Hannah Madden

Abstract

Prey resources in oligotrophic tropical marine environments are often scattered and unpredictable. Tracking studies of tropical seabirds can provide critical information about ocean habitat affinities, prey choice and the utilisation of surrounding areas, which can be useful for conservation managers. Foraging studies of Red-billed Tropicbird populations in the Caribbean are scarce but increasing. We sought to expand on this by tracking chick-rearing adults using GPS devices and subsequently linking these tracking data to remotely-sensed environmental variables. We related our spatial data to opportunistic sampling of regurgitates in a globally significant nesting colony on Saba, Caribbean Netherlands. Diet samples were dominated by flying fish (Exocoetidae; numerical frequency: 70.73%), but prey items from the squid family (Loliginidae; 9.76%) and the families of flying gurnards (Dactylopteridae; 2.44%) and the ray-finned fish (Carangidae; 2.44%) were also identified, although we were unable to identify 14.63% of samples due to digestion. An additional goal of our study was to compare the foraging ecology of Tropicbirds on Saba with those nesting on St. Eustatius, located circa 25 km south-east. As expected, Tropicbirds nesting on Saba exhibited diurnal foraging patterns, travelling a maximum distance from the colony of 553.7 km, with an average trip length of 117.2 ± 144.6 km (±SD). Adults foraged in shallower, cooler waters with higher chlorophyll a concentrations and higher Exocoetidae species richness compared to travelling points. Despite the proximity of Saba and St. Eustatius, this is contrary to what was found for Tropicbirds nesting on St. Eustatius, where adults foraged in deeper waters with a low Exocoetidae species richness. However, Tropicbirds from Saba and St. Eustatius did exhibit some similarities in their foraging behaviour; specifically, foraging adults traversed multiple exclusive economic zones and marine protected areas, reinforcing our recommendation for nature managers in the Caribbean to create a transboundary network in order to effectively protect and conserve this species.

Full article available here: https://bioone.org/journals/ardea/volume-111/issue-2/arde.2022.a14/Forag...

AbstractUrbanization has introduced novel materials for nesting birds, including anthropogenic debris that may be dangerous to adults and nestlings (e.g., entanglement or ingestion leading to injury or mortality). We present two observations of incorporation of man-made materials into passerine nests on St. Eustatius, Caribbean Netherlands. This is the first publication of such observations for St. Eustatius, and addresses a gap in literature that acknowledges the use of anthropogenic litter by landbirds in the wider Caribbean.

Abstract

Prey resources in oligotrophic tropical marine environments are often scattered and unpredictable. Tracking studies of tropical seabirds can provide critical information about ocean habitat affinities, prey choice and the utilisation of surrounding areas, which can be useful for conservation managers. Foraging studies of Red-billed Tropicbird populations in the Caribbean are scarce but increasing. We sought to expand on this by tracking chick-rearing adults using GPS devices and subsequently linking these tracking data to remotely-sensed environmental variables. We related our spatial data to opportunistic sampling of regurgitates in a globally significant nesting colony on Saba, Caribbean Netherlands. Diet samples were dominated by flying fish (Exocoetidae; numerical frequency: 70.73%), but prey items from the squid family (Loliginidae; 9.76%) and the families of flying gurnards (Dactylopteridae; 2.44%) and the rayfinned fish (Carangidae; 2.44%) were also identified, although we were unable to identify 14.63% of samples due to digestion. An additional goal of our study was to compare the foraging ecology of Tropicbirds on Saba with those nesting on St. Eustatius, located circa 25 km south-east. As expected, Tropicbirds nesting on Saba exhibited diurnal foraging patterns, travelling a maximum distance from the colony of 553.7 km, with an average trip length of 117.2 ± 144.6 km (±SD). Adults foraged in shallower, cooler waters with higher chlorophyll a concentrations and higher Exocoetidae species richness compared to travelling points. Despite the proximity of Saba and St. Eustatius, this is contrary to what was found for Tropicbirds nesting on St. Eustatius, where adults foraged in deeper waters with a low Exocoetidae species richness. However, Tropicbirds from Saba and St. Eustatius did exhibit some similarities in their foraging behaviour; specifically, foraging adults traversed multiple exclusive economic zones and marine protected areas, reinforcing our recommendation for nature managers in the Caribbean to create a transboundary network in order to effectively protect and conserve this species.

