In places around the edge of Bird Island’s Sooty Tern colony, a filamentous and leafless plant, vary
ing from light green through to orange, can be found. This sometimes completely smothers other vegetation, including beach crest bushes like Velutye (Scaevola sericea), Bwamatlo (Suriana maritima) and Bwatabak (Tournefortia argentea), even killing them in the process. The leafless plant is Lalyann san fan, or “Vine without end” (Cassytha filiformis). It is parasitic on other plants but can extend from them to carpet nearby areas of sand, with entwining stems several metres long. It has small white flowers and produces seeds that can be dispersed by birds, sea and waves, and even by strong winds.
In 1958 two British scientists, Viscount Ridley and Sir Richard Percy, were sent to Seychelles to investigate reports that Sooty Tern eggs were being harvested in excessive numbers. During their studies they noted that this plant was hazardous to Sooty Terns, especially on Bird Island. This was due to the birds becoming entangled in the vine and, being unable to escape, they died of heat stress or starvation. This still happens and every year we record fatalities due to entanglement in the vine. Island staff try to reduce the risk by cutting the plants, rolling up the mats of stems and leaving them to dry. This is effective but labour intensive and its variety of dispersal methods leads to the plant colonising new areas unpredictably.
Experience has shown, however, that Sooty Terns can nest quite happily beneath bushes covered in the vine, entering and leaving via pathways they have made through the tangles of stems. The main problem arises when the birds are disturbed unexpectedly, and the main cause of this is people approaching the colony. During disturbances Sooty Terns try to move away from the source of potential danger. Attempts to do so, by forcing their way through the stems by routes other than those with which they are familiar, risks them becoming entangled. When this happens they struggle, which can make the entanglement more severe.
The cutting back of the vine at the beginning of each Sooty Tern breeding season certainly helps to resolve the problem of entrapment, but avoidance of disturbance by visitors or island workers in parts of the colony affected by the plant is also essential to protect the birds from this unpleasant death.
The vine is common in the tropics and in many places is called “Love Vine”.
This stems from the widespread belief that the plant has medicinal and aphrodisiac properties – some Sooty Terns would doubt the latter!