After a few hours of searching and reading I've just come across a brilliant article which might explain what I witnessed. It's called pooling, where a vixen will mix her cubs with that of another and help raise them. My claim wasn't that of assumption, I watched a vixen and seven cubs clearly a good number of weeks apart four/three split play and hunt for three days solid on a public/petting farm. Maybe at the time I should have looked in to it, but with the lack of any other adult fox in the surrounding area over so many days just thought both must have been hers.!I've seen a couple of instances where two litters have joined up, not necessarily of the same age. I can imagine that if one of the vixens was shot then the other would continue bringing up the two litters.
The family set up of foxes is far more complex than most people realise and certainly, there are overlaps especially where cubs are involved.
