Geai de Santa Cruz

Identification

The island scrub jay is closely related to the California scrub jay (the coastal population found on the adjacent mainland), but differs in being larger, more brightly colored, and having a markedly stouter bill. They will bury, or cache, the acorns in the fall and may eat them months later. They also eat insects, spiders, snakes, lizards, mice and other birds' eggs and nestlings.

Taxonomie

The island scrub jay was first described by American ornithologist Henry Wetherbee Henshaw in 1886 and an archaeological specimen at site SCRI-192 dating from the 1780s-1812 on Santa Cruz Island is the earliest evidence of the bird in the historic period. This bird is a member of the crow family, and is one of a group of closely related North American species named as scrub jays. These were formerly treated as a single species, the scrub jay (as Aphelocoma coerulesens), with five subspecies, but are now considered four species: the Florida scrub jay (A. coerulescens), the island scrub jay, the California scrub jay (A. californica), and Woodhouse's scrub jay (A. woodhouseii). DNA studies indicate that the island and coastal forms have long been isolated from their relatives inland. The relationships within the genus have been studied in several papers (e.g.)<p>Island scrub jays seem to be incapable of crossing to the mainland. However they were once present on three of California's northern Channel Islands, San Miguel Island, Santa Rosa Island, and Santa Cruz Island where they persist today. Reliable historical observer records for island scrub jays in addition to Santa Cruz Island include only a single 1892 account on neighboring Santa Rosa Island, only about 10 km (6 mi) away. The historic observation on Santa Rosa Island is supported by a Pleistocene archaeological record of a single island scrub jay femur from a Late Pleistocene-Holocene site (SRI-V-3) found by Paul Collins of the Santa Barbara Museum of Natural History. There are also two Late Holocene archaeological remains found in San Miguel Island cave sites. There are no definite occurrences of a scrub-jay on any other of the Channel Islands, or on the Coronado Islands, only 13 km (8 mi) from the mainland. <p>Early studies suggested that the ancestor of the present population was storm-borne or carried on driftwood to Santa Cruz, or that the colonization occurred during a period of glaciation 70,000 to 10,000 years ago, when sea levels were much lower and the channel between the coast and the islands was correspondingly narrower. More recent DNA studies show that, although other island endemics such as the island fox and the Santa Cruz mouse may have diverged from their mainland relatives around 10,000 years ago, the scrub jays separated in a period of glaciation around 151,000 years ago. The most recent analysis indicates that the island scrub jay has been evolving in isolation for approximately one million years, i.e. over multiple glacial cycles. Up to about 11,000 years ago, the four northern Channel Islands were one large island, so island scrub jays must have been present on all four islands initially, but became extinct on Santa Rosa, San Miguel and Anacapa after they were separated by rising sea levels.

Nom scientifique

Aphelocoma insularis

Autorité

Henshaw, 1886

Ordre

Passériformes

Famille

Corvidés

Genre

Aphelocoma

Répartition et habitat

The island scrub jay is found today only on Santa Cruz Island, the largest of California's Channel Islands with an area of 250 km2 (96 mi2). The island is a nature reserve, the eastern 24% being administered by National Park Service as the part of the Channel Islands National Park and the rest of the island by the Nature Conservancy. Fossil remains for island scrub jays have been found on Santa Rosa and San Miguel Islands.<p>Island scrub jays occur in oak chaparral and bishop pine (Pinus muricata) woodland on Santa Cruz Island. Island scrub jays in pine habitat have longer, shallower bills than individuals in oak habitat; variation in bill shape is heritable, and individuals mate nonrandomly with respect to bill morphology.

Conservation

Le Geai de Santa Cruz est classé comme vulnérable dans la Liste rouge de l'UICN, car sa petite aire de répartition le rend potentiellement exposé à une catastrophe telle qu'une maladie ou un grand incendie détruisant son habitat. En 2008 et 2009, ses effectifs étaient estimés entre 1 700 et 2 300 individus, ce qui en fait l'un des passereaux les plus rares des États-Unis. L'ensemble de son aire est actuellement protégé dans le parc national des Channel Islands et l'espèce ne court pas de risque imminent d'extinction. Toutefois, l'installation du virus du Nil occidental dans le sud de la Californie en 2003 pourrait constituer une menace s'il atteint l'île Santa Cruz depuis le continent. Les corvidés sont particulièrement sensibles à ce virus. De plus, la fréquence croissante des incendies dans le sud de la Californie fait craindre un incendie catastrophique sur l'île. La couverture arbustive a augmenté depuis l'élimination des moutons, entre 1980 et 1991, puis des cochons retournés à l'état sauvage, entre 2005 et 2007, ce qui pourrait accroître ce risque. Le surpâturage par des ongulés non indigènes pourrait avoir provoqué la disparition du geai sur l'île Santa Rosa. Le rétablissement d'une seconde population sur les îles Santa Rosa et San Miguel pourrait accélérer la restauration des plantes et des arbres indigènes grâce au comportement des Aphelocoma, qui dispersent leurs réserves de graines dans de nombreuses caches.