Explanations for birth seasonality can be classified under two main headings: sociocultural and environmental. Within the former category, the role of social class, marriage, holidays, and agricultural cycles have been implicated as determinants of the seasonality phenomenon. Within the environmental class of hypotheses, temperature (heat) and photoperiod (length of daylight over the seasons) have received much attention in the biometeorological literature.

The above is from: Frank Trovato and Dave Odynak, (1993), The seasonality of births in Canada and the Provinces, 1881-1989: theory and analysis, Canadian Studies in Population 20,1:1-41

So......how do the Outman's compare to the patterns described above?
Below are some graphs so you can see for yourself.