Socorro Mockingbird
Encyclopedia
The Socorro Mockingbird, Mimus graysoni, is an endangered
mockingbird
endemic
to Socorro Island
in Mexico
's Revillagigedo Islands
. The specific epithet commemorates the American ornithologist Andrew Jackson Grayson
.
Mimus graysoni shows its close relationship to the Northern
and Tropical Mockingbird
s rather subtly. It is a much stouter bird, resembling some thrasher
s in habitus
. It also has a distinct juvenile
plumage, more rufous above and with heavy pattern, especially below. This uncannily resembles, e.g., the Gray Thrasher
(Toxostoma cinereum) from Baja California
, but is apparently a case of convergent evolution
.
mockingbird and was for some time placed into a distinct genus
, Mimodes. This was revealed to be incorrect based on analysis of mtDNA NADH dehydrogenase
subunit
2 sequence
s.
Rather, the present species is closely related to the Northern
and Tropical Mockingbird
s. Its distinctiveness is the result of the strong selective
pressure on its island home, which enforced the evolution
of conspicuous adaptational autapomorphies. The juvenile plumage might also have been the result of genetic drift
enforced maybe by resource partitioning in this aggressive bird. The standard model of molecular clock
s (which in any case is fundamentally flawed) cannot be applied for mimid
s as their rates of mutation
seem to vary much over time. While it is the most phenotypically distinctive bird on Socorro, it also has the strongest ecological change from its ancestors; therefore its distinctiveness is not informative except supporting the theory that it is one of the older Socorro endemics.
Thus, and because the adaptation to the peculiar conditions on Socorro may even have accelerated not only morphological
but also molecular evolution - see also founder effect
-, it cannot be said with any certainty whether or not among Mimus, the Socorro species is a quite recent island offshoot of either of the mainland species. In any case, the three taxa are very close relatives. This serves to show that evolution does not move on at a constant speed; certainly not on a morphological level, and often neither on a molecular level either. Rather, the rate of evolutionary change varies, sometimes considerably, depending on the circumstances and the strength with which natural selection acts upon a founding population.
above 600 m, where it prefers groves of Oreopanax xalapensis and the endemic Guettarda insularis, with an understorey dominated by Triumfetta socorrensis and the endemic Eupatorium pacificum. As late as March 1953 it was still "common" at lower elevations, foraging in arid
open areas of the Croton masonii-prickly pear
shrubland
. In November of the same year, the birds had retired to the more humid forest in the uplands and were busy singing and defending territories. Today, they mainly remain in Ficus cotinifolia stands when visiting the lower elevations. The breeding season is extended, with nests in attendance between November and July, with the peak laying occurring in March and April. Three eggs are laid, which take no more 15 days to hatch.
The birds are generally reluctant to fly and as late as the mid-20th century were still fatally unwary; if pressed they will rather hop away than fly and if they take wing, it is usually for a few meters only. This may be an adaptation to the fact that Socorro has no native terrestrial
predators, but Red-tailed Hawk
s and Great Frigatebird
s that not infrequently prey on mockingbird-sized birds.
This species feeds on small invertebrate
s, the remains of land crab
s, and fruit, namely of the endemic shrubs Ilex socorrensis and Sideroxylon socorrense
, the latter of which has also become rare. Flies are pecked up but do not seem to be snatched out of the air. Like many Mimidae, the Socorro Mockingbird is an aggressive, solitary species living alone or with its partner. When they come together at a plentiful food source - e.g. blowflies on a carcass
- there is a marked social hierarchy between birds and rarely are more than two or three actively feeding; the less dominant birds hang around nearby, waiting for their turn.
The call is two medium followed by one lower whistle. Less often, a full song is given, consisting of a variable warbling tune, repeated several times as in many mockingbird
s. It is not clear whether this species imitates other birds like its relatives do; in any case only the Socorro Tropical Parula
, Socorro Towhee and Socorro Wren
would seem to possess songs that might serve as models for the mockingbirds, and except the parula these are not often found in the mockingbird's core habitat
.
by the IUCN. Mimus graysoni is mostly threatened by habitat loss caused by feral
sheep and the locust
Schistocerca piceifrons, and predation by feral cat
s which became established after 1953, probably in the early 1970s. It is not believed that the Northern Mockingbird
which has colonized Socorro in the late 20th century is limiting the recovery of its relative; the two Mimus do not occupy the same habitat and even if they did, the native bird is larger and more powerful and would probably simply outcompete its mainland relative in native vegetation at least. The extermination
of the sheep is underway in the hope of restoring the island ecosystem.
