The treating of birds as something "completely different" from other dinosaurs, that is. There is no magical "division" that makes avians fundamentally different from other dinosaurs in general any more than sauropods or ceratopsians are different from non-sauropod and non-ceratopsian dinosaurs. Even going only by superficial similarities, there's nothing that all non-avian dinosaurs share that they don't also share with avians. And deinonychosaurs (for example) are more similar to birds than to (for example) ceratopsians, so any group of related animals that includes both deinonychosaurs and ceratopsians must also include birds. To say otherwise is ridiculous. Besides, no one has the slightest problem with bats and whales both being mammals. The only way you could even come close to logically arguing that birds aren't dinosaurs is to make a case for birds not being of dinosaurian origin, period, but at this point you're more or less out of luck with that, too. Birds are dinosaurs. Fact. It's not any stranger than bats being mammals. There isn't a difference.
I've talked about this (baffling) issue previously here