Montserrat oriole

Montserrat oriole
Male at London Zoo
Conservation status
Vulnerable  (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Passeriformes
Family:	Icteridae
Genus:	Icterus
Species:	I. oberi
Binomial name
Icterus oberi (Lawrence, 1880)

The Montserrat oriole (Icterus oberi) is a medium-sized black-and-yellow icterid (the same family as many blackbirds, meadowlarks, cowbirds, grackles, and others, including the New World orioles) endemic to the island of Montserrat.

It inhabits the Centre Hills and South Soufriere Hills Important Bird Areas on the island of Montserrat in the Lesser Antilles of the West Indies, and is the national bird of this British territory. It is threatened by habitat loss, and was classified by BirdLife International as Critically Endangered, with a current estimated population of between 200 and 800. Much of its habitat was destroyed by deforestation, Hurricane Hugo and the volcanic activity between 1995 and 1997. However, a reduction in volcanic activity on the island has allowed the population of the Montserrat Oriole to rebound. As a result, the bird has been reclassified to the status of Vulnerable.

Female at Frankfurt Zoo, Germany

The oriole once was found in three main areas: the bamboo forest east of Galways Soufrière, the leeward slopes of the Chances Peak mountain and the Centre Hills (especially the Runaway Ghaut area).

The diet of the bird consists mainly of insects and fruits. The birds usually lay two spotted eggs. All models indicate that they begin breeding at the age of one year. Most of them were almost wiped out during the volcano eruptions and only about 200 of them are still surviving.

The binomial name of this bird commemorates the American naturalist Frederick Albion Ober.

Description

Among the Caribbean members of the genus Icterus, Montserrat Oriole is the only one that shows substantial sexual dimorphism in plumage coloration. The male Montserrat Oriole has a full black hood, with black eye and silver-black bill. The black reaches to the middle of the breast, leaving the appearance of a small yellow spur near the wing. The wings are all black with no wing bars and the tail is all black as well. The belly, flanks and rump are rich yellowish-orange. The legs are blackish gray.

The female Montserrat Oriole has an overall greenish olive drab appearance. The head has a greenish yellow appearance, with the back and tail being more green. The wings are greenish brown with a yellow leading edge. The belly, flanks, breast and throat are a uniform greenish yellow. Juveniles are very similar to females, but have an overall duller appearance.

Habitat

Historically

A forest and edge species, the Montserrat oriole probably occurred throughout the main hill ranges prior to anthropogenic forest clearance. The Montserrat oriole is believed to have formerly had a total range of around 30 square kilometres. In the lowlands, it probably occurred in the wetter forests of the west and south of the island, particularly along the steep river valleys, known locally as ghauts. Most forest habitat, including the hill forests, was destroyed by clearance for plantation agriculture during the eighteenth and nineteenth centuries (Beard, 1949). However, with the decline of the plantations, secondary forest developed over most of the hill ranges.

Present day

Today, the Montserrat Oriole inhabits mesic to wet neotropical forests, and these forests are now restricted to two areas of the island: the Centre Hills (1110 ha) and the South Soufriere Hills (250 ha). It formerly occurred across the island's three main forested hill ranges (the Centre, Soufrière and South Soufrière hills), and has also been observed in agricultural and residential areas of lowland Montserrat, but this is likely due to individuals wandering from core population areas. Today, the Montserrat Oriole is confined to hill forests on the small island of Montserrat. However, volcanic eruptions between 1995 and 1997 destroyed around two-thirds of its habitat, all but removing this species from the Soufrière and South Soufrière hills, and largely restricting it to just the Centre Hills. In total, the Montserrat oriole is thought to presently occupy an area of less than 13 square kilometres.

Behavior

Because of the small range of this species, information relating to their behavior is limited. What we do know, is that the bird tends to move around the forest solitarily, or frequently in pairs. The extent of daily movements and home range size unknown, but believed to exceed radius of 50 m. There is evidence that seasonal movements to higher elevation may occur, especially during dry season when the forest at higher elevations retains some moisture offers more food. Orioles can be inquisitive and will frequently approach humans, but they can also be shy and inconspicuous and are easily overlooked. Because the species has few natural predators, no predator-avoidance behavior is known. However, Pearly-eyed Thrashers (Margarops fuscatus) are aggressive competitors at fruiting trees, and may be potential predators of young and possibly adult orioles. In the presence of Pearly-eyed Thrashers, Montserrat Orioles often reduce vocalizations and become more secretive.

