patterns. We then carried out simulations of future scenarios based on the loss of PAFS to predict likely changes in the movement patterns of both non-breeders and breeders.

RESULTS

Our results show that foraging strategies in non-breeders and breeders differ significantly: non-breeders performed more dispersal movements than breeding birds across a spatial-use network. Non-breeding and breeding networks were found to be vulnerable to the removal of central foraging areas containing landfill sites, a highly predictable resource, while perturbation analysis showed dissimilar foraging responses to the gradual reduction of other predictable resources. Under a context of the non-availability of landfills for breeders and non-breeders, vultures will increase their use of extensive livestock as a trophic resource.

CONCLUSIONS

Future environmental policies should thus extend the areas used by scavengers in which livestock carcasses are allowed to remain in the wild, a strategy that will also mitigate the lack of food caused by any reduction in available waste if landfills close. In general, our results emphasize the capabilities of a spatial network approaches to address questions on movement ecology. They can be used to infer the behavioural response of animal species and, also demonstrate the importance of applying such approaches to endangered species conservation within a context of changing humanized scenarios.

Second-generation anticoagulant rodenticides in the blood of obligate and facultative European avian scavengers

The widespread use of second-generation anticoagulant rodenticides (SGARs) and their high persistence in animal tissues has led to these compounds becoming ubiquitous in rodent-predator-scavenger food webs. Exposure to SGARs has usually been investigated in wildlife species found dead, and despite growing evidence of the potential risk of secondary poisoning of predators and scavengers, the current worldwide exposure of free-living scavenging birds to SGARs remains scarcely investigated. We present the first active monitoring of blood SGAR concentrations and prevalence in the four European obligate (i.e., vultures) and facultative (red and black kites) avian scavengers in NE Spain. We analysed 261 free-living birds and detected SGARs in 39.1% (n = 102) of individuals. Both SGAR prevalence and concentrations (ΣSGARs) were related to the age and foraging behaviour of the species studied. Black kites showed the highest prevalence (100%), followed by red kites (66.7%), Egyptian (64.2%), bearded (20.9%), griffon (16.9%) and cinereous (6.3%) vultures. Overall, both the prevalence and average ΣSGARs were higher in non-nestlings than nestlings, and in species such as kites and Egyptian vultures foraging in anthropic landscapes (e.g., landfill sites and livestock farms) and exploiting small/medium-sized carrions. Brodifacoum was most prevalent (28.8%), followed by difenacoum (16.1%), flocoumafen (12.3%) and bromadiolone (7.3%). In SGAR-positive birds, the ΣSGAR (mean ± SE) was 7.52 ± 0.95 ng mL^-1; the highest level detected being 53.50 ng mL^-1. The most abundant diastereomer forms were trans-bromadiolone and flocoumafen, and cis-brodifacoum and difenacoum, showing that lower impact formulations could reduce secondary exposures of non-target species. Our findings suggest that SGARs can bioaccumulate in scavenging birds, showing the potential risk to avian scavenging guilds in Europe and elsewhere. We highlight the need for further studies on the potential adverse effects associated with concentrations of SGARSs in the blood to better interpret active monitoring studies of free-living birds.

Urban waste no replacement for natural foods—Marabou storks in Botswana

We compared diets of marabou storks Leptoptilos crumenifer foraging from urban landfills and natural areas in northern Botswana using stable isotope analyses and inductively coupled plasma mass spectrometry on moulted feathers. There were significant differences in the diet of marabous foraging from natural areas compared to urban waste sites, reflected by lower δ13C and less enriched δ15N concentrations in those feeding at landfills, suggesting a shift in trophic niche. Feathers from birds foraging at landfills also had significantly higher concentrations of chromium, lead, nickel, and zinc and lower levels of cadmium and potassium than feathers sampled from natural areas. We also analysed marabou regurgitant (42 kg, naturally expelled indigestible food resources) from the Kasane landfill site. More than half was plastic, with single regurgitants weighing up to 125 g. Urban waste stored in open air landfills is altering some marabou diets, affecting their natural trophic niche, resulting in the consumption (and regurgitation) of large amounts of plastic, and exposing marabou to potentially chronic levels of trace metals. Despite the marabou’s apparent resilience to this behavioural shift, it could have long-term effects on the population of the marabou stork, particularly considering Botswana has some of the few regular marabou breeding colonies in southern Africa.

