I have a partial result in terms of visit-statistics and have posted two plots here.
The first plot depicts the number of visits made by each species on a particular day. The second plot depicts the number of visits made one or more Eurasian Tree Sparrow(s) per hour each day (the images can be enlarged by clicking on them).
Obvious is the increase subsequent to the beginning of nesting (09.03.2010) and well that since nesting, the other two bird species have visited less frequently if not at all. Visits made by the Sparrow after the midday day hours were all made post 9th of March, i.e. beginning of nesting (I defined this as the date on which the Sparrow brought the first twigs in the box, "heavy" nesting actually began on the 15th of March)