Madden, H.

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.

The Lesser Antillean Iguana, Iguana delicatissima is the largest extant native land vertebrate of St. Eustatius and was recently lost (~1990s) from the only other Dutch Caribbean island where it was native (St. Maarten). It is an IUCN Red List Critically Endangered (CR) species that has disappeared from most islands in the Lesser Antilles. A recognized principal cause for its endangerment throughout the islands is displacement by and hybridization with invasive alien Green Iguanas, Iguana iguana (IAGI) (Knapp et al. 2014). On 22nd February 2016 an adult female IAGI was caught in Princess Estates, St. Eustatius. The possibility that the animal had been present on the island for a longer period, that it may have laid one or more nests, or the possibility that there may have been other IAGIs on the island, represented an imminent danger to the continued existence of the Lesser Antillean Iguana. We conducted a Rapid Response Extermination Campaign (RREC) with the goal of wiping out the IAGI at an early stage. Three thorough visual surveys were conducted during 2016-2017 in key risk areas in an attempt to detect and eliminate all IAGIs and their hybrids. In total 409.5 observer hours were spent during three dedicated surveys in and around areas where IAGIs or hybrids had been captured, seen or reported. Searches were conducted over a total of 40 days and covered a total of trajectory of 114.2 km. Only a single detection was realized during these directed surveys. This suggests that the RREC took place at an early stage of the invasion. Nevertheless, due to local publicity via newspapers and radio programs, several records were reported by the public. Thanks to these reports, and opportunistic encounters by park management staff, five captures of IAGIs or their hybrids were realized between February 2016 and January 2017. Since then eight additional captures have been realized, demonstrating that the RREC, even when augmented by public support and extra vigilance by park management staff, was insufficient to purge the island of the IAGI. Our study documents three distinct IAGI introductions between 2013 and 2020, one of which was likely intentional and two of which were incidental stowaways on container ships. Our results show that, even though it is a relatively large animal, due to its relatively secretive nature, camouflage, and high fecundity, eliminating the IAGI from an island will require a more intensive and sustained effort than we provided, even by means of an RREC in the early stages of the invasion. Informing stakeholders and the public in an early stage of the campaign can clearly make a critical contribution towards an RREC. Even four years after the campaign, the numbers of the IAGI and its hybrids still appear to be limited and concentrated in and around inhabited areas, their likely main point of entry being the island’s harbour. We conclude that it may not be too late to quell the invasion before the critically endangered, largest surviving island-endemic vertebrate is permanently lost from St. Eustatius. Additional IAGI extermination campaigns need to be launched as soon as possible. The harbour of St. Maarten was identified as the source on the most recent 2020 introduction. As St. Maarten serves as a major inter-island trans-shipment hub in the Lesser Antilles, and the Lesser Antilles are rich in endemic iguanas vulnerable to the IAGI, it is essential that St. Maarten ports cull all Green Iguanas in and around their grounds to prevent the spread of this major pest to the islands with which they trade.

Structural vegetation damage and food limitation are important effects of major hurricanes, particularly

for fruit/seed-eating, forest-dependent Caribbean birdswith restricted distributions and small

populations, such as the Bridled Quail-dove Geotrygon mystacea. Motivated by the lack of abundance

estimates, corrected for detection probability, we conducted distance-sampling surveys inside

and outside theQuill National Park eachMay in 2016-2019.Detection mode was the most important

covariate, with others receiving no support fromthe data. Detectability of available single individuals

and clusters of individuals within 60mof transect centrelines averaged 0.957 0.114 standard error

for audio detections, 0.434 0.052 for visual detections, and 0.693 0.064 for detection modes

combined. Availability averaged 0.475 0.138 and the product of detectability and availability

averaged 0.329 0.098. Density averaged 1.459 0.277 individuals ha-1 and population size

averaged 642 122 individuals in 440 ha. Density did not differ along and away from forest trails,

but was higher inside than outside the park and at elevations within 201-400 m than 100-200mand

401-600 m.Density declined by 76%after hurricanes IrmaandMaria in 2017.We suggest thatmajor

hurricanes together with free-ranging livestock overgrazing degraded foraging habitats, limited food

supply, and caused a population bottleneck. Our methodology can be implemented across the

distribution range to assess population status and trends and evaluate the result of management

actions at key conservation sites. Bridled Quail-dove populations probably were declining on most

islands before the 2017 hurricanes and population status warrants revision.