Latitudinal patterns in the diet of Andean condor (Vultur gryphus) in Chile: Contrasting environments influencing feeding behavior

Human-dominated environments alter the availability and quality of resources for many species, especially for scavengers that have large home ranges and plastic foraging behaviors that enable them to exploit novel resources. Along the western slope of the Andes, the modification of natural landscapes have resulted in significant declines in native prey, the introduction of non-native species, and an increase in the availability of anthropogenic resources. These factors have likely influenced the resources available to Andean condors (Vultur gryphus), however, data are lacking as to how condor's diet vary along their large latitudinal range. We evaluated differences in Andean condor diet along a ~2500 km latitudinal gradient in Chile from the heavily modified Central zone (32-34°S) to the more pristine Austral zone (44-56°S). We assessed diet composition through the identification of prey remains in condor pellets, and carbon and nitrogen isotope analysis of condor feathers and their primary prey identified from pellet analysis. Our results identified medium- and large-bodied domesticated mammals (ungulates) and introduced exotic species (lagomorphs) as common prey across the study area. Condors from the Central zone had the largest isotopic niche width, probably related to consumption of anthropogenic resources with distinctly high carbon isotope values indicative of C₄-based foods likely acquired from landfills or corn-fed livestock. Isotopic niches for condors from the Southern and Austral zones almost completely overlapped. Andean condor diet is strongly influenced by local conditions determining differential access to prey sources. The high dependence of Andean condors on livestock across a large geographical area, and landfills in more (sub)urban areas, may help stabilize their populations via anthropogenic resources subsidies. Long-term dependence on such resources, however, may have health costs including contaminant exposure and greater mortality risk. These data will help identify potential threats related to resource availability and use, and better inform management and conservation decisions.

What do minerals in the feces of Bearded Vultures reveal about their dietary habits?

The diet of Bearded Vultures Gypaetus barbatus consists mainly of bones, which are completely digested in the gastrointestinal tract, unwanted bone minerals being discarded via the feces. Chemical analyses of feces therefore provide a noninvasive technique for studying the diet of this species. We analysed the inorganic and organic remains in feces collected from Bearded Vulture nests in the Spanish Pyrenees and discussed these results with the diet of individuals determined by video camera observations. Of the food items delivered to the nest, taxonomically 65% were bone fragments of Ovis/Capra spp. (range 56-75%) and anatomically 76% (74-81%) bones from the extremities, indicating a selective preference. At least 15% of the diet was meat based, mainly originating from small prey (e.g. small carnivores, birds). The fecal analyses show that calcium and phosphorus are the most abundant mineral constituents, accounting for 41.3-44.4% of the mineral part of the feces. Among the minor elements identified, the variation in the concentrations of iron, silicon and zinc suggest differences in food selection between territories, although this could be related to varying amounts of accidentally ingested soil particles present in the food. We found variation in the content of uric acid in the feces, ranging between 0.5 and 4.6%. Higher values of uric acid might be due to a more meat or marrow bone-based diet. However, no relationship was found between the amount of calcium and uric acid levels, suggesting that the metabolites of meat digestion (uric acid) and those of bone digestion (calcium) are not negatively correlated as expected. In conclusion, our chemical analyses of feces collected from the nests of Bearded Vultures confirm that their diet consists mainly of bone remains and that these bones are digested completely. However, the direct observations of the prey items delivered to the nest produced more detailed information than the chemical analyses.

Combining stable isotopes, morphological, and molecular analyses to reconstruct the diet of free-ranging consumers

Accurate estimates of animal diet composition are essential to untangle complex interactions in food webs. Biomarkers and molecular tools are increasingly used to estimate diet, sometimes alongside traditional dietary tracing methods. Yet only a few empirical studies have compared the outcomes and potential gains of using a combination of these methods, especially using free-ranging animals with distinct foraging preferences.We used stable isotopes, morphological, and molecular analyses to investigate the diet of free-ranging consumers with two distinct diet types, that is, carnivore and omnivore. By combining the three analytical methods to assess the diet of consumers during the same period, we aimed to identify the limits of each method and to assess the potential benefits of their combined use to derive diet estimates.Our results showed that the different methods led to a consistent diet description for carnivores, which have a relatively simple diet mixture, but their outcomes somewhat differed for omnivore, which have a more complex diet. Still, the combined use of morphological and molecular analyses enhanced the diversity of food sources detected compared to the use of a single method independently of diet types. Precision of diet estimates derived from stable isotope analyses was improved by the addition of priors obtained from morphological and molecular diet analyses of the same population.Although we used free-ranging animals without a known diet, our empirical testing of three of the most widely used methods of diet determination highlights the limits of relying over a single approach, especially in systems with few or no a priori information about the foraging habits of consumers. The choice of an appropriate approach of diet description should be a key step when planning dietary studies of free-ranging populations. We recommend using more than one dietary determination methods especially for species with complex diet mixtures.

