Birdseed or Bird Seed (children’s trail mix)

Birdseed (also written as “bird seed”) is a mixture of seeds used to feed birds. By World War I, “birdseed” was used as slang (especially military canteen slang) for cereal that resembles the real “bird” article.
 
“Birdseed” meaning a trail mix (also called “gorp”) for children—containing ingredients such as raisins, peanuts and M&Ms candies—is cited in print from at least 1971.
   
   
Cooks.com
BIRD SEED (Trail Mix) 
4 c. Cheerios
2 c. M&M’S®
2 c. raisins
2 c. nuts
Mix well. Place in zipper bags for each hiker.
   
Cooks.com
CHILDREN’S BIRD SEED  
2 c. Sugar Pops cereal
1/2 c. raisins
1/2 c. peanuts
1/2 c. M & M’s candies
Combine ingredients in large bowl and serve as a snack. Makes 3 1/2 cups. Watch carefully, M & M’s go first.
   
Merriam-Webster Dictionary
birdseed
Main Entry: bird·seed
Pronunciation: \ˈbərd-ˌsēd\
Function: noun
Date: 1840
: a mixture of seeds (as of hemp, millet, and sunflowers) used for feeding caged and wild birds
   
(Historical Dictionary of American Slang)
birdseed n. breakfast cereal that resembles birdseed.
1919 Witt Riding to War 158: Submarine chicken. Birdseed. Corned Willy. Slum. Goldfish.
1930 American Speech VI 203: Bird-seed: dry breakfast cereals.
1936 American Speech (Feb.) 42: Bird Seed. Cereal.
1968 Spradley Drunk 31: Had bowl of “birdseed” cracked wheat mush, powdered milk, two slices of bread/oleo, ersatz coffee.
 
16 August 1971, Chicago (IL) Daily Herald, “Eating Is Part Of Camp Fun” by Joann Van Wye, pg. 3, col. 4:
Birdseed, ants on a log and somores are favorites with the girls. Birdseed consists of coconut, M & M’s, raisins, marshmallows and peanuts mixed together. It comes in handy for girls who get the “munchies” on long hikes.
 
Google Books
Cooking the Dutch Oven Way
By Woody Woodruff, Ellen Anderson and Jane Woodruff
Guilford, CT: Globe Pequot
2000
Pg. 111:
“BIRD SEED”
Here’s a simple between-meal snack recipe:
Just mix equal quantities of M&Ms, peanuts, and raisins.
     
Fredericksburg (VA) Free Lance-Star
Kids contest a piece of cake
Dessert Competition: Show Your Sweets
By COLLETTE CAPRARA
Date published: 1/13/2005
(...)
Past prize-winning children’s entries have included a brightly colored “Sunshine Cake,” decorated with mandarin oranges and pineapple, and a gorp-type creation called “Birdseed,” with peanuts, cashews, raisins and M&M candies.
 
20 December 2006, Milwaukee (WI) Journal Sentinel:
Veronica Pendell, Racine, sent her recipe for a snack mix called “bird seed,” which was requested by Marilyn Krueger, South Milwaukee. It includes nuts, M&M’s, raisins, pretzel and sesame sticks, and yogurt-covered raisins and peanuts. She wrote: “Most of these ingredients are available in a variety of weights and sizes. Don’t worry if you can’t find the exact amounts given. A little more or less of one or two…”