Introduction
The Hooded Vulture Necrosyrtes monachus is an African endemic vulture species widely distributed south of the Sahara (del Hoyo et al. Reference del Hoyo, Elliot and Sargatal1994). It is the only old-world vulture species that lives close to human settlements, exhibiting a mutualistic relationship with humans due to its scavenging role (Henriques et al. Reference Henriques, Granadeiro, Monteiro, Nuno, Lecoq and Cardoso2018; Kibuule Reference Kibuule2016; Mundy Reference Mundy, Harrison, Allan, Underhill, Herremans, Tree and Parker1997). This association makes the species more susceptible to anthropogenic threats compared with other vulture species (Ogada and Buij Reference Ogada and Buij2011; Thompson et al. Reference Thompson, Davies, Daboné, Tate and Therrien2023). A study by Buij et al. (Reference Buij, Nikolaus, Whytock, Ingram and Ogada2015) found the species ranked highest in trade for traditional medicine in West Africa. The Hooded Vulture, like most African vulture species, has experienced large population declines since the 1990s and is currently listed as “Critically Endangered” by the International Union for the Conservation of Nature (IUCN) Red List (Birdlife International 2022; Ogada and Buij Reference Ogada and Buij2011).
Anthropogenic causes that contribute to vulture declines have been extensively studied (Ogada et al. Reference Ogada, Keesing and Virani2012). They include poisoning (targeted and non-targeted), belief-based use of vultures and their parts, collision with structures, and electrocution (Angelov et al. Reference Angelov, Hashim and Oppel2013; Buij et al. Reference Buij, Nikolaus, Whytock, Ingram and Ogada2015), as well as habitat and nest destruction (Manja et al. Reference Manja, Tende, Ottosson and Deikumah2021). Also, nest removal and egg harvesting are common practices in many West African countries (Daboné et al. Reference Daboné, Buij, Oueda, Adjakpa, Guenda and Weesie2019), which cause reproductive failure and, in some cases, individual mortality. However, natural causes of mortality in wild birds, including predation, disease, pollution, and natural extreme events resulting from climate change impacts have not received the much needed attention. These causes of mortality are, however, important to explore for a species with a low reproduction rate, as they may affect nest success. For example, studies have shown that predation is a common cause of juvenile mortality among most species (Hill et al. Reference Hill, DeVault and Belant2019).
In recent times, camera traps have afforded researchers the ability to monitor nests and investigate predation events at different stages of juvenile development. In vultures, causes of nest failure have included predation, diseases, and nests falling to the ground due to poor weather, among other things (Adhikari et al. Reference Adhikari, Bhandari, Baral, Lamichhane and Subedi2022; Thompson et al. Reference Thompson, Hoffman and Brown2013, Reference Thompson, Davies, Daboné, Tate and Therrien2023). Hooded Vulture pairs typically lay one egg per breeding session (Kemp et al. Reference Kemp, Christie, Marks, Sharpe, del Hoyo, Elliott, Sargatal, Christie and De Juana2020), usually once a year. As a result, reproductive success and population turnover rates are naturally slow, suggesting that nest failures or any juvenile loss could have a cumulative impact on population recovery. It is therefore imperative to investigate the frequency and magnitude of causes of nest failure and mortality at the individual level to inform research and further conservation interventions. This will provide a comprehensive understanding of the overall threats facing this threatened group.
Little is known about the nesting ecology of the Hooded Vulture across its range in Africa. Research on the nesting ecology of vultures is limited in West Africa compared with Southern and Eastern Africa, where much more work has been carried out, although predominantly in protected areas (Bamford et al. Reference Bamford, Monadjem, Anderson, Anthony, Borello and Bridgeford2009a; Fern et al. Reference Fern, Thompson and Downs2022; Monadjem and Garcelon Reference Monadjem and Garcelon2005; Monadjem et al. Reference Monadjem, Wolter, Neser and Bildstein2016; Virani et al. Reference Virani, Kirui, Monadjem, Thomsett and Githiru2010). In Ghana, only one published study exists on the nesting ecology of Hooded Vultures (Goded et al. Reference Goded, Annorbah, Boissier, Rosamond, Yiadom and Kolani2023). This study was, however, conducted in a protected area. Given the significant population of Hooded Vultures in urban areas of Ghana, studies on the nesting ecology are essential in urban habitats where vultures are under constant threat from humans.
