Time switches can only roughly accommodate the changing time of dusk and dawn, and can't handle dark weather at all. Said otherwise: if you're trying to approximate a light sensor, a clock is a worse approximation than a live chicken.

Or to give a specific counterexample to "The idea that [a chicken could be] more effective than using a clock seems ridiculous": during a solar eclipse, a chicken may go in to roost, but it would have to be a pretty sophisticated (or broken) clock to handle that case. (I'm not saying that solving for the solar eclipse case is necessary or useful, just that it demonstrates the point.)