Foraging preferences of Swift Parrots on Tasmanian Blue-gum: tree size, flowering frequency and flowering intensity

Abstract

The Swift Parrot (Lathamus discolor), is a migratory species that forages primarily on Blue-gum (Eucalyptus globulus) during its Tasmanian breeding season. We investigated the foraging preferences of Swift Parrots for Blue-gum trees of different sizes and the effect of tree size on flowering frequency and flowering intensity over three seasons (1998, 1999, 2001). Swift Parrots showed a clear preference for larger Blue-gum trees: Blue-gum trees in which Swift Parrots foraged were ∼40% larger than surrounding (non-forage) trees, while the size-class distribution of forage trees was significantly skewed towards larger tree-size compared with surrounding non-forage trees. The mean flowering intensity of forage trees was also significantly greater than the mean flowering intensity of non-forage trees. Both flowering frequency and flowering intensity increased with tree size, although there was a trend for both flowering frequency and intensity to decline in the largest tree size-classes. Past clearing has resulted in the loss of over 50% of the original (pre-European) grassy Blue-gum forest in Tasmania. Remnant grassy Blue-gum forest as well as artificially planted Blue-gum trees typically occur in small patches of <1 ha, including many solitary and scattered trees in pasture, parkland and gardens. These isolated patches of Blue-gum frequently include trees of large stature, making them an important food resource for the endangered Swift Parrot.