Abstract

Aim: Mutualistic interactions between plants and animals are fundamental for the maintenance of natural communities and the ecosystem services they provide. However, particularly in human-dominated island ecosystems, introduced species may alter mutualistic interactions. Based on an extensive dataset of plant–frugivore interactions, we mapped and analysed a meta-network across the Caribbean archipelago. Specifically, we searched for subcommunity structure (modularity) and identified the types of species facilitating the integration of introduced species in the Caribbean meta-network.

Location: Caribbean archipelago (Lucayan archipelago, Greater Antilles, Lesser Antilles).

Methods: We reviewed published scientific literature, unpublished theses and other nonpeer-reviewed sources to compile an extensive dataset of plant–frugivore interactions. We visualized spatial patterns and conducted a modularity analysis of the cross-island meta-network We also examined which species were most likely to interact
with introduced species: (1) endemic, nonendemic native or introduced species, and (2) generalized or specialized species.

Results: We reported 3060 records of interactions between 486 plant and 178 frugivore species. The Caribbean meta-network was organized in 13 modules, driven by a combination of functional or taxonomic (modules dominated by certain groups of frugivores) and biogeographical (island-specific modules) mechanisms. Few introduced species or interaction pairs were shared across islands, suggesting little homogenization of the plant–frugivore meta-network at the regional scale. However, we found evidence of “invader complexes,” as introduced frugivores were more likely to interact with introduced plants than expected at random. Moreover, we found generalist
species more likely to interact with introduced species than were specialized species.

Main conclusions: These results demonstrate that generalist species and “invader complexes” may facilitate the incorporation of introduced species into plant–frugivore communities. Despite the influx of introduced species, the meta-network was structured into modules related to biogeographical and functional or taxonomic affinities. These findings reveal how introduced species become an integral part of mutualistic systems on tropical islands.

Investigating the foraging patterns of tropical seabirds can provide important information about their ocean habitat affinities as well as prey choice. Foraging studies of Red-billed Tropicbird Phaethon aethereus populations in the Caribbean are lacking. We sought to rectify this by opportunistically sampling regurgitates at nest sites on the island of St. Eustatius, Lesser Antilles, and by linking the GPS tracks of foraging adults to remotely sensed environmental variables. Diet samples were dominated by Exocoetidae (59.5%) and Belonidae (14.9%), although we were unable to identify 25.5% of samples due to digestion. Tropicbirds nesting on St. Eustatius exhibited diurnal foraging patterns, foraged in deeper waters with higher chlorophyll concentration, and consumed fewer Exocoetidae species compared to travelling behaviour. The maximum distance travelled from the colony was 953.7 km, with an average trip length of 176.8 (± 249.8) km. The biologged birds crossed multiple exclusive economic zones and marine protected areas, and on that basis, we suggest that efforts to protect and conserve this species may require transboundary collaboration throughout the wider Caribbean.

Abstract

The daily nest-survival rates of Red-billed Tropicbirds (Phaethon aethereus) were estimated over six breeding seasons on St. Eustatius in the Caribbean. We analyzed 338 nesting attempts between 2013 and 2020. The daily survival rate (DSR) of tropicbird nests was modeled as a function of nest initiation date, sea surface temperature (SST), elevation, vegetation in front of the nest, and year. Yearly nest survival rates (± SE) of the best fitting models ranged from 0.21 ± 0.06–0.74 ± 0.13 (n = 338 nests). DSR of the most parsimonious models averaged 0.39 ± 0.04 during the incubation period, 0.83 ± 0.05 during the chick-rearing period, and 0.30 ± 0.04 during the nesting period (incubation through fledging) when data were pooled across all years. Models with linear and quadratic trends of nest initiation date combined with SST and elevation received strong support in the incubation and nesting periods. Nests initiated in peak nesting season, when SSTs were lower, had higher DSR estimates than nests initiated early or late in the season. Compared to studies of the same species from Saba and the Gulf of California, survival probability on St. Eustatius was lower during the incubation stage but higher during the chick-rearing period. Similar to populations in the Gulf of California, tropicbird reproduction differed and laying date varied among years, and survival was influenced by SST. Our results are consistent with a study on White-tailed Tropicbirds (Phaethon lepturus) in Bermuda which found that survival was affected by temporal factors rather than physical site characteristics. Our study contributes to a better understanding of the factors that influence Red-billed Tropicbird survival on a small Caribbean island

https://www.researchgate.net/publication/365346690_Reproductive_Success_...

