Because the first analysis showed no effect for nest box volume or altitude, we could then add 334 more observations. The results described below are based on the 945 observations for which we had information on clutch size, egg-laying date, latitude, and longitude. The mean clutch size was 4.49 eggs, with extremes of 2 and 7 eggs. We calculated whether latitude, longitude, egg-laying date, and year in which the clutch was laid had significant effects on clutch size. The quick answer is yes.
Clutch size increases from south to north and from east to west.
Figure 1 Figure 1. These four graphs illustrate different models for describing latitudinal variation in clutch size. In all cases, clutch sizes vary across the season. The red lines represent clutch sizes farther south, and the black lines represent clutch sizes farther north. In these hypothetical examples, we do not consider possible effects of year or of longitude.