On one hand, it seems that the Socorro Mockingbird is a prolific species and would be able to increase in numbers quickly if habitat improves. On the other hand, its terrestrial habits make it vulnerable to cat predation and this may limit its recovery even if sheep are contained; it is not known for example in how far foraging in the lowlands – now cat-ridden – was important for robust breeding success. In addition, it can be expected that predation on this species by both the native Red-tailed Hawk as well as the feral cats has increased since the Socorro Dove
– formerly a preferred prey item – has become extinct in the wild
. Cerro Evermann, Socorro's main volcano
, is still active and erupts on a limited scale every few decades; as the mockingbirds seem to depend on upland forest habitat, a major eruption could place the species in jeopardy (see also San Benedicto Rock Wren
). This threat is presently not considered significant compared to the problem of introduced species however.
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...
mockingbird
Mockingbird
Mockingbirds are a group of New World passerine birds from the Mimidae family. They are best known for the habit of some species mimicking the songs of other birds and the sounds of insects and amphibians, often loudly and in rapid succession. There are about 17 species in three genera...
endemic
Endemic (ecology)
Endemism is the ecological state of being unique to a defined geographic location, such as an island, nation or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, all species of lemur are endemic to the...
to Socorro Island
Socorro Island
Socorro Island is a small volcanic island in the Revillagigedo Islands, a Mexican possession lying some 600 kilometers off the country's western coast at 18°48'N, 110°59'W. The size is 16.5 by 11.5 km, with an area of 132 km².- Geology :...
in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
's Revillagigedo Islands
Revillagigedo Islands
The Revillagigedo Islands or Revillagigedo Archipelago are a group of four volcanic islands in the Pacific Ocean, known for their unique ecosystem...
. The specific epithet commemorates the American ornithologist Andrew Jackson Grayson
Andrew Jackson Grayson
Andrew Jackson Grayson was an American ornithologist and artist.Grayson was the author of Birds of the Pacific Slope , which he considered to be a completion of John James Audubon's Birds of America...
.
Mimus graysoni shows its close relationship to the Northern
Northern Mockingbird
The Northern Mockingbird, Mimus polyglottos, is the only mockingbird commonly found in North America. This species was first described by Linnaeus in his Systema naturae in 1758 as Turdus polyglottos....
and Tropical Mockingbird
Tropical Mockingbird
The Tropical Mockingbird, Mimus gilvus, is a resident breeding bird from southern Mexico south to northern Brazil, and in the Lesser Antilles and other Caribbean islands. The birds in Panama and Trinidad may have been introduced. The Northern Mockingbird is its closest living relative, but the...
s rather subtly. It is a much stouter bird, resembling some thrasher
Thrasher
Thrashers are a New World group of passerine birds related to mockingbirds and New World catbirds. Like these, they are in the Mimidae family. There are 15 species in one large and 4 monotypic genera.These do not form a clade but are a phenetic assemblage...
s in habitus
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
. It also has a distinct juvenile
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
plumage, more rufous above and with heavy pattern, especially below. This uncannily resembles, e.g., the Gray Thrasher
Gray Thrasher
The Gray Thrasher is a medium-sized passerine bird belonging to the family Mimidae like the other thrashers. It is endemic to the Baja California peninsula in Mexico where it occurs from the tip of the peninsula north to 31°N on the west coast and 29°N on the east coast. It inhabits desert...
(Toxostoma cinereum) from Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
, but is apparently a case of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
Systematics and taxonomy
This is a rather distinct MimusMimus
Mimus is a bird genus in the family Mimidae. It contains the typical mockingbirds. In 2007, the genus Nesomimus was merged into Mimus by the American Ornithologists' Union.The following species are placed here:...
mockingbird and was for some time placed into a distinct genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, Mimodes. This was revealed to be incorrect based on analysis of mtDNA NADH dehydrogenase
NADH dehydrogenase
NADH dehydrogenase is an enzyme located in the inner mitochondrial membrane that catalyzes the transfer of electrons from NADH to coenzyme Q...
subunit
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
2 sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...
s.