The Caribbean Netherlands island of Saba is home to a unique but relatively unknown iguana population. Given the many threats faced by islands worldwide, including the Lesser Antilles and Saba, fieldwork was conducted in 2021 in order to bring attention to this unique species and the threats it faces. As on St. Eustatius, the most urgent threats appear to be low survival of hatchlings, limited nesting sites, and the presence of non-native iguanas.

Precise taxonomic status in discussion

Saba black iguana. Photo source: Thijs van den Burg

The iguana population on Saba forms part of a recently described species, Iguana melanoderma. Although there is no doubt about the unique appearance of the iguanas, experts are still deliberating over its exact taxonomic status. Despite this, it is clear that the population requires urgent protection given the number of threats it faces. The biggest concern is the presence of and possible further influx of non-native iguanas from nearby St. Maarten.

Thankfully more iguanas than originally thought

A recent study estimated the iguana population at just 200-300 individuals. This number is worrying and would be extremely concerning for the long-term survival of the population. However, because the data in the aforementioned study were preliminary, additional fieldwork was conducted on Saba in 2021. During this time, 38 transects were surveyed multiple times using distance sampling methodology, which allowed the population to be estimated more accurately. An unexpected positive result of this analysis is that the population is at least ten times larger than originally estimated, in the thousands rather than hundreds.

Saba black iguana. Photo source: Thijs van den Burg

More melanism at higher altitudes?

It is widely believed that black reptile species have evolved this color in order to be able to get warmer faster in colder climates, given that they are cold-blooded. This may also apply to the iguanas on Saba, which boasts the highest mountain in the Kingdom of the Netherlands: Mount Scenery (887m). This high volcano ensures that Saba is often cloudy, which could make it difficult for reptiles to get warm. However, we did not find any relationship between the extent of melanism and the elevation at which iguanas were found. In other words, iguanas that live higher up the volcano are not darker than iguanas at lower elevations.

Is there another explanation for why the iguanas on Saba are black? One hypothesis is that the original iguanas that established on Saba were already black. This is in line with the existence of partially black iguanas in Venezuela, which appear to be closely genetically related to the iguanas on Saba.

Few nest sites or juvenile iguanas

In order for successful reproduction and conservation of an iguana population, good quality nest sites are essential. It is well known that goats can destroy iguana nest sites through overgrazing, which leads to habitat destruction. Furthermore, goats can trample iguana nests and destroy the embryos inside eggs.

On Saba there is an enormous goat population (several thousands) which negatively impacts the island’s vegetation and causes erosion. During fieldwork we searched for iguana nest sites to evaluate their availability, distribution and quality. While this was not the main goal of the research, we were disappointed to find only four nest sites.

Juvenile iguana. Photo source: Thijs van den Burg

Another area of concern was the low number of juvenile iguanas found during fieldwork. Despite finding over 600 individuals, just 2.4% of these were juveniles or hatchlings. While the exact reason for this is unknown, one possible cause could be the feral cat population. Previous research on Saba demonstrated that feral cats exist in the lower altitudes of the island where they hunt for prey, which are exactly the same areas where iguanas prefer to nest. In fact, iguana remains were found in 9% of the cat scats examined.

Unfortunately, the situation for the melanistic iguana on Saba appears to be similar to that of the Lesser Antillean iguana on St. Eustatius, where the presence of cats and goats and an absence of suitable nest sites are well-known issues.

Remaining areas of concern

Despite the larger than expected iguana population on Saba, there are still many causes for concern. Clearly the species needs better protection as well as continued knowledge-building. Pressing concerns for the long-term existence of this species are the presence of free-roaming goats and feral cats, as well as the low number of juveniles and absence of suitable nest sites. However, by far the biggest threat is the presence of non-native iguanas which, though competitive hybridization, can mate with and thereby suppress the native population. During fieldwork, some individuals were found that looked different to the native iguana, and preliminary genetic research has confirmed that these animals were indeed invasive. This situation therefore requires the authorities to take immediate action in order to halt the invasion of non-native iguanas.

