A feather is any one of the protruding flat soft growths making up the total plumage of a bird.
Feathers degrade over time because of sunlight and bumps with physical objects. Few feathers last more than two years, and flight is less efficient with older feathers so birds must be constantly replacing their feathers. Growing new feathers takes a lot of energy, and sometimes is spread over two years in a feather by feather exchange in larger birds, while, for smaller birds, feathers are replaced quickly (in 3 to 6 weeks) at other times than breeding seasons. Though feathers grow with barbs so they hook together into an aerodynamic system, small birds can fly with some gaps in their wings because their greatest power is much more than that needed for simple flying. Larger birds do not have this margin of effectiveness so they would not be able to fly if they lost too many feathers at once. Presumably small birds are less maneuverable and fly less high while they are molting.
One study suggests that the main limit on size of birds is the time needed to replace feathers. The larger the bird, the larger the feather, and the longer it takes to mature completely. Flightless birds can grow larger because their feathers do not need to be so aerodynamically perfect.
Even Ernst Mayr, who some would hail as a father of modern evolutionary biology stated in 1942: "It must be admitted, however, that it is a considerable strain on one’s credulity to assume that finely balanced systems such as certain sense organs (the eye of vertebrates, or the bird’s feather) could be improved by random mutations." 
Creationist scientists also cite the March 2003 issue of Scientific American which stated:
"Of all the body coverings nature has designed, feathers are the most various and the most mysterious...The origin of feathers is a specific instance of the much more general question of the origin of evolutionary novelties--structures that have no clear antecedents in ancestral animals and no clear related structures (homologues) in contemporary relatives. Although evolutionary theory provides a robust explanation for the appearance of minor variations in the size and shape of creatures and their component parts, it does not yet give as much guidance for understanding the emergence of entirely new structures, including digits, limbs, eyes and feathers...." 
- Allometry of the Duration of Flight Feather Molt in Birds Rohwer S., Ricklefs R.E., Rohwer V.G., Copple M.M., 2009. PLoS Biol 7(6): e1000132. doi:10.1371/journal.pbio.1000132
- Online Edition of - In the Beginning: Compelling Evidence for Creation and the Flood, References and Notes 8 Complex Molecules and Organs, by Dr. Walt Brown, Center for Scientific Creation. Accessed 19 June 2010
- Scientific American admits creationists hit a sore spot - Need for a ‘new paradigm’ in bird evolution, by Michael Matthews, AiG–US, 13 March 2003. Accessed 19 June 2010
- Which Came First, the Feather or the Bird? ( Preview ) A long-cherished view of how and why feathers evolved has now been overturned. By Richard O. Prum and Alan H. Brush, Scientific American, March 2003, Accessed 19 June 2010.
- The Feather Atlas U.S. Fish and Wildlife Service Forensics Lab. High-resolution scans of flight feathers of major groups of North American birds as an aid to species identification.