The blue jay (Cyanocitta cristata) is a passerine bird in the family Corvidae, native to North America. It is resident through most of eastern and central United States, although western populations may be migratory. Resident populations are also found in Newfoundland, Canada, while breeding populations can be found in southern Canada. It breeds in both deciduous and coniferous forests, and is common near and in residential areas. It is predominantly blue with a white chest and underparts, and a blue crest. It has a black, U-shaped collar around its neck and a black border behind the crest. Sexes are similar in size and plumage, and plumage does not vary throughout the year. Four subspecies of the blue jay are recognized.
The blue jay mainly feeds on nuts and seeds such as acorns, soft fruits, arthropods, and occasionally small vertebrates. It typically gleans food from trees, shrubs, and the ground, though it sometimes hawks insects from the air. Like squirrels, blue jays are known to hide nuts for later consumption.  It builds an open cup nest in the branches of a tree, which both sexes participate in constructing. The clutch can contain two to seven eggs, which are blueish or light brown with brown spots. Young are altricial, and are brooded by the female for 8–12 days after hatching. They may remain with their parents for one to two months.
The blue jay was first described as Pica glandaria cærulea cristata in English naturalist Mark Catesby's 1731 publication of Natural History of Carolina, Florida, and the Bahamas.  It was later described as Corvus cristatus in Carl Linnaeus' 1758 edition of Systema Naturae.  In the 19th century, the jay was described by French ornithologist Charles Lucien Bonaparte in 1838 as Cyanocorax cristatus in A geographical and comparative list of the birds of Europe and North America,  and given its modern scientific name Cyanocitta cristata by Hugh Edwin Strickland in 1845.  The genus name Cyanocitta derives from the Greek words 'kyaneos' (blue) and the 'kitta' and 'kissa' (chattering bird, jay), and the term 'blue chatterer' refers to the bright blue plumage of the head, nape, back, and tail of the bird. The specific name cristata (crested, tufted) derives from Latin referring to the prominent blue crest of the jay. 
The blue jay measures 22–30 cm (9–12 in) from bill to tail and weighs 70–100 g (2.5–3.5 oz), with a wingspan of 34–43 cm (13–17 in).   Jays from Connecticut averaged 92.4 g (3.26 oz) in mass, while jays from southern Florida averaged 73.7 g (2.60 oz).   There is a pronounced crest on the head, a crown of feathers, which may be raised or lowered according to the bird's mood. When excited or aggressive, the crest will be fully raised. When frightened, the crest bristles outwards, brushlike. When the bird is feeding among other jays or resting, the crest is flattened on the head. 
Its plumage is lavender-blue to mid-blue in the crest, back, wings, and tail, and its face is white. The underside is off-white and the neck is collared with black which extends to the sides of the head. The wing primaries and tail are strongly barred with black, sky-blue and white. The bill, legs, and eyes are all black. Males and females are almost identical, but the male is slightly larger. 
As with most other blue-hued birds, the blue jay's colouration is not derived from pigments but is the result of light interference due to the internal structure of the feathers;  if a blue feather is crushed, the blue disappears because the structure is destroyed.  This is referred to as structural colouration.
Distribution and habitat
The blue jay occurs from southern Canada through the eastern and central United States south to Florida and northeastern Texas. The western edge of the range stops where the arid pine forest and scrub habitat of the closely related Steller's jay (C. stelleri) begins. Recently, the range of the blue jay has extended northwestwards so that it is now a rare but regularly seen winter visitor along the northern US and southern Canadian Pacific Coast.  As the two species' ranges now overlap, C. cristata may sometimes hybridize with Steller's jay.  The increase in trees throughout the Great Plains during the past century due to fire suppression and tree planting facilitated the western range expansion of the blue jay   as well as range expansions of many other species of birds.   
The northernmost subspecies C. c. bromia is migratory, subject to necessity. It may withdraw several hundred kilometres south in the northernmost parts of its range. Thousands of blue jays have been observed to migrate in flocks along the Great Lakes and Atlantic coasts. It migrates during the daytime, in loose flocks of 5 to 250 birds. Much about their migratory behavior remains a mystery. Some are present throughout winter in all parts of their range. Young jays may be more likely to migrate than adults, but many adults also migrate. Some individual jays migrate south one year, stay north the next winter, and then migrate south again the next year. To date, no one has concretely worked out why they migrate when they do. Likely, it is related to weather conditions and how abundant the winter food sources are, which can determine whether other northern birds will move south. 
The blue jay occupies a variety of habitats within its large range, from the pine woods of Florida to the spruce - fir forests of northern Ontario. It is less abundant in denser forests, preferring mixed woodlands with oaks and beeches.  It has expertly adapted to human activity, occurring in parks and residential areas, and can adapt to wholesale deforestation with relative ease if human activity creates other means for the jays to get by. 
Four subspecies are generally accepted, though the variation within this species is rather subtle and essentially clinal. No firm boundaries can be drawn between the inland subspecies. The ranges of the coastal races are better delimited.
- Cyanocitta cristata bromia – Northern blue jay
- Canada and northern United States. The largest subspecies, with fairly dull plumage. Blue is rather pale.
- Cyanocitta cristata cristata – Coastal blue jay
- Cyanocitta cristata cyanotephra – Interior blue jay
- Inland USA, intergrading with C. c. bromia to the north. Mid-sized, quite dark blue on mantle contrasting cleanly with very white underside.
- Cyanocitta cristata semplei – Florida blue jay
- Southern Florida. The smallest subspecies, much like C. c. bromia in colour.