More info

For more info, e.g. considering student projects please contact dolfi.debrot@wur.nl and thijs.burg@gmail.com

More info in the Dutch Caribbean Biodiversity Database

Published in BioNews 56.

Caribbean flora and fauna have always coped with the destructive forces of hurricanes. However, climate change leading to an increase in their frequency and strength, and because many species have declined in abundance due to anthropogenic causes, a better understanding of how hurricanes effect local populations is essential.

The Quill before and after Hurricane Irma. Photo credit: Hannah Madden

2017 Hurricane Season

The 2017 Caribbean hurricane season was the most intense recorded to date. Both Irma and Maria, category-5 hurricanes, closely passed Sint Eustatius and caused major destruction on the island; reported in this Nature Today article. Although immediately after the storms it was clear that trees were heavily affected and mostly defoliated, understanding which species were affected and to what extent requires time for data collection and comparison. Since 2017, several studies have provided pieces of information in order to understand how local populations coped, or not, with the 2017 hurricane season.

Since 2017, researchers found that >90% of all trees were defoliated by more than ¾, and that especially trees at higher elevations (such as on the Quill volcano) were affected more severely. Another study that focused on the Bridled Quail-dove (Geotrygon mystacea), demonstrated that the population declined by 77% in 2019 compared to pre-hurricane levels. A follow-up study in 2021 (not yet published) recorded a further decline to just 125 individuals, and the Bridled Quail-dove will likely be re-assessed by the IUCN.

Reptiles

Focusing on reptile species, a novel study further aids our understanding of the ecosystem-wide impact that the 2017-hurricane season had on Statian biodiversity:

The new study, focusing on the Critically Endangered Lesser Antillean iguana (Iguana delicatissima), shows that its population decreased by at least 20% during 2017. Comparing sighting and survey data from 2017–2018, the authors found a decrease in both the occupancy and population size of the iguana species. Importantly, no recovery was observed in 2019, suggesting that this already small population needs multiple consecutive years without major hurricanes to recover. Interestingly, similar to Statia’s forests, iguanas at higher elevations were found to have been affected more severely.

Letter Antillean Iguana. Photo credit: Philippa King

Importance

Small islands such as Sint Eustatius are home to declining populations of rare and endangered species. In many cases, these isolated populations are unable to migrate between islands and thus populations can only increase in size locally. These new studies highlight the need to improve habitat quality and lower anthropogenic threats to optimize the natural recovery of these species. Ideally, at least for population increase could be aided by a local head-starting project where baby iguanas are nourished in temporary captivity and released once they are larger and more likely to survive.

You can find the full study here entitled “Hurricane-induced population decrease in a Critically Endangered long-lived reptile” using the DCBD link below.

More info in the Dutch Caribbean Biodiversity Database
 

Downloads & links >

Published in BioNews 54

The Red-bellied Racer (Alsophis rufiventris) is a harmless snake species that is native to the St. Christopher and Saba Banks. Today, it unfortunately only occurs on the islands of Saba and St. Eustatius, which represent just 11 % of the species’ original range. Previously the snake was also found on St. Kitts and Nevis, but was driven to extinction by the mongoose (Herpestes javanicus) following its introduction in the early 1900s. Little is known about the racer, which makes it an important species to study. However, studying snakes in their natural environment can be challenging due to their camouflage, secretive nature and cryptic behaviour.

On Saba and St. Eustatius, the racer plays a key role in the islands’ ecosystems by regulating small reptile and amphibian populations. Anoles, especially the endemic and highly abundant Anolis sabanus on Saba, and whistling frogs (Eleutherodactylus johnstonei) form an important food source for racers. 

Red-bellied Racer on Saba (Photo: L. Mielke)

Despite the presence of invasive species such as black rats (Rattus rattus) and domestic cats (Felis catus), racer populations on Saba and St. Eustatius were previously described as “robust” and “abundant”, and were considered stable. For that reason, the species was downgraded from Endangered to Vulnerable by the International Union for the Conservation of Nature in 2016. However, in September 2017, category 5 hurricanes Irma and Maria caused extensive damage to the natural ecosystems on both islands, resulting in habitat loss, reduced prey abundance and increased racer mortality, which raised concerns for the conservation status of this species.  

