Introduced social bees reduce nectar availability during the breeding season of the swift parrot (Lathamus discolor)

Abstract

Numerous pollinators are declining across the world. One of these, the swift parrot (Lathamus discolor) is a critically endangered Australian bird that feeds largely on the nectar and pollen of Eucalyptus trees. The Swift Parrot Recovery Plan includes competition for food from introduced social bees as a threatening process, although little evidence exists in support of this. Here, we present the strongest evidence yet to support this theory. We examined nectar standing crops in the species of trees that are important to swift parrots during their breeding season, Tasmanian blue gum (Eucalyptus globulus) and black gum (E. ovata). By comparing the amounts of nectar between flowers exposed to visitors and those bagged to exclude visitors throughout the day, we discovered that introduced honey bees (Apis mellifera) and bumble bees (Bombus terrestris) consumed most of the nectar and that exposed flowers often contained little nectar. Honey bees were the more common species, but bumble bees had greater per capita rates of nectar consumption. However, at low densities these bees had no effect on standing crops of nectar, and in such situations some nectar could be harvested by managed honey bees without reducing nectar availability for swift parrots. Although this study suggests that introduced social bees may pose a threat to swift parrots, further work is needed to determine whether our results are indicative of the impacts of bees across greater scales of time and space and whether these affect the reproductive success and conservation status of the swift parrot.