A flank refers to the fleshy area on the side of an animal’s body between the ribs and hip. In birds, the flank is located between the wing and leg on each side of the body. It contains large muscles that control wing movement. Understanding bird flank anatomy helps provide insight into flight mechanics and avian physiology.
Bird Flank Anatomy
The flank of a bird contains several important muscles involved in powering wing movement:
Pectoralis Muscles
The pectoralis major and pectoralis minor are large, fan-shaped muscles that originate on the breastbone (sternum) and insert on the humerus bone of the wing. When contracted, these pull the wing downwards. They make up the bulk of the fleshy breast area.
Supracoracoideus Muscle
The supracoracoideus originates on the coracoid bone of the shoulder and inserts on the humerus near the pectoralis minor. It raises the wing when contracted.
Latissimus Dorsi Muscle
The latissimus dorsi runs from the vertebrae to the humerus. It draws the wing backwards.
Scapulohumeralis Muscle
This muscle connects the scapula with the humerus. It helps to coordinate wing elevation.
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Pectoralis major | Sternum | Humerus | Depresses wing |
| Pectoralis minor | Sternum | Humerus | Depresses wing |
| Supracoracoideus | Coracoid | Humerus | Elevates wing |
| Latissimus dorsi | Vertebrae | Humerus | Draws wing backward |
| Scapulohumeralis | Scapula | Humerus | Coordinates elevation |
In addition to these muscles, the flank contains:
Ribs
Birds have ribs that encase the thoracic cavity and protect internal organs. Ribs articulate with vertebrae and attach to the sternum.
Air Sacs
Extensions of the respiratory system called air sacs may overlay or infiltrate some flank muscles. Birds have a complex air sac system interconnected with the lungs.
Kidneys
The kidneys are located in pockets of the flank called renal fossae. Avian kidneys function to filter blood and regulate water balance.
Fat Deposits
Stores of fat in the flank provide energy reserves. The amount of fat deposition varies by species, diet, and metabolic status.
Functions of the Bird Flank
The main functions of the flank in birds include:
Powering Wing Movement
Contraction of the large pectoral muscles allows birds to flap their wings and generate the aerodynamic forces required for flight. The supracoracoideus, scapulohumeralis and other muscles coordinate complex wing motions.
Breathing
The flank muscles wrap around ribcage to facilitate breathing by changing thoracic volume. The close association with air sacs also aids pulmonary oxygen exchange.
Temperature Regulation
Fat deposits in the flank help insulate birds against heat loss. Feather coverage over the flank also contributes to this function. Blood flow to the flank skin allows heat dissipation when required.
Protecting Organs
The flank helps protect vital organs like the heart, lungs, liver and kidneys located in the coelomic body cavity. Overlying muscles and ribs shield organs from trauma.
Energy Storage
Fat stored in the flank provides an energy reserve that allows birds to endure periods of prolonged fasting and high energy expenditure, such as migration.
Water Balance
The kidneys in the flank allow birds to regulate water retention and excretion, maintaining fluid balance. This is especially important for desert-dwelling species.
Evolution of the Bird Flank
The flank anatomy of modern birds evolved from theropod dinosaurs. Key adaptations include:
Expanded Breast Muscles
As forelimbs evolved into wings, pectoral muscles increased in size and power to enable flight. The pectoralis major makes up 15-25% of a bird’s full body mass.
Reduced Ribs
Primitive birds evolved a slightly compressed thoracic region to improve aerodynamics. This involved fusing and reducing the number of dorsal ribs compared to dinosaurs.
Skeletal Pneumatization
Many bones in the bird flank (vertebrae, sternum, humerus) became hollow and reinforced internally by bony struts. This air sac system lightened the skeleton for flight.
Growth of Kidneys
As birds became more active, kidneys enlarged and shifted position to facilitate water regulation. Renal systems adapted to rapid excretion.
Feather Development
Feathers insulated and streamlined the body profile. Flight feathers on the wings enabled aerodynamics. Body contour feathers covered the flank.
In summary, bird flank anatomy evolved over millions of years to meet the specialized demands of powered flight. Flank adaptations provide a window into the physiology and evolution of dinosaurs and birds.
Varying Flank Anatomy in Birds
While all living birds share the same basic flank structure, there are some variations in form and function between different avian species and orders. Key differences include:
Flightless Birds
In flightless birds like ostriches, flank muscles are greatly reduced since they do not power wing beating. The pectoralis muscles in particular are much smaller than flying species. Ribs may also be more robust.
Wading Birds
Waders like herons have enlarged pectoral muscles for generating thrust during takeoff from water. Their wide sternum provides attachment points for large muscle masses.
Swimming Birds
Strong swimmers like penguins have adapted their flank for underwater propulsion. Their wings became modified into flippers with truncated muscle attachments. The breastbone is also extensively pneumatized.
Birds of Prey
Raptors like eagles and hawks have large latissimus dorsi and scapulohumeralis muscles for stability and maneuvering during hunting. Their flank facilitates powerful wing beats.
Running Birds
Ostriches and other flightless running birds have more laterally positioned legs to increase stride length. This moved the flank muscles closer to the pelvis and tail.
Hummingbirds
Hummingbirds have incredibly enlarged pectoral muscles accounting for 25-30% of their total mass to enable rapid, sustained wing beats required for hovering. Their tiny flank oscillates rapidly.
These examples demonstrate how natural selection has fine-tuned flank anatomy in different bird groups for specialized lifestyles and flight modes. The flank provides great morphological flexibility.
Conclusion
In conclusion, the flank is the side region between the wings and legs of birds. It contains important muscles that control wing movement necessary for avian flight. The flank also facilitates breathing, temperature regulation, organ protection and energy balance. Bird flank anatomy has evolved from theropod dinosaurs to become highly adapted for meeting the demands of flight. Different species demonstrate specific modifications of the flank reflective of their ecology and behavior. Analysis of the flank provides key insights into the physiology, evolution and biodiversity of Aves.
