Here's the scoop on iceberg:
This food is low in Sodium, and very low in Saturated Fat and Cholesterol. It is also a good source of Thiamin, Vitamin B6, Iron and Potassium, and a very good source of Dietary Fiber, Vitamin A, Vitamin C, Vitamin K, Folate and Manganese.
This food contains known anti-inflammatory nutrients, including Vitamin C, Vitamin K, Folate and Beta Carotene.
You can feed iceberg lettuce if you have it on hand, but darker leaf lettuce and darker more colorful veggies have more nutrients obviously than the lighter green iceberg (so iceberg won't harm them-just has less nutritional value than darker greens). Keep in mind when choosing treats, not all things that have a lower nutritional value or no nutritional value are harmful-just as humans consume "empty" calories. Offering treats in limited quantities in conjunction with a good nutritional feed should be perfect for happy healthy ducks.