In 2011, a group of students from the University of Puerto Rico visited St. Eustatius for one week, where they conducted some racer surveys in the Quill and Boven National Parks. In 2018 and 2019, RAVON interns Kevin Verdel and Brent Kaboord conducted extensive surveys in the parks to monitor racer populations following hurricane impacts. Until this year, however, no quantitative data had been collected on Saba. Between July and September 2021, Hannah Madden from the Caribbean Netherlands Science Institute and volunteer Lara Mielke joined forces to conduct racer surveys on Saba. Replicating the methods used on St. Eustatius allowed us to compare the results from both islands and create a baseline for future research and conservation efforts.  

Important food source for racers on Saba – the endemic Anolis sabanus (Photo: L. Mielke).

In cooperation with the Saba Conservation Foundation, Lara Mielke conducted line transect surveys along existing hiking trails, covering six different vegetation types and a range of elevations, from 100m to the top of Mount Scenery (870m). Occupancy modelling revealed that racers were present in all vegetation types and were more likely to be present at elevations over 400m. Distance analysis revealed a density estimate of 10.9 racers/hectare on Saba, which across the entire survey region (438.6 ha) gives an estimate of 4,917 racers. These results are similar to those from St. Eustatius, where racers were present in all vegetation types surveyed and mean abundance increased in line with elevation. However, density estimates from St. Eustatius were lower (9.9/ha in 2018 and 7.3 in 2019), with a population estimate of 3,915 racers across the study region (540 ha) in 2019. This suggests that the racer population on Saba is healthier than that on St. Eustatius, but this may be due to the fact that surveys on Saba were conducted later, giving the population more time to recover.  

Red-bellied Racer along the Sandy-Cruz Trail on Saba (Photo: L. Mielke).

Unfortunately, few detailed studies exist on the ideal pre-hurricane density of A. rufiventris, however a 2016 study from Guana Island, British Virgin Islands, on the Puerto Rican Racer estimated 19/ha. While racer densities are likely to be species- and habitat-specific, based on literature describing pre-hurricane populations on Saba and St. Eustatius as “thriving”, it is highly likely that these suffered significant declines as a result of the hurricanes. The encounter rate of racers from St. Eustatius based on fieldwork in 2011 was estimated at 16/hour, which dropped to just 0.41/hr in 2019. On Saba, the current encounter rate estimate is 1.28/hr, thus while the species may have recovered since 2017, it is unlikely to be close to pre-hurricane levels.  

Besides hurricanes, racer populations face a suite of additional threats on both islands, including ongoing habitat destruction and predation by invasive species. We are extremely concerned about the impacts of free-roaming domestic cats on racers, which appear to be especially pervasive on Saba. Saba residents continue to import pet cats from St. Maarten, despite this being prohibited by a local invasive species ordinance. Consequently, enforcement of legislation and prevention of local extinction should be a top conservation priority on both islands, especially given the species’ extremely limited range.  

Domestic cat preying on a Red-bellied Racer on Saba (Photo: B. Noort).

Article published in BioNews 49

Two years ago we shared the worrying news that the Bridled Quail-dove population had suffered a significant decline on St. Eustatius (Statia) following hurricanes Irma and Maria in 2017. Almost four years on, has the situation improved? It is time for an update. 

Bridled Quail-dove in the Quill National Park (photo by Hannah Madden)

The Bridled Quail-dove (Geotrygon mystacea) is a shy, ground-dwelling species that is endemic to the Caribbean. It spends its day foraging the forest understory for fruits, seeds and the occasional gecko. Quail-doves prefer undisturbed forests with a closed canopy, and are sensitive to changes in their natural habitat. Unfortunately, the impacts of human-induced climate change mean hurricanes are becoming more intense and are occurring more often, which could spell trouble for this vulnerable species.  

I began surveying Bridled Quail-doves in the Quill National Park in 2016. The Quill is a dormant volcano that rises to 600 meters with a large, accessible crater. It is the only habitat on Statia where the species occurs. Pre-hurricane, quail-doves were relatively common and observant birders could almost certainly spot one or two while hiking along the trails. The species breeds in May, when its mournful and unmistakable ‘whooo’ can be heard echoing through the forest.  

