Chickens drops only 4 kind of seeds: Cantaloupe, Apple, Graph and Pear seeds.
To get a Mango seeds you must compound (seed + anything).
From that recipe you can get same or other kind of seed.
Even I compound 100x times, I have never seen a pumpkin seed.
I also think it will be active at the Halloween Event.
But when is the Halloween, I wonder... since my home land doesn't have Halloween.