I'm not suggesting that this applies to any of the examples given in this thread, but bear in mind that birds aren't always doing/eating what you think they are:
Seeing many Sparrowhawk kills in my garden over the years, means I've also seen a few spooked from their meal by neighbours etc. Sometimes the Sparrowhawk flies off with what remains of it's prey (I won't go into the reasons why/why not), other times the meal is left behind. The Sparrowhawk will usually return later or the next day to finish the meal.
But this isn't about Sparrowhawks, but about Pigeons. A few times I've seen Feral Pigeons return after the Sparrowhawk has left, go to the prey (now dead), and apparently eat the flesh. However, looking closely I saw that they wasn't eating flesh at all; they was feeding on the seeds that was in the prey's crop and stomach, that was now exposed.
I can't say this any other way really; the bloody mess was just a seed container to the Pigeons.