59 
NOTES ON THE CHRONOLOGY OF THE 1950 WATERFOWL NESTING 
SEASON IN SOUTHERN MANITOBA 
Lyle K. Sowls 
The proper interpretation of breeding ground surveys in the exceptionally 
cold and late spring of 1950 partly depends upon an understanding of the lateness of 
the season and its effects upon nesting chronology. The spring of 1950 was one of 
the latest and coldest ever recorded in central Canada. What effect did this abnormal 
weather have in delaying nesting? Did it shorten the nesting season and thus reduce 
production by. limiting the number of opportunities for renesting? 
Along with studies of nesting behavior, migrational homing, nesting ecology, 
and renesting, an effort to compare the chronology of the nesting seasons is being 
made at the Delta Waterfowl Research Station through intensive study. The starting 
date for all nests found has been determined by the aging of the embryos and back- 
dating by the age of the embryo plus the number of eggs in the nest to arrive ata 
beginning date. The plotting of these beginning dates then gives a curve of the nesting 
chronology for the season. To accomplish the aging of the embryos, one egg from 
each nest was taken and its embryo compared with an embryo of known age in the 
station collection which has been made up from incubator eggs. 
It is the purpose of this report to compare the years 1949 and 1950 from the 
standpoint of nesting dates and to speculate on the meaning of the comparison. 
For this purpose the nesting curves of three species (mallard, pintail, and 
blue-winged teal) are shown in Figure 1. These curves give the percentage of the 
total number of nests found which were started in each of the week-periods of the 
nesting season. In the construction of these curves only one assumption is made that 
the amount of effort in finding nests was the same for each year. To be certain that 
the assumption could safely be made, repeated nest-hunts were made in the same 
manner for both years beginning as soon as the ducks arrived in spring and continuing 
until after repeated searches yielded no nests at all. The majority of the records used 
were gathered on the special study area at Delta where other intensive work is being 
done. 
For some species there are other means of determining the end of the nesting 
season, and it is also possible to get information about the percentage of hens which 
were unsuccessful nesters, In 1950 the end of the nesting season, as indicated in 
Figure 1, was followed closely by an influx of pintails into the Delta marsh, This 
population did not consist entirely of males as would have been the case had all hens 
been successful in hatching broods or had unsuccessful hens been still renesting. 
For one small water area within the study area counts of pintails were begun as soon 
as the influx became apparent and were continued until the drakes were so far into 
eclipse plumage that they could no longer be identified from females. These counts 
over a period of 10 days are given in Table 1. 