This study, therefore, investigates the nesting ecology, nest-site characteristics, and causes of nesting failure, including chick mortality, in Hooded Vultures in Ghana. Specifically, (1) we identify and monitor active Hooded Vulture nests within the Cape Coast Metropolitan area to determine causes of nest failure, (2) we determine variations in environmental variables between nesting and non-nesting sites to understand nest-site selection criteria, and (3) we determine the breeding success rate of Hooded Vultures in the Cape Coast Metropolitan area.
Methods
Study area
The study was conducted in the Cape Coast Metropolitan district, located in the Central Region of Ghana, at coordinates (5°6′23.4″N 1°14′29.04″W). The area is located in the Upper Guinea forest region, a significant tropical biodiversity hotspot. The area experiences annual rainfall ranging from 800 mm to 1,500 mm. Temperatures vary from 24°C to 34°C, and relative humidity ranges from 50% to 85%. The area experiences two seasons: a wet season from March to October, and a dry season which lasts from November to February. The elevation of the area lies between -2 m and 117 m (Figure 1). The vegetation consists of coastal savannah, pockets of native forests, and tree species such as African mahogany Khaya ivorensis, silk cotton tree Ceiba pentandra, and African oil palm Elaeis guineensis.
Figure 1. Map of the study area (Cape Coast Metropolitan district). Generated on ArcMap 10.8.2. For the safety of the species, specific locations of the studied nesting sites have been omitted from the map.
Nest search surveys
A team of three (two researchers and a field assistant) searched for breeding pairs of Hooded Vultures using breeding behaviours such as courtship activities, carrying of nesting materials, and territorial behaviour. Although territoriality is usually less intense in Hooded Vultures compared with some other raptors, nesting territories are especially clearly maintained and defended. We therefore used territorial behaviours such as individuals consistently chasing others around a particular tree as a clue to search for a nest on that tree or within proximity of that tree. The search for nests was carried out during the breeding seasons when these behaviours were obvious. The team walked and travelled along road networks, searching areas around roost and feeding sites to look for breeding pairs using the aforementioned clues. The search was carried out between the hours of 07h30 and 11h00 hours local time (GMT+0) daily. The team monitored individuals that elicited any of the behaviours above from a distance of about 30–70 m until nest locations were revealed and confirmed. Once a nest was located, the nest was given a unique identification code (Nest I through to Nest VIII) and the coordinates were marked with a handheld GPS device (Garmin® Etrex HC) to aid further visitations. Data collection on nest locations was carried out from June 2023 to March 2024, which encompasses most of the breeding season (del Hoyo et al. Reference del Hoyo, Elliot and Sargatal1994; Mundy Reference Mundy, Harrison, Allan, Underhill, Herremans, Tree and Parker1997; Mundy and Cook Reference Mundy and Cook1975).
Nest monitoring
Once a nest was found, we inspected the contents of the nest with a DJI Mavic Mini drone. Drones were only used when parents were off the nest to reduce disturbance to the bird (Junda et al. Reference Junda, Greene and Bird2015, Reference Junda, Greene, Zazelenchuk and Bird2016). Although the use of drones for monitoring nesting birds has shown that some solitary birds elicit aggressive behaviour in response to drones during breeding (Lyons et al. Reference Lyons, Brandis, Callaghan, McCann, Mills and Ryall2018), there is also evidence that breeding raptors surveyed with drones were without such issues (Gallego and Sarasola Reference Gallego and Sarasola2021), suggesting that birds are unlikely to be negatively affected by drones during wildlife monitoring research. When the nest was inaccessible via drone, we used a spotting scope to monitor the nest’s activity.
