Ghiandaia messicana

Identificazione

The Mexican jay is a medium-large (~120 g) passerine similar in size to most other jays, with a blue head, blue-gray mantle, blue wings and tail, and pale gray breast and underparts. The sexes are morphologically similar, and juveniles differ only in having less blue coloration and, in some populations, a pink/pale (instead of black) bill that progressively becomes more black with age (Brown and Horvath 1989). Some field guides misreport this color as yellow because the pale bill becomes yellow in museum study skins. The iris is brown and legs are black. It is most readily distinguished from the similar Woodhouse's scrub-jay by the plain (unstreaked) throat and breast, and the mantle contrasting less with the head and wings. Its range somewhat overlaps with the Woodhouse's scrub-jay, but, where they co-occur, the two species seem to show ecological and morphological character displacement (Curry et al. 2002).

Tassonomia

A recent decision by the American Ornithologists' Union Check-list Committee elevated some populations of the Mexican jay to a separate species, called the Transvolcanic jay (A. ultramarina), based on diagnosable phenotypic differences in plumage and morphology, millions of years of genetic divergence and no evidence for interbreeding with Mexican jays. The Transvolcanic jay inhabits montane forest in the Transvolcanic Belt of central Mexico. Populations to the north retained the common name Mexican jay, but the Latin name changed to A. wollweberi. This was because the type specimen was a Transvolcanic jay, meaning that this species had precedent for the original Latin name A. ultramarina.<p>Thus, as of this decision, there are now five described subspecies of the Mexican jay that are divided into three divergent groups (see below). Marked differences in size, color, vocalizations, and genetics have led some authors to consider at least two of these groups as separate species (Eastern and Western; Navarro-Sigüenza and Peterson 2004). The three groups inhabit three distinct mountainous regions in northern and central Mexico. Genetic breaks in mitochondrial and microsatellite DNA occur abruptly between the groups, indicating some barriers to genetic exchange (McCormack et al. 2008). Size variation among the groups does not always follow Bergmann's rule, with more southerly populations in the Sierra Madre Oriental being larger than populations to the north. Mexican jays do not seem to follow Gloger's rule either, as populations in arid habitat in southwestern Texas are very blue. On the other hand, Mexican jays in Arizona - also arid habitat - have a washed-out appearance, in accordance with Gloger's rule.<p>Western groupSierra Madre Occidental in northern [Jalisco] north to central Arizona and southwestern New Mexico. Southern and eastern limits in Jalisco deserve further study. Juveniles have a pink/pale base to the otherwise black bill for up to two years. Eggs are pale green-blue and unspeckled, unlike Eastern group where speckled eggs are common.<p>Eastern groupSierra Madre Oriental in Nuevo León and western Tamaulipas north to Texas (Chisos Mountains). Juveniles have an all-black exterior to the bill after fledging, but roof of inner upper mandible can remain partially white for up to two years. Reports of less social behavior compared to other groups are over-stated and credible accounts of cooperative breeding (Ligon and Husar 1974) and large flock sizes (Bhagabati and Horvath 2006) exist. Plain, speckled, and even white eggs have been observed in a single study area (McCormack and Berg 2010).<p>Central Plateau groupCentral Plateau in Queretaro, Guanajuato, San Luis Potosi, and eastern Jalisco. Similar to Eastern group but larger in most features. Distinguishable in morphology and plumage in ~80% of specimens. There is an area of apparent hybridization in San Luis Potosi that deserves study.

Nome scientifico

Aphelocoma wollweberi

Autorità

Kaup, 1855

Ordine

Passeriformi

Famiglia

Corvidi

Genere

Aphelocoma

Areale e habitat

It is native to the Sierra Madre Oriental, Sierra Madre Occidental, and Central Plateau of Mexico as well as eastern Arizona, western New Mexico and western Texas in the United States. Its preferred habitat is montane pine-oak forest.

Migrazione

Resident species

Comportamento

In the winter, the Mexican jay's diet consists mainly of acorns and pine nuts, which are stored in the autumn. However, they are omnivorous in all seasons and their diet includes a wide variety of plant and animal matter, including invertebrates, small amphibians and reptiles, and birds' eggs and nestlings (McCormack and Brown 2008).<p>It has a cooperative breeding system similar to that of the related Florida scrub-jay, with several birds helping at a nest; these "helpers" are usually immature offspring of the dominant pair from the previous 1–2 years, but also include apparently unrelated flock members.

Conservazione

Tendenza della popolazione

declining