Is this normal behaviour?

On my way back from my hunt for bees and butterflies I was struck by a loud ruckus coming from a tree. There were about 7 or 8 magpies making their loud clacking noise along with an angry sounding crow.

crow's nest-4

There was a nest in the tree, although I am not sure who’s and it certainly didn’t look like a magpie’s nest

crow's nest

It took me a while to work out what the fuss was, but as usual it was someone’s precious pet that had set the birds off:

crow's nest-3However, this isn’t a rant about too many cats in too small an area, but I wondered if this was normal behaviour for the corvids?  The crow was on it’s own and I would have thought that if it was defending a nest it would have been in a pair, as the rooks in a nearby rookery already seem to be showing signs of getting ready to nest.  I also can’t remember the last time I saw more than 3 or 4 magpies together, certainly not at this time of year, and, would they really help out a crow?

Maybe they were just not happy about a cat being in their territory.  After a while the magpies flew off to a nearby tree, leaving the crow to remain angry alone for a little bit longer before he (or she) gave up and flew away as the cat seemed distinctly unperturbed.

I did note some group mobbing by some crows a few days later, this time against another crow – thus overturning my notions that crows, unlike rooks and jackdaws, tend to be less gregarious and more solitary.