“If it looks like a duck, walks like a duck, swims like a duck and quacks like a duck, it’s a duck”

Emil Mazey (1913-1983), the secretary-treasurer of the United Automobile Workers for 33 years, said at a labor meeting in 1946:
 
“I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I’m certainly going to assume that he IS a duck.”
 
Walter Reuther (1907-1970), the UAW president from 1946-1970, used the “like a duck” saying, as did labor leader James B. Carey (1911-1971). “Like a duck” has extended beyond its Communist-finding origins to become a part of general legal reasoning.
 
Wikipedia cites a similar phrase from Indiana poet James Whitcomb Riley, but no such phrase exists in any of his work.
 
 
Wikipedia: Duck test
The duck test is a humorous term for a form of inductive reasoning. This is its usual expression:
 
“If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.”
 
The test implies that a person can identify an unknown subject by observing that subject’s habitual characteristics. It is sometimes used to counter abstruse arguments that something is not what it appears to be.
 
History
Indiana poet James Whitcomb Riley (1849–1916) may have coined the phrase when he wrote “when I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.”
 
The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala during the Cold War in 1950, who used the phrase when he accused the Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:
 
“Suppose you see a bird walking around in a farm yard. This bird has no label that says ‘duck’. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he’s wearing a label or not.”
 
Later references to the duck test include Cardinal Richard Cushing’s, who used the phrase in 1964 in reference to Fidel Castro.
 
Variants include Douglas Adams’ Dirk Gently’s Holistic Detective Agency:
 
If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family anatidae on our hands.
         
Walter P. Reuther Library 
UAW Secretary Treasurer’s Office: Emil Mazey Records
Posted December 4th, 2008 by drice
Date:
1942-1955, bulk 1947-1952
A long-time member of the Socialist Party, Emil Mazey began his union career with the United Auto Workers at the Briggs Manufacturing Co. He would play leading roles in wage negotiations, organizing locals, and gaining equality for African-American workers. He served as secretary-treasurer of the UAW for over 30 years in addition to stints on the executive boards of both the UAW and the Congress of Industrial Organizations, and directorships of several UAW departments. His records reflect primarily his long tenure as secretary-treasurer and include appeal board cases, regional and local files, and officer, department, and council files.
 
Google News Archive
29 September 1946, Milwaukee (WI) Sentinel, pg. 1, col. 1:
Communist Expose
The Case of the Duck

By JOHN SENTINEL
WHAT is the Sentinel trying to do—promote a witch-hunting expedition?
 
Not at all. The Sentinel is encouraging a duck hunting expedition, and thereby hangs a tale.
 
Emil Mazey, a Detroit labor leader, recently rose at a labor meeting to object to a proposed resolution as Communist. The man who introduced the resolution challenged Mazey to prove he was a Communist. Mazey retorted:
 
“I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I’m certainly going to assumer that he IS a duck.”
 
Since that classic pronouncement, the duck is reported to have become a symbol in Detroit labor circles, and “Quack! Quack!” the literal rallying cry of the anti-Communist forces there whenever something that smells Communistic raises its head in union meeeting halls.
 
Newspapers.com
30 September 1946, Sheboygan (WI) Press, “Mary Brendzej Denies She Is Communist In Interview,” pg. 8, cols. 1-2:
Meanwhile, John O. McGlashan, Baraboo, special UIU organizer, declared: “we’re standing on the statements that we made Saturday to The Press:
   
“When I see a bird that quacks like a duck, has feathers and webbed feet and associates with ducks, I’m certainly going to assume that he is a duck.”
   
Newspapers.com
12 October 1946, The Financial Post (Toronto, ON), “How to Recognize a Red” by Ronald Williams, pg. 15, col. 1:
“How do you know I’m a Communist?” the mover of the resolution asked.
 
“Now, fellas,” said Emil (Mazey—ed.), turning to the meeting. “When you see a bird that walks like a duck, quacks like a duck and looks like a duck and hangs around other ducks, you say, ‘There is a duck!’”
 
Google Books
Newspapers.com [Nashville (TN) Banner, March 18, 1947, pg. 4, col. 1.]
Plain Talk
Volume 1
1946
Pg. 45:
Proof Positive
Emil Mazey, the Detroit labor leader, when challenged to prove that a certain individual in his union was a Communist said: “I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks, I’m certainly going to assume that he is a duck.”
 
Google Books
Traitors in the Pulpit and Treason Toward God
By Kenneth Goff
Englewood, CO: Kenneth Goff
1946
Pg. 53:
Mazey retorted: “I can’t prove you are a Communist, but when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet, and associates with ducks — I’m certainly going to assume that he is a duck.”
     
