A bird's nest is a situation that occurs underneath the needle plate of an embroidery machine. It is when a tangle or knot of threads bunch up making it impossible to continue sewing until the “nest” has been removed.
Some of the causes include:
The top tension thread is too loose or the bottom bobbin tension thread is too tight. When the bobbin thread is too tight, it puts excess pressure on the top thread and pulls it down into the needle plate. An excessive amount of thread builds up inside the bobbin case and causes the bird's nest. The solution is to loosen the bobbin thread tension by turning the small screw on the bobbin until proper tension is achieved.
If your top tension thread is too loose, the same problem will be created. There must always be a proper relationship between thread tension and bobbin tension. The best way to ensure that both thread and bobbin tension are correct is to use a thread tension gauge.
To remove a bird's nest, it helps to have the right tools. Standard scissors will be too large and short to reach underneath where you need to clip away at the knot. Surgical scissors work well in this situation. A thread hook can be handy for extracting threads out from under the needle plate where they can be snipped. After removing the bird's nest, use canned air and ensure that the bobbin and bobbin case are free of excess thread.
Source: Ed Levy