The basic building blocks of a cell membrane are phospholipids. Phospholipids contain a hydrophobic (insoluble in water) end made up of two fatty acid chains of nonpolar molecules such as carbons and hydrogens. The other end is hydrophilic (soluble in water) and contains polar phosphate molecules.
The finches adapted over time to better suit their environment that they were in -- the size and shape of their beaks changed as a result of natural selection.