Incorrect upper threading is the cause of the vast majority of bird's nesting problems. Even if you've already re-threaded, it's worth doing it again slowly and carefully, referring to your manual rather than going from memory.

• Raise the presser foot before re-threading - this opens the tension discs so the thread seats properly
• Follow your machine's threading path step by step, making sure the thread passes through every guide and tension disc in the correct order
• Missing a single guide is enough to cause repeated bird's nesting

If the thread is catching or snagging on its way to the machine, it creates uneven tension that can cause bunching underneath the hoop.

• Make sure the thread is unwinding smoothly and isn't catching or getting wrapped around the spool pin
• Check the spool is sitting correctly and that you're using the right spool cap for your thread size

Upper tension that's too loose is one of the most common causes of bird's nesting in embroidery - when the tension is too light, excess thread gets pulled through and dumps underneath the hoop instead of locking into the stitch.

• Increase the upper tension slightly and test on scrap fabric
• Make small adjustments and test after each one rather than making large changes

While the upper thread is usually to blame, a poorly inserted or unevenly wound bobbin can also contribute to bunching.

• Remove and re-insert the bobbin, making sure it's going in the correct direction for your machine
• Make sure the thread is pulled through the tension slot properly
• Check the bobbin is wound evenly - if it looks uneven or messy, rewind it before continuing

Loose thread tails at the very start of a design are a common trigger for bird's nesting, they get pulled down underneath the hoop on the first few stitches and create an instant tangle.

• Hold the top thread tail for the first few stitches so it can't be pulled through
• Alternatively, trim the thread tail close to the fabric before starting the design

Fabric that's hooped too loosely can shift during stitching, which disrupts stitch formation and can cause thread to bunch underneath.

• Check the fabric feels taut in the hoop with no give or movement
• Re-hoop if necessary, making sure the fabric is secure but not stretched or distorted

Under-stabilising is a common cause of all sorts of embroidery problems including bird's nesting. If the fabric is moving more than it should, the thread can't form stitches cleanly and bunching is the result.

• Make sure your stabiliser choice is appropriate for your fabric type and the density of the design
• If in doubt, add an extra layer of stabiliser and test on scrap before returning to your project

Before returning to your project, run a test on scrap fabric of the same type. Check the underside of the hoop after the first few stitches to confirm there's no bunching before you commit to your actual piece.