//Density_Image is the image of bird densities I want to reduce.
//ROI is a FeatureCollection of regions within my region of interest.
print(Density_Image.reduceRegion({
    reducer: ee.Reducer.sum(),
    geometry: ROI.geometry().geometries().get(3),
    maxPixels: 1e11,
    tileScale: 16}));

var pop_est_table = table.map(function(f) {
    print(f);
    // line above throws an error that argref = null.
    Density_Image = ee.Image(remap2);
    print(Density_Image);
    var pop_est = Density_Image.reduceRegion({
        reducer: ee.Reducer.sum(),
        geometry: f.geometry(),
        bestTry: true,
        maxPixels: 1e12,
        tileScale: 16});
    return ee.Feature(f).set('pop_est', pop_est);
});