To avoid the destruction of a Hooded Vulture nest during the study, we used the double-rope technique, as described in Thompson et al. (Reference Thompson, Davies, Gudehus, Botha, Bildstein and Murn2017), to access the nest of vultures in the canopy of the nesting trees. At each nest, one (in some cases two) high-resolution Solaris Weapon 4K trail camera(s) were mounted on a branch approximately 1.0–1.8 m from the nest to monitor nesting activities. These trail cameras are designed to operate using a combination of solar power and built-in lithium batteries, which allows for extended deployment without the need to frequently change batteries. Each camera operates with an infrared motion sensor and was programmed to trigger when movement is detected to capture a sequence of two successive photographs and record a 10-second video clip. The Passive Infrared (PIR) interval between activations was set at 10 seconds to ensure comprehensive documentation of activities without excessive data storage issues. Cameras were set to operate daily between 04h30 and 22h00 hours local time, using infrared light to capture images under low-light conditions. As a precautionary measure to minimise disturbance, cameras were only installed at nests that were still under construction at the time of discovery. Nests that already contained eggs or chicks when first located were not monitored with trail cameras. Instead, every week (seven-day interval), the team of three monitored the nest until the chick fledged. In these instances, nests were observed using drones and spotting scopes, but only when the parent birds were absent to avoid causing stress or abandonment. We also maintained silence as much as possible to reduce disturbance resulting from our presence. Cameras were removed once we could confirm that the young had successfully fledged or if the nest became inactive. All recorded footage was subsequently reviewed to document and analyse nesting behaviour. Nest content and composition were assessed using photographs obtained from drone imagery and camera traps.
Environmental data collection
At each confirmed Hooded Vulture nest, a laser rangefinder (Bushnell Bone Collector 850 yards, 6 × 25 mm) was used to measure both the height of the tree on which the nest was found (tree height) and the height at which the nest was placed (nest height). The circumference at breast height (CBH) of the nesting tree was measured using a surveyor’s tape, and the tree species was recorded. Tree crown coverage was estimated as percentage canopy cover to the nearest 5%, following Manu (Reference Manu2003). Within a 50 × 50 m plot with the nesting tree at the centre, all other woody trees were counted and recorded as “other trees”. To estimate mean tree height, three additional trees with similar CBH and canopy cover as the nesting tree were randomly selected within the plot and their height measured. From these measurements, the mean tree height was calculated.
Other environmental variables around each nest-site were further assessed using QGIS and current Google Maps imagery. These included distance from the nest to the nearest human building or settlement, food source, water source, and forest patch (Bamford et al. Reference Bamford, Monadjem, Anderson, Anthony, Borello and Bridgeford2009a). To compare nest-site parameters with unused but potential nest-sites, 10 random locations with similar characteristics, such as tree density and canopy cover, were selected in QGIS. In the field, the nearest tree to each of these points was identified, and the same environmental variables were measured. These sites served as “absence” locations (i.e. sites without nests), allowing for a comparison of environmental conditions between selected and non-selected nesting areas and to identify variables that may predict nest-site selection in Hooded Vultures.
Data analysis
We defined nest success as the proportion of nesting attempts (laying an egg) in which a pair successfully fledge a chick, while nest failure is the proportion of nesting attempts in which a pair lays an egg but is unable to fledge a chick, expressed as a percentage. The percentage of nest success and failures was then calculated. We also defined fledgling rates as the number of nestlings to reach fledgling age relative to the brood size for each nest (White et al. Reference White, Perlut, Travis and Strong2021). A multicollinearity test was conducted to eliminate highly correlated environmental variables (above 0.8). In such cases, the variable with the least biological significance was removed from the analysis, ensuring that only relevant and independent variables were included. A t-test was used to analyse the variations in environmental variables between nesting sites and areas with similar characteristics but without active nests. All analysis was carried out using RStudio 2023.06.0, working on base R version 4.2.3 (R Core Team 2022, RStudio Team 2023).
