Two main factors comprise population change. First, population can increase or decrease through net migration. That is, over the year, people either move into or out of the area in question. The second factor is natural increase or decrease. An area naturally increases in population if more births than deaths occur in a given year.
Following a dramatic decline in gains from migration during the recessionary years of 2007 to 2009, an increasing number of people have moved to Oregon during the past three years. Net migration increased from 14,027 in 2012 to 23,280 (+66%) in 2013. While net in-migration increased, natural increase stayed roughly the same, at about 12,000. This means net migration gains made up about two-thirds of Oregon’s 2013 population growth. The combination of natural increase and net in-migration (+35,300) led to the largest increase in population since 2008.
For more information on Oregon's population change, read the full article here: In-Migration Makes Up Two-Thirds of Oregon's 2013 Population Growth.