To precisely follow fine, intricate outlines you require small brushes.
On the other hand, you'll need large brushes for smooth gradients in large areas, which are, preferably, continuous.
The most practical solution is to use a masking fluid, which is available in any art supply store.
A masking fluid is a fluid that can be applied by painting or pouring, and which will solidify after a short time.
This gives you the opportunity to both follow fine lines, and cover a large area in two, separate steps: after having masked the bird, you can paint the background in one large gesture. After the paint has dried, you can peel off the mask, and you're done.
With this method you can keep on using pure watercolours (as in the example painting)—you won't need to change your watercolour opacity, consistency, or technique, and it won't muddy the colours of the bird by having the background colours shine through (watercolours are never opaque). Neither would you hazard peeling off part of the pre-existing painting, as DIY masks might do.
