Why is the population of Europe expected to decrease?

Europe's death rate is currently greater than that of it's birth rate, however this does not apply to ALL of Europe, just Europe altogether. Western Europe's population is increasing (with the exception of Germany) while Eastern Europe's is decreasing for the most part. For example, Ireland is growing at a rate of 1.06% while Bulgaria is declining at a rate of -0.78%. The death rate is generally higher than the birth rate for every country, which is the problem. However, Western Europe has a large Immigration rate, which accounts for most of the positive growth, while Eastern Europe has a negative migration rate for the most part, which accounts for most of the negative growth. However, there are signs of improvement, such as in Russia. Just in their 2010 census, Russia experienced the largest birth rates since the dissolving of the Soviet Union and are is expected to switch to a positive growth by the time the next census comes around in 2018 (Russia is even thought to be growing presently, but can not be confirmed).