The character names "Bluebird", "Lake", "Big Bird", "Hex", etc. are trail names. It is part of the "hiking culture" to have a trail name, e.g., a nickname that is bequeathed to you by your hiking comrades, usually after you pull some stunt or tell a story that should have been left untold. How Bluebird and Lake came by their trail names is never explained.
At 28:20, there is a hint to her trail name "Bluebird" as there is a tattoo of a bird on her right hand. (she rolls over after he moves her hair so it doesn't get burned by the camp stove).
In the opening scenes, Bluebird brushes her teeth with a toothbrush whose handle has been cut off. This shot establishes that Bluebird is an experienced hiker who knows backpacker tricks to minimize the weight she has to carry on the trail.