Voice

The Montserrat Orioles song is a loud series of melodious whistles, but slow and methodical. Single notes given are given every 2 – 3 seconds. Notes are composed of single syllables, or two syllables, usually short, sharp whistles or lower pitched gurgled whistles. One predominant note is a sharply descending whistled 'tseew'. The song is mostly heard during breeding season, but does not sing consistently at high rate. Both sexes can produce song. The sound of the call is similar to other orioles, a harsh and sharp chic or chuck note, often repeated incessantly when bird is alarmed. It is said that the call is the easiest way to detect the presence of this species.

Feeding

Like the behavioral data, limited information exists on the feeding habits of the Montserrat Oriole. Research we do have suggests that the Montserrat Oriole has a varied diet that consists of Arthropods (insects, spiders), snails, and to a lesser extent fruits, and possibly nectar and flowers. Foraging occurs frequently in flowers, and it is likely that both nectar and associated arthropods are being consumed. Larvae of aquatic and semi-aquatic invertebrates that are found in water-filled Heliconia floral bracts presumably are an important diet item. Wild individuals have been observed to consume mango (Mangifera indica); captive individuals have consumed kiwifruit (Actinidia) and papaya (Carica).

Foraging behavior

The oriole forages mostly in pairs. It uses a strong bill to pry bark and leaf-stems open to extract arthropods, but also gleans arthropods from foliage. When foraging on flowers, it relies on eyesight to obtain invertebrates. Prey items that are too large for immediate ingestion are held with claws and manipulated with the beak while the bird is perched on a branch. Although fly-catching has been observed, leaf-gleaning is the more dominant mode of foraging. Frequently, orioles search hanging dead leaves, vine tangles, and leaf covered branches for arthropods.

Measurements

MALE (n = 37 individuals): tarsus length: 22.0 mm (25.8; range 24.9-26.4); body mass: 36.6 g 38.0, range 36.0-39.0). FEMALE (n = 11 individuals): tarsus length: 20.9 mm (24.3, range 22.0-27.2); body mass: 33.6 g (33.7, range 31.5-35.4). JUVENILE (n = 25 individuals of both sexes): tarsus length: 20.9 mm; body mass: 34.3 g.

Population

In 1984, Arendt and Arendt estimated that about 1000-1200 Montserrat Orioles were present on the island, but that number was likely an underestimate (Evans 1990). Initial surveys suggested that much of the population had survived the eruptions, with healthy numbers remaining in the Centre Hills and a smaller relict population in the South Soufrière hills. These surveys were confirmed by scientists publishing in the journal Bird Conservation International who extrapolated that the population must contain around 4,000 individuals. But between 1997 and 2003, the population declined very rapidly. Thankfully, later surveys discovered a remnant population occupying an area of just one to two square kilometres in the South Soufrière hills. Since 2010 there has been a reduction in volcanic activity, and improved efforts in surveying since 2011 have shown that the oriole's population is currently stable or mildly increasing. In 2012, abundance estimates derived from repeated point counts coupled with the assumption of birds using a 100 m radius of forest around each sampling station yielded a total population estimate of 765 birds (95% CI: 492 - 1345), with 540 birds (95% CI: 377 - 902) in the Centre Hills and 225 birds (95% CI: 115 - 442) in the South Soufriere Hills.

Effects of human activity on population

The direct effects of human activities on the Montserrat Oriole's population currently are low. However, the conversion of forest habitat to housing and agricultural plantations in the past has severely reduced the extent of suitable forest habitat on the island of Montserrat. Ongoing housing developments may encroach on the margins of the protected Centre Hills forest, and lead to further, albeit very small, loss of forest habitat.

