The Yellow-headed Amazon complex includes several forms that are regarded as subspecies by some authorities and as full species by others. Juniper and Parr (1998) recognized three distinct species: the Yellow-crowned Amazon (A. ochrocephala), the Yellow-headed Amazon (A. oratrix), and the Yellow-naped Amazon (A. auropalliata). However, they noted that the status of these taxa was unresolved and that, for example, the presence of forms intermediate between the Yellow-headed Amazon and Yellow-naped Amazon suggests that they might better be treated as belonging to a single species. They also pointed out that clarifying the situation is made more complex by possibly age-related plumage variation and substantial individual variation within currently recognized subspecies. Although Juniper and Parr provisionally recognized three distinct species, for similar reasons Collar (1997) and others have provisionally treated the various forms as members of a single highly variable species. Subsequent molecular phylogenetic studies have revealed even more taxonomic complexity. Investigations by Eberhard and Bermingham (2004) and Russello and Amato (2004) indicated that the Blue-fronted Amazon (A. aestiva) may in fact be nested within the Yellow-headed complex (Russello and Amato concluded that the Yellow-shouldered Amazon, A. barbadensis, also falls within this group). Further sampling and analysis by Ribas et al. (2007) provided additional evidence indicating that actual evolutionary relationships among genetic lineages in this group may not be well reflected by the plumage variations by which the traditional taxa are defined (see below). (Ribas et al. and references therein should be consulted for more details.)
As traditionally defined, "Yellow-crowned Amazons" are mainly green with yellow feathers on the forehead and forecrown, sometimes extending onto the lores (the area between the bill and the eyes) and around the eyes. and often with a red spot at the base of the upper mandible (birds in the western Amazon basin have a green forehead). "Yellow-headed Amazons" are similar in appearance but have yellow extending over the entire head (immatures are largely green with little or no yellow on the head and little or no red and yellow on the wing). "Yellow-naped Amazons" have yellow limited to the nape and, sometimes, the forehead and forecrown.
Yellow-headed Amazons move quietly in the treetops and fly well above the canopy with rapid, shallow wingbeats. They are found in savanna, tropical deciduous forest (including clearings), dense thorn forest, Pacific swamp forest, evergreen floodplain forest, dense gallery woodland, woods with Pinus caribea (in Belize), and cultivated areas with trees. They occur mostly in lowlands below 500 m. Nests are in tree cavities at 6 to 15 m. The Yellow-headed Amazon is endangered, although some of the other forms are locally quite common (e.g., "Yellow-crowned Amazon" in parts of Peru and Brazil). This species is reputed to be among the best "talkers" of all parrots, increasing its popularity in the pet trade.
"Yellow-headed Amazons" are confined to Middle America in Mexico, Belize, extreme eastern Guatemala, and extreme northwestern Honduras. Feral populations are established in Miami (Florida, U.S.A.) and Puerto Rico. These birds are local and uncommon throughout most of their range, with populations severely depleted by habitat loss and trapping for the pet trade both within and outside the native range. "Yellow-naped Amazons" occur in Middle America in the eastern Pacific lowlands of Mexico, Guatemala (possibly), El Salvador (lower arid tropical zone), Honduras, and Nicaragua to northwestern Costa Rica (from the southern end of the Gulf of Nicoya northward). The "Yellow-crowned Amazon" is found in Panama (and possibly Honduras) in Middle America and in South America south to eastern Brazil and northern Bolivia (it is rare in Trinidad).
(Collar 1997 and references therein; Juniper and Parr 1998 and references therein)
- Collar, N.J. 1997. Yellow-crowned Amazon (Amazona ochrocephala). Pp. 473-474 in: del Hoyo, J., Elliott, A., and Sargatal, J., eds. Handbook of the Birds of the World. Volume 4. Sandgrouse to Cuckoos. Lynx Edicions, Barcelona.
- Eberhard, J.R. and E. Bermingham. 2004. Phylogeny and biogeography of the Amazona ochrocephala (Aves: Psittacidae) complex. Auk 121: 318–332.
- Juniper, T. and M. Parr. 1998. Parrots: A Guide to Parrots of the World. Yale University Press, New Haven, Connecticut.
