Oil glands are small protrusions on the skin near the base of the tail that secrete oil. This oil is spread over the feathers during preening, providing waterproofing and allowing the feathers to remain flexible and durable. However, some groups of birds, particularly parrots, lack oil glands entirely. This raises an interesting evolutionary question – why did oil glands disappear in certain avian lineages?
The function of oil glands
All birds have feathers, but not all birds have oil glands. Feathers are made of keratin and are essential for flight, insulation, waterproofing and communication. However, feathers are prone to becoming brittle and worn. Oil glands produce a waxy, fatty substance made of triglycerides and fatty acids that birds spread over their feathers to maintain their flexibility and water repellency.
The oil coats the barbs and barbules of the feathers, keeping the feathers arranged in an orderly, flexible and hydrophobic form. Without oil glands, feathers are more prone to matting, clumping and breakage from waterlogging. Birds spread oil over their feathers by preening – using their beak to distribute oil from the gland over their plumage. This maintains the feather structure and function.
So in most birds, oil glands are crucial for keeping feathers in excellent condition. But some avian groups, including parrots, lack oil glands entirely.
Which birds lack oil glands?
Most birds possess oil glands, but there are a few groups that lack them:
- Parrots
- Pigeons and doves
- Mesites
- Kagu
- Sunbittern
Parrots are especially notorious for their lack of oil glands. All 350-400 species completely lack these glands. Pigeons and doves, their close relatives, are also missing oil glands. Several other taxa like mesites, kagus and sunbitterns, represent examples of oil gland loss in other parts of the avian family tree. But parrots are by far the largest group of birds lacking oil glands.
Why did parrots lose their oil glands?
This raises the question – why did parrots and some other groups lose their oil glands in the course of evolution? There are a few leading hypotheses:
Unique feather structure
Parrot feathers may be specially structured to not require oil. The details of the feather structure may allow flexibility and interlocking even without continuous oiling.
Special powder down feathers
All parrots have powder down feathers – special feathers that continuously disintegrate into a fine powder. This powder may substitute for oil in maintaining feather structure and waterproofing.
Preening with dust
Parrots rub themselves in dusty substrates, which may provide a substitute method of feather maintenance in the absence of oil. The dust particles become incorporated into the feathers.
Reduced reliance on feathers for waterproofing
Parrots have relatively short and thin plumage compared to other birds. With less surface area covered in feathers, parrots may rely less on feathers for waterproofing. Their skin may be more inherently water-resistant than in other birds.
Higher metabolism
Parrots have very high metabolic rates, so the absence of oil glands may help save energy and reduce unnecessary physiological processes.
Let’s examine the evidence for these hypotheses in greater detail:
Unique feather structure in parrots?
Do parrot feathers have a special structure that eliminates the need for oil glands? There are some unusual features of parrot plumage compared to other birds:
Short, rigid feathers
Parrot feathers are generally shorter and more rigid than the long, flexible feathers of many birds. The short, stiff structure may allow the feathers to remain aligned and flexible even without continuous oiling.
Smaller, simpler barbules
The barbules of the feather (the small hooks that interconnect adjacent barbs) are smaller and less complex in parrots. Simpler barbules may interlock adequately even without softening from oil.
Tighter feather attachment
Parrot feathers attach more securely and deeply into the skin via a strong follicle anchor system. More robust feather attachment may help compensate for a lack of oiling.
However, parrot feathers are not radically different from other birds in these aspects. Overall, unique feather structure may contribute to, but does not seem to fully explain, the lack of oil glands in parrots.
The function of powder down in parrots
All parrots have special feathers called powder down feathers. These feathers continuously disintegrate into a fine, waxy powder. This powder may help to waterproof and maintain the plumage even without oil:
Waxy substance
The powder consists of a waxy substance made of fat-like lipids, similar to the preen oil of other birds. This waxy quality suggests it could help repel water.