The blue jay is a noisy, bold and aggressive passerine. It is a moderately slow flier (roughly 32–40 km/h (20–25 mph)) when unprovoked.  It flies with body and tail held level, with slow wing beats. Due to its slow flying speeds, this species makes easy prey for hawks and owls when flying in open areas. Virtually all the raptorial birds sympatric in distribution with the blue jay may prey upon it, especially swift bird-hunting specialists such as the Accipiter hawks. Diverse predators may prey on jay eggs and young up to their fledging stage, including tree squirrels, snakes, cats, crows, raccoons, opossums, other jays and possibly many of the same birds of prey who attack adults. 
The blue jay can be beneficial to other bird species, as it may chase predatory birds, such as hawks and owls, and will scream if it sees a predator within its territory. It has also been known to sound an alarm call when hawks or other dangers are near, and smaller birds often recognize this call and hide themselves away accordingly. It may occasionally impersonate the calls of raptors, especially those of the red-tailed and red-shouldered hawks, possibly to test if a hawk is in the vicinity, though also possibly to scare off other birds that may compete for food sources.  It may also be aggressive towards humans who come close to its nest, and if an owl roosts near the nest during the daytime the blue jay mobs it until it takes a new roost. However, blue jays have also been known to attack or kill other smaller birds and sleeping, foliage-roosting bat species such as Lasiurus borealis.  Jays are very territorial birds, and they will chase others from a feeder for an easier meal. Additionally, the blue jay may raid other birds' nests, stealing eggs, chicks, and nests. However, this may not be as common as is typically thought, as only 1% of food matter in one study was bird material.  Despite this, other passerines may still mob jays who come within their breeding territories.
Blue jays, like other corvids, are highly curious and are considered intelligent birds. Young individuals playfully snatch brightly coloured or reflective objects, such as bottle caps or pieces of aluminum foil, and carry them around until they lose interest.  While not confirmed to have engaged in tool use in the wild, blue jays in captivity have been observed using strips of newspaper as tools to obtain food,   while captive fledglings have been observed attempting to open the doors of their cages. 
Blue jays have strong black bills which they use for cracking nuts, usually while holding them with their feet, and for eating corn, grains and seeds. Its food is sought both on the ground and in trees and includes virtually all known types of plant and animal sources, such as acorns and beech mast, weed seeds, grain, fruits and other berries, peanuts, bread, meat, small invertebrates of many types, scraps in town parks, bird-table food and rarely eggs and nestlings.  Blue jays will sometimes cache food, though to what extent differs widely among individuals.  Although seemingly contentious in their general behavior, Blue jays are frequently subservient to other medium-sized birds who visit bird-feeders. In Florida, Blue jays were dominated at feeders by Eastern gray squirrels, Florida scrub-jays, common grackles and red-headed woodpeckers, all of which were occasionally observed to aggressively prevent the jays from feeding. 
The mating season begins in mid-March, peaks in mid-April to May, and extends into July. Any suitable tree or large bush may be used for nesting, though an evergreen is preferred. The nest is preferentially built at a height in the trees of 3 to 10 m (9.8 to 32.8 ft). It is cup-shaped and composed of twigs, small roots, bark strips, moss, other plant material, cloth, paper, and feathers, with occasional mud added to the cup.
Blue jays are not very picky about nesting locations. If no better place is available – e.g. in a heavily deforested area – they will even use places like the large mailboxes typical of the rural United States.  They also appropriate nests of other mid-sized songbirds as long as these are placed in suitable spots; American robin nests are commonly used by blue jays, for example.
Blue jays typically form monogamous pair bonds for life. Both sexes build the nest and rear the young, though only the female broods them. The male feeds the female while she is brooding the eggs. There are usually between 3 and 6 (averaging 4 or 5) eggs laid and incubated over 16–18 days. The young fledge usually between 17–21 days after hatching. 
After the juveniles fledge, the family travels and forages together until early fall, when the young birds disperse to avoid competition for food during the winter. Sexual maturity is reached after one year of age. Blue jays have been recorded to live for more than 26 years in captivity and one wild jay was found to have been around 17 and a half years old.  A more common lifespan for wild birds that survive to adulthood is around 7 years. Beyond predation and the occasional collision with man-made objects, a common cause of mortality in recent decades has been the West Nile Virus, which corvids as a whole seem especially susceptible to. However, despite several major local declines, overall blue jays have not seemed to have been depleted by the disease. 
Blue jays can make a large variety of sounds, and individuals may vary perceptibly in their calling style. Like other corvids, they may learn to mimic human speech. Blue jays can also copy the cries of local hawks so well that it is sometimes difficult to tell which it is. Their voice is typical of most jays in being varied, but the most commonly recognized sound is the alarm call, which is a loud, almost gull -like scream. There is also a high-pitched jayer-jayer call that increases in speed as the bird becomes more agitated. This particular call can be easily confused with the chickadee's song because of the slow starting chick-ah-dee-ee. Blue jays will use these calls to band together to mob potential predators such as hawks and drive them away from the jays' nests.
Blue jays also have quiet, almost subliminal calls which they use among themselves in proximity. One of the most distinctive calls of this type is often referred to as the "rusty pump" owing to its squeaky resemblance to the sound of an old hand-operated water pump. The blue jay (and other corvids) are distinct from most other songbirds for using their call as a song.
Cultural depiction and interpretation
In old African American folklore of the southern United States, the blue jay was held to be a servant of the Devil, and "was not encountered on a Friday as he was fetching sticks down to Hell; furthermore, he was so happy and chirpy on a Saturday as he was relieved to return from Hell".