Keywords: Bridled Quail-Dove, distance sampling, hurricanes, population assessment, St.

Eustatius

We studied the micro- and macrohabitat preferences of black rats (Rattus rattus) and house mice (Mus musculus) on St. Eustatius, a small, inhabited Caribbean island. Our study builds upon a preliminary assessment of invasive alien rodents on St. Eustatius, which has no extant native rodent species. We deployed tracking tunnels (baited cards with ink left overnight to track animal visitors to the card) in 12 macrohabitats to determine the presence and relative abundance of rodent species. Data were collected between July 2017 and May 2019. We collected data in 5 Å~ 5 meter (n = 120) plots to determine whether grass cover, leaf litter cover, number of living trees, and other microhabitat components influenced habitat use by rats and mice. Rats were more likely to occur in microhabitats with more live trees, but with fewer logs and less grass and bare ground. Mice were more likely to occur in microhabitats with more bare ground. Rat tracks were recorded in all macrohabitats, whereas mouse tracks were detected in all but four, namely: C, M2, M3, and M5. Based on tracking tunnel data, the relative abundance of rats and mice ranged from 0% to 70%, respectively, in all macrohabitats over the study period. Rat relative abundance in forest macrohabitats differed significantly from that in other macrohabitats. Rat relative abundance in regenerating grassland differed significantly from that in other macrohabitats. There is evidence of habitat selection by rats at the micro- and macrohabitat scale, which could be linked to food availability. However, our study revealed only weak or no patterns of habitat selection by mice. Rat relative abundance differed significantly between the second and third assessment, and between the fourth and fifth assessment. Mouse relative abundance differed significantly between the first and second assessment.

We present the first confirmed evidence of plastic ingestion by a Red-billed Tropicbird on the Caribbean Netherlands island of St. Eustatius, which supports a regionally important nesting population. With our observations, all species of tropicbird have now been documented ingesting marine plastic pollution.

Free-roaming livestock constitute a major threat to the terrestrial and marine ecosystems of St. Eustatius. In anticipation of a government-led culling program, we repeated population surveys of feral livestock from November 2019 to February 2020. Our goal was to compare current population estimates with those presented from 2013 by Debrot et al. (2015). Population densities of goats, sheep and chickens were estimated along 33.5 km of permanent roads and trails, representing six different habitats. Each of the 13 transects was surveyed twice. The results show that densities of goats and sheep have increased significantly compared with 2013, and chickens have increased slightly. The island population estimate (± SE) based on habitat-specific detection curves for goats is 7,602 ± 1,555; for chickens the island population estimate is 2,668 ± 417. Detections of cows were too low to be included in the analysis, however these were present in town, lower Quill and grasslands. Given that sheep were primarily restricted to grasslands, the island population estimate is less exact at 4,316 ± 2,140. Nevertheless, the densities of freeroaming goats and sheep rose significantly between 2013 and 2020, and are now at levels considered extremely unsustainable considering the island’s size. Our estimates for goat densities per hectare and combined population size in the terrestrial protected areas are D = 5.93 ± 1.35 and N = 5,171 ± 1,182, compared with D = 1.09 ± 0.27 and N = 1,323 ± 329 in 2013. This suggests that the numbers of goats have increased fourfold in the parks over the past seven years, especially the northern hills. Such excessive densities of feral goats increase soil erosion and degradation, reduce organic matter, and reduce water retention in vulnerable landscapes. Feral chickens were present in all habitats but especially prevalent in urban areas. In addition to being aggressive omnivores, chickens can transmit diseases and therefore constitute a public health risk. Our results suggest that there is an urgent need to drastically reduce numbers and implement effective management of free-roaming livestock on St. Eustatius. 

Premise of research. Small populations on small islands are intrinsically more vulnerable to population decline and extinction. Nevertheless, small native populations that occur on multiple islands may have life history characteristics that buffer impacts from novel disturbance regimes, and rather than populations contracting, they may be expanding. We monitored three populations of the orchid Brassavola cucullata from two Caribbean islands and asked what is the likelihood of population persistence.