- Ribas, C.C., E.S. Tavares, C. Yoshihara, and C.Y. Miyaki. 2007. Phylogeny and biogeography of Yellow-headed and Blue-fronted Parrots (Amazona ochrocephala and Amazona aestiva) with special reference to the South American taxa. Ibis 149: 564-574.
- Russello, M.A. and G. Amato. 2004. A molecular phylogeny of Amazona: implications for Neotropical parrot biogeography, taxonomy, and conservation. Molecular Phylogenetics and Evolution 30: 421-437.
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) RESIDENT: Pacific slope of Mexico from Colima south to Oaxaca; Caribbean slope from southern Nuevo Leon and Tamaulipas south to Veracruz and Tabasco; and Belize. Introduced and established in southern Florida (Dade County), and Puerto Rico (AOU 1998).
Length: 36 cm
Belizean Pine Forests Habitat
This species is found in the Belizean pine forests along the Central America's northwestern Caribbean Sea coast; the ecoregion exhibits relatively well preserved fragments of vegetation as well as a considerable abundance of fauna. This ecoregion comprises a geographically small portion of the total land area of the ecoregions of Belize. There is relatively low endemism in the Belizean pine forests, and only a moderate species richness here; for example, only 447 vertebrate taxa have been recorded in the ecoregion. The ecoregion represents one of the few examples of lowland and premontane pine forests in the Neotropics, where the dominant tree species is Honduran Pine (Pinus caribaea var. hondurensis), which requires periodic low intensity burns for its regeneration. The vegetation is adapted to the xeric, acidic and nutrient-poor conditions that occur primarily in the dry season.
In the forest of the Maya Mountains, vegetation reaches higher altitudes, the topography is more rugged and crossed by various rivers, and nighttime temperatures are lower. The pine trees are larger and numerous, and the pine forest intersects other formations of interest such as rainforest, Cohune Palm (corozal), cactus associations, and others. About eleven percent of Belize is covered by natural pine vegetation. Only two percent represents totally closed forests; three percent semi-closed forests; and the remaining six percent pine savannas, that occupy coastal areas and contain isolated pine trees or stands of pine trees separated by extensive pastures. In addition to human activity, edaphic factors are a determining matter in this distribution, since the forests on the northern plain and southern coastal zone are on sandy soils or sandy-clay soils and usually have less drainage than the more fertile soils in the center of the country.
At elevations of 650 to 700 metres, the forests transition to premontane in terms of vegetation. At these higher levels, representative tree species are Egg-cone Pine (Pinus oocarpa), which crosses with Honduras Pine (P. hondurensis), where distributions overlap, although belonging to subsections of different genera; British Honduras Yellowwood (Podocarpus guatemalensis) and Quercus spp.; moreover, and in even more moist areas there is a predominance of Jelecote Pine (Pinus patula), together with the palm Euterpe precatoria var. longivaginata and the arboreal ferns Cyathea myosuroides and Hemitelia multiflora.
A number of reptilian species are found in the Belizean pine forests, including: Guatemala Neckband Snake (Scaphiodontophis annulatus); Indigo Snake (Drymarchon corais); On the coasts, interior lakes and rivers of Belize and by extension in this ecoregion there are two species of threatened crocodiles: American Crocodile (Crocodylus acutus) and Morelet's Crocodile (C. moreletii), while observation of the Central American River Turtle (Dermatemys mawii CR) is not uncommon in this ecoregion.
Also to be noted is the use of this habitat by the Mexican Black Howler (Alouatta pigra), which can be considered the most endangered howler monkey of the genus, and the Central American spider monkey (Atteles geoffroyi). Both species experienced a decline due to the epidemic yellow fever that swept the country in the 1950s. The five feline species that exist in Belize: Jaguar (Panthera onca), Puma (F. concolor), Ocelot (Leopardus pardalis), Margay (Leopardus wiedii) and Jaguarundí (Herpailurus yagouaroundi) are in appendix I of CITES, as well as the Central American tapir (Tapirus bairdii) can been seen with relative frequency. Belize has the highest density of felines in Central America. The tapir is abundant around rivers. The White-lipped Peccary (Tayassu pecari) is also present in the ecoregion.