Distributed during preening
Parrots distribute the powder throughout the plumage by preening, just like birds distribute oil from glands. This suggests the powder serves a similar function.
Powder clings to feathers
The fine powder is thought to cling statically to the feathers. This may provide similar waterproofing and feather maintenance as oil does in other birds.
Limited to parrots
Powder down is only found in parrots, consistent with it compensating for the loss of oil glands.
However, more research is still needed to demonstrate the functional role of powder down. Overall, it likely contributes to feather maintenance but may not fully substitute for the roles of oil.
Preening with dust by parrots
Many parrot species rub themselves in dusty, clay-like substrates. The dust particles become incorporated into the feathers:
- African parrots utilize mounds of powdery soil
- Cockatoos use decayed, dusty wood
- Parakeets cover themselves in volcanic clays
This dusting behavior likely helps maintain plumage in the absence of oil:
Water resistance
The dust is thought to confer some water repellency to the feathers, similar to oil.
Filling space
The dust may fill in gaps between feather barbules where oil would normally be distributed.
Maintains flexibility
The layer of dust is thought to help feathers remain flexible, preventing clumping and breakage.
Antiparasitic
The dust may also help remove feather parasites like lice and mites.
However, dusting is not equivalent to oil production and preening. For example, the benefit is temporary as dust particles are dislodged. Still, dusting likely contributes to plumage maintenance in parrots lacking oil.
Reduced reliance on feathers for waterproofing?
Parrots have relatively thinner, shorter plumage compared to many other birds. With less surface area covered in feathers, parrots may rely less heavily on feathers for waterproofing and instead use alternative mechanisms:
Smaller feathered surface area
With fewer and shorter feathers, there is less plumage that needs waterproofing.
Lower feather density
Parrots have fewer feathers per unit skin area. Less dense plumage implies a reduced need for feather waterproofing.
Greater exposed skin
More skin is exposed relative to feather surface area. Exposed skin may have inherent water resistance.
Avoidance of swimming
Parrots spend little time swimming and diving like water birds. A dry lifestyle reduces reliance on plumage for waterproofing.
However, parrots still depend extensively on their feathers for insulation and resisting rain. Complete loss of oil glands is still puzzling given the remaining importance of feather waterproofing.
High metabolism of parrots
Parrots have unusually high metabolic rates, even after accounting for their small body size. Their constant high energy expenditure may favor the loss of unnecessary physiological processes like oil secretion:
Elevated baseline metabolism
Even at rest, parrot metabolism is intrinsically higher than expected based on body mass. This creates pressure to eliminate expendable processes.
Active foraging lifestyle
Parrots have energy-demanding lifestyles, constantly climbing and foraging. Reduction of non-essential metabolic functions may help allocate energy to motion.
Heat loss in warm climates
Many parrots live in warm tropical regions. Loss of oil glands may facilitate heat dissipation in hot climates.
Expensive production of oil
Producing oil requires energy and nutrients. Elimination of oil glands conserves the resources required for their function.
However, the energy savings from losing oil glands seems modest. Still, parrot physiology is highly energy optimized, which may favor shedding glands.
Conclusion
In summary, the loss of oil glands in parrots and some other birds remains somewhat puzzling. There are some leading hypotheses:
- Feather characteristics may facilitate flexibility and interlocking even without continuous oil application.
- Powder down feathers may help substitute for oil in parrot plumage.
- Preening with dust provides an alternate route to feather maintenance.
- Shorter, thinner plumage reduces reliance on feather waterproofing.
- High parrot metabolism favors losing expendable processes like oil secretion.
The true explanation is likely a combination of these factors. But more research is still needed to determine the relative contribution of each idea. Going forward, detailed analyses of parrot feather micromorphology, powder down biochemistry, effects of dusting, skin properties, and energy budgets can shed further light on why parrots lost their ancestral oil glands. The loss of oil glands provides an intriguing example of how even complex, useful traits can disappear without necessarily impairing function or survival.
