The Okinawa rail (Gallirallus okinawae) is a flightless bird endemic to the Japanese island of Okinawa. As its species name suggests, the Okinawa rail is unable to fly. But how did this bird lose its ability to fly? And could it ever regain the power of flight? In this article, we’ll explore the evolution, anatomy, and behavior of the Okinawa rail to understand why it can no longer fly.
Quick Facts on the Okinawa Rail
– Scientific Name: Gallirallus okinawae
– Listed as Endangered by IUCN
– Endemic to Okinawa Island, Japan
– Flightless bird, unable to fly
– Grows up to 30 cm long
– Average mass around 170 g
– Dark grey or brown plumage with white spots
– Short wings relative to body size
– Lives in dense forest and scrub habitat
– Omnivorous; eats insects, spiders, slugs, seeds
– Population estimates around 2,000 individuals
Evolutionary History
The Okinawa rail belongs to the rail family Rallidae, which includes other flightless island rails along with more widespread flying rails. It is most closely related to the Lewin’s rail of Australia and Papua New Guinea. The ancestor of the Okinawa rail likely flew to the islands of Japan, but once established there, the rails evolved to lose their flying ability.
Island species often evolve flightlessness due to lack of predators. With fewer threats, the ability to fly becomes less crucial for survival. Flight is metabolically costly, so birds may adapt to save energy by reducing their flight muscles over time. The Okinawa rail’s wings shortened, its keel became less prominent, and its breast muscles atrophied as the species adapted to its island habitat.
The Okinawa rail’s flightless condition developed over thousands of years. Fossil evidence shows that a flying ancestor colonized the island 400,000 years ago. This ancestor gave rise to three rail species on Okinawa, two of which (the Okinawa woodpecker and Lidth’s jay) went extinct within the last few hundred thousand years. Only the Okinawa rail remains today.
Anatomy and Physiology
Several anatomical adaptations enable the Okinawa rail’s flightlessness:
Smaller Wings
The Okinawa rail has proportionally small wings relative to its body size. Its wings measure only around 6.3 cm, while the rail’s body can grow over 30 cm long. The wings are rounded rather than pointed like the wings of flying rails. The smaller, blunter wings reduce drag and energetic costs for a species that spends its life on the ground.
Reduced Flight Muscles
Within the wing, the flight muscles that power flying in other birds have substantially reduced in the Okinawa rail. The pectoralis and supracoracoideus muscles, which make up the bulk of a flying bird’s breast, have atrophied as they are no longer needed for strenuous flight.
Less Prominent Keel
The keel is a prominent breastbone ridge that anchors flight muscles in flying birds. The Okinawa rail’s keel is much shorter and less deep than in its flying relatives. Without large flight muscles, the rail does not require an extensive keel for muscle attachments.
Dense, Heavy Bones
While most birds have lightweight, hollow bones to minimize body weight, the Okinawa rail’s bones are solid and heavy. This may help stabilize the rail for walking rather than flight. Heavy bones are common in other flightless island birds as well.
Behavior and Ecology
The Okinawa rail spends its whole life on the ground inhabiting dense forest and scrub habitat. It runs rapidly between dense vegetation and hides when threatened. Its flightlessness limits its habitat range and ability to escape predators like mongooses.
The Okinawa rail is most active in mornings and evenings when it forages for invertebrate prey like insects, spiders, slugs and worms. It also consumes seeds and berries. Breeding occurs from February to July. Nests are built on the ground concealed within vegetation. Parents care for the young after hatching.
Population numbers have declined due to habitat loss from development and predation by invasive species. Surveys estimate around 2,000 individuals left on Okinawa today. Conservation efforts aim to protect and restore native forest for this endangered flightless rail.
Could the Okinawa Rail Regain Flight?
Is it possible for the seemingly permanent loss of flight in the Okinawa rail to be reversed? Could this flightless bird ever regain the power of flight? Theoretically, evolution could act on the species to select for increased flight ability again. However, the rail would face major obstacles in re-evolving flight:
Lack of Selective Pressure
For flight to re-evolve, natural selection needs to favor individuals with improved flight ability until the ability is regained. But on Okinawa, flight offers little advantage that would drive this change. The main predators (mongooses, cats, snakes) hunt on the ground rather than from the air, so flight offers little escape advantage. And the island has abundant food on the ground, so flying to new foraging sites provides minimal benefit.
Long Timescales Required
Evolution of complex traits takes a long time. The Okinawa rail took hundreds of thousands of years to lose its ancestral ability to fly. Re-evolving that ability would likely require a similarly lengthy timescale. Small incremental changes would need to accumulate over many generations. The entire population would also need sufficient genetic diversity to evolve increased flight muscle and wing size.
Developmental Constraints
Bird embryos develop flight muscles and feathers in a certain sequence timed by gene expression. The Okinawa rail’s development patterns likely no longer program growth of large flight muscles and wings. These developmental constraints would have to be unlocked again. Changing such core developmental pathways requires major genetic changes.
Bioenergetic Costs
Developing bigger flight muscles and wings has a high energetic cost. The nutrients and energy budget of an individual bird may be insufficient to grow these costly structures. For regaining flight to spread through the population, the benefits of flight would need to exceed the developmental costs.
Conclusion
While theoretically possible, the evolution of regained flight in the Okinawa rail currently seems very unlikely. The species has fully adapted to its flightless existence over hundreds of thousands of years. For an inactive wing to transform back into a functioning wing would require substantial changes throughout the rail’s anatomy, development, genetics and behavior. Such complex changes seem improbable without a strong selective advantage favoring flight. Barring a major new threat from aerial predators or competitors, the Okinawa rail appears destined to remain flightless into the foreseeable future. Though it has lost the power of flight, this fascinating island-dweller persists by making the most of life on the ground.