Weight.

The weights of the owls varied considerably during the year, being highest in winter, when in some cases an extraordinary amount of fat was to be found not only around the viscera but also forming a thick layer below the skin. The two sexes are similar in weight, as they are in size. The weights of 93 males ranged from 139 to 273 grms., while the limits in the case of 104 females were 139 and 298 grms.

The birds were weighed just as they were received, so that some variation in weight is due to the amount of food in the stomach. In order to find out how far this affected the result, the stomach contents of two birds with unusually full stomachs were weighed. The total weights of the birds were 174 and 170 grms, and the weights of their stomach contents were 4.3 and 4.5 grms. respectively. Clearly the weight of the food in the stomach will not interfere with the use of gross body weights in the study of the seasonal variations, which are of the order of 100%.

The changes in weight from month to month are shown in Table IV and the graph. The figures plotted are the mean weights, and it will be seen from the table that the monthly number of birds varies from 1 to 17, the mean being about 4. In spite of the smallness of the samples the shapes of the graphs for the two years are similar, and so it seems probable that the true changes are shown.

In the male the weight falls steadily from a maximum about July until November, when it remains low and more or less the same for two or three months. Then it rises again towards the winter maximum. There is, however, a minor loss in weight about April or May, interrupting the upward trend. The curve for the female is of similar shape except for a sudden and very marked increase in the spring, about October or November. The minor loss in weight in the autumn occurs one or two months earlier in the female than in the male. As the moult takes place in autumn it is reasonable to assume that this loss in weight is associated with it. This is confirmed by the fact that of the birds recorded as being in moult before March 1, eight were females and four were males; of those after March 1, seven were females and 15 were males. The high average weights of the females in October and November are partly due to the presence of a few very heavy birds, but even if these are excluded the average weight of the remainder is above that of the males. This increase in weight of the females is clearly connected with the changes in metabolism during the development of the ovary, and perhaps also to the period of enforced inactivity during incubation.

The same general form of curve showing the seasonal changes in weight has been found in a number of different birds. Nice, working with the Song Sparrow, Melospitza melodia, found a winter maximum for both sexes in January. The weight decreased during the spring in both sexes, but the females showed an increase during laying and incubation, as in the owl. Similar seasonal changes are recorded in other passerine species.