Although most of the amphibians and reptiles are found in humid premontane and lowland forests, the only endemic frog in this ecoregion, Maya Mountains Frog (Lithobates juliani), is restricted to the Mountain Pine Ridge in the Maya Mountains. Salamanders in the ecoregion are represented by the Alta Verapaz Salamander (Bolitoglossa dofleini NT), whose males are arboreal, while females live under logs. Anuran taxa found in the ecoregion include: Rio Grande Frog (Lithobates berlandieri); Sabinal Frog (Leptodactylus melanonotus); Northern Sheep Frog (Hypopachus variolosus); Stauffer's Long-nosed Treefrog (Scinax staufferi); and Tungara Treefrog (Engystomops pustulosus).
Present in the ecoregion are a number of avian species, including the endangered Yellow-headed Amazon Parrot (Amazona oratrix EN), although this bird is adversely affected by ongoing habitat destruction. Of particular interest is the presence in this ecoregion of Central America's highest procreative colony of Jabiru (Jabiru mycteria), a large migratory bird, particularly in the Crooked Tree sanctuary, on the country's northern plains.
- C. Michael Hogan & World Wildlife Fund. 2013."Belizean pine forests". Encyclopedia of Earth, National Council for Science and the Environment, Washington DC ed.Mark McGinley.
- E. Dinerstein, D.M. Olson, et al. 1995. A Conservation Assessment of the Terrestrial Ecoregions of Latin America and the Caribbean. The World Bank in association with WWF, Washington, D.C. ISBN: 0821332961
Habitat and Ecology
Comments: ALL SEASONS: Deciduous forest, open woodland, and pine ridges in native range; suburban areas where introduced. BREEDING: Nests in cavities in trees.
Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: Unknown
2500 - 10,000 individuals
Comments: Estimated to be 7000 in 1994 (BirdLife International 2001).
Life History and Behavior
Nests February-June in Puerto Rico (Raffaele 1983).
Molecular Biology and Genetics
Barcode data: Amazona oratrix
No available public DNA sequences.
Download FASTA File
Statistics of barcoding coverage: Amazona oratrix
Public Records: 7
Specimens with Barcodes: 12
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
National NatureServe Conservation Status
Rounded National Status Rank: NNA - Not Applicable
NatureServe Conservation Status
Rounded Global Status Rank: G2 - Imperiled
Reasons: Precipitous decline since mid-1970s, as a result of both aggressive capture for the pet trade and extensive habitat loss. Probably fewer than 7000 individuals remain.
Global Short Term Trend: Decline of 70 to >90%
Comments: An estimated decline of 90% from mid-1970s to 1994; equivalent to 68% in ten years. The population is now so small that lower, but still significant, rates of decline are likely in the future (BirdLife International 2001).
Degree of Threat: Very high - high
Comments: Threatened by capture for the pet trade, by habitat loss, and by persecution in response to crop damage (BirdLife International 2001). Eighty percent of the Tamaulipas lowlands have been cleared for agriculture, and there is increasing settlement along the Western Highway in Belize (BirdLife International 2001).
CITES Appendix I. In Mexico it occurs in nine protected areas (T. Monterrubio-Rico et al. in litt. 2007). The race magna occurs in El Cielo, Los Tuxtlas, Pantanos de Centla and Laguna de Terminos Biosphere Reserves. The race tresmarieae is protected in the Islas Marias Biosphere Reserve. The race oratrix occurs in Chamela-Cuixmala Reserve, on the Lagunas de Chacahua, Huatulco National Park, and on the recently created Zicuiran-Infiernillo Biosphere Reserve in Michoacan (T. Monterrubio-Rico et al. in litt. 2007), as well as seven protected areas in Belize (E. C. Enkerlin-Hoeflich in litt. 1994, Miller and Miller 1997, Snyder et al. 2000). The only breeding population known in Guatemala was declared a wildlife refuge in 2005, but effective protection is difficult due to organised crime in the area (Eisermann in litt. 2007). There are several country-wide awareness campaigns in Mexico (Roberson and Carratello 1997). It is bred in captivity, but the reintroduction of captive-bred birds is unfeasible (Low 1995b). Conservation Actions Proposed
Carry out surveys to obtain an up-to-date estimate of the population size. Monitor rates of habitat loss and degradation. Monitor levels of hunting, capture and trade. Enforce trade restrictions. Effectively protect key sites such as Las Colorados Ranch, Soto La Marina/La Pesca, (Tamaulipas), rÃo Naranjo, centred on Las Abritas (San Luis PotosÃ) and Punta de Manabique. Survey to identify additional important sites. Research habitat use and local movements. Continue to expand awareness campaigns. Develop structured captive-breeding programmes and research the possibility of future release.
