Translation:Now the birds are flying back north.
"Сейчас" is a general-purpose "now", with the same range of meanings from "at this very moment" to "at present".
"Теперь" is "now in contrast to what has been the case before/earlier". In that it better corresponds to English "and now".
They are generally not interchangeable (without changing the meaning that is).
Also, сейчас is a joint word consisting of two parts: сей (an outdated this) and час (an hour). So, сей час would be this hour, which coins the meaning of сейчас as a general-purpose now.
"Теперь" can't replace "Сейчас" , it is more specific indeed, but the other way around is possible.
"теперь" is not erally a contrast, but it is always used if you discussed what happen before, it can be just a current step in some sequence: "Cначала включи ноутбук, потом введи имя пользователя и пароль, теперь ты можешь им пользоваться " - First switch on the laptop, then enter the username and the password, now you can use it.