Ghiandaia siberiana

Identificazione

The Siberian jay is the smallest of the western Palearctic corvids, weighing 75 to 90 g (2.6 to 3.2 oz) and measuring about 30 centimetres (12 inches) in length. The adult plumage is greyish brown, with a dark brown head, paler forehead and buff breast. The rump is yellowish and the chin and throat are grey. There is also rufous streaking on the outer feathers, and the bill and legs are black. Their overall colouration is fairly inconspicuous to visually conceal them from predators within their forest habitat. The plumage is also very soft and downy for insulation against extreme cold in winter. There is one moult per year, which lasts from mid-June to mid-September. The longest recorded lifetime is 13 years 4 months for a bird ringed in Finland, although the average lifespan has been reported as 7.1 years.<p>Siberian jays appear to be specially adapted to navigate in flight through dense forest despite being rather cumbersome flyers across open terrain. This may explain their vulnerability to predation by raptors outside forests.<p>The Siberian jay is mostly silent but can utter a loud screech resembling that of a buzzard (Buteo spp.). The song, which is performed by both sexes, mainly during the breeding season and heard only from a short distance, comprises a wide repertoire of sounds. These range from sequences of separate soft and harsh notes to bouts of whistling, creaking and trilling, sometimes incorporating mimicked song of other birds. Siberian jays also engage in nepotistic alarming calling, which may serve to warn conspecifics of an approaching predator. Warning calls have been shown experimentally to decrease reaction time in response in an approaching predator and improve survival within non-breeding individuals. However, females seem more able than males to differentiate between kin and non-kin. Alongside direct warning to family group members, nepotistic alarm calling by a breeding female may also function to divert predator attention away from her offspring.

Tassonomia

The Siberian jay was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Corvus infaustus. Linnaeus specified the location as "Europae alpinis sylvis" but the type location was restricted to Sweden by Ernst Hartert in 1903. The specific epithet infaustus is Latin meaning "unlucky" or "unfortunate" as Siberian jays were formerly considered a bad omen. The Siberian jay is now placed together with the Canada jay and the Sichuan jay in the genus Perisoreus that was introduced in 1831 by Charles Lucien Bonaparte.<p>Five subspecies are recognised:<p>The Siberian jay differs from the other two species in its group-living behaviours. Unlike the other two species, where group individuals unrelated to breeding parents may help provision the young, the group individuals accompanying a breeding pair of Siberian jays do not help raise the offspring. The lack of extra-parental care within the groups may be due to historic selection against cooperative breeding in the Siberian jay; probably as an anti-predator strategy to avoid predator attention.<p>With the colonisation of coniferous forests in Scandinavia just after the last ice age, the Siberian jay probably expanded its range from east to west in response to the newly formed suitable habitat resulting from climatic warming.

Nome scientifico

Perisoreus infaustus

Autorità

(Linnaeus, 1758)

Ordine

Passeriformi

Famiglia

Corvidi

Genere

Perisoreus

Areale e habitat

The Siberian jay is resident in the northern boreal spruce, pine, cedar and larch forests stretching from Scandinavia to northern Russia and Siberia. The species has an extensive range estimated at 19,300,000 square kilometres (7,500,000 square miles) and is native to Norway, Sweden, Finland, Russia, Mongolia, Kazakhstan and China. It is vagrant in Belarus, Estonia, Latvia, Poland, Slovakia and Ukraine. Despite being largely sedentary, some southward movement may occur in winter by individuals in the east part of the range.<p>This jay prefers dense, mature forest habitat with closed canopy within lowlands and foothills. Spruce forest is the preferred foraging and nesting habitat because the denser foliage of spruce than other local conifers better conceals the bird from the watchful eyes of predators. Indeed, high breeding success has been linked to high foliage density, since eggs and nestlings are better hidden from view and so less likely to attract predators. Additionally, the benefit of increased predator evasion through more hiding space would probably outweigh the cost of making predators more difficult to see by the jays within the dense foliage. The Siberian jay is notably selective in its choice of territory, with a typical territory comprising old dense spruce swamp with ample vegetation cover. Territories also tend to be structurally diverse, comprising variably aged scrub, groves and flood meadows; so that active territories may be regarded as an indicator of high ecological diversity within the forest.