Global Protection: Several (4-12) occurrences appropriately protected and managed
Comments: Populations occur in El Cielo Biosphere Reserve, Sanoval Ranch (Mexico), and seven protected areas in Belize (BirdLife International 2001).
Relevance to Humans and Ecosystems
Stewardship Overview: See Beissinger and Snyder (1991).
The Yellow-headed Amazon (Amazona oratrix), also known as the Yellow-headed Parrot and Double Yellow-headed Amazon, is an endangered amazon parrot of Mexico and northern Central America. Measuring 38–43 centimetres (15–17 in) in length, it is a stocky short-tailed green parrot with a yellow head. It prefers to live in mangrove forests or forests near rivers or other bodies of water. It is sometimes considered a subspecies of the Yellow-crowned Amazon. It is a popular pet and an excellent talker.
This species is part of the Amazona ochrocephala complex, which also includes the Yellow-naped Amazon (A. auropalliata). This complex, "a taxonomic headache", is considered a single species by some authorities and split into three species by others. The split is mainly based on the amount of yellow in the plumage, the color of the legs and bill, the proximity of A. oratrix and A. auropalliata in Oaxaca, Mexico, without apparent interbreeding, and the presence of both A. ochrocephala and A. auropalliata in northern Honduras. This evaluation has, however, been confused by misunderstandings regarding the plumage variations in the populations in northern Honduras, where birds vary greatly in amount of yellow on the head, crown and nape, but have pale bills and a juvenile plumage matching A. oratrix, but neither A. ochrocephala nor A. auropalliata. In 1997, the population from the Sula Valley in northern Honduras was described as a new subspecies, hondurensis, of A. oratrix. A. auropalliata caribaea on the Islas de la Bahía, which is relatively close to the recently described A. oratrix hondurensis, may have a relatively pale lower mandible, indicating that gene flow may occur between the two. If confirmed, this could suggest that the two are better considered conspecific. Alternatively, it has been suggested that caribaea and parvipes, both typically considered subspecies of A. auropalliata, may be closer to A. oratrix than they are to the nominate A. auropalliata. Both are relatively small and have red on the shoulder like A. oratrix, but unlike nominate A. auropalliata. The members of this complex are known to hybridize in captivity, and recent phylogenetic analysis of DNA did not support the split into the three "traditional" biological species, but did reveal three clades, which potentially could be split into three phylogenetic species: a Mexican and Central American species (including panamensis, which extends slightly into South America), a species of northern South America, and a species from the southern Amazon Basin. The Central American clade can potentially be split further, with panamensis (Panama Amazon) and tresmariae (Tres Marías Amazon) recognized as two monotypic species.
According to the traditional split, A. oratrix includes the taxa tresmariae (from Tres Marías Islands), belizensis (from Belize) and hondurensis (from Sula Valley, Honduras) as subspecies. An additional subspecies, magna, has sometimes been recognized for the population on the Gulf slope of Mexico, but today most authorities consider it invalid, instead including this population in oratrix, which also occurs on the Pacific slope of Mexico. In contrast, the population in north-western Honduras and adjacent eastern Guatemala (near Puerto Barrios), which resembles A. oratrix belizensis and commonly is included in that subspecies, may represent an undescribed subspecies. It has sometimes been referred to as guatemalensis, but until this population is officially described, the name remains provisional.
The origin of the common epithet "Double Yellow-headed" is that this species is differentiated from the others in the Yellow-headed Amazon complex by possessing both the yellow nape and yellow crown of its two close relatives, hence a "double-yellow" head.
