The color set you have used is actually not suitable to produce such fine art on paper like the bird. Rather you would better understand if you take a look on the leaves (greenish bulgings), the color has not been equally distributed. This happens when the pencil is pushed forward and backward with pressure and the nip of the pencil breaks in small portions. This happens with everyone, at least when they have just started their art.
Now going back to your point. If you want to have darker arts with your current set of pencils here's a trick you can apply. Take two drops of water in a pallete or container. Take your pencil and use it as a brush. Just plunge it for 2 seconds on that drop and apply on paper. The first 2-3 shades will be darker.
Or for better results: try using darker shares of colors and merge it as it seems fit. Like with dark green you might apply brown or black there.
Or change your color pencil set. (not preferred if you are a novice). Many pro people grab Staedtler/Maped/Copic. Although the first two color brands are available on medium range. The third one is definitely heavy on pocket. So if you can compromise with accuracy, you surely will get precise results with your current set of colors.