The historical value of the above picture is immense. Taken in 1886, it shows what is believed to be the first photographed instance of someone flipping the bird. It also documents what is believed to be one of the earliest birds flipped in America.
The finger-giver is Charles Radbourn, a pitcher for The Boston Beaneaters. He's giving the finger to the opposing team, the New York Giants. Charles is an interesting figure in his own right; he pitched a whopping 678 2/3 innings, and the leg cramp known as the Charley Horse may be named after him. He died, as most great baseball players do, after a long battle with syphilis.
The middle finger has a long and illustrious history. We think the Greeks invented it; the outstretched finger not only gives the appearance of an engorged penis resting atop a bed of enormous balls, it was also used by the Greeks as a gesture for anal sex.
That's the truth, but a better origin story for the middle finger as obscene gesture comes from the Battle of Agincourt in 1415. This story tells us that the French cut off the middle fingers of British archers, making it impossible for them to use their longbows. The Brits showed their middle fingers to the French in an act of defiance; the real story seems to be that Agincourt is where the peculiarly British gesture of 'flicking the V' originated. See, you needed the index and middle fingers to draw the longbow.
Historians believe that the middle finger gesture didn't come to the United States until the 1890s, making Radbourn a real outlier. The official histories have Italian immigrants bringing it over with them. I still haven't been able to figure out why it's called 'flipping the bird,' but I remain impressed that this gesture has maintained its basic, satisfying obscenity for thousands of years.