Google Books
Labor Relations Program : hearings before the Committee on Labor and Public Welfare, United States Senate, Eightieth Congress, first session, on S. 55 and S.J. Res. 22, and all other bills and resolutions referred to the committee having the object of reducing industrial strife in the United States. January 23,28,29,30,31, February 4,5,6, and 7, 1947.
By United States. Congress. Senate. Committee on Labor and Public Welfare.
Washington, DC: U.S. Government Printing Office
1947
Pg. 1684:
These are strong indicators—and, when all these indicators point the same way, we are inclined to adopt the statement of a prominent labor leader when in angry debate he was asked to prove a man a Communist:
 
“I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks — I’m certainly going to assume that he is a duck.”
   
Newspapers.com
12 March 1947, Wisconsin State Journal (Madison, WI), “Nailing the Liars” (editorial), pg. 6, col. 1:
Another witness before the house labor committee probably pointed a better way just last week. The record reveals his testimony like this:
 
“The head of a union local once accused a member of being a Communist and the latter challenged him to prove it. That union head replied, ‘When I see a bird that looks like a duck, walks like a duck and quacks like a duck—I know h is a duck. You’re a Communist.’ And that member he had labeled a Communist sat down.”
 
Newspapers.com
27 March 1947, Moline (IL) Dispatch, ‘Treason Can Be Outlawed” (editorial), pg. 6, col. 1:
You can’t swear that any certain man on the federal payroll is a Communist. But, as one witness at Washington stated, when you see a fowl that walks like a duck, looks like a duck and quacks like a duck, you know it is a duck even if it doesn’t admit it.
 
3 September 1948, New York (NY) Times, pg. 1:
“A door-opener for the Communist party is worse than a member of the Communist party.  When someone walks like a duck, swims like a duck, and quacks like a duck, he’s a duck.”
(James B. Carey, secretary-treasurer of the Congress of Industrial Organizations—ed.)
 
Time magazine
National Affairs: He’s a Duck
Monday, Sept. 13, 1948
(...)
James B. Carey, secretary-treasurer of the C.I.O. and onetime U.E. president, was much better informed. Carey had been ousted from the U.E. presidency by the men behind Fitzgerald—Secretary-Treasurer Julius Emspak and Organizing Director James Matles. He named them, along with Fitzgerald and the whole U.E. executive board, as men who “sacrifice the interests of the U.E. to promote the foreign policy of the Soviet Union.”
 
Did Carey think that the U.E. was a front for the Communist Party? “On many questions, yes sir,” he answered.
 
Did he believe that any of the U.E. bosses he named were members of the C.P.?
 
“I don’t think that makes any difference,” said Carey. “When someone walks like a duck, swims like a duck, and quacks like a duck, he’s a duck.”
     
Google Books
Un-American Activities in the State of Washington;
The work of the Canwell Committee

By Vern Countryman
Ithaca, NY: Cornell University Press
1951
Pg. 108:
“But he did their business and other work, and as Mr. Walter Winchell says, ‘When you see a bird that looks like a duck, walks like a duck, and quacks like a duck, it’s a duck.’”
 
Google Books
No Wonder We Are Losing
By Robert Morris
New York, NY: The Bookmailer
1958
Pg. 7:
One noted lawyer put it this way: “If a creature walks like a duck, quacks like a duck, waddles like a duck and flies like a duck, I can assume he is a duck.”
 
Google Books
Men in Battle:
A story of Americans in Spain

By Alvah Cecil Bessie
New York, NY: Charles Scribner’s Sons
1975 (1939 original, but this passage with “John Birch Society” could not have been written in 1939)
Pg. 367:
For it is really very easy to convict people who have committed no crime if you make use of that classic diagnosis-by- parallel beloved of the John Birch Society and American Legion brass hats, and both the government and the many witch-hunting committees throughout the land have used it regularly: “If it looks like a duck, walks like a duck, swims like a duck and quacks like a duck, it must be a duck.”
 
New York (NY) Times
EMIL MAZEY, LEADER OF AUTO WORKERS, IS DEAD OF CANCER
AP (The New York Times); Obituary
October 11, 1983, Tuesday
Late City Final Edition, Section D, Page 31, Column 1, 222 words
[ DISPLAYING ABSTRACT ]
Emil Mazey, who was secretary-treasurer of the United Automobile Workers for 33 years, has died of cancer, the union said today. He was 70 years old.