Here, let me tell you excatly what it is.
It's spring, time when almost all bird species nest and produce young ones. You're probably seeing multiple blackbirds, starlings, finches, pigeons ....... protecting and trying to scare away crows, jackdaws, magpies, jays, rooks which are all very smart opportunistic predators who specifically search and prey on the smaller bird's nest and eat their eggs and young ones. That's why you don't see ''big'' birds fighting back because they don't give a damn about adult blackbirds or starlings, they are focused only on their nests.
And yea that's universal it's happening all around the world right now