"What do ducks eat for…

A duck riddle is:

Q: What do ducks eat for breakfast?
A: Quacker Oats.

“Quacker Oats” puns were cited in print in 1978 and 1982, and the full riddle has been cited in print since at least 1993. “What is a duck’s favorite snack?”/“Quackers” is another duck “quacker” riddle.

Wikipedia: Quaker Oats Company
The Quaker Oats Company, known as Quaker, is an American food conglomerate based in Chicago. It has been owned by PepsiCo since 2001.

History
Quaker Oats was founded in 1901 by the merger of four oat mills: ...

4 February 1978, Star-Gazette (Elmira, NY), “Postmaster banks on his collection,” pg. 10, col. 4:
A third bank, presumably dedicated to Donald Duck’s favorite breakfast food, is in the shape of an oatmeal container and carries the inscription “Quacker Oats.”

24 September 1982, Tampa (FL) Tribune, “Et Cetera,” Friday Extra, pg. 23, col. 3:
Quacker Oats—makes breakfast food for ducks.
Charlie Brignac, Tampa

Google Books
Kids’ Funniest Jokes
Edited by Sheila Anne Barry
New York, NY: Sterling Publishing Company, Inc.
1993
Pg. 68:
What do ducks eat for breakfast?
Quacker Oats.
MICHELLE QUA, 11

Google Books
Nutty Good Clean Jokes for Kids
By Bob Phillips
Eugene, OR: Harvest House Publishers
1995
Pg. 55:
Jeff: What do ducks eat for breakfast?
Joel: Who knows?
Jeff: Quacker Oats.

Google Groups: alt.humor.puns
A duck pun.
James D. Davis
3/28/95
(...)
Q. What do ducks eat for breakfast?
A. Quacker Oats.

26 November 1995, Newsday (Long Island, NY), Kidsday sec., pg. 1:
What do ducks eat for breakfast?
Quacker Oats.
Jodi Abramowitz, Commack

Twitter
Felicity Todd‏
@_Flik_
What do Ducks have for breakfast?? Quacker oats Ha!!
1:35 AM - 22 Apr 2009

Reddit—Jokes
What did the duck eat for breakfast?
submitted January 17, 2018 by maritimeskrew
Quacker Oats

“What do ducks eat for breakfast?”/“Quacker Oats.”