Results
Overview
The search team covered ≥112 km in an area of 122 km2 during the study period. A total of eight Hooded Vulture nests, predominantly located in the southern part of the metropolis, where tall trees were scarce compared with the northern area. Four of the nests were at the construction stage, two had eggs, and two had chicks at the time they were found. The tallest tree on which a nest was found was 28.82 m, with a nest placed at a height of 21.82 m above ground level, while the shortest nesting tree was 17.82 m with a nest placed at 12.82 m. Nests were typically found on trees of 24.94 ± 3.68 m (mean ± SD) height and placed approximately 18.7 ± 4.73 m above ground level at a fork of at least three branches (Table 1).
Table 1. Hooded Vulture Necrosyrtes monachus nests identified within the study sites and summary of nest characteristics. CBH = circumference at breast height
Nest-site characteristics
The eight nests were found on six different tree species, five of which are currently native and listed as “Least Concern” on the IUCN Red List. Two species, rosy trumpet Tabebuia rosea, Least Concern, and Manila palm tree Adonidia merrillii, Vulnerable, are introduced tree species. Two nests were found on Indian almond trees Terminalia catappa, and another two nests were on two rosy trumpet trees, one of which had been parasitised by Ficus spp. The other four nests were placed on silk cotton, ackee tree Blighia sapida, Manila palm tree, and giant cola tree Cola gigantea. Mean tree CBH was 2.5 m. Also, nests were placed between the forks of branches but were exposed to above-ground sunlight. Nest materials were mostly dead and dry branches and leaves, but when a pair renested, nests were made with both dry and fresh branches.
Additionally, Nests I and VII were clustered together, just 42 m apart, while Nests II, III, and IV formed another cluster located 536 m away from Nests I and VII. Nests VI and VIII were separated by 980 m. Nest V, the only nest located outside the University of Cape Coast, was 4 km away from the nearest nest on the campus.
Variations in environmental variables at nest areas and non-nesting areas
Among the seven variables tested, distance from human settlement and circumference of the tree were found not to vary significantly between areas with nest present and areas with nest absent. However, five variables (nest tree height, canopy cover, proximity to water, mean tree height, and number of other trees present) were found to vary significantly between the two areas. Nest tree height, canopy cover, mean tree height, and number of other trees present were found to be higher in areas with nests compared with areas without nests, while distance from nesting tree to water source was found to be less than in areas without nests (Table 2).
Table 2. Summary t-test results for environmental variables between areas with nest present and areas with a nest absent
Nesting success
In all eight identified nests, vulture pairs successfully laid an egg, with seven out of the eight hatching a chick, and six nests successfully fledged a juvenile, resulting in a 75% success rate for the identified nests (Table 3). However, considering the nesting attempts by pairs, a total of 10 nesting attempts with 10 eggs laid, out of which two did not hatch, two hatchlings out of the eight hatched eggs could not fledge, only six chicks fledged. This resulted in an 80% hatch rate and a 60% fledgling rate.
Table 3. Activities, and nest and chick status of the eight identified nest-sites during the study
An additional nest, which was abandoned after a pair started building, was not included in this analysis. The first eggs of Nests II and IV failed due to seemingly unintentional cracking of the egg by the adults as they moved nesting material around, but the pair at Nest IV successfully re-laid, with the chick successfully fledging. Nest III failed due to the chick being eaten, either alive or nearly dead, by the parents.
Camera trap account of failed nests
Egg crushing at Nest II
Between 12 November 2023 and 3 February 2024, the camera trap monitoring Nest II captured 1,812 photographs and 906 videos. The photographs revealed that the Hooded Vultures returned to their nest 13 minutes after the installation of the camera and continued building it. The adult Hooded Vulture pair was observed to visit the nest together twice daily. The Hooded Vulture egg was first photographed on 27 November at 17h36 (Figure 2). Photographs from the camera showed that the vulture spent the night of 26 November in the nest. The female adult vulture stayed in the nest throughout the day and night of 27 November after laying the egg without leaving the nest.
Figure 2. A Hooded Vulture and its egg photographed on a nest (Nest II) in Cape Coast, Ghana, on 27 November at 17h36 GMT from Solaris weapon 4k Trail Camera.
