This is one hefty tome, being the result of decades of birding and a monumental amount of compiling. Birds of Ontario lists every known (wild) bird to have been observed in the province of Ontario and privides information concerning identification, measurements, and seasonal residency. For migrating birds, average and earliest/last arrival/departure date is provided based on observations by naturalist clubs and individual researchers. For the common birds of Ontario a grid-marked map of the province is provided for Christmas Bird Counts and Breeding Bird Surveys; the former lists (per grid) the average number of birds observed per 100 party-hours, while the latter the average number per 50 stops. Both are based on information collected between 1968 and 1977. Given this, and the fact that the book was published in 1985, suggests that the information could be now dated considering the amount of habitat destruction that has occurred since then, not to mention other ecologically-related matters. For the average birder, this book is not necessarily essential--especially for $150--but if you are fortunate enough as I was to come across one cheap, it is certainly a useful volume to add to one's library.