Mistletoes are commonly seen growing on our wayside trees. Their fruits are small, oval, one-seeded and berry-like. The seeds are enclosed by a viscid layer of mucilage that pass out intact after being swallowed by sunbirds and flowerpeckers. Usually these sticky seeds need to be scraped off the bottoms of these birds when they defecate.
Angie Ng describes in detail how a male Scarlet-backed Flowerpecker (Dicaeum cruentatum) deposited a string of sticky seeds on her sui mei (Wrightia religiosa) plant thus:
“After comfortably positioning itself transversely across the branch, it turned its head… then it awkwardly stretched apart its legs, lowered its little body for a second or two and with a swagger, it moved a few steps to the left. With that quick swaggering action it wiped off a string of six gluey mistletoe seeds onto the branch of my sui mei.”
Other reports state that these birds deposit the seeds by wiping them off their beaks after pecking on the fruits.
In Australia the main dispersal agent of mistletoes is the Mistletoe Bird (Dicaeum hirundinaceum). This bird has a modified gut that allows the seeds to be excreted within 25 minutes of ingesting without any damage. The birds are said to perch along branches, rather than across them like most other birds, so that when the seed is excreted, it falls onto the branch upon which the bird is perched. The seeds are excreted in a stringy mass of three to five seeds.
Lim Kim Chuah has sourced a Tropical American reference that claims “A sticky layer of viscin surrounds each seed, and it is impregnated with toxic chemicals that offer some protection to the seed and at the same time help to speed passage through the gut… As long as the viscin layer is undamaged, the seeds remain viable and the bird is not poisoned.” I have not been able to locate any work of this nature on our tropical species of mistletoes nor any reference on their poisonous nature. However, there is reference to the toxicity of the American species, especially the leaves, stems and fruits.
Once stuck to the surface of a developing branch, the mistletoe seed will send out a small shoot with a bulbous tip. This tip expands to form a ‘holdfast’ that eventually enters the host’s internal tissues to reach the sapwood, from where moisture and minerals are extracted. As the mistletoe grows, the primary holdfast enlarges in size to send out runners that develop secondary holdfasts at intervals.
Image of sticky seed by Angie Ng, others by YC