On 29 November at 08h10, the egg was seen broken (Figure 3). A video recording showed that while the adult bird continued to arrange the contents of the nest, it accidentally stepped on the egg, causing it to break open. The bird then proceeded to drink the contents of the egg. Before this incident, no other animals, including bird species, were captured on camera, however, Yellow-billed Kites Milvus aegyptius and Pied Crows Corvus albus were occasionally heard calling close to the camera. After the incident, on 5 January 2024, a Senegal Coucal Centropus senegalensis was seen visiting the nest. Pied Crow was also seen on 22 January 2024, collecting nesting materials from the nest. Photographs from 3 February 2024, when the camera was retrieved, showed that the pair continued to visit the nest, but failed to relay an egg.
Figure 3. A Hooded Vulture and its broken egg in Nest II photographed in Cape Coast, Ghana on 29 November 2023 from Solaris weapon 4K Trail Camera.
Egg crushing at Nest IV
Between 8 November 2023 and 4 April 2024, the camera trap monitoring at Nest IV captured 9,126 photographs and 4,564 videos. The Hooded Vulture egg was first photographed on 12 November at 16h06. The egg succumbed to the same fate as that in Nest II. Due to movements in the nest, the egg laid on 12 November 2023 (Figure 4a), was crushed on 12 December 2023 (Figure 4b). The pair successfully laid a new egg on 10 January 2024, almost a month after the first was destroyed (Figure 4c), which was successfully incubated and fledged a chick (Figure 4d).
Figure 4. Figures showing the sequence of events at Nest IV revealed by camera traps. (a) The pair of Hooded Vulture laid an egg, (b) unfortunately, it was broken while a parent was tending to the nest, (c) but the pair re-laid another egg, which (d) progressed to juvenile fledging.
Cannibalism on Nest III
We located Nest III on 9 June 2023, when the pair had just started building the nest. A camera trap was mounted to monitor the nest on 12 November. Between 12 November 2023 and 3 February 2024, the camera trap monitoring this nest captured 7,748 photographs and 3,872 videos. The female laid the egg on 10 December 2023 (Figure 5a). A similar incident occurred where an adult, while moving, stepped on the egg briefly, but fortunately, in this instance, the egg was not crushed. The egg hatched into a chick on 24 January 2024 (Figure 5b). The chick seemed healthy initially but was sluggish in later days until it was devoured either alive or dead by the adults on 1 February 2024 (Figure 5c). Photographs as of 3 February 2024 when the camera was retrieved show that the pair continued to visit the nest (Figure 5d).
Figure 5. Figures show events at Hooded Vulture Nest III. (a) The pair successfully laid, and (b) the egg hatched. (c) The chick was eaten either alive or dead by the parents (d). Photographs from 3 February 2024, when the camera was retrieved, showed that the pair continued to visit the nest.
Discussion
Nest-site selection
Our findings revealed that Hooded Vultures nested on trees with a mean height of 24.94 ± 3.68 m (mean ± SD), and nests were placed at a mean of 18.7 ±4.73 m above ground level. Results from this study showed that Hooded Vulture nests were placed at heights approximately 6 m higher than those recorded in Kogi State, Nigeria (12.8 m; Adang et al. Reference Adang, Tanko, Saliu and Abdulwahab2019), about 9 m higher than those in Kaduna State, Nigeria (9.5 m; Ayuba et al. Reference Ayuba, Tanko, Peter, Adang and Bakam2020), and about 6 m higher than those reported in Mole National Park, northern Ghana (12.3 m; Goded et al. Reference Goded, Annorbah, Boissier, Rosamond, Yiadom and Kolani2023). This high difference in trees and nest height could be attributed to variation in habitat types and tree species. This study was conducted in southern Ghana, characterised by fast-growing tree species like African mahogany, silk cotton tree, and African oil palm, which typically reach heights of 30–60 m, whereas the earlier studies were conducted in savanna ecosystems characterised by slower-growing and generally shorter trees. Although the results of this study confirm the preference of Hooded Vultures for tall trees, this is consistent with findings from other studies in other West African countries, such as Burkina Faso, where a mean nest height of 18.1 m was recorded (Daboné et al. Reference Daboné, Buij, Oueda, Adjakpa and Weesie2023), as well as from studies conducted in South Africa (Fern et al. Reference Fern, Thompson and Downs2022; Thompson et al. Reference Thompson, Davies, Gudehus, Botha, Bildstein and Murn2017). Collectively, these studies suggest a trend of Hooded Vultures placing