Methodology. Over 3-4 years, we recorded growth, fruit production, herbivory, recruitment and mortality for all plants in each of our populations. We assessed persistence and predicted possible population changes using both population projection models (PPM) and integral projection models (IPM). Our results include a mixture of traditional (lambda, elasticities) and more recent indices (transient dynamics and non-linear sensitivities).

Pivotal results. Growth, reproduction and predicted population persistence varied among years and islands. IPM and PPM gave similar results. The overall trend is for a reduction in population growth rates, although population reactivity may buffer this pattern in the short-term (λ > 1). Populations would be extremely vulnerable to reduction if small plants dominated, yet even with an abundance of large plants, substantial reductions in population density are possible. Medium and larger plants contribute more to the persistence of the population, yet survival and growth of small individuals might have greater effect on lambda if retrogression is observed. To attain population stability, effective recruitment rates must increase dramatically.

Conclusions. Populations of perennial plants on small islands can fluctuate substantially suggesting a degree of vulnerability. While B. cucullata shows a general trajectory of decline, there are some signs of stability despite deforestation and herbivore activity. The outlook is precarious for the Saba population given the predominance of younger plants, and all three populations could decline if spasmodic recruitment fails to occur, which may happen if disturbance regimes change and the ongoing warming and drying trends persist.

Keywords: Brassavola cucullata - population dynamics - Lesser Antilles - transient dynamics - island conservation - Orchidaceae 

Based on four years of butterfly monitoring in four contrasting natural habitats on St. Eustatius, we document large and consistent differences in the butterfly species assemblages in the different habitats and compare the butterfly assemblages of the three windward Dutch islands to those of other islands of the Lesser Antilles. Seven new species records were established for St. Eustatius, thereby updating the butterfly list to a total of 32 species. Pieridae were the most numerically abundant group of butterflies (48%), followed by Lycaenidae (26%), Hesperiidae (12%), and smaller numbers of both Heliconiinae (6%) and Charaxinae (5%). Heliconiinae and Charaxinae both showed a significant dependence on the moister, wind-sheltered habitats of the volcanic slopes and crater of the Quill, but this dependence was particularly strong for Heliconiinae. The butterfly faunas of the windward Dutch islands numbered a total of 44 species. The presence of larval host plants needed for local reproduction was confirmed for all but two species. Cluster analysis separated the butterfly faunas of these and the surrounding islands into two groups. The more speciose butterfly assemblages of Saba, St. Eustatius, and St. Martin clustered together with the those of the surrounding higher islands of Antigua, Montserrat, Nevis, and St. Kitts, while the poorer faunas of the low-lying islands of Anguilla, St. Bartholomew, and Barbuda formed a separate cluster and had a lower species richness particularly in the Heliconiinae and Charaxinae. Based on consistent effects of elevation on butterfly faunas, at both geographic scales (between areas on St. Eustatius and between islands), our results suggest that island maximum elevation is the overriding factor explaining the distribution of butterfly faunal richness in the northern Lesser Antillean islands studied.

The Caribbean is a biodiversity hotspot due to its rich biodiversity and wholesale loss of primary vegetation. Yet, there is a paucity of studies on the status and trends of terrestrial avifauna populations in the region. We combined survey data from six habitats (Quill and Gilboa Hill, Town, Botanical Garden, Garden Road, and Venus Bay) on a small Lesser Antillean island over a seven-year period. Species abundance and richness varied among habitats, with Town having the highest species richness. Logistic regressions revealed significant differences among habitat and year (P = 0.008), but not foraging guild, season, rainfall or elevation (P > 0.05). The Quill (P = 0.0003) and Town (P = 0.001) differed significantly from the other habitats surveyed. Granivorous + frugivorous and omnivorous species were most commonly detected in the Quill, whereas nectarivores and granivores were most commonly detected in Town. The influence of total annual precipitation on bird detection rates was unsubstantial. However, site-specific climate data for the different habitats are not available. Though resident landbird populations on St. Eustatius were stable over the survey period, we recommend that annual monitoring continue in the same habitats, especially following extensive vegetation damage caused by hurricanes Irma and Maria in September 2017 and in light of predicted global climate change. The results of our monitoring proved useful in evaluating protected areas and Important Bird Areas compared with non-protected areas. We encourage island researchers and conservation stewards to initiate similar long-term monitoring in order to determine the status of their resident landbird populations.

