Impact on Birds
The behaviour of at least three species of native bird is affected by this reduction in honeydew, but it is unknown if this affects the survival or reproductive success of these birds.
Wasps reduce the standing crop of honeydew by more than 90% for 5 months of the year and so compete with native species (such as birds and invertebrates), which also consume honeydew.
There are several species of native birds, including kaka (Nestor merdionalis) and honeyeaters (tui, Prosthemadera novaeseelandiae, and bellbirds, Anthornis melanura) that use honeydew as an easily obtained carbohydrate resource.
 Tui |
 Kaka |
 BellBird |
The behaviour of kaka changed when it became unprofitable for them to forage on honeydew. Tui and bellbirds also changed their behaviour when honeydew became scarce. Tui spent nearly all their foraging time on honeydew (usually more than 80%) except on days when there was little honeydew available. They reduced their feeding on honeydew or left beech forest reserves when the standing crop of honeydew fell below a threshold level (2500 J m-2). Bellbirds remained in the forest on days when the standing crop of honeydew was low, but reduced the time spent feeding on honeydew (non-breeding season only); or reduced their non-foraging activities such as singing, flying, social interactions and grooming (during both the breeding and non-breeding season).
In addition to honeydew, wasps are likely to compete with a range of native birds for invertebrate food. Standard five-minute bird counts were carried out along a 2-km altitudinal transect for 10 years before the arrival of common wasps (but not before the arrival of German wasps). These counts were repeated in 1996, once common wasps had been established for about 10 years. Preliminary results indicated that the abundance of many bird species, including insectivorous ones, had declined. This is only circumstantial evidence that population changes were due to wasps - indeed it is probable that other variables will also have influenced bird abundance (e.g., predation, climate). Further work is required to determine the effect wasps have on birds at the population level.
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Wasp numbers |
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Impact on invertebrates |