Winter use by takahe (Notornis mantelli of the summer-green fern (Hypolepis millefolium in relation to its annual cycle of carbohydrates and minerals

The winter diet of the takahe (Notornis mantelli), an endangered flightless gallinule, which inhabits the mountain grasslands and forests of the Fiordland region of southwestern New Zealand is described. A close correlation exists between their use of rhizomes of the summer-green fern (Hypolepis millefolium), a major item in the winter diet, and the peak in the rhizomes' annual cycle of carbohydrates and certain minerals. It is suggested that the high carbohydrate concentrations in the rhizomes are required by the birds to meet the metabolic requirements of thermoregulation in the subfreezing temperatures of mid-winter. The likely adverse effects of competition from introduced red deer (Cervus elaphus), both directly in limiting the extent to which the winter diet of takahe can be supplemented with other favoured plants, and indirectly in reducing availability of Hypolepis rhizomes by increasing the incidence of soil freezing through depleting subcanopy layers of the evergreen Nothofagus spp. forest, are also discussed.