The Bridled Quail-dove is a regionally endemic species that, on Statia, is only found in upper elevations of the Quill (above ~150m) and inside the crater.  Since 2017, annual surveys have highlighted an alarming decline in populations.  At an estimated population decrease of ~77% since 2017, this species has caught the attention of conservationists and requires immediate protection.

The Plight of the Dove

Last year we reported the results of a post-hurricane assessment of the Bridled Quail-dove (Geotrygon mystacea) population on Statia. Once thought to have been a common resident of the West Indies, declining populations are now isolated within coastal dry forest patches of the eastern Caribbean. Introduced, non-native predators such as feral cats, mongooses and rats are thought to negatively impact Bridled Quail-dove populations by preying on adults, eggs and/or chicks. Other external factors that contribute to population declines include hunting, volcanic activity, hurricanes, and habitat loss and alteration. Furthermore, this species is sensitive to openings in the forest canopy, which also affects nesting. Despite its current classification as a species of Least Concern by the IUCN (in 1992 it was classified as Near Threatened), the Bridled Quail-dove is likely of conservation concern due to data deficiency and population declines across its entire habitat. The only surveys we are aware of took place in Montserrat (2007) following a volcanic eruption, St. Croix (1992) following hurricane Hugo, and Guana Island, British Virgin Islands (2018). Populations on other islands such as Puerto Rico are thought to be so limited that the species was excluded from a Columbid study (1995). Calling and breeding activity are dependent on rainfall, therefore the dove is sensitive to hurricanes and extended periods of drought. Similar to other Columbids, the Bridled Quail-dove lays clutches of two eggs in a flimsy nest made of twigs up to six meters above the forest floor. Furthermore, Bridled Quail-doves do not fare well in areas of human activity. 

Current Study Highlights Rapid Decline in Population

Our pre-hurricane assessment in May 2017 was initially encouraging, with an estimated 1,039 (minimum 561 -maximum 1,621) quail-doves across its local habitat of 440 hectares, possibly the highest known density in the region. Post-hurricanes season, in November 2017, we repeated the surveys and recorded a decrease of 22% to 803 (minimum 451 -  maximum 1,229). Furthermore, in May 2018, we recorded a decline to 253 individuals (minimum 83 - maximum 486). 

We repeated surveys across the entire Quill (440 hectares) during May 2019, coinciding with the quail-dove’s peak breeding season. Estimations for detection probability, density and population size were calculated by measuring the perpendicular distance of the quail-dove from the transect centerline during repeated surveys. The results are very concerning since the population has continued to decline to 238 individuals (minimum 118 - maximum 390). The surveys of May 2018 and 2019 showed that little if any successful post-hurricane reproduction has occurred. Additionally, the majority of detections were recorded inside the crater and near the crater rim, with very few detections at lower elevations. This means that the population is highly clumped at low numbers, which increases the chance of local extinction.  

Possible Causes for Population Decline

Indirect effects of hurricanes, human-induced habitat degradation and increased predation continued to affect quail-dove survival and reproduction in 2019. Rat and other invasive predator species may increase in density following hurricane-induced changes in foraging resources, affecting quail-dove survival and reproduction even further. A feral cat was detected during surveys inside the crater, probably as a result of forest openness after the hurricane.  Due to the fact that members of the Columbidae family have early maturity and short lifespans, conservation efforts should focus on successful reproduction through invasive species management. The integrity of the Quill should be improved to help forest-dependent birds and other wildlife recover in order to enhance their prospects for long-term survival on Statia.

Uncertain Future for Quail-Doves on Statia

Unfortunately, the frequency and intensity of hurricanes are predicted to increase as a result of climatic change. Furthermore, Caribbean islands are expected to see more frequent and severe droughts. Between 2013 and 2016, the region experienced a widespread drought due in part to El Niño. Large scale trends are difficult to estimate since precipitation has been very inconsistent over the past century.  However, there does appears to be a regional trend towards an increase in variability of precipitation.   This increase in variability will continue to threaten the local quail-dove populations.

We are grateful to BirdsCaribbean for funding Frank’s travel expenses in 2017, to St. Eustatius National Parks for allowing us to conduct surveys in the Quill National Park, and to Caribbean Netherlands Science Institute (CNSI) for facilitating this ongoing project. This July, Hannah Madden presented the results of this research at BirdsCaribbean’s regional meeting in Guadeloupe.