their nests higher than in previous years, likely as a strategy to reduce human interference. This behaviour may be explained by the ease with which humans can climb shorter trees. As noted by Majgaonkar et al. (Reference Majgaonkar, Bowden and Quader2018) and Daboné et al. (Reference Daboné, Buij, Oueda, Adjakpa and Weesie2023), taller trees offer increased protection against both terrestrial predators and human disturbance. Also, tall trees provide a vantage point for adult vultures to monitor their surroundings and detect potential threats, which is crucial for the survival of both adults and their offspring. The availability of tall trees for roosting and nesting in the Cape Coast Metropolitan district is limited, posing a significant challenge for the Critically Endangered Hooded Vultures. The southern part of the metropolitan area, including the University of Cape Coast campus, has experienced considerable targeted tree removal, similar to reports on the University of Ghana campus (Boakye et al. Reference Boakye, Agyemang, Wiafe, Dossou-Yovo and Ziekah2021). Recently, a major roosting tree on the campus was cut down to ensure the safety of commuters, leading to the dispersal of the local vulture population that had been established there. Although a few tall trees remain on the campus that vultures could potentially use, it is uncertain how long they will persist before being removed as well.
Hooded Vultures in the study also showed strong selection for nest-sites with greater canopy cover, taller surrounding trees, and higher numbers of other woody trees, while also nesting closer to water sources. These features likely enhance concealment, microclimatic stability, and structural support, patterns consistent with other African vultures and tree-nesting raptors (Bamford et al. Reference Bamford, Monadjem and Hardy2009b; Daboné et al. Reference Daboné, Buij, Oueda, Adjakpa and Weesie2023). Proximity to water may indirectly increase foraging opportunities by attracting livestock, wildlife, and human activity, a pattern also noted in White-backed Vultures Gyps africanus (Bamford et al. Reference Bamford, Monadjem and Hardy2009b; Virani et al. Reference Virani, Kirui, Monadjem, Thomsett and Githiru2010). Distance from human settlements and tree circumference did not differ between nest and non-nest-sites, suggesting tolerance of moderate human presence. These findings collectively highlight the importance of conserving tall, broad-canopied trees, particularly near water sources, as a key component of Hooded Vulture nesting habitat.
Nest success
Our study revealed that all eight identified nests produced an egg, with seven nests successfully hatching a chick, and six nests successfully fledging a chick. This translates to a fledgling success rate of 75% (six out of eight nests), indicating a relatively high level of reproductive success compared with some other vulture populations, although the caveat is that this is a very small sample size. Our study’s relatively high fledgling success rate can be attributed to several factors. The selection of nest-sites in tall trees with mean heights of 24.95 m and the placement of nests at a mean height of 18.82 m above ground level likely reduces terrestrial predation risk and human disturbance. The proximity of nests to abundant food resources in the form of discarded fisheries, landfill sites, and slaughter slabs within the highly populated southern part of the metropolis may also play a role in ensuring an adequate food supply for the chicks.
In South Africa, Monadjem et al. (Reference Monadjem, Wolter, Neser and Bildstein2016) reported nest success rates ranging from 0.44 to 0.89 offspring per pair per year in 2013 and 0.50–0.67 offspring per pair per year in 2014. In Sokoto, Nigeria, Peter Mundy recorded a mean nest success rate of 41% in the local Hooded Vulture population. This lower success rate compared to our study may be attributed to differences in habitat quality, predation pressure or human disturbance, as reported in studies by Mundy and Cook (Reference Mundy and Cook2024). It underscores the importance of site-specific factors in determining reproductive success and highlights the need for localised conservation efforts. Daboné et al. (Reference Daboné, Buij, Oueda, Adjakpa, Guenda and Weesie2019) conducted a study on vulture populations in Burkina Faso, recording breeding success rates of 0.68 in 2013 and 0.70 in 2014. Additionally, Dabone noted that fledgling success (0.88 fledglings per chick) was higher than hatching success (0.80 chicks per egg). In our study, hatching success was 0.8 (8 out of 10 nests), and fledging success was 0.6 (6 out of 10 nesting attempts).
