I'm not one to get all upset over "songbirds" such as the fetishizers on the Audubon websites seem to be: it's a nomenclature based on brain structure (aka passeriformes), like with many things, used for emotional effect. Many of the Songbirds in my backyard, in fact, are what's known as both Nuisance Wildlife and Invasive.
This said: had the upsetting case today, where the black cat brought in an Invasive bit of Nuisance Wildlife* to finish off. Didn't occur to me that the Poor Thing wasn't a toy until I separated her from it.
Since the injuries were such that Heroic Measures would have only prolonged his misery, I had to take him outside and kill him myself, before giving the little guy a decent burial.
Came back in and had a good cry.
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* http://waiting-for-april.blogspot.com/2010/04/even-further-inspired-by-aforementioned.html
* http://waiting-for-april.blogspot.com/2010/04/even-further-inspired-by-aforementioned.html
* * Male House Finch - invasive to the NE USA; crowd out the other, native, less aggressive finches