I'm am going to disagree with everyone. When there is birds nests, I find the top tension is to loose and is being pulled down under the quilt sandwich forming the nest. The bobbin may be too tight also. 90% of thread problems is caused by the upper thread.

Patchwork, if you go back, you'll actually see that 5 of us agreed that it was a top tension issue. :)
I think the confusion stemmed from the fact that ladyof2 did a pieced back so people originally made assumptions that it was the front of her quilt that had problems.

It just happened to me, I took the bobbin and the plate out, brushed out all the lint and it was a lot and replaced everything, it works great now!

I had this kind of "thing" recently and after lots of searching took a look at how thread was feeding off the spool. It had slipped out of one of the tension guides so basically no upper tension and a big mess on the back. Just another thing to check out. FYI, this happened while I was quilting. All was well, then, suddenly a mess - not sure why it popped out but as soon as I rethreaded and made certain all was as it should be the problem went away.

(Also check your needle size?)

This is a top tension problem. Many people think when the birdsnest is on the bottom that Itis the bobbin thread. May I explain why it is the top tension. When the threaded needle goes through your material it needs to connect with bobbin thread to make a stitch. If top tension is loose it will leave extra thread on the bottom of your material. the top tension cannot hold the thread correctly. To help yourself understand this on a sample stitch with two distinct different colors of thread in the bobbin and top. Observe your stitches. If the thread from the top shows too much on the bottom the top tension is too loose. If the tread color from bobbin shows on the top then the top tension is too tight. Most all machines recommend adjusting the top tension only. It gets a bigger problem if you try adjusting the bobbin tension. I am a retired shop owner and have had much experience with this concern. Hope this will help you and others in your upcoming projects and great stitching

Hold your breath, adjust the tension on your bobbin, rethread the machine and all else fails cry a lot

Well now I am scared & admit to being a beginner & not a very smart one at that. I have been SID a queen quilt & haven't even looked at the back yet & I'm almost done except for the boarders. Now I guess I'll take the big plunge tomorrow & look at the backing after I say a prayer that it's all ok. I saved all the expert advice on this post...just in case.
Jeri

I agree with pekish. It is the top tension. If the problem is on the bottom it is top that is causing the problem. If the problem is on the top it is the bobbin that is causing the problem. Good luck.

So, which "experts" are you going to believe? There were a lot each way. I think Peckish is correct as is quiltingsunshine. A few others joined in later. The nests were on the back, so the trouble is with the top tension.

Re-thread the top, making sure the thread goes through all guides and tension disks with the presser foot up.
Then put the presser foot down and you should be fine.