The large Common Birdwing (Troides helena), with its distinctive black and yellow wings, is among the most beautiful butterflies in Hong Kong. Its beauty, unfortunately, makes it sought by greedy profiteers who turn it into specimens for sale. Over catching has caused a decline in its population. The government therefore placed it, together with one other butterfly species, under legal protection.
Common Birdwing (Troides helena) belongs to the Genus Troides of the Papilionidae (Swallowtail) Family. Swallowtails are often large, and Common Birdwing is particularly outstanding. Its wingspan is up to 14 cm and the butterfly looks like a small bird from a distance, as indicated in its English common name Birdwing.
Locally there is another close relative of Common Birdwing, the Golden Birdwing (Troides aeacus), which is even rarer. The two look much alike, with red head and thorax, black forewings, and golden yellow hindwings with black veins. What distinguishes the two is that the male Golden Birdwing has fewer black marks on the hindwings, while the female Golden Birdwing has many more black marks on the hindwings. It is indeed hard for novice butterfly watchers to observe this slight difference between the two species!
Pretty Sign of Warning
Like many other living things in nature, bright colours are used not to attract people but to warn predators! In Common Birdwing, both caterpillars and adults are toxic. The caterpillars feed on leaves and young shoots of India Birthwort (Aristolochia tagala) which contain carcinogenic Aristolochic acids. The poisons have no effect on the caterpillars, but instead are accumulated inside the body until the caterpillars turn into adults.
Apart from the poisons in their body, the caterpillars have other self-protection mechanisms. Firstly, they are covered with fleshy dorso-lateral processes that make them appear inedible. Secondly, when threaten they will protrude from their prothoracic segment an osmeteria – a fleshy forked structure like a snake's tongue with a foul odour to deter predators. Upon transforming into pupae, they look like dry leaves that are hard to detect. If disturbed, the pupae can emit a sound to scare away potential predators. These all increase the chances for survival of the species!
With all the tricks and defence, Common Birdwing (Troides helena) still has to face another threat – this time by us human. Profiteers catch the beautiful butterflies in large numbers and make them into specimens for sale, forcing the species into near extinction. On the other hand, the caterpillars consume more food plants than other butterfly species, and only feed on India Birthwort (Aristolochia tagala). The plant has a narrow distribution in Hong Kong. Adding to the pressure is city development: habitats for the butterflies and its food plants are become rarer and rarer. And the survival of the species is at risk.
The silver lining is that the situation of Common Birdwing (Troides helena) is noted. The species and others in the same genus have already been listed in CITES Appendix II and trade in them is prohibited by law. Locally, Common Birdwing is protected by the Wild Animals Protection Ordinance, which prohibits the catching and trading of the species. The Protection of Endangered Species of Animals and Plants Ordinance also restricts the export of its specimens. The Forestry Regulations protects the food plant India Birthwort (Aristolochia tagala). It is hoped that these measures can effectively protect the pretty yet vulnerable butterfly.