a tendency for polyandry, where breeding groups consist of multiple males and a single female (though monogamous pairs were also observed). It was also shown that the sex ratio in the population is consistently biased towards males according to historical counts. A 2008 genetic study by the British biologists Tiawanna D. Taylor and David T. Parkin showed that the sex ratio was equal among echo parakeet chicks and embryos and that the male-biased sex ratio among adults is therefore not due to, for example, inbreeding. A preliminary 2009 genetic study by Taylor and Parkin showed that matings of "auxiliary males" with the female of a breeding pair do occur, and that the echo parakeet is therefore not strictly monogamous. Such a mating system is beneficial to the conservation of the species, as it increases genetic diversity, but it is unclear why such breeding groups form. Diet and feeding The echo parakeet mainly feeds on native Mauritian plants, though small amounts of introduced plants are also eaten, and eats parts such as fruits (53%), leaves (31%), flowers (12%), buds, young shoots, seeds, twigs, and bark or sap (4%). It feeds in the trees and rarely or never lands on the ground, and returns to favoured trees, which have been used for generations in some cases. According to one study, more than 25% of the plant species taken consisted of Calophyllum tacamahaca, Canarium paniculatum, Tabernaemontana mauritiana, Diospyros sp., Erythrospermum monticolum, Eugenia sp., Labourdonnaisia sp., Mimusops maxima, Mimusops petiolaris, Nuxia verticillata and Protium obtusifolium. Some species, such as Calophyllum parviflorum, are more important than others, while fruits of Syzygium contractum and Sideroxylon cinereum were often ignored. Echo parakeets never forage on the ground, in contrast to the rose-ringed parakeet, and may have been pushed into an arboreal niche because other parrots of Mauritius were already adapted for ground feeding. Species that are now rare may have been favoured in the past, such as Olax psittacorum, known as bois perroquets (parakeet trees), perhaps because the birds were fond of it. Parakeets must have had an impact on seed production of favoured plants in the past; some fruits have a very hard epicarp (the tough outer skin) resistant to parrots, which may have evolved for protection. Some species have a hard epicarp surrounded by a fleshy pericarp which is eaten by echo parakeets, after which they reject the former, which probably contributes to seed dispersal.