The reference of course is to the legend that ostriches when frightened bury their head in the sand. It is pure legend and a canard on a very distinguished bird.
Mr. Bashman's comment:
[W]hile we are debunking canards (which, by contrast, are birds that can fly), allow me to question the use of the singular "head" in the following sentence from Judge Posner's opinion: "The reference of course is to the legend that ostriches when frightened bury their head in the sand."
On June 26, Mr. Bashman posted an email from Judge Posner:
Dear Prof. Bashman, to say "ostriches hide their heads in the sand" would imply that each ostrich had more than one head.
In this case, my own intuitions are on Mr. Bashman's side. Under the theory that each ostrich has an individual and unique head, different from those of other ostriches, the phrase "Ostriches hide their heads in the sand" seems entirely appropriate, whereas "Ostriches hide their head in the sand" raises distracting questions. What is this head? Perhaps it's the shrunken head of a lion, handed down from their heroic ancestors; or perhaps this phrase refers to their elected or hereditary leader, the Head Ostrich, who must be protected in a siliceous bunker.
But as you know, we don't privilege any one person's linguistic whims, not even mine. Instead, let's look at the precedents.
What we'll learn is that the distributed meaning of the plural "their heads" — one per individual — is sanctioned by the habitual usage of many esteemed writers. The singular version "their head" has two interpretations, one that is semantically singular (meaning "their leader" or the like), and one where again there are many heads, distributed one per individual. In the distributed meaning, where each individual has a unique and individual head, the plural heads is substantially more common than the singular head; use of the singular is roughly equally divided between its two meanings.
As a point for further investigation, I note that some examples of "their heads" seem to work equally well with the singular "head" substituted, but in other cases, this substitution is more problematic or even grammatically impossible.
I interpret the evidence to mean that ostriches may be said to hide either "their heads" or "their head", but "their heads" is more widely used and is clearer.
Let's start with a couple of examples from the writings of Thomas Jefferson. This example is from his autobiography:
They took all the arms, discharged the prisoners, and such of the garrison as were not killed in the first moment of fury, carried the Governor and Lt. Governor to the Place de Grève (the place of public execution) cut off their heads, and sent them thro' the city in triumph to the Palais royal.
And from a letter to Philip Mazzei, Monticello, Apr. 24, 1796:
It would give you a fever were I to name to you the apostates who have gone over to these heresies, men who were Samsons in the field & Solomons in the council, but who have had their heads shorn by the harlot England.
All examples in Jefferson's works of "their head" are cases where the head is indeed singular:
Possessing the confidence and intimacy of the leading patriots, & more than all of the Marquis Fayette, their head and Atlas, who had no secrets from me, I learnt with correctness the views & proceedings of that party; while my intercourse with the diplomatic missionaries of Europe at Paris, all of them with the court, and eager in prying into it's councils and proceedings, gave me a knolege of these also.
(Note again how lucky it is that Lynne Truss was not alive in the 18th century.)
More recent politicians seem to agree with Jefferson's pattern of usage, for example John F. Kennedy's Remarks at the American Farm Bureau Federation National Convention, 12/12/1956:
But my intention has been not to deprecate your importance as farmers but to appreciate your importance as citizens — citizens who are interested in more than farm issues, affected by more than farm profits and convinced by more than expensive farm promises — citizens who vote not as a bloc but as thoughtful individuals, not with their pocketbooks but with their heads and their hearts.
Turning to judicial language, and searching U.S. Supreme Court opinions, we find examples of "their heads" in a large number of opinions. Thus in Lee v. Weisman, Justice Scalia wrote:
Most recently, President Bush, continuing the tradition established by President Washington, asked those attending his inauguration to bow their heads, and made a prayer his first official act as President.
There's certainly no reason to believe that all (or even any) of the guests at President Bush's inauguration were polycephalous.
The King James version of the bible also has a large number of instances, for example:
Genesis: And they bowed down their heads, and made obeisance.
Kings: So they girded sackcloth on their loins, and put ropes on their heads, and came to the king of Israel, and said, Thy servant Ben-hadad saith, I pray thee, let me live.
