It’s called the ‘inner platform effect’. They are basically replicating parts of the underlying platform (the OS in this case) inside their own application, until the application turns into a platform itself, one crappier than the one below it. You see this happening with web browsers and ‘web apps’.
It’s actually quite rare for cats to kill birds, it’s a skill very few cats have. None of our cats has ever brought back a dead bird, while they have gone on an absolute killing spree among the local mouse population.
The birds around our house (mainly jackdaws, which are wicked smart) like to taunt the cats though.