Rather, the present species is closely related to the Northern
Northern Mockingbird
The Northern Mockingbird, Mimus polyglottos, is the only mockingbird commonly found in North America. This species was first described by Linnaeus in his Systema naturae in 1758 as Turdus polyglottos....
and Tropical Mockingbird
Tropical Mockingbird
The Tropical Mockingbird, Mimus gilvus, is a resident breeding bird from southern Mexico south to northern Brazil, and in the Lesser Antilles and other Caribbean islands. The birds in Panama and Trinidad may have been introduced. The Northern Mockingbird is its closest living relative, but the...
s. Its distinctiveness is the result of the strong selective
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
pressure on its island home, which enforced the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of conspicuous adaptational autapomorphies. The juvenile plumage might also have been the result of genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
enforced maybe by resource partitioning in this aggressive bird. The standard model of molecular clock
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
s (which in any case is fundamentally flawed) cannot be applied for mimid
Mimid
The mimids are the New World family of passerine birds, Mimidae, that includes thrashers, mockingbirds, tremblers, and the New World catbirds...
s as their rates of mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
seem to vary much over time. While it is the most phenotypically distinctive bird on Socorro, it also has the strongest ecological change from its ancestors; therefore its distinctiveness is not informative except supporting the theory that it is one of the older Socorro endemics.
Thus, and because the adaptation to the peculiar conditions on Socorro may even have accelerated not only morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
but also molecular evolution - see also founder effect
Founder effect
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall...
-, it cannot be said with any certainty whether or not among Mimus, the Socorro species is a quite recent island offshoot of either of the mainland species. In any case, the three taxa are very close relatives. This serves to show that evolution does not move on at a constant speed; certainly not on a morphological level, and often neither on a molecular level either. Rather, the rate of evolutionary change varies, sometimes considerably, depending on the circumstances and the strength with which natural selection acts upon a founding population.
Ecology and status
The Socorro Mockingbird today lives mainly in unmodified low forestForest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
above 600 m, where it prefers groves of Oreopanax xalapensis and the endemic Guettarda insularis, with an understorey dominated by Triumfetta socorrensis and the endemic Eupatorium pacificum. As late as March 1953 it was still "common" at lower elevations, foraging in arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...
open areas of the Croton masonii-prickly pear
Opuntia
Opuntia, also known as nopales or paddle cactus , is a genus in the cactus family, Cactaceae.Currently, only prickly pears are included in this genus of about 200 species distributed throughout most of the Americas. Chollas are now separated into the genus Cylindropuntia, which some still consider...
shrubland
Shrubland
Shrubland, scrubland, scrub or brush is a plant community characterized by vegetation dominated by shrubs, often also including grasses, herbs, and geophytes. Shrubland may either occur naturally or be the result of human activity...
. In November of the same year, the birds had retired to the more humid forest in the uplands and were busy singing and defending territories. Today, they mainly remain in Ficus cotinifolia stands when visiting the lower elevations. The breeding season is extended, with nests in attendance between November and July, with the peak laying occurring in March and April. Three eggs are laid, which take no more 15 days to hatch.
The birds are generally reluctant to fly and as late as the mid-20th century were still fatally unwary; if pressed they will rather hop away than fly and if they take wing, it is usually for a few meters only. This may be an adaptation to the fact that Socorro has no native terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
predators, but Red-tailed Hawk
Red-tailed Hawk
The Red-tailed Hawk is a bird of prey, one of three species colloquially known in the United States as the "chickenhawk," though it rarely preys on standard sized chickens. It breeds throughout most of North America, from western Alaska and northern Canada to as far south as Panama and the West...
s and Great Frigatebird
Great Frigatebird
The Great Frigatebird is a large dispersive seabird in the frigatebird family. Major nesting populations are found in the Pacific and Indian Oceans, as well as a population in the South Atlantic....
s that not infrequently prey on mockingbird-sized birds.
This species feeds on small invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s, the remains of land crab
Land crab
Gecarcinidae is a family of true crabs that are adapted for terrestrial existence, commonly known as land crabs. Similar to all other crabs, land crabs possess a series of gills. In addition, the part of the carapace covering the gills is inflated and equipped with blood vessels. These organs...
s, and fruit, namely of the endemic shrubs Ilex socorrensis and Sideroxylon socorrense
Sideroxylon socorrense
Sideroxylon socorrense is a plant species in the family Sapotaceae. It is endemic to Mexico, occurring only on Socorro in the Revillagigedo Islands.On its island home, this small tree grows in habitat that is at least seasonally humid...