Isaiah: He is gone up to Bajith, and to Dibon, the high places, to weep: Moab shall howl over Nebo, and over Medeba: on all their heads shall be baldness, and every beard cut off.
Lamentations: The elders of the daughter of Zion sit upon the ground, and keep silence: they have cast up dust upon their heads; they have girded themselves with sackcloth: the virgins of Jerusalem hang down their heads to the ground.
Matthew: And they that passed by reviled him, wagging their heads, And saying, Thou that destroyest the temple, and buildest it in three days, save thyself.
Turning to a professional writer, in the works of Mark Twain we find 102 instances of "their heads" in 26 works. There are 17 instances in Innocents Abroad, starting with these three:
There is not a wheelbarrow in the land — they carry everything on their heads, or on donkeys, or in a wicker-bodied cart, whose wheels are solid blocks of wood and whose axles turn with the wheel.
Here are five thousand Jews in blue gabardines, sashes about their waists, slippers upon their feet, little skullcaps upon the backs of their heads, hair combed down on the forehead, and cut straight across the middle of it from side to side — the selfsame fashion their Tangier ancestors have worn for I don't know how many bewildering centuries.
Nine-tenths of them wear nothing on their heads but a filmy sort of veil, which falls down their backs like a white mist.
And finally, in the Oxford English Dictionary, we find that the example sentences include 873 instances of "their heads". The first three, in alphabetical order of head word, are:
1703 MOXON Mech. Exer. 4 The About Sledge is the biggest Hammer of all, and..they hold the farther end of the Handle in both their Hands, and swinging the Sledge above their Heads, they..let fall as heavy a Blow as they can upon the Work.
1668 EVELYN Mem. (1857) III. 209 Those, therefore, who..accustom to wash their heads, instead of powdering, would doubtless find the benefit of it.
1880 R. BROUGHTON Sec. Th. I. I. xii. 206 Most of them are standing acrobatically on their heads.
The phrase "their head" occurs 173 times. I haven't checked them all, but the first six are evenly divided:
1912 S. A. LAFONE QUEVEDO Great Chanca Confederacy iii. 9 The Abipón nation..claimed nobility for their head men.
1658 J. ROBINSON Eudoxa v. 142 They dare not seem to worship the bread, by kneeling before it; yet will they reverence it with their head bare; which is no gesture befitting familiar accumbency, and fraternal communion.
1696 PHILLIPS, Acephalists, a sort of Hereticks, whose first founder is unknown; also Vagabond Clergymen, having neither King nor Bishop for their Head.
1671 MILTON Samson 192 In prosperous days They swarm, but in adverse withdraw their head.
1607 TOPSELL Four-footed Beasts (1673) 7 Yet is their head and tip of their tail yellow, so that the Martins before mentioned, seem to be affianced to these.
1635 E. PAGITT Christianogr. App. 18 They had submitted to the Pope of Rome, and agnized him their Head.
If this is the overall percentage, then 873/((873+(173/2))) = 91% of distributed meanings for "their head(s)" use the plural.
But in fact, the percentage in many sources seems to be higher than this. Searching Shakespeare at Literature Online, I find that he used "their heads" 85 times in 52 works (though many are duplicates), for example this passage from King Henry the Fifth:
Constable of France: If the English had any apprehension, they would run away.
Duke of Orleans: That they lack; for if their heads had any intellectual armour, they could never wear such heavy head-pieces.
Rambures: That island of England breeds very valiant creatures; their mastiffs are of unmatchable courage.
Duke of Orleans: Foolish curs, that run winking into the mouth of a Russian bear and have their heads crushed like rotten apples.
All of these examples seem to be the distributed reading, in which each of a number of individuals has a unique head. In contrast, Shakespeare used "their head" only once, in Romeo and Juliet, where the Prince says to Montague:
Seale vp the mouth of outrage for a while,
Till we can cleare these ambiguities,
And know their spring, their head, their true descent,
And then will I be generall of your woes,
And lead you euen to death?
And in this case, there is indeed just one head for all the ambiguities.