1893.] The Flight of Birds. 23 
I have called this region the “neutral zone,” for want of a 
better name. The bodies of all birds are almost entirely con- 
fined to this zone. 
The original question is now in order. Why do not all 
birds soar? For many reasons, among which are the follow- 
ing: 
1. A bird may be of such shape that to throw it over to an 
inclination that gives a parallel factor small enough to be 
neutralized by the forward thrust would get part of the ani- 
mal out of the neutral zone. The gray pelican of the Gulf 
coasts is a case in point; especially when its gullet is full of 
fish. It must flap to get thrust for its large parallel factor. If 
its wings were one foot longer on each side, it could doubtless 
soar continuously, even if its body did encroach on resisting 
air to some extent. 
2. Weight may be too great for surface. In this case con- 
densations escape too readily to be utilized. A wild turkey, 
prairie-chicken, or pheasant, are examples. 
3. Weight may be too little for surface. Here condensation 
is too weak to give thrust. A species of sea-gull found on the 
lower Florida peninsula is a good example. In May and 
June, when food is scarce, they flap continually. In Novem- 
ber and December, when food is plenty, they rarely move a 
wing. They will put into an empty craw their weight in food. | 
4. Life habits may prevent suitable feather construction. I 
presume this reason may apply to many small birds. 
In the case of bats, the small factor may be entirely neu- 
tralized by flapping, as there seems to be at least no surface 
provision for utilizing the escaping condensations. 
Flying squirrels do not sustain themselves. 