Migrazione

Resident species

Comportamento

The species has a complex and unusual social structure. Siberian jays live in small flocks of 2–7 individuals, with the dominant breeding pair at the centre of the group; alongside retained multigenerational offspring and unrelated immigrants. Within a group, there is a dominance hierarchy; whereby males are dominant over females and breeders are dominant over non-breeders; with some male non-breeders being dominant over female breeders. Flock composition varies, with some comprising only family members, families associated with nonrelated immigrants, and others containing only nonrelated individuals. Immigrated unrelated individuals can be tolerated within the territory outside nesting areas.<p>Siberian jays are aggressive to non-related intruders on their territory. Two different aggressive responses from territory holders within feeding grounds have been observed: (1) the intruder is approached and forced away, (2) or is chased in flight; although the latter behaviour is more costly to the aggressor. Although breeders are considerably more aggressive toward immigrants than retained offspring, aggressive responses appear to be modified by social dominance within groups. For example, females have been found to receive significantly more aggression than males because males show more resistance on account of their higher social ranking, therefore resulting in a higher energy cost on the part of the aggressor. Siberian jays also appear to recognise their own young through associative learning as opposed to genetic cues, as shown by experiments in which Siberian jays did not differentiate between own and fostered offspring.<p>Siberian Jays are strictly monogamous, with an established pair staying together and holding the same territory for life. Mate guarding in both sexes has been observed, whereby males and females become increasingly aggressive toward same-sex conspecifics. This may prevent extra-pair mating opportunities for the partner and thereby preserve inclusive fitness for both pair members. The size of the territory is 1 to 4 km2 (0.39 to 1.54 sq mi), which is slightly enlarged in autumn and winter. Although territories are firmly established, the jays can move to a neighbouring site if this is a better quality habitat where breeding success will be higher. Widowed individuals have been observed to establish new pair bonds.<p>The nest is situated in a pine or spruce tree 4–6 m (13–20 ft) above ground within dense foliage for optimal concealment from nest predators. The nest comprises a loose cup of dry twigs broken off trees by the jays and is thickly lined with beard lichen moss, down feathers, cobwebs, reindeer fur and wasp nest fragments; which serves as necessary insulation against the extreme winter cold. Nest material is hoarded in winter long before building takes place. Nest building proper begins in late March and lasts for about three weeks. Both partners collect nest material; but only the female builds the nest.<p>The eggs are pale green, blue or grey and spotted with grey and brown. The average size is 31.6 mm × 22.9 mm (1.24 in × 0.90 in). They are laid 31 March – 22 April, with a variable clutch size of 1–5 and an incubation period of about 19 days. The eggs are incubated entirely by the female, whilst the male provides all the food for the brooding female and the chicks. The Siberian jay is single brooded and does not relay in a breeding season even after nest failure, but will wait until next year. A new nest is also built for every breeding attempt. Newly hatched young are almost bare and are closely covered by the female. Juveniles look similar to adults, but have paler heads.<p>A high proportion of the diet of the young comprises insect larvae, which is collected by the male and stored in his oesophagus until he returns to the nest to regurgitate it to feed to the young. In the first week of brooding, the male provides the female and young with all their food, and the female takes an increasing share of the work thereafter.<p>The young fledge in mid-May to early June and leave the nest around this time, 18–24 days after the first chick has hatched; although they usually hide within tree branches on the territory until they are able to properly fly. Parents continue to feed the young for about three weeks after fledging, with all of them remaining in a family group throughout summer, autumn and winter. Most mature young disperse from the natal territory in their first summer 4–8 weeks after fledging to join new groups which are usually situated more than four territories away from the natal one. These early dispersers are generally subordinates which have been out-competed and displaced by one or a few socially dominant offspring that are retained. If offspring have not yet dispersed by the time they are eight weeks old, they usually remain in the natal territory at least through the first winter. The retained, socially dominant offspring remaining on the natal territory may wait for up to five years before dispersing, whenever a suitable breeding opportunity arises. Retained offspring can also claim their natal territory, but this happens only in the rare event that both parents disappear from the territory within a relatively short time. Offspring may inherit the new territory to which they disperse and at which they breed. However, if there are no vacant territories to occupy, juveniles can join a different family group to gain feeding advantages and may even attempt to establish their own territory at the age of 2–3 years.<p>Retention of some offspring in the natal territory after fledging is probably best explained by the nepotism (favoritism granted by individuals to related individuals of the same species) which parents show towards them. Parents provide retained offspring with reliable access to resources and antipredator protection, thereby imparting to them a survival advantage (in turn with an inclusive fitness advantage to the parents). Although delayed dispersal of offspring may be explained by “queueing” for available high-quality territories for the offspring to occupy, the influence of nepotism has been shown experimentally. In removal experiments in which a father alpha male in a group was replaced with a new male unrelated to the offspring, the offspring were more likely to disperse prematurely; probably because the new male did not impart the same nepotistic advantages to the now unrelated young. Although retained offspring may incur an initial cost of postponing breeding, this may be offset by enhanced breeding success later in life as a result of resource advantages gained from nepotistic parents; and late dispersers have been found to have higher lifetime reproductive success than early dispersers.<p>At least one offspring usually remains with the parents after successful reproduction and regularly accompanies the pair for at least a year before dispersing; although some retained offspring may delay their dispersal for up to five years. These are the dominant offspring which out-compete and expel their subordinate siblings. The dispersed subordinate individuals (which disperse in their first summer) settle as non-breeding immigrants in other existing flocks and tend to disperse much further than their dominant siblings, which more often move straight into a breeding position in a new territory. Unusually for a gregarious corvid species, members in a group of Siberian jays do not help the parents raise younger siblings in future cohorts (i.e., there is no cooperative breeding) and so offspring retention is not explained by cooperative breeding. Groups also stay together outside the breeding season, which is another unusual phenomenon.<p>The Siberian Jay is omnivorous, feeding mainly on berries, seeds, insects and spiders. Flocks will also feed on large carcasses killed by mammalian predators such as wolves and wolverines. Other occasional food items include eggs of small birds, tit nestlings, snails, slugs, small mammals and lizards. In autumn and winter, berries (especially bilberries and cowberries) are typically collected and stored behind loose bark or in hanging beard lichen and between forked twigs. Siberian jays distribute many different hidden food caches over a large area and are therefore known as scatter hoarders. However, unlike in other corvids, this hidden food stash is not purposefully shared with siblings to increase inclusive fitness but is consumed by the hoarder or a pilferer for selfish use. These food caches are critical to this species' winter survival since foraging time is greatly restricted by the few hours of daylight. To securely store food, Siberian jays have developed special saliva glands which they use to form sticky food clumps which they can adhere to beard moss or holes in tree bark where they are readily accessible throughout the winter. Because of their reliance on food stores during winter, these birds are territorial and hold food reserves scattered around the territory, especially near to the nest.<p>When foraging, Siberian jays often stay within the closed-canopy forest to avoid visual detection by predators, although they may occasionally forage for insects in wet, tussock-strewn open areas. They forage in flocks comprising 3–5 individuals within their large territory. Especially in winter, Siberian jays will also venture into increasingly open areas to take and store small rodents for food when the latter are abundant.

Conservazione

Tendenza della popolazione

declining