10 Causes for Thread Bunching and Tangling.

1. It is immediately assumed that thread bunching is a  bobbin problem, most of the time it has nothing to do with the bobbin. If your bobbin is tightly and evenly wound, and inserted in the bobbin case correctly, then the bobbin is not the problem. 
2. If the top thread isn’t threaded correctly, the upper thread that passed through the fabric cannot be firmly pulled up and the upper thread becomes tangled in the bobbin case, causing a rattling noise. The thread can also get caught around the spool holder, next to your spool. Turn off your machine, and remove the tangled thread. Check for threads that are left in the bobbin case. If possible, don’t cut the thread. Remove the upper thread and re-thread the machine again. Make sure the thread is in the take-up lever!
3. If the bobbin tension is too loose: Extra thread may unwind from the bobbin, which causes a buildup of thread underneath the needle plate. Try tightening the bobbin tension by turning the tension screw on the bobbin case a quarter turn clockwise. Tighten it in bits, and test it until you get the desired results. Follow the instructions in your sewing machine’s manual. Also, always make sure to use the correct bobbins for your machine’s make and model.
4. If the top tension is too loose: If your top tension is too loose, you will see excess thread build up under the needle plate. To remedy this, increase your top tension in small increments.
5. Change your needle. A damaged needle can be the cause of many sewing problems. Also make sure you are using the proper needle for your machine, the fabric you are sewing, and the job at hand. The needle is part of the upper threading. If the needle is too thin for your fabric and thread it is more likely to jam than a thicker needle.
6. If the bird nesting always happens at the beginning of a line of stitching, then make sure to bring the bobbin thread to the top before you start stitching and leave a decent amount of thread (tails) both on the bobbin and upper thread. If you sew with short threads, it will get pulled back into the machine. Leave a tail of at least 6 inch (15 cm) of thread. Apply a little tension, hold the threads as you start to sew. Don’t start sewing at the very edge of your fabric, the thread will easily form knots and jam.
7. Make sure you lower the presser foot. Forgetting this can happen every once in a while, especially when you have a very thick fabric underneath the foot, when you have pressure from the foot but the lever is still up.
8. Make sure you use good quality thread.
9. If using thin fabrics, be careful with securing stitches or back stitches. They can sometimes lead to the fabric being pulled down in the needle plate.
10. Maintaining your machine and keeping it clean can also solve your thread bunching problems. Remove dust underneath the throat plate and along the thread path frequently to avoid build up. Check your sewing machine’s manual for guidelines on cleaning and maintenance. Get your machine serviced annually by a professional.

Make sure to try one solution at a time from this list to find the exact cause of your bird nests. If all this isn’t giving the desired results, or the same problem keeps on occurring, have it checked by a professional.