Nesting, also known as bird's nesting, refers to the problem of thread bunching when you use a sewing machine. Specifically, the top stitches form properly, but the bottom thread gets bunched up and tangled, and too much thread is fed through the bobbin. Nesting can be caused by a number of possible problems with the machine. Correct each potential problem individually until the thread no longer nests.
Ensure that you have threaded your sewing machine properly. Rethread it. If the thread is still nesting, consult the instruction manual for your particular sewing machine model.
Check that the needle is installed properly. Consult the instruction manual if it is loose and unstable.
Make sure the bobbin is properly installed. The bobbin and bobbin case are found beneath the needle. Check that the bobbin thread runs in the appropriate direction for your sewing machine (usually clockwise), and ensure that the thread runs through the tension piece of the bobbin case correctly. Consult your instruction manual for the correct bobbin set-up for your machine.
Adjust the upper tension. The tension controls the release of thread, and if the upper and lower tensions are not balanced, stitches will not be even. Upper tension can usually be adjusted using a dial on the machine, often found on the upper arm of the machine head or on the faceplate. Consult your instruction manual if you are unsure of its location.
Check the lower tension. The lower tension can be adjusted by turning the screw nearest to the centre of the spring on the bobbin case. The lower tension should not be adjusted if possible, but through cleaning and previous adjustments, it may be improperly set.
Rethread the machine while the pressure foot is up and the needle is as far up as it will go. The pressure foot is the piece that presses down on the fabric to keep it in place. Threading the machine with the pressure foot down is a very common cause of nesting.
Contact a professional seamstress or tailor if none of the above steps work. They are qualified to address any of the more specific, unusual problems that may cause nesting.
Test each step with a piece of scrap fabric until the nesting has stopped.
Be careful when making adjustments to your machine so you don't damage it.