Possible nest destruction
Nest I was located on 8 June 2023 from the territorial breeding activity of a pair of Hooded Vultures with pictures taken the next day (Figure 6a). The nest was placed on an Indian almond tree that was 25.82 m above ground. The nest was placed in a fork formed by four branches of the tree. The nest was placed 20.82 m above ground. The nest was examined with the DJI Mavic mini drone the next day and revealed an empty nest. A follow-up exploration on 6 September 2023, also revealed an empty nest (Figure 6b). On 4 November 2023, the whole nest had disappeared from the tree (Figure 6c) when we made further investigation with the drone the next day. Prior to that date, a severe weather incident occurred, hence we initially theorised that the nest had been blown away. However, further investigation revealed that some individuals climbed the tree to the nest. Although the witness could not state categorically if the nest was taken, he testified to seeing an individual climb to the nest some days before, suggesting a possible nest destruction or harvest by humans.
Figure 6. Sequence of events at Nest I, showing the nest present on 9 June 2023 (a) and 6 September 2023 (b), the whole nest is found destroyed on 4 November 2023 (c), but the pair renests and lays an egg, picture taken on 13 December 2023 (d).
Reports from several West African countries highlight the coveted status of vulture eggs and nests among traders for traditional and ritual purposes. In pursuit of these eggs and the entire nests, hunters often scale trees with nests to retrieve them. In Nigeria, Williams et al. (Reference Williams, Ottosson, Tende and Deikumah2021) revealed that hunters create graduated steps to facilitate easy climbing for egg collection. The study further noted instances where the entire nest was harvested for belief-based or traditional medicinal purposes. Cameroon, also identified by Saidu and Buij (Reference Saidu and Buij2013), has been reported to supply vulture eggs to neighbouring countries. However, the most commonly sold and used vulture parts in Ghana remain the body parts of an adult bird (Boakye et al. Reference Boakye, Wiafe and Ziekah2019, Reference Boakye, Agyemang, Wiafe, Dossou-Yovo and Ziekah2021; Gbogbo and Daniels Reference Gbogbo and Daniels2019).
This observation, the first of its kind reported in Ghana, raises concerns about the growing attention vulture eggs and nests may be receiving from ritualists and healers. The implications for the already threatened local population of vultures in urban areas are substantial. Harvesting the only eggs of Hooded Vultures has the potential to impact their breeding success significantly. Observations have shown that Hooded Vultures rarely attempt to renest if the initial attempt has failed. Although vultures in our study did not show preference for particular tree species over others due to the low sample size, the re-attempt of the same individuals or other individuals to nest at the same spot on the same tree could be due to preference for these tree species and signifies the lack of suitable nesting sites and the significant role that these tree species play in the nesting of the species (Figure 6d). We, therefore, propose further research to investigate this occurrence. We believe this will give us a better picture of activities occurring in and around the nests. This will also help us know where to target our conservation education and awareness creation.
Causes of nest failures
The first cause of nest failure in our study was the unintentional crushing of eggs by adult vultures. The egg crushing occurred as adults moved around the nest and repositioned nesting materials. Eggs are particularly vulnerable during the incubation period, and the weight and movements of adult vultures can easily result in accidental damage. The incident at Nest II occurred two days after the egg was laid; however, the incident at Nest IV occurred a month later. Therefore, we cannot attribute the crushing at Nest II solely to the freshly laid state of the egg, but we can attribute it to possible thinning of the eggshells due to environmental pollutants. Eggshell thinning occurs when the calcium content of the eggshell is reduced, leading to thinner, more fragile shells (Orłowski et al. Reference Orłowski, Hałupka, Klimczuk and Sztwiertnia2016). This phenomenon can be caused by several factors, including exposure to environmental pollutants such as pesticides, heavy metals, and other contaminants. Pesticides such as DDT and its derivatives are well-documented causes of eggshell thinning in various bird species, as they interfere with calcium metabolism in female birds during egg formation (Albert et al. Reference Albert, Barcenas, Ramos, Iñigo, Meyburg and Chancellor1989; de Solla et al. Reference de Solla, King and Gilroy2023; Hernández et al. Reference Hernández, Colomer, Pizarro and Margalida2018). Eggshell thinning affects nest success because it results in easily cracked eggs, ending in the loss of eggs. In our study, the unintentional crushing of eggs by adult Hooded Vultures may be partly attributed to such shell thinning, as more fragile shells are less capable of withstanding the weight and movements of the incubating birds.
