The magpies were scolding incessantly for several minutes. It finally got my attention, so I got up and looked out my living room window. I saw the problem almost immediately: Settled in the crook of the tree, looking rather comfy, was a white cat I've seen around the neighborhood before, and I'm sure, the same cat that once made its way into the unfinished magpie nest now above it.
The magpies who built the nest still see this birch tree as theirs. They scolded and chattered at the cat, and, amusingly, attracted a number of other birds, like a couple of great tits and blackbirds. It took a while but the birds finally got the cat's attention; this blackbird (thrush) came very close. (I apologize for the blurriness.)
It occurred to me that my first assumption about the birds being attracted to the cat in the birch tree was for the entertainment value, was wrong. I now concluded that the birds were gathering across species to rid the tree of a common enemy. The magpies absolutely did not want the cat in their tree, and other birds were helping in pestering the cat to leave (though I still suspect entertainment mattered, too). They managed, instead, at one point to arouse the cat's hunting instinct (see magpie out on a limb to the left of the cat), but still the cat, although it could no longer stay in peace in the tree, was very reluctant to leave it. Behind on the right, next to a boulder, I spotted the reason why: Another cat.
It finally became obvious to me why the white cat went up the tree in the first place: To avoid the tuxedo cat. I have seen this white cat intimidate other cats, actually behaving either brashly or retardedly in that it more or less ignored the other cat's body language, which was screaming "Stay the f**k away from me!!!", and would try to approach the other cat, anyway. Now here was a cat that the white one wanted to avoid.
Whitey ultimately had no choice and reluctantly got down from the tree, attracting the other cat's attention. Of course, they denied me an end to this story by disappearing behind a building.
High entertainment value!