Straw spew

Apologies to the sensitive souls amongst you, but this had to be done today.  For the last month I've walked past this straw bale left in the fields and it periodically looks like something (a fox?) has been sick on it.  

It looks like this one day and then the next it goes brown and eventually crunchy...and then a few days later another yellow spot appears.

Walking with my friend Liz we noticed another yellow patch and, how funny and brilliant, some bird has walked across it...!  We think it must actually be some sort of fungus - can anyone enlighten me?  

Lots of photos from the Bookfestival here.