I'm torn between joking about how birds need to learn their place (their songs are about making territorial claims, so essentially their sweet chirps are telling each other--and maybe us--to frack off) to wondering why cars considered that valuable aren't put into garages (though my GUESS would be so they can show it off, like when Giles was eager to show off his new car in season 5).
A car left outside has a lot more to worry about than birds (and I've heard of at least 2 cats killed because they decided to sleep by a wheel under cover while outside, though usually I mean the animals that leave their prints on car as they walk over them, sometimes with muddy paws), and a prized car left outside seems to be inviting car thieves to try to get it as well, either for the car itself or just to troll the owner so proud of it. And branches (especially those that have been messed with as in this case) can also fall off when the wind gets bad enough, and I'd think many would not want a branch full of spikes falling upon their beloved car.