Island Scrub-Jay

Identificazione

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.

Tassonomia

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.

Nome scientifico

Aphelocoma insularis

Autorità

Henshaw, 1886

Ordine

Passeriformi

Famiglia

Corvidi

Genere

Aphelocoma

Areale e 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.

Conservazione

La ghiandaia insulare è classificata come vulnerabile nella Lista rossa IUCN perché il suo areale ridotto la rende potenzialmente esposta a eventi catastrofici, come una malattia o un grande incendio che distrugga l'habitat. Nel 2008 e nel 2009 la popolazione era stimata tra 1.700 e 2.300 individui, rendendola uno dei passeriformi più rari degli Stati Uniti. L'intero areale è attualmente protetto all'interno del parco nazionale delle Channel Islands e la specie non è a rischio imminente di estinzione. Tuttavia, l'insediamento del virus del Nilo occidentale (WNV) nella California meridionale nel 2003 potrebbe costituire una minaccia qualora il virus raggiungesse l'isola di Santa Cruz dalla terraferma. I corvidi sono particolarmente vulnerabili al WNV. Inoltre, la crescente frequenza degli incendi nella California meridionale potrebbe preannunciare un incendio catastrofico sull'isola. La copertura arbustiva è aumentata dopo la rimozione delle pecore (1980–1991) e dei maiali inselvatichiti (2005–2007) da Santa Cruz, con un possibile aumento del rischio di incendio. Il pascolo eccessivo da parte di ungulati non autoctoni potrebbe aver causato la scomparsa della ghiandaia insulare da Santa Rosa. Ricostituire una seconda popolazione sulle isole Santa Rosa e San Miguel potrebbe accelerare il recupero delle piante e degli alberi autoctoni grazie all'abitudine delle specie di Aphelocoma di accumulare semi in numerosi nascondigli sparsi.