Physics of flight
A wing has to:

• generate lift: tendency to rise up
• minimize drag: air resistance
• avoid stalling: loss of lift when air around wing becomes too turbulent

• direct air flow downward as they move through the air
• results in air moving slowly BELOW the object, rapidly ABOVE the object
• air moving rapidly exerts less pressure than air moving slowly
• so overall, the surface is pushed upward
• objects that generate lift are shown in figure 1.

To decrease drag:

• streamlining: curved, tapered surfaces like airfoil to keep air moving smoothly over wing
• fairing: have wings taper into body

• air must flow smoothly over the wing; depends on wing speed and wing angle
• slow moving wing: air becomes turbulent; lift is lost; bird stalls
• speed at which a bird stalls = stalling speed
• stalling also depends on angle of attack: steeper angle of attack results in stalling
• alula, slots at the end of a wing help maintain smooth flow over the wing, avoid stalling

• aspect ratio: length/ width.
• longer wings generate more lift because leading edge generates lift; also have reduced drag; fewer problems with wing-tip turbulence
• wing loading: weight/wing area
• W=weight, A = area
• assume a bird is a sphere with radius r
• W  proportional to  r³, A proportional to r²  so as we look at larger and larger birds, W gets larger faster than A
• large birds would need proportionally larger wings to have same wing loading
• in general, large birds do not have proportionally larger wings, they have higher wing loading (see Table 1)

Table 1. Wing loadings of selected North American Birds (information taken from Welty and Baptista (1988))

SPECIES	WEIGHT (grams)	WING AREA (cm2)	WING LOADING (g/cm2)
House Wren	11.0	48.4	0.24
Barn Swallow	17.0	118.5	0.14
Red-winged Blackbird	70.0	245.0	0.28
Peregrine Falcon	1222.5	1342.0	0.91
Mallard	1408.0	1029.0	1.37
Golden Eagle	4664.0	6520.0	0.72
Canada Goose	5662.0	2820.0	2.01
Mute Swan	11602	6808.0	1.70

• size in flying birds may be constrained by how big wings have to be to lift off combined with the fact that flapping very large wings is energetically very expensive.

• wing loading not just related to size (see Table 1); also depends on shape
• wings have been divided into 4 categories, with different functional properties, as shown in figure 2

Figure 2.  The four main categories of wing shape, and their functional properties

Movement of wings, tail during flight:

• flapping flight: physics much harder to understand (not fully understood)
• downstroke: down and foreward; pulls bird through air
• upstroke in small birds: up and backward; partly folded (does little work)
• upstroke in larger birds: up and backward; tilts back; primary feathers push bird foreward
• wing movements in hummingbirds: figure eight pattern; wing rotates so lift on both strokes.  See figure 3.

Figure 3.  Positions of hummingbird wings on each stroke; wing is "upside down" on upstroke so lift is created on upstroke as well as downstroke

• tail: acts in concert with wings; spreads as wings spread on landing, takeoff

• loss of wings, tail. Sternum has no carina ("ratite" sternum instead of "carinate" sternum)
• reduction in toe number: adaptation for running

• few predators on islands; less need for flight
• flight energetically costly
• extinction of flightless birds when humans introduce predators to islands
• example: dodo