The Yellow-headed Amazon averages 38–43 centimetres (15–17 in) long. The shape is typical of amazons, with a robust build, rounded wings, and a square tail. The body is bright green, with yellow on the head, dark scallops on the neck, red at the bend of the wing, and yellow thighs. The flight feathers are blackish to bluish violet with a red patch on the outer secondaries. The base of the tail also has a red patch, which is usually hidden. The outer tail feathers have yellowish tips.
The bill is horn-colored, darker in immatures of the Belizean subspecies. The eye ring is whitish in Mexican birds and grayish in others. The most conspicuous geographical difference is the amount of yellow. In adults, the head and upper chest are yellow in the subspecies of the Tres Marías Islands (tresmariae); just the head in the widespread subspecies of Mexico (oratrix); just the crown in Belize (belizensis); and the crown and nape in the Sula Valley of Honduras (hondurensis, which thus resembles the Yellow-naped Parrot). Immatures have less yellow than adults; they attain adult plumage in 2 to 4 years.
The variety "Magna" (or "Magnum") is bred for more yellow and commands a premium price as a pet. Some "extreme" Magnas have as much yellow as Tres Marías birds, but are distinguished from them by heavier barring on the chest and a less bluish tint to the green plumage.
Wild birds give low-pitched, sometimes human-sounding screams, but often fly silently (unlike many other parrots). The calls can be described as "a rolled kyaa-aa-aaah and krra-aah-aa-ow, a deep, rolled ahrrrr or ahrhrrrr," etc. Young birds make a "clucking" sound to indicate that they are hungry.
Distribution and habitat
This species lives in riparian forest and areas with scattered trees, as well as evergreen forest in Belize and mangroves in Guatemala. A notable ecoregion of occurrence is the Belizean pine forests. It occurs singly or in pairs, in small groups, and occasionally in big flocks. The range formerly included both coastal slopes of Mexico from the Tres Marías Islands and Jalisco to Oaxaca and from Nuevo León to northern Chiapas and southwestern Tabasco, as well as a disjunct area including most of Belize, and another comprising a small part of northeastern Guatemala and northwestern Honduras. However, their numbers have been reduced drastically—by 90 percent, to 7,000, from the mid 1970s to 1994, and by 68 percent from 1994 to 2004—because of capture for the pet trade and habitat destruction.
Introduced populations can be found in Stuttgart, Germany where a population of over 50 individuals resides . Smaller introduced populations are to found at Imperial Beach, Santa Ana, Loma Linda, California and Pasadena, California; All in Southern California.
The Yellow-headed Amazon is considered endangered by the IUCN, and is on the CITES Appendix I, which by international treaty, has made export, import and trade of wild-caught Yellow-headed Amazons illegal and the trade in birds bred in aviculture subject to controls in most of the world. Captive-bred Yellow-headed Amazons can be sold and owned legally subject to checks and regulations. Populations range from Central America, through Mexico, and even into the Southmost region of Texas. Generally, throughout the world, sale of Appendix I species bred in aviculture must be accompanied with official certification which is provided by the breeder, and they must have a closed ring on one leg.
The popularity of Yellow-headed Amazons as a pet continues to fuel poaching efforts, which have nearly driven it to extinction in the wild. Their wild population has declined from 70,000 to 7,000 in the past two decades alone. An estimated 40-60% of poached Yellow-headed Amazons die before they are sold. Yellow-headed Parrots nest in holes in tree trunks or fallen branches. Poachers usually hack at the nest site with a machete to steal parrots, which is especially destructive because habitat is lost at the same time that the wild parrot population is reduced. The situation for tresmariae, which potentially can be treated as a separate species, is unclear, but its very small range gives cause for concern and some reports indicate it is under considerable threat.
Though only captive-bred Yellow-headed Amazons may be owned, these are widely available (if somewhat expensive) and their personalities make them highly desirable pets; they have been kept as such for centuries because they are among the parrots that "talk" best. Their vocal abilities are generally bested only by the African Grey Parrot and matched by similar species, such as the Yellow-naped Parrot. Yellow-headed Amazons in captivity appear to have an affinity for both singing and the learning of song - and a naturally powerful, operatic voice.