, the latter of which has also become rare. Flies are pecked up but do not seem to be snatched out of the air. Like many Mimidae, the Socorro Mockingbird is an aggressive, solitary species living alone or with its partner. When they come together at a plentiful food source - e.g. blowflies on a carcass
Carrion
Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...
- there is a marked social hierarchy between birds and rarely are more than two or three actively feeding; the less dominant birds hang around nearby, waiting for their turn.
The call is two medium followed by one lower whistle. Less often, a full song is given, consisting of a variable warbling tune, repeated several times as in many mockingbird
Mockingbird
Mockingbirds are a group of New World passerine birds from the Mimidae family. They are best known for the habit of some species mimicking the songs of other birds and the sounds of insects and amphibians, often loudly and in rapid succession. There are about 17 species in three genera...
s. It is not clear whether this species imitates other birds like its relatives do; in any case only the Socorro Tropical Parula
Tropical Parula
The Tropical Parula is a small New World warbler. It breeds from southernmost Texas and northwest Mexico south through Central America to northern Argentina, including Trinidad and Tobago. This widespread and common species is not considered threatened by the IUCN.This passerine is not migratory,...
, Socorro Towhee and Socorro Wren
Socorro Wren
The Socorro Wren is a species of bird in the Troglodytidae family.It is endemic to Socorro Island, Mexico. It was formerly placed in Thryomanes but was moved to Troglodytes considering "manners, song, plumage, etc." and by biogeography and mtDNA NADH dehydrogenase subunit 2 sequence analysis.Its...
would seem to possess songs that might serve as models for the mockingbirds, and except the parula these are not often found in the mockingbird's core habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
.
Status and conservation
This species presently numbers fewer than 400 individuals altogether and is considered Critically EndangeredCritically Endangered
Critically Endangered is the highest risk category assigned by the IUCN Red List for wild species. Critically Endangered means that a species' numbers have decreased, or will decrease, by 80% within three generations....
by the IUCN. Mimus graysoni is mostly threatened by habitat loss caused by feral
Feral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
sheep and the locust
Locust
Locusts are the swarming phase of short-horned grasshoppers of the family Acrididae. These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory...
Schistocerca piceifrons, and predation by feral cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s which became established after 1953, probably in the early 1970s. It is not believed that the Northern Mockingbird
Northern Mockingbird
The Northern Mockingbird, Mimus polyglottos, is the only mockingbird commonly found in North America. This species was first described by Linnaeus in his Systema naturae in 1758 as Turdus polyglottos....
which has colonized Socorro in the late 20th century is limiting the recovery of its relative; the two Mimus do not occupy the same habitat and even if they did, the native bird is larger and more powerful and would probably simply outcompete its mainland relative in native vegetation at least. The extermination
Island restoration
The ecological restoration of islands, or island restoration, is the application of the principles of ecological restoration to islands and island groups. Islands, due to their isolation, are home to many of the world's endemic species, as well as important breeding grounds for seabirds and some...
of the sheep is underway in the hope of restoring the island ecosystem.
On one hand, it seems that the Socorro Mockingbird is a prolific species and would be able to increase in numbers quickly if habitat improves. On the other hand, its terrestrial habits make it vulnerable to cat predation and this may limit its recovery even if sheep are contained; it is not known for example in how far foraging in the lowlands – now cat-ridden – was important for robust breeding success. In addition, it can be expected that predation on this species by both the native Red-tailed Hawk as well as the feral cats has increased since the Socorro Dove
Socorro Dove
The Socorro Dove is a dove that is extinct in the wild. It was endemic to Socorro Island in the Revillagigedo Islands off the west coast of Mexico. The last sighting in its natural habitat was in 1972. There are not more than 200 and probably fewer than 100 purebred birds in captivity...
– formerly a preferred prey item – has become extinct in the wild
Extinct in the Wild
Extinct in the Wild is a conservation status assigned to species or lower taxa, the only known living members of which are being kept in captivity or as a naturalized population outside its historic range.-Examples:...
. Cerro Evermann, Socorro's main volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
, is still active and erupts on a limited scale every few decades; as the mockingbirds seem to depend on upland forest habitat, a major eruption could place the species in jeopardy (see also San Benedicto Rock Wren
San Benedicto Rock Wren
The San Benedicto Rock Wren is a small extinct passerine which was endemic to San Benedicto Island in the Revillagigedo Islands off Mexico. It was a subspecies of the Rock Wren.-Extinction:...
). This threat is presently not considered significant compared to the problem of introduced species however.