Why do YOU think some birds can fly?

They're small enough.
% (38 votes)
They're light enough.
% (117 votes)
They have big wings.
% (74 votes)
They have big feathers.
% (44 votes)
They have hollow bones.
% (165 votes)
They have a special muscle.
% (145 votes)
They have hollow bones.
28.3% (165 votes)
They have a special muscle.
24.87% (145 votes)
They're light enough.
20.07% (117 votes)
They have big wings.
12.69% (74 votes)
They have big feathers.
7.55% (44 votes)
They're small enough.
6.52% (38 votes)