What does 'full-summed' indicate regarding a bird's feathers?

The term "full-summed" refers to a bird that has completely molted and regrown its feathers. Molting is the natural process through which birds shed old or damaged feathers to make way for new ones, which can be essential for maintaining flight efficiency and overall health. When a bird is described as full-summed, it indicates that the recent molt has been completed successfully, resulting in a full set of new feathers that are often more vibrant and effective for flight.

In the context of the other options, worn out feathers are not indicated by being full-summed since that term signifies new growth. A juvenile stage of a bird does not correlate with full-summation, as juveniles often have different feather patterns and quality compared to adults after molting. Lastly, preparing for migration involves behavioral and physiological changes but does not specifically denote the status of the feathers in terms of whether they have been recently molted or not. Thus, the term "full-summed" distinctly highlights the condition of having entirely renewed feathers through the molting process.