Certain bird species, such as robins, cedar waxwings and thrushes, are the most prone to drunken antics since they eat more berries than other species like warblers and flycatchers that primarily eat insects. And as they prepare to fly south for winter, they may overindulge in an attempt to store up fat for the journey, Dodder said. The birds go from bush to bush trying to find more berries, their balance getting progressively worse.