The Papuan Mountain-pigeon, like the other 300 species in the pigeon and dove family, is an ecosystem engineer, unique parent and clever opportunist.
By Rex Graham
New research about this species and others in Papua New Guinea, Kenya, Jamaica, Europe and the U.S. paint a compelling picture of pigeons at odds with urban prejudices and “rats with wings” pejoratives.
Monogamous pairs of Papuan Mountain-pigeons make flimsy nests, but both male and female parents produce highly nutritious milk for the 1 or 2 squabs in each one. They are vegetarian fruit-eaters that sow the seeds of ecologically important Tropical trees, regenerating rainforests during bathroom breaks.
Many non-birders call pigeons nuisances or “rats with wings.” However, they are loved as pets by many others. Domestic pigeons are bred as racers, show birds and even for meat production in parts of Asia. There are roughly 1,100 kinds of domesticated pigeons, from the Antwerp to the Zitterhal, engineered by breeders over centuries.
Darwin crossbred breeds of fancy pigeons to understand genetic variation. Today, Islamic State militants are using homing pigeons to deliver messages, according to Jordanian border officials. The scientific name of the pigeon and dove family, Columbidae, is derived from the French word for dovecote, which are stylish structures to house pigeons and doves that first appeared in Medieval Europe.
The ability of wild pigeons to act as ecosystem engineers can be seen in the seed-dispersing behavior of species such as the Papuan Mountain-pigeon. An important season for the pigeon unfolds April through June. That’s a lean time of year before most fruit trees on Papua New Guinea begin producing. This is when a common rainforest canopy tree in the Magnolia family called Elmerrillia tsiampaca explodes with high-energy fruit that the pigeon exploits. The pigeon swallows the fruit whole.
Researchers saw 26 species of fruit-eating birds eat the fruit of 28 Elmerrillia trees growing in a 4-square-kilometer area of Papua New Guinea with a 27-year-old moratorium on hunting and tree cutting. “The only bird species that consistently appeared in large flocks was Gymnophaps albertisii [Papuan Mountain-pigeon],” scientists from the University of Alaska and the Carnegie Museum of Natural History wrote in the journal Biotropica.
Most Topical trees rely on fruit-eating birds and mammals for seed dispersal, especially birds-of-paradise, doves and pigeons. Not much is known about when the fruit-eating pigeons defecate fruit seeds. Most birds poop when they take off and while roosting, and some pigeons lighten their load while flying.
Pigeon ‘crop milk’
Papuan Mountain-pigeons are dark gray with a silvery green sheen and bright red skin around their eyes. They tend to nest in the hills and mountains of New Guinea, but seek Elmerrillia and other fruiting trees opportunistically.
Even though some seasonal fruit may be low in nutritional value, parents compensate by providing highly nutritious “crop milk” to their squabs for roughly 28 days. The milk is actually made up of cells sloughed from the interior surface of the crop. This cheesy fluid is a mixture of protein, fats, minerals and even growth factors and antibodies. The milk is easily absorbed by the immature digestive system of squabs.
The more spectacular bird-of-paradise species on Papua New Guinea use their larger bills to grab insects to feed to their young. Insects are also highly nutritious. However, their larger bills and food-handling abilities also enable them to remove fruit from the protective structures of fruiting trees.
Papua New Guinea pigeon fruit feasts
The Papuan Mountain-pigeon and many other pigeons avoid such hard-to-remove fruits and focus instead on unprotected ones. Individual Great Cuckoo-doves go one step further on Papua New Guinea. They sometimes guard fruit-laden shrub-like trees called Schefflera chaetorrhachis and drive away all comers “by clapping its wings loudly while attempting to alight on the visitor,” Thane Pratt, a then-research fellow of the Smithsonian Institution, reported in The Condor.
Fruit-eating birds and fruit-producing trees have co-evolved over millions of years. The trees in Tropical rainforests, especially those in New Guinea and the Amazon region of South America, have a faster tempo of evolution than trees in cooler parts of the earth. For example, the Schefflera chaetorrhachis plant is a member of the Araliaceae, a family of 254 flowering trees, shrubs and perennials. The Araliaceae is closely related to 2 other large families of plants.
Pigeons and doves and other Tropical bird species have evolved to exploit fruit opportunities. For example, there are about 45 species of pigeons and doves in New Guinea, but only about 2 in Canada.
The diversity among pigeons and doves is as great as any family of birds. Indeed, there is an 88-fold difference in body mass between the 2.4 kg (5.3 lb) Victoria Crowned-pigeon and the 28 g (0.06 lb) Diamond Dove. The extinct Dodo weighed 21 kg (47 lb).
Infrequent fruit fliers
The key role that pigeons and doves play in reforestation is seen in Kenya where agriculture has eliminated or fragmented many forests and grasslands. Forest-dependent pigeons and doves in the Kakamega Forest in western Kenya will fly more than 1,000 km (621 mi) to feed on isolated fig trees that have been planted at corn, bean, tea and sugar-cane farms. Farmers often plant trees for shade, fruit and timber. Fig trees often sprout on their own. Kenyan farmers told researchers that 70 bird species, including 30 forest-dependent species, ate the figs in their trees between September and December. This is a time when fruit availability is low in Kakamega Forest.
In Jamaica, White-crowned and Plain Pigeons seek the fruit of a tall spindly palm tree called Thrinax parvifora that grows in the threatened Portland Ridge dry forest. The palm produces large amounts of fruit during the November-to-March dry season. This is a time when Jamaica’s other fruit trees are not producing and outside the Jamaica hunting season. The tree story is actually more complex, because the 2 species of pigeons prefer to temporarily roost in the safety of mangrove forests adjacent to the fruiting palm trees. Like the fig-eating pigeons of Kenya, Jamaica’s pigeons may be vitally important in regeneration of the island’s dry forests.
“Frugivorous birds are significant seed dispersers and can play a prominent role in transporting seeds into disturbed areas and setting the stage for the regeneration of these systems,” researchers from the University of Cape Town, the National Museums of Kenya and Johannes Gutenberg Universitat said in the Journal of Tropical Ecology.
Conservation drum roll
The same seed-dispersal services are provided, but mostly unseen by the Papuan Mountain-pigeons. Pigeons and doves, Cassowaries and megapodes are hunted, possibly without knowledge that they play an essential role in the regeneration of key rainforest trees such as Elmerrillia tsiampaca.
Incidentally, large Elmerrillia tsiampaca trees are felled and the trunk is cut into sections to make slit-gong drums. These traditional instruments are used to communicate between hamlets in New Guinea via a code system of beats to announce ceremonies, other events and important information, according to Porer Nombo and James Leach, authors of Reite Plants: An Ethnobotanical Study in Tok Pisin and English.
Obviously, by protecting pigeons that disseminate tree seeds, conservationists argue that we also protect forest and ecosystem diversity. This applies not only to pigeons and Elmerrillia, fig and palm trees, but also to other families of birds and thousands of other fruit-producing trees, shrubs and other flowering plants.
I believe that the more we watch, photograph, protect and study Papuan Mountain-pigeons, the better for the species, all birds, all wildlife and all humans.