Why do roosters wake up so early?

That is a most interesting question and one that probably has several factors. One that is most significant in my opinion has to do with the hormone that makes all birds sing, or in this case crow. The hormone is testosterone. In the early hours of morning a rooster's testosterone level probably goes up- along with some others.

Biologists have a theory that birds sing or crow to let other birds know where they are so they can space themselves far enough apart so they don't overcrowd each other. You could think of this a defending their territory, and rooster are very protective of their hens. They often fight to the death with other roosters.

So we might interpret the rooster crowing at the beginning of the day to clear out rival males by crowing really loud and scaring them away. They do this in the daytime because they sleep at night and would not engage in battle in the darkness since they are diurnal, active in the daytime.

Roosters crow to announce their presence to possible rivals, they communicate their whereabouts to both their own hens who may be off foraging and to entice hens from other flocks to join them.