The Socorro Dove (Zenaida graysoni) is a dove that is extinct in the wild. It was endemic to Socorro Island in the Revillagigedo Islands; the last sighting in its natural habitat was in 1972.
There are not more than some 200 and probably fewer than 100 purebred birds in captivity. A reintroduction program is in the early stages of preparation.
In captivity, it hybridizes vigorously with the former and almost all privately-owned birds as well as several of the captive breeding programme that lacked a pedigree are known or strongly suspected to be such hybrids; they are for the time being excluded from the reintroduction programme 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 long and weighs 190 g on average.
In overall color pattern, it agrees with its relatives. It may be considered a stronger-colored insular representative of the Mourning Dove; the male is deep cinnamon on head and underparts, with an ear streak like in the two closest relatives. Its 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 to 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 a constant threat from Red-tailed Hawks and maybe Great Frigatebirds was 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 miles) 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 Above Mean Sea Level (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 today 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 gominate its habitat; one of which, Sideroxylon 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; the 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 being taken care of. 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 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 turned out to be a she. 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.