In The Grapes of Wrath:
I learned to write nice as hell. Birds an' stuff like that, too; not just word writin'. My ol' man'll be sore when he sees me whip out a bird in one stroke. (Tom Joad)
What is this 'bird' he is talking about?
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In advanced penmanship/calligraphy, skill is exemplified (among other things) by good control of the width of the line (by pressure on downward nib strokes), the spacing between lines, the precision of curves, the angle of your strokes, and especially by being able to repeat these exactly.
A common way to demonstrate one's skill was with swashes (easier) and flourishes (more difficult) that required all these elements executed to perfection. One common flourish was a bird. There was even a book (Spencerian Bird) devoted to this flourish alone. Other flourishes included deer, feathers, frames, etc.
Since America's golden age of ornamental penmanship was approximately 1850 to 1925, a character in Grapes of Wrath (set in the '30's) would be familiar with this style of ornamentation.
Examples (two antique and one modern):