What do you mean by general population numbers? I doubt that any state (let alone the entire country) has numbers for how many Common Ravens or Virginia Opossums there are. Wildlife action plans include statewide relative abundance’s (coded by how imperiled they are). Trapping harvest data may be available for opossums for various states, but they aren’t necessarily corrected for trapping effort (e.g. the average trapper is retirement age and the number of trappers is falling in many states). Bird data are commonly collected as part of the Christmas bird count and breeding bird count. I haven’t played with those data to know if there are “population numbers.”

Often ecologists who are interested in population trends try to study isolated populations (e.g. moose and wolves on an island) or species with limited ranges and minimal dispersal between patches (eg . Karner Blue butterflies). It’s easier in those cases to define the population boundaries and get a more accurate count of individuals. You have picked two species that are common, very widespread, urban adapted, and which have limited barriers to dispersal, so even if you find numbers (Researcher X found an opossum population density of x individuals per square km in a rural area and y individuals per square km in the suburbs in the midwestern US), it may be hard to extrapolate to their full range.

If you have the opportunity to change your species to either more commonly hunted/trapped species or range-limited endangered species, you may have more luck finding data.