Yes, in rotation — spinach is nutritious but contains oxalates that bind calcium, which matters especially for egg-laying hens.
Keeper's noteFine occasionally — the trick is rotation. Alternate with kale, bok choy, and dandelion greens, which deliver the leafy-green nutrition without the oxalate load.
What to watch out for with spinach
Spinach is rich in vitamins A and K, but its oxalic acid binds dietary calcium and can reduce absorption. For most pet parrots an occasional leaf is harmless; for laying hens and birds with calcium issues, frequent spinach is a real concern.
How to serve it
Offer a leaf or two, once or twice a week, washed well.
Rotate with low-oxalate greens like kale and bok choy.
Limit further for laying hens or birds on calcium supplements.