I've noticed that not all Blue Jays appear to have a crest. I know that at certain times the crest may be down and less visible, but I notice in most images of juvenile Blue Jays they don't appear to have a crest. I'm curious if the crest forms later on or if Blue Jays are born with a crest?
It seems like blue jays form their crests as adolescents, after their first post-juvenal molt before their first winter.
The top of the head of a bird is called the pileum, with the very top called the crown. As far as I can tell there is no particular distinction between "crown feathers" and "crest feathers", rather the presence of a "crest" is because the crown feathers are longer and stand out from the rest of the head.
Blue jays born with their natal down will molt within the first two weeks to get their juvenal plumage. Dwight (1900) is quite descriptive of the juvenal plumage, but does not mention any crest, only that the pileum is a "flax-flower blue separated from the blue-tinged white forehead and white superciliary line by a narrow black line".
Crown feathers are longer after the first prebasic molt, forming a crest
The first prebasic molt refers to the (partial) loss of the juvenal feathers. Bancroft & Woolfenden (1982), examining young birds captured in Florida, found 8% in the first prebasic molt of the crown/head area in the first half of August, 89% in the second half of August, 78% in the first half of September, and 54% in the second half of September. So it seems most birds are getting through this molt by mid-September.
Dwight (1900) writes that "young birds become practically indistinguishable from adults" around this time and that compared to the juvenal plumage, bold mine:
the blue of head, back and wing coverts now distinctly barred with black and much brighter, and the crest feathers longer
I haven't found mention of crest feathers rather than crown feathers in descriptions of the juvenal plumage, with one exception I'll get to below, so I'll tentatively say that a blue jay forms its crest as part of this first prebasic molt in which the first winter plumage is evident.
Blue jay crest in behavior
The crest position of blue jays differs across behavioral contexts, ranging from erect to pressed against the head. Conant (1972) mentions the crest specifically as part of several different postures/behavioral contexts (parentheses are my paraphrasing):
In neutral posture, the crest and plumage are relaxed
While a Blue Jay is asleep, the crest is relaxed and the body plumage is usually fluffed
(while "investigating"): the crest is fluffed
(during "mild aggression"): the crest is fluffed
(during "intense aggression"): crest is fully erect or ruffled
(during avoidance/submissive posturing): the crest is relaxed or sleeked
(during "courtship feeding solicitation"): the crest is relaxed or sleeked
And finally, the one section that makes me a little uncertain, Conant (1972) describes the "nestling alarm freeze" behavior of nestlings when their nest is disturbed or they hear an alarm call:
The crest and body plumage are sleeked, wings closely appressed to the body, and tail closed.
My guess is that she's using "crest" here to mean the crown feathers, and just referring to them as the "crest" since that's the word used when describing the other behaviors that primarily involve birds with adult plumage. However, it raises a bit of doubt and perhaps the other descriptions missed that there is also a crest in the juvenal plumage, hidden in the alarm/sleeked position under the watchful presence of
predators scientists. Dr. Sheila Conant is now Professor Emerita at the University of Hawai'i at Manoa; one could ask her, but I think it has been awhile since she studied blue jays closely.
Bancroft, G. T., & Woolfenden, G. E. (1982). The molt of scrub jays and blue jays in Florida. Ornithological Monographs, (29), iii-51.
Conant, S. (1972). Visual and acoustic communication in the blue jay, Cyanocitta cristata (Aves, Corvidae). The University of Oklahoma.
Dwight, J. (1900). The sequence of plumages and moults of the passerine birds of New York. New York Academy of Sciences.