That's fur and feather. I've never known a squirrel rob a Marsh Tit nest or, at least, I have no evidence that it's happened in the 2 study sites I'm looking at. The top 3 seem to to be GSW, Jay and Weasel. Stoats, rats, feral cats and possibly mice are also predators. In fact, I've never known of a squirrel robbing a tit nest, but what i have seen is a stoat 9m up a tree, attacking a MT nest. One wonders how many times squirrels get blamed when it may be something else, just because the nest was high? Weasels will also happily rob nests up to 2m off the ground, and rats even higher. It is, however, often difficuly to be absolutely certain what has robbed a nest, as a lot of the time the evidence is circumstantial (GSW are the easiest, for obvious reasons). A squirrel attack on live wood would be quite easy to infer, I'd imagine, as you'd have clear knaw marks. But I've never seen it yet.