Given the increase in pesticide use in the Cape Coast Metropolitan area, we predict that this might be affecting the eggshell integrity of the vulture populations (Deikumah Reference Deikumah2020). Understanding the current extent to which pesticides are contributing to eggshell thinning could help inform conservation strategies aimed at reducing the impact of environmental contaminants on vulture reproductive success.
The second notable cause of nest failure observed was the apparent eating of a chick by an adult vulture. It was unclear whether the chick was alive or dead at the time it was fed upon, which raises concerns about both infanticide and scavenging behaviours. Infanticide, where an adult kills and eats a chick, can occur under certain stressful conditions, such as food scarcity or high competition, as pointed out by Stanback and Koenig (Reference Stanback, Koenig, Elgar and Crespi1992). This recorded behaviour, while a worry for conservationists, has been recorded in other parts of the world in many raptor species (Allen et al. Reference Allen, Inagaki and Ward2020), including both Old and New World Vultures. For instance, Margalida et al. (Reference Margalida, Bertran, Boudet and Heredia2004) reported adult Bearded Vulture Gypaetus barbatus feeding chicks with remains of their sibling. Evans et al. (Reference Evans, Humphrey, Tillman and Kluever2022) also reported observing Black Vultures Coragyps atratus feeding on a juvenile Black Vulture carcass. Together, these studies point to the same fact that cannibalism is a result of increased stress and competition due to food and point to the relevance of behavioural studies for vulture conservation (van Overveld et al. Reference van Overveld, Blanco, Moleón, Margalida, Sánchez-Zapata and de la Riva2020)
However, we can rule that out in this particular incident as the presence of food was high, and adults were provisioning the chick before the incident occurred. Alternatively, scavenging on a dead chick may occur if the chick died of other causes, such as disease or malnutrition. We prefer this explanation as the camera records showed the chick moving sluggishly.
Conclusions and Recommendations
This study on the nesting ecology of Hooded Vultures in the Cape Coast Metropolitan district provides valuable insights into their nest-site selection, breeding success, and the natural causes of nest failures. Our findings indicate a 60% reproduction success rate, which is higher than that reported in other studies, although several interventions could improve this success rate. The unintentional crushing of eggs by adults and the observed instance of chick consumption highlight the complexity and challenges of natural nesting environments and the need for the integration of behavioural studies for vulture conservation. Furthermore, the potential impact of pesticide use on eggshell thinning is a pressing concern that warrants future investigation, despite the current lack of specific data. This study contributes to a broader understanding of vulture conservation needs and underscores the importance of addressing anthropogenic threats such as persecution and removal of nests, nesting trees, and roosting trees to support the survival of this Critically Endangered species.
Acknowledgements
The authors thank the editors and reviewers for their helpful comments on the manuscript. JKD extends his gratitude to Kimberly Armah-Agyeman for her support, Mansa for hosting him during the period of the study, and to the individuals who permitted him to conduct the research. This work was funded by A.P. Leventis Ornithological Institute (APLORI), University of Jos, Nigeria and The Rufford Foundation (grant number 40846-1) and Idea Wild (grant numbers AFRIGHAN0921 DANIGHAN0623). Authors’ contributions: all authors conceived the idea for the research; JKA and JKD collected data and wrote the manuscript; JPD, JKA, and LAB edited and wrote the manuscript; KAM helped with project design and review of manuscript drafts. Disclaimer: publications remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. The data used to generate the results of this study are deposited at Zenodo and a GitHub repository.
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