Dragon Broodmother [Alara Reborn] Serpent The "Sheep Tokens" cannot be

The "Sheep Tokens" cannot be used as a Tribute for a Tribute Summon (or Set)

Set: Return to Ravnica Type: Land — Swamp Forest Rarity: Rare ({T}: Add {B} or {G}

Set: Urza's Saga Type: Creature — Shapeshifter Rarity: Rare Cost: {3}{U}{U} {U}: Untap Morphling

Set: Sword & Shield: Chilling Reign Type: Grass Rarity: Ultra Rare Retreat cost: Colorless

and their effects

Dragon Broodmother [Alara Reborn] Serpent The "Sheep Tokens" cannot beSet: Alara Reborn Type: Creature Dragon Rarity: Mythic Cost: {2}{R}{R}{R}{G} Flying At the beginning of each upkeep, create a 1 1 red and green Dragon creature token with flying and devour 2. (As the token enters the battlefield, you may sacrifice any number of creatures. It enters the battlefield with twice that many +1 +1 counters on it.)