Birds fly for four main reasons: feeding young, breeding, avoiding predators, and migration. The mechanism of flight in birds is based on Bernoulli's principle of fluid dynamics and the aerodynamic properties of wings. When air flows over and under the wing, the convex upper surface and concave lower surface cause lower pressure and higher velocity over the wing, generating lift. A bird can fly when the lift force is greater than or equal to its weight and the thrust force is greater than the drag force. Morphological adaptations like feathers, wings, lightweight skeletons, and strong flight muscles enable avian flight.

2.  Feeding (feeding to there young
ones)
 Breeding.
 Avoiding Predators.
 Migration.
BIRD FLY DUE TO 4 REASONS

3. MECHANISM OF FLIGHT IN BIRD
Mechanism of flight is based on two main
point-
Sir George Cayley
Bernoulli’s principle Aerodynamics Properties

4. This principle was first worked out by
Swiss mathematician DANIEL BERNOULLI in
1738.

5.  Bernoulli’s principle can be defined mathematically
i.e. V ∞ 1/P [where, V= velocity, P=Pressure.
 Velocity is inversely proportional to pressure. When the
pressure is least where velocity is greatest and vice versa.
 Bernoulli’s principle apply for the adult fluid flow. But in
case of bird there is no fluid flow but there is air that
interact with wing. Wing act as a solid and the air act as
a fluid flow.

6.  When the air flow just comes towards the bird the
air bifurcate in two ways-
o Over the wing &
o Below the wing.
 There is a displacement of air towards the wing that
cause the lifting of the bird.

7.  It is says that the upper surface of the wing is
convex and the lower surface of the wing is
concave.
 The concave side of the wing has the lower
surface area, so there will be less displacement.
 Whereas, the convex side of the wing has more
surface area, so there will be more displacement
i.e., there will be less pressure and more velocity
and vice versa.

8. Due to this pressure difference above or beneath
the wing-
o It lift the wing up.
o The drag force is less there.

9. o There is a forward movement of the bird.
o lifting of the bird.
This all causes the bird to fly.

10.  The Aerodynamics property was first discovered by SIR GEORGE CAYLEY
IN 1809.
 He called both as “THE FATHER OF AEROPLANE” and also “THE FATHER
OF AERODYNAMICS”.
 He was born in 27th Dec 1773.
Sir George Cayley

11.  Aerodynamics is a branch of science which deals with the interaction of
air with solid object and how air moves around this solid object.
The air exhibit different types of force on the wing and this force can be
resolved into-
 Vertical lift component perpendicular to the air.
 Backward drag component parallel to the air sting.
 We see the force due to gravitation that universally denoted by Fmg.
Lift force denote by = FL
Thrust force denoted by = FT
Drag force denoted by = FD
Gravitational force = FG

12.  Condition for the bird to fly :-
First Condition
Force of the lift must be greater or equal to gravitational
force.
FT > FD 2nd Condition.
Force of thrust must be greater than drag force otherwise
bird cannot fly.

13. More+
MoreLess
Less
 The lift and thrust
force that are the two
+ve force due to
which the bird fly and
weight and drag are
two –ve force due to
which the bird cannot
fly because they resist
the flight of the bird.

14. Modification of body in flight for aerial life –
Morphological modification -

15.  Body contour –
o Spindle shaped of fusiform body to ensure least wind
resistance.
o Low center of gravity.
 Presence of feathers –
o Feathers, very light, hardly adds weight to the bird.
o Feathers also serves as a blanket enveloping air around the
body and adds buoyancy.
 Forelimbs are modified into wings –
o Forelimbs becomes modified into unique and powerful
propelling organs, i.e., wings covering with feathers.

16. Mobile neck and beaks –
o Mobile neck and beak help them for feeding, nest-
building, preening, etc.
 Bipedal locomotion –
o Hind limbs supports the body weight and acts as
locomotory organs in the ground.
 Perching –
o Strongly developed muscles so modified that when a
bird sits in a perch, the toes automatically grips the
perch.

17. Perching muscles are of two types –
o Flexor Muscles &
o Extensor Muscles.
Flexor muscles –
There occurs 8 flexor
muscles on the back of
the tibio-tarsus bone of
hind limb.
Extensor muscles –
These muscles are in
front of the tibio-tarsus.
The toes are unlocked by
extensor muscles.
PERCHING MUSCLES

18.  Important flexor muscles are –
• Ambiens
• Peroneus medium
• Gastrocnemius
• Flexor perforans (above the knee).
 Important extensor muscles is –
• Dermal muscles.
PERCHING MUSCLES

19. Short tail and tail feather-Short tail with
feathers help in lifting, and counterbalancing.

20.  ENDOSKELETON-
o Endoskeleton are light and provides large surface for
attachment of muscles.
o Bones are pneumatic (hollow).
o Teeth absent. Synsacrum present.
o T-Shaped sternum
o The fusion of distal tarsal with metatarsals to make the
body light.

21. Muscles on the back are much reduced and flight
muscles on the breast are strongly developed.
There are three types of flight muscles are as
follows-
oPectoral muscles.
oAccessory muscles &
oTensor muscles.

23. They are of two Types-
i. Pectoralis Major Muscles- This muscles is very
large, triangular and most powerful flight muscles.
Weight is 1/5th of the body, dark red in color.
i. Pectoralis Minor Muscles- this muscles is small
and elongated which help in lifting the wing
during flight.
PECTORAL MUSCLES

24. ACCESSORY MUSCLES
Accessory muscles also help in lifting and dragging
the wing during flight. It arises from coracoids.
TENSOR MUSCLES
Three muscles are together called as tensor
i.e.,-
o Tensor Longus.
o Tensor brevis.
o Tensor accessorius.
 This muscles keep the propatagium fully
stretched when the wing is extend in the flight.

25. DIGESTIVE SYSTEM-
o Rectum reduced in size. Therefore metabolism very fast and
waste elimination is very fast.
o Excretory waste product is uric acid.
RESPIRATORY SYSTEM-
o Some alveoli modified into a large air sac present
throughout the body in thorax, abdomen which function as
2nd supply of oxygen.

26. CIRCULATORY SYSTEM-
o Birds are warm-blooded animal.
o Heart is large and had four chambers.
o Heart functions very efficiently with a double
circulation of blood.
REPRODUCTIVE SYSTEM-
o Single ovary and an oviduct is present in female, is a
weight reducing device.
TEMPERATURE-
o Body temperature is 40-42°C.