Lost in...navigation?

Since a few days I can observe (or better hear) a weird thing going on in the frontyard of my dorm. Every night around eleven I hear birds starting to chirp all over the place. That phenomenon is going on till the early morning hours (which I noticed because I stood up at 0730, a very uncommon thing for me). I don't know why they don't chirp during the day, probably they have to go to work :-) or just sleep. I'm not exactly an ornithologist, but wheren't they supposed to be on holiday somewhere in the south at that time of the year?

What evil lurks in the dark behind this sinister thing? Is it just another implication of the phenomena we know as "El Nino"? Is it due to globalization, maybe other birds, who do the job in the south for less payment? Or is it just a different species of birds, who are just accustomed to the temperatures here (although it's not that different from the one's we have in Austria, at least here down in the south)

Feel free to elucidate me!