|Socorro dove collecting nesting material
Burgers' Zoo, Netherlands
Zenaida macroura graysoni
The Socorro dove (Zenaida graysoni) 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. A reintroduction program is in the early stages of preparation.
It is a close relative of the mourning and eared doves, particularly the former and was at one time considered a subspecies. In captivity, it hybridizes with the former and almost all privately owned birds as well as several of the captive breeding programs are known or strongly suspected to be hybrids. These are excluded from the reintroduction program as there is evidence of unique adaptations in the Socorro species. The scientific name commemorates Zénaïde Laetitia Julie Bonaparte and the American ornithologist and artist Andrew Jackson Grayson.
The Socorro dove is a medium-sized, principally terrestrial dove with long legs for a zenaida dove. It is 26.5–34 cm (10.5–13.25 in) long and weighs 190 g (7.5 oz) on average. In overall color pattern, it agrees with its relatives. It is considered to be a stronger-colored insular representative of the mourning dove. The male is deep cinnamon on its head and underparts, with an ear streak like its relatives. The nape is blue-grey and the neck has an iridescent pink patch, most prominent after moult. The upperparts are rufous brown. Female and juvenile coloration is slightly duller. The most conspicuous differences from the mourning dove – darker coloration and more well-developed feet – were useful amid the dark lava rock and the shady forests of its island home, where native mammalian predators were absent but constant threats from red-tailed hawks and great frigatebirds were present. The advertising call begins with a disyllabic coo, followed by three single calls, and ends with another disyllabic coo: "Coo-oo, OO, OO, OO, Coo-oo". Each of these 5 elements takes a little less than one second.
The arid and fairly barren Clarión Island, some 400 km (250 mi) west of Socorro, has an endemic subspecies of the mourning dove, Zenaida macroura clarionensis. These birds are roughly intermediate in appearance between the Socorro dove and the mainland mourning doves. It is highly likely that they represent a later colonization by mourning doves (rather than "proto-Mourning Dove" stock), which evolved its phenotype independently from but in parallel with the Socorro birds. This would mean that predation by great frigatebirds is significant enough to select towards a cryptic darkening of the plumage, as these are the only predators of Z. m. clarionensis. On a semidesert island such as Clarión, birds would be expected to evolve towards lighter plumage – to better withstand the heat and lack of reliable freshwater sources – in accordance with Gloger's Rule.
There is marked behavioral difference to the mourning dove. When Andrew Jackson Grayson discussed the species, he called it the "solitary dove" because he never saw more than one male and one female together. The doves, particularly the adult males, chase away their young as soon as these can fend for their own and the partners split for the time being. This too is believed to be in adaptation to the former dominance of aerial predators, lest local concentrations of birds, let alone young, unexperienced ones, would present easy targets for the hawks. Typical of many birds on mammal-less islands, Socorro doves show little fear of humans or, fatally, cats.
The last habitat in which this dove was found is the low seasonally humid forest above 500 m ASL. Before the introduction of cats, it seasonally descended into the lowlands where it was "common" in March 1953 for example. It may be that this coincided with the peak of the breeding season, when many birds had dependent young and dispersed widely to gather more varied food. This was the case in the Socorro Mockingbird, the other mid-sized native landbird of Socorro, which apparently has very similar habitat preferences.
Its last refuge was dominated by endemic Guettarda insularis, Ilex socorroensis and Sideroxylon socorrense, as well as black cherry (Prunus serotina), Ficus cotinifolia fig trees and Psidium socorrense guavas, apparently all native on Socorro. At least at the time of the last record, it seemed to be dependent on intact understorey of Euphorbiaceae (spurges and relatives) and ferns, maybe because cats hunt less efficiently in these dense tangles. It is frugivorous and, as is often the case in Columbidae, might have played an important role in the reproduction of the trees which dominate its habitat. One of these (S. socorrense) is classified as vulnerable by the IUCN. In particular, it was found to associate with the fig trees.
Virtually nothing is known about breeding in the wild. Parallels in altitudinal migration with the equally solitary mockingbird might be taken as indication that the breeding activity peaked around March–April. In captivity, the female generally lays two white eggs in a nestbox 1–2.5 m above ground. The incubation lasts 14–17 days. The young fledge after 14–20 days.
Extinction in the wild
The Socorro dove is thought to have been extirpated mainly by feral cats, but high levels of understorey grazing by sheep may have limited the population. Human hunting might have temporarily brought down numbers, though little permanent settlement exists on Socorro. Cats were introduced apparently in the early 1970s. They do not seem to have been present in 1953.
The species reproduces no less willingly in captivity than other Zenaida doves, provided its different ecological needs are addressed. Most of the surviving birds are in zoos and other facilities in Germany and the USA. On October 30, 2006, the Socorro dove was successfully bred at the London Zoo. The bird was named "Arnie", after Arnold Schwarzenegger, with reference to his famous line "I'll be back" (on Socorro Island). However, when Arnie was sexed he was discovered to be female. It is hoped that the descendants of Arnie and her relatives can someday soon be reintroduced into the wild. As of early 2006, it was being prepared to remove the sheep and to rid the island of cats. In the meantime, with the maintenance of the remaining birds, stock for reintroduction is being provided.
- BLI (2004), Schmechel (2006)
- Johnson & Clayton (2000)
- Schmechel (2006)
- Brattstrom & Howell (1956)
- BLI (2007)
- Probably ssp. capuli according to biogeography, Brattstrom & Howell (1956) contra CMICD (2007)
- Endemic Acalypha umbrosa and Croton masonii, native Euphorbia anthonyi, E. incerta and manchineel (Hippomane mancinella), and perhaps also introduced Mexican fireplant (Euphorbia heterophylla), E. hirta var. hirta, E. hyssopifolia and E. thymifolia: CMICD (2007)
- Adiantopsis radiata, Asplenium formosum, Asplenium sessilifolium, Mexican resurrection fern (Pleopeltis polypodioides var. aciculare), Polypodium alfredii, Polystichum muricatum, Pteridium caudatum, the Socorro endemic Botrychium socorrense and the Revillagigedos endemic Cheilanthes peninsularis var. insularis
- WCMC (1998)
- Brattstrom & Howell (1956), BLI (2007), CMICD (2007)
- Brattstrom & Howell (1956), BLI (2004, 2007)
- BirdLife International (BLI) (2007). Socorro Dove – BirdLife Species Factsheet. Retrieved 2007-NOV-24.
- Brattstrom, Bayard H. & Howell, Thomas R. (1956). The Birds of the Revilla Gigedo Islands, Mexico. Condor 58(2): 107–120. doi:10.2307/1364977 PDF fulltext DjVu fulltext
- California/Mexico Island Conservation Database (CMICD) (2007). Plant accounts: Socorro. Retrieved 2007-NOV-13.
- Johnson, Kevin P. & Clayton, Dale H. (2000). A Molecular Phylogeny of the Dove Genus Zenaida: Mitochondrial and Nuclear DNA sequences. Condor 102(4): 864–870. PDF fulltext
- Schmechel, Ria (2006). Socorro Dove at World Associations of Zoos and Aquariums. Version of 2006-APR-06. Retrieved 2007-NOV-24.