Is it berries or insects?
For birds, it's both actually. As a food source, that is.
I just finished reading a very interesting book called "Bringing Nature Home" by Douglas Tallamy who is a Professor of Entomology.
So many people focus on planting berry producing plants "for the birds". Actually, birds eat a lot of INSECTS. And when it comes time to feeding their young, they are feeding them insects (and spiders).
So the premise of this book is to plant a wide variety of native plants to attract a variety of native insects for the birds to feast on. Berries are good too ... it is just that they are not the primary source of food for birds over the course of a year.
He explained that non-native plants are not so attractive to native insects so they will not be drawn to eat them. Many insects are specialists (eat only certain things), although there are generalists (think of Japanese Beetles which eat so many different plants). The specialists are geared to only eat certain plants, so planting crepe myrtle and nandina and other non-natives will not attract them. While you might think that is good (fewer pests), if you are trying to attract birds, you have lessened your chances.
Non-native plants therefore reduce the diversity of the whole food chain by reducing the source of food for the first guy in the chain: the insect herbivores.
He also pointed out that a damaging by-product of importing non-natives is the parade of pests (Japanese beetle, hemlock adelgid) and diseases (the fungus that killed the chestnuts, the Sudden Oak Death pathogen that affects more than just oaks) that have unknowingly come over with the plants.
It's a very good book, has lots of facts, references and tables to back up what he's saying. He and his students are currently performing even more studies on non-native plants and the ability/willingness of native insects to eat them.
By the way, the number one plant family in terms of nourishing a wide variety of insect herbivores in the US? The oaks (Quercus family). They support 517 different insect herbivores. By contrast, the invasive Melaleuca tree (a problem in Florida), supports just 8 insect herbivores in this country. But in it's native habitat, it supports 409. After 150 years in this country, our insects are by and large not interested in it. This plant has taken over thousands of acres from native vegetation and has obviously contributed very little to the biodiversity of the area in return. In fact, I'd say it has lessened the biodiversity.
Here is a link that might be useful: Link to book on amazon