In 1928 or so, the coastal towns and farmlands of Hawai'i were bereft of native bird species. Non-native species brought to the islands for a number of reasons had carried in their blood the parasite that caused avian malaria. Mosquitos transferred the disease to the native birds, and they were largely extirpated from the coastal lowlands. Most of the natives only survive above 4,000 feet where it is too cold for the mosquitos to thrive.
It wasn't that there were no birds at all in the lowlands. The Common Myna, introduced in the 1850s to control pests in the sugar cane fields, was...uh...common. There were also Spotted Doves, House Finches, and European Sparrows. But these weren't enough for some of the residents. In the 1930s, a bird club called Hui Manu devoted itself to introducing colorful and interesting birds to the islands. The effort was well-meaning, but misguided as one could imagine. Some of their introductions spread well beyond the coastal forests and spread into the higher forests where the native birds struggled to hang on. The new competition didn't help matters.
In any case, a number of the introduced birds were naturalized, and have spread widely. These are the birds most likely to be seen by tourists who stay in the coastal cities. And some of them are very colorful, literally a delight to the eye. Among them most be counted the Red-crested Cardinals (Paroaria coronata). The red head and crest seem to throw off the color balance on my camera sometimes. I saw them all over Oahu, but got the closest when we visited Waimea Valley on the North Shore, and at Kualoa Regional Park west of Kailua. Kualoa is one of the most beautiful shorelines on Oahu.
The Red-crested Cardinals rarely venture above 1,200 feet, so they probably can't be regarded as one of the "bad" invasive species that threaten the livelihood of the native species of Hawai'i. They certainly add a splash of color to the urban and developed areas of the islands.