Here below is the full list of harmful plants to Cockatiels:

Aconite

Ghostweed

Peach (Leaves, twigs, seeds)

Agaric Mushroom

Glory Bean

Pear seeds

Alacia

Ground Cherry

Pencil Tree

Amaryllis bulbs 

Periwinkle

American Yew Apple (Seeds)

Philodendron

Apricot (Pits and Bark)

Henbane Fly

Pigweed

Autumn Crocus

Honeysuckle

Pine (Needles, twigs, sap)

Avocado

Horsetail

Pointsetta

Azalea Balsam

Hyacinth

Poison Ivy

Hydrangea

Poison Oak

Pokeweed

Baneberry root

Indian Licorice

Potato (eyes, new shoots)

Bird of Paradise

Indian Turnip

Pyracantha

Black Locust

Inkberry

Bleeding HeartIris

Ranunculus

Blue Flag

Red Maple

Bluebonnet

Jack-in-Pulpit

Rhubarb

Blue-green Algae

Jasmine

Rosary Peas

BoxwoodJava Bean

Bracken Fern

Jimsonweed

Saffron

Broomcorn Grass

Juniper

Sandbox Tree

Buckeye

Scarlet

Buttercup

Lantana

Skunk Cabbage

Laurel

Snowdrop

Lily of the Valley

Spindle Tree

Cabbage

Lima bean

Sweet Pea - seeds

Caffeine

Lobelia

Caladium

Locoweed Lords

Taro

Calla Lily

Lupines

Thornapple

Candelabra Tree

Tobacco

Cardinal Flower

Mandrake

Tomato All green parts

Castor Bean

Mango Tree - wood,leaves,rind-(fruit safe)Trumpet

Chalice Vine

Marijuana

Cherry (Leaves, twigs, seeds, pits)

Marsh Marigold

Virginia Bower Coral

Chestnut

Mayapple

Virginia Creeper

Chinaberry Tree

Meadow

Christmas Candle

Mescal Beans

Water Hemlock

Clematis

Mistletoe

Western Yew

Cowslip

Mock Orange

Wisteria

Croton

Monkshood 

Crown of Thorns

Morning Glory 

Mountain Laurel 

Mushrooms 

Daffodil

Daphne

Narcissus 

Datura

Nightshade 

Deadly Amanita

Nutmeg 

Death Camas

Dumb Cane

Oak (Foliage, acorns) 

Oleander 

Onion 

Eggplant  

Elderberry  

Elephants Ear  

English Ivy  

Ficus  

Firethorn  

Foxglove