Accumulation of pollutants in nestlings of an endangered avian scavenger related to territory urbanization and physiological biomarkers

We monitor pollutant accumulation and investigate associated changes at the physiological level within the population of an obligate avian scavenger, the Egyptian Vulture (Neophron percnopterus), from Catalonia (NE Spain). This population is expanding its range, presumably because of the use of human waste disposal sites as food resource. We hypothesized that habitat urbanization, presumably associated with feeding from human wastes, could influence the accumulation of persistent organic pollutants and metals. The aim of this study was to explore the relationship between accumulated pollutants and biochemical parameters in nestling blood. We used the proportion of urban surface within an 8 km radius of each nest as a proxy to study the relationship between anthropic influence and pollutant accumulation. Observed blood levels of metals, organochlorine pesticides, polychlorinated biphenyls (PCBs), per- and polyfluoroalkylated substances (PFAS) and polybrominated diphenyl ethers (PBDEs) were relatively low, as expected for nestling individuals due to short-term exposures. CB-180 and PBDEs were associated with variations in blood biochemistry parameters; hexa-BDEs appeared positively associated with activities of the enzymes aspartate aminotransferase and lactate dehydrogenase, whereas CB-180 accumulation was associated with an increased activity of creatine phosphokinase and elevated glutathione levels. Increased CB-180 levels were also related to decreased blood concentrations of calcium, cholesterol, α-tocopherol and lutein. A proportion of urban surfaces as low as 6.56% within a radius of 8 km around the nest appears related to the accumulation of CB-180, the majority of analysed PFAS and of PBDE congeners 99 and 209, and increased urbanization was also associated with decreased plasma levels of α-tocopherol and carotenoids. These associations suggest that changes in blood profiles of vitamins, carotenoids or other analytes, despite related to increased plasma levels of CB-180, would be consequence of exploitation of artificial food sources rather than of a direct effect of the pollutants.

Domestic waste disposal sites secure food availability but diminish plasma antioxidants in Egyptian vulture

Some domestic waste disposal sites (DWDS) accumulate enormous amounts of food that humans discard. This food resource is exploited by different species whose population dynamics have as a result been altered. The effects of feeding on human waste on individual health parameters such as body condition are still unknown. Traditionally, body condition in wildlife has been assessed using morphological indices based on size and weight. Expanding the range of indices to include the analysis of indicators at the physiological level can provide a more comprehensive picture of the animals' health status. In this work, we studied how different amounts of waste consumption (assessed using stable isotope analysis) affect nestling health and body condition in an expanding population of the endangered Egyptian Vulture (Neophron percnopterus). We used morphometric and physiological approaches, including plasma biochemistry, antioxidant defences and oxidative stress biomarkers. The results of the multivariate redundancy analysis and variance partitioning showed that the proportion of diet originating from DWDS is the main factor explaining physiological indicators in the studied vulture population. Nestlings that were fed on food from DWDS had lower levels of vitamins (e.g. α-tocopherol) and carotenoids (zeaxanthin and lutein), which have important functions as antioxidant defences, immunostimulant properties and ornamental purposes. Nevertheless, nestlings that were fed on food from DWDS were better nourished and experienced fewer food shortages than individuals not fed using this resource, which had lower levels of cholesterol. Possibly due to the easy access to food, breeding pairs using DWDS could have fewer difficulties in feeding their nestlings and thus could engender more regular brood sizes than those not exploiting this type of resource. Our integrative approach using diverse components was crucial for a reliable assessment of health and body condition in Egyptian Vulture nestlings and its relationship to the consumption of food derived from human-altered environments.