As in most amazons, nervous plucking of plumage is rare among this species. A generally recognized disadvantage of the Yellow-headed Amazon and its close relatives (such as the Yellow-naped Amazon) is hormonal aggressiveness, most notable among males in the breeding season. It is a member of the "Hot Three" (referring to the male bird's 'hot' temper), along with the Yellow-naped and Blue-fronted.
- BirdLife International (2013). "Amazona oratrix". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Steve N. G. Howell and Sophie Webb (1995). A Guide to the Birds of Mexico and Northern Central America. Oxford University Press. ISBN 0-19-854012-4.
- Binford, L. 1989. A distributional survey of the birds of the Mexican state of Oaxaca. Orn. Monographs. 43: 1-418.
- Monroe, B., JR., & T. Howell. 1966. Geographic variation in Middle American parrots of the Amazona ochrocephala complex. Occasional Papers of the Museum of Zoology, no. 34. Louisiana State University, Baton Rouge.
- Lousada, S., & S. Howell. 1996. Distribution, variation, and conservation of Yellow-headed Parrots in northern Central America. Cotinga 5: 46-53.
- Lousada, S., & S. Howell. 1997. Amazona oratrix hondurensis: A new subspecies of parrot from the Sula Valley of northern Honduras. Bull. BOC 117: 203-223.
- Lousada, S. 1989. Amazona auropalliata caribaea: A new subspecies of parrot from the Bay Islands, northern Honduras. Bull. BOC 109: 232-235.
- Eberhard, J., & E. Bermingham. 2004. Phylogeny and Biogeography of the Amazona ochrocephala (Aves: Psittacidae) Complex. Auk 121(2): 318-332
- Russello, M. A., & Amato, G. (2004). A molecular phylogeny of Amazona: implications for Neotropical parrot biogeography, taxonomy, and conservation. Molecular Phylogenetics and Evolution. 30(2): 421-437
- Ribas, C. C., Tavares, E. S., Yoshihara, C., & Miyaki C. Y. (2007). Phylogeny and biogeography of yellow-headed and blue-fronted parrots (Amazona ochrocephala and Amazona aestiva) with special reference to the South American taxa. Ibis 149: 564-574
- Juniper, T., & M. Parr. 1998. A Guide to the Parrots of the World. Pica Press, East Sussex. ISBN 1-873403-40-2
- Clements, J. 2007. The Clements Checklist of the Birds of the World. Christopher Helm. ISBN 978-0-7136-8695-1
- "Species factsheet: Amazona oratrix". BirdLife International (2008). Retrieved 12 August 2008.
- "Double Yellow-Headed Amazon Parrot". Aves International. Retrieved 2006-08-23. A commercial site. Shows many photographs including captive-bred young.
- "Where are they now?". The Feather Tree. 2003. Retrieved 2006-08-23. A commercial site. Shows many photographs comparing "extreme Magna" to tresmariae
- C. Michael Hogan & World Wildlife Fund. 2012. Belizean pine forests. ed. M. McGinley. Encyclopedia of Earth. Washington DC
- "Yellow-headed Parrot (Amazona oratrix)" (PDF). Defenders of Wildlife. Retrieved 2006-08-23.[dead link]
- Stuttgart Amazon Parrots - Jonker R.M.V., City Parrots
- "Yellow-headed Amazon Parrot". Hogle Zoo. 2002–2006. Archived from the original on 2008-04-17. Retrieved 2006-08-23.
- Larry Lachman, Diane Grindol, and Frank Kocher (2003). http://books.simonandschuster.ca/9780743227049 Birds Off the Perch: Therapy and Training for Your Pet Bird. Simon and Schuster. p. 7. ISBN 0-7432-2704-2.
- "The Amazon Parrot". lafebercares.com.
- "The Amazing Amazon Parrots". The Parrot Post.
- "Amazon Parrot Update". Animal and Pet Adventures.
Names and Taxonomy
Comments: Frequently considered conspecific with A. auropalliata and A. ochrocephala, but lack of evidence of interbreeding in areas of near or actual overlap in Oaxaca and Honduras suggests allospecies treatment (Sibley and Monroe 1990).