Comparing BirdsEye with other Custom Maps Images
BirdsEye image on a 78sc    USAPhotoMaps .kmz on a 78sc     Google Earth .kmz file on a Dakota
The USAPhotoMaps "Urban" images are from the 4-pixels-per-meter Microsoft MSR Maps.

Other Solutions:
There are two types of free map programs that can create higher-resolution Custom Maps .kmz files than BirdsEye.  

The easiest is the free USAPhotoMaps to create a "Big" .jpg image of the
Microsoft MSR 'Urban' color maps. (These maps look much better when enhanced by Photoshop Auto Color Correction.)  The created .jpg file is accompanied by a .txt and .jgw file (similar to a DRG map's .tfw file) in order to register the coordinates.  With this information, G-Raster (another free program) can convert the .jpg file in one operation into a .kmz file needed by Custom Maps for Garmin's latest units.  

Google Earth can also be used to create even higher-resolution .kmz maps (as high as 8 pixels per meter).  However, at present, is a very time-consuming (or an expensive task if using Global Mapper).

Click on Thumbnails
BirdsEye                                                               TerraServer

USAPhotoMaps  MSR Maps    (4 pixels/m)                                        Google Earth  (8 pixels/m)

This USAPhotoMaps MSR Map is from a 2002 database when the roofs were painted blue.
Google Earth has the best resolution but is much more difficult to create Custom Maps.