Conservation status

Multiple zoological expeditions have brought individuals for conservation in the last thirty years. Back in the late 1990s, the RSPB worked with international partners, notably Durrell Wildlife and the Royal Botanical Gardens, Kew, to mount a rescue of some orioles into ex situ care, ensuring the species was not lost forever. In 1999, at the height of the volcanic eruption, some birds were also taken into captivity to safeguard the species against extinction in the wild. Captive breeding at Jersey Zoo (UK) was successful, and in February 2012 there were 48 individual birds alive in 14 different zoos. Due to persistent threats in the only remaining wild habitats, captive population may have to be maintained for the next 25 years.

Threats

No information exists on predation on adult birds. Eggs and chicks are vulnerable to predation by introduced mammals (rats, mostly Rattus rattus) and native birds (Pearly-eyed Thrashers, Margarops fuscatus). Pearly-eyed Thrashers are opportunistic omnivores, and occasionally pursue adult or immature orioles, but it is unlikely that such attempts are successful. All birds of prey that regularly occur on Montserrat are mostly associated with open habitats, and are therefore unlikely predators of Montserrat Orioles. American Kestrels (Falco sparverius) occasionally hunt in forest habitat, but it is unknown whether they prey on orioles. Its populations are also at risk of further volcanic eruptions, as well as other natural destructive events, such as hurricanes, which are a frequent disturbance in the Caribbean region.

Invasive animals

Non-native mammals such as rats (Rattus rattus and R. norvegicus), pigs (Sus scrofa), and goats (Caprus hirca) are abundant and widespread in the forests on Montserrat. Rats act as nest predators and kill both eggs and chicks. Pigs and goats affect the forest undergrowth and alter the structure of the forest vegetation, which may reduce the availability of optimal nesting habitat. Pigs also uproot Heliconia plants, which are an important foraging substrate for Montserrat Orioles.

Territoriality

The Montserrat Oriole defends territory aggressively against conspecific intruders during the breeding season. Although birds are present in territories year-round, territories appear to be less important outside the main breeding season, and the territorial response can be much reduced or completely lacking. Offspring are tolerated in territories for up to 3 months, but length of post fledging care and tolerance depends on the re-nesting decision of the territorial pair.

Inter-specific aggression is rare. Orioles sometimes displace smaller forest bird species from Heliconia flowers in the process of foraging. Occasionally, territorial adults defend flowering trees and aggressively chase off both conspecifics and other bird species.

Reproduction

The Montserrat Oriole has a smaller clutch, more extended parental care, and higher adult survival than most orioles nesting in the North Temperate Zone. A clutch of usually two or three eggs is laid with an average annual productivity of 1 fledged chick per pair, and successful pairs may occasionally rear up to three broods per year. Unsuccessful pairs may attempt up to five clutches in a single season. Incubation duties, which may be shared equally by both parents or have little input from the male, usually last for around 14 days. Both adults feed the chicks for the 13 days that they are in the nest, with an average of 40 days taken for post-fledgling parental care. However, this relationship will not last much longer than three months.

Reproductive investment

Montserrat Orioles are multiple-brooded, and can lay up to five clutches per season when some of these nests fail, and raise a maximum of three broods per year. No information exists about the factors controlling the cessation of breeding activity, but last clutches are generally initiated before the end of August. In each year, a small number of pairs may occupy territories and construct nests without attempting to lay eggs. After nest failure, new clutches are generally laid within 3 weeks, whereas after successfully raising a brood a new clutch is generally initiated within 5 weeks.

Sexual behavior

The social system of the Montserrat Oriole is characterized by social monogamy and high mate fidelity. Pairs occur in breeding territories year-round, and show strong fidelity to the nesting territory in successive years. Divorce of partners is uncommon, but does occur. In most cases, the female retains control over the breeding territory.

Phenology

Montserrat Orioles breed mostly between late March and September, coinciding with the peak rainfall season on Montserrat. The lay date of first clutches is around mid April (± 30 days). A single active nest was found in January 1999 and recently fledged juveniles were observed in December 2004, but such unseasonal nesting events are rare.

