This aspect of biology is fascinating. Hummingbirds stop at our ranch before flying over the Gulf of Mexico. Those tiny guys gain just the right amount of weight to go 600 miles over water. Too little or too much and its in the drink!

Here is a discussion of geese and muscle size. What makes the muscles get bigger without exercise, but at the right time?:

http://www.physorg.com/news/2011-11-migratory-birds-dont-migrations.html

In developing horses for halter horse competition, they are bred for large muscle mass and come that way without exercise.