Green’s Dictionary of Slang

birdseed n.

1. as food.

(a) chocolates or other sweets [play on the attraction of SE birdseed to a bird n.1 (1b)].

[UK](con. WWI) Fraser & Gibbons Soldier and Sailor Words 23: Birdseed: Chocolate, sweets: — something nice for the ‘Bird’.
[Aus]D. Niland Call Me When the Cross Turns Over (1958) 122: Wait till I eat my birdseed.

(b) any breakfast cereal seen as resembling birdseed.

[US]H.W. Bentley ‘Linguistic Concoctions of the Soda Jerker’ in AS XI:1 42: BIRDSEED. Cereal.
[US]Wentworth & Flexner DAS.
[US] in J.P. Spradley You Owe Yourself a Drunk (1988) 53: Breakfast: Birdseed mush.

(c) (US) any food.

[US]L. Dills CB Slanguage.

2. (US) rubbish, nonsense; thus as adj.

[US]R. Chandler ‘Guns At Cyrano’s’ in Red Wind (1946) 211: I’ve been getting threats. Maybe it’s a lot of birdseed.
[US]B. Schulberg On the Waterfront (1964) 113: O.K. O.K. Without the bird seed.
[US]J. Brosnan Long Season 75: Smith might have been excused for cursing his luck. If he cursed. Which he doesn’t; although he said ‘bird seed’ once, I recall.
[US] in P.R. Runkel Law Unto Themselves 238: Or, if I chose to be a lawyer, I wouldn’t need to be a mere ambulance chaser, shyster, or birdseed wiseguy.

3. (US teen) an unpopular individual.

Pittsburgh Post-Gaz. (PA) 15 Jan. 8/6: Birdseed — Another word for ‘Drip’.