There is a big difference between Serb birthrates/population, and Serbian birthrates/population.
Serb birthrates can only be calculated on a global scale, because Serbs are an ethnic group and can be found practically everywhere.
Serbian birthrates, i.e. the birthrates of the population/citizenry of the Republic of Serbia is another matter. In relation to the issue of Serb birthrates, Serbian birthrates only cover the Serb birthrates within Serbia, completely ignoring the Serbs in Bosnia, Montenegro, Croatia, Western Europe and the rest of the world, while also incorporating the birthrates of non-Serbs within the Republic of Serbia, such as Hungarians, Romanians, Albanians, and others.
The difference is important.
You're correct though; the demographic trends are definitely not good. Serbs/Serbia are not unique in this though. Practically all of Europe, the West and even Japan has these issues.
A lot of people mention the bad economic conditions in Serbia, but this only goes so far as an explanation. There are many places in the world that are far, far worse than Serbia and yet are doing better demographically. I think Serbia suffers from the disappointment of unmet high and possibly unrealistic expectations. They're not as wealthy as they want to be, but they're also not poor enough to just not even try any more.
There are many reasons Serbian demographics are terrible, but I think a lot have to do with expectations and disappointment. I remember reading a UN report on world happiness from 2013, and Iraq scored higher on happiness than Serbia. FUCKING IRAQ! Iraqis are/were happier than Serbians!
Serbia has more than just economic issues. I also believe that the entrenched cultural tales of the golden age of Yugoslavia, where everyone has a steady job, decent pay, holidays and worldwide respect, has left a hard-to-deal-with impression on the younger generation of Serbians who have none of those things, yet exist in a world (i.e. EU/West … "world" always means the EU/West when Serbians talk of non-Serbia… it doesn't refer to Pakistan or Mexico or Vietnam or anything else that exists… "world" is only the good parts) in which they do, and Serbia wants to be part of it.
Serbia definitely has many problems, but the idea that it's all because it has a poor economy and is always around the corner from a war, has become a lazy cliche which acts more as as an excuse than a real reason.
I'm probably going to be downvoted as hell for this, ali iskreno bmk.