Overgrazed understory in the Quill National Park, St. Eustatius (photo by Hannah Madden)

Hannah Madden during fieldwork in May 2021. Some 100 meter transects took up to 20 minutes to complete due to the steep and challenging environment (photo by Oliver Jones)

In September 2017 the forest habitat of the Quill suffered extensive damage from hurricanes Irma and Maria. Trees were stripped of leaves and fruits, branches were broken, and some trees did not survive. The effects of the hurricanes are still evident in some parts of the forest. In addition, we have a goat problem in the park; a very serious goat problem that has been pervasive for decades. Non-native, free-ranging goats overgraze already degraded habitats, resulting in a limited food supply and reduced understory cover for the quail-dove (as well as other forest-dependent species). Feral chickens disturb ground cover and compete for the same food source. Nest predators (invasive black rats and feral cats, both of which are present in the Quill) prey on eggs and chicks. This leads to lower reproduction and/or survival for the Bridled Quail-dove. There is no evidence of immigration of adults from nearby islands, which means populations are physically and genetically isolated. Based on the above we became concerned for the welfare of the Bridled Quail-dove and, thanks to support from many generous donors, BirdsCaribbean was able to provide funds to conduct post-hurricane surveys.  

I conducted surveys of the Bridled Quail-dove in the Quill National Park. This involved walking previously established transects within the dove’s range (~150 to 600 m). We surveyed during peak breeding season (May) to allow for audio as well as visual detections. Once I saw or heard a quail-dove, I measured the distance of the bird from the center of the transect. I also recorded elevation and canopy height to assess their influence on Bridled Quail-dove presence. Once surveys were complete, I pooled all data from 2016 – 2021 to obtain abundance and density estimates per year. 

Results 

As shown in the graph below, effort has increased from 1,200 m of transects in 2016 to over 15,000 m in 2021. This means survey effort has increased 13-fold to detect around one third of the number of doves that were counted in 2016. As you can imagine, repeating so many surveys is physically demanding. 

Number of detections (left axis; red line) and survey effort (right axis; blue line) between 2016 and 2021 (no surveys were conducted in 2020).

Unfortunately, abundance estimates of the Bridled Quail-dove on St. Eustatius have declined significantly since 2016, and the current estimate is just 123 individuals (min 72 – max 210). This is less than 50% of the 2019 estimate, and less than 5% of the 2016 estimate.  

After assessing the influence of covariates on Bridled Quail-dove presence, I found that doves were more likely to be present at higher elevations in habitats with a higher canopy (such as inside the crater, along the rim, and on the upper outer slopes of the Quill). In contrast, I found a strong negative effect of year on dove presence. This means that in the years following hurricanes Irma and Maria, doves were less and less likely to be present in the survey area.  

What now for the Bridled Quail-dove on Statia? 

We are extremely concerned about the Bridled Quail-dove on St. Eustatius. The combination of habitat degradation and predation by invasive species is likely responsible for the continued decline of this species. Its long-term survival is now uncertain unless urgent conservation actions are implemented. Even if the dove does persist, such a small and isolated population faces additional risks such as inbreeding. Since hurricanes are likely to increase in frequency and intensity, this could be catastrophic for the Statia population, and others in the region. Recently the International Union for the Conservation of Nature proposed that the Bridled Quail-dove be reassessed based on our work on Statia. This means the classification could be changed from Least Concern to Near Threatened or Vulnerable, but data are still lacking from many islands. We encourage enthusiastic birders to conduct their own surveys so that local populations can be assessed.  

Passive acoustic monitoring device (source: www.wwf.org.uk)

We will be working with local conservation NGO St. Eustatius National Parks (STENAPA) to create an Action Plan for the Bridled Quail-dove. To effectively protect the species locally, it is likely that multiple conservation actions will be required: goat and cat removal, rodent control, and feral chicken removal from the park. In order to achieve this we need the support of the local government and the community. Because of the tremendous effort now required to monitor Bridled Quail-doves on Statia, we also suggest trialing passive acoustic monitoring (PAM) devices during the breeding season. This involves the placement of recording units that can be placed in the field for up to a month to record and interpret calls. Using these devices will allow us to collect data in less accessible areas, as well as increasing the scale of our study. Hopefully by combining conservation efforts with field surveys and PAM we will have better news in the coming years.  

Hannah Madden is a terrestrial ecologist with the Caribbean Netherlands Science Institute, based on St. Eustatius. She is also a member of the IUCN Pigeon and Dove Specialist Group.  

Article published in BioNews 47