The Kawainui Marsh trail is actually part of the flood control efforts to protect Kailua. The "trail" is a walking path the follows the flood control wall. Don't expect to see a lot of wildlife near the trail, most of the birds probably prefer to inhabit the more central parts of the marsh that are away from humans and the trail.
Views of the Koolaus and all the greenery are the best part of the walk.
I highly recommended visiting the heiau which is part of the marsh and is being lovingly restored by local residents. It is quite an impressive monument to Hawaiian antiquities and the auwai (waterways) that feed the taro and banana patches can give you a real sense of how Hawaiians ingeniously conducted their farming.
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