Demographics

Adult survival is higher in Montserrat Orioles than for temperate orioles, and ranges from 60-76% per year (Allcorn et al. 2012). There appears to be substantial annual variation in adult survival, which is likely a consequence of environmental conditions. In years when a volcanic dome collapse led to heavy ash fall over existing forest areas, adult survival was reduced compared to years when no ash fall occurred. This reduction is presumably a result of food shortage, as the volcanic ash suppresses insect availability (Marske et al. 2007). Rainfall, which may also affect the availability of food, has a comparatively minor influence on adult survival. Because of the high adult survival, Montserrat Orioles are long-lived birds and population dynamics depend heavily on adult survival and to a smaller extent on juvenile survival and recruitment. Although nesting success is low, it is possible that Montserrat Orioles could maintain stable populations if re-nesting and long parental care of fledglings compensate for the high mortality of eggs and chicks.

Nesting

Construction characteristics

The Montserrat Oriole nests are a hanging basket structure sewn onto the underside of horizontal leaves or leaf clusters, which is typical for the genus. The nest is built entirely by the female. Males indicate potential nest sites to their mates by hanging upside down from the underside of chosen leaves. The female begins building the basket by shredding strands out of the leaf frond. These form the "ribs" of the structure, and the female oriole then weaves the basket around these ribs with fine strands of vegetation obtained from Heliconia and other forest plants. Nests are typically constructed 1.5 – 20 m above the ground, with nests in Heliconia usually around 3–4 m, and nests in woody tree species higher above ground. Nests are not conspicuously lined. Heliconia leaves are the most commonly used nest substrate (83% of 229 nests) at present, but because Heliconia is an early successional plant that has benefited from human forest modification and hurricanes, this apparent preference may be fairly recent. Alternative nest substrates are leaves of banana (Musa acuminata), Cordia sulcata, Cecropia spp., and Coccoloba spp. These broad leaves provide shelter from heavy rainfall events, which are common during the breeding season.

Nesting period

Eggs are laid in daily intervals, and incubation commences once the clutch is complete. Only the female incubates. The male remains generally within sight of the nest and responds aggressively to both intra- and interspecific intrusion into the nest's vicinity throughout the incubation period. Mean length of incubation is 13.9 ± 0.2 days (n = 70 nests). Mean time from hatching to fledging is 13.1 ± 0.3 days (n = 56). Both parents feed nestlings, but no information exists on feeding rates and diet delivered to nestlings.

Success rate

Nesting success is generally low. Reproductive output averages slightly over one chick per pair per year, but is unequally distributed among pairs and varies substantially among years. Nest failures are mostly a result of nest predation, with smaller numbers of nests failing after ash fall or structural collapse of the nest-supporting leaves.

Post-fledgling parental care

Both parents share the duty of feeding fledglings. Mean length of post-fledging parental care is 40 ± 5 days (n = 35 broods), but parental care can be very long (up to 88 days after fledging). Parental care gradually diminishes over time after fledging, and can be 1–2 weeks shorter when the pair subsequently starts a new nest in the same season. Brood division is common in pairs that feed more than one fledgling, and each parent appears to preferentially feed one of the fledglings. The female provides most of the care for solitary fledglings. After about 55 days post-fledging, males start chasing fledglings to encourage them to leave their natal territory.

Priorities for future research

Currently no information exists on the survival and recruitment of juvenile birds. This critical parameter is important for the development of more precise population models that can predict the species' response to future climatic changes, and research is needed to describe juvenile movements and the range of survival probabilities among years with differing environmental conditions. Very little is known about the diet of Montserrat Oriole, and the role that dietary abundance plays in increasing reproductive output. Fecundity appears to increase with higher pre-breeding season rainfall, but a mechanistic link has not been established yet. Furthermore, no information currently exists on genetic bottleneck effects as a result of the dramatically reduced population size following the volcanic eruption. Work is ongoing to assess the genetic health of the species across temporal and spatial scales. Samples have been collected from the extant wild population, from captive birds, and from museum specimens, and will be analysed for genetic structure and overall diversity.

External links

• BirdLife Species Factsheet.