Northern Prairie Wildlife Research Center
Spring Arrival Dates. The average date when resident SY mallard hens were first seen was 5 May, which was significantly (F = 92.9; 1, 54 df; P < 0.001) later than the average arrival date of 20 April for ASY hens. There was no significant difference (F = 1.21; 1, 54 df; P = 0.27) between the date when ASY hens and ATY hens were first seen nor between hens that were previously successful or unsuccessful. The date when 10% of all resident hens were first seen was 1 April, the median date was 22 April, and 90% were seen by 14 May.
The mean return date of SY gadwall hens was significantly (F = 134.6; 1, 69
df; P < 0.001) later than ASY and ATY hens (Fig. 2). Successful hens as a group
returned significantly earlier (F = 3.76; 1, 69 df; P = 0.06) than unsuccessful
hens. Ten percent of all resident gadwall hens were first seen by 16 April,
and 50% were first seen by 9 May.
|Fig. 2. Age and previous success of resident gadwall hens related to mean spring arrival dates on the Koenig study area, North Dakota, 1980-81.|
Nest Initiation Dates.-The timing of nest initiation by resident mallard hens did not vary greatly between 1978-81 (Fig. 3). Over the 4 years, an average of 10% of all mallard nests were initiated by 26 April. The average date of nest initiation in 1979 was 5 days later than the 4-year norm, possibly because April temperatures were 9 C below the monthly average. The median nest initiation date in 1978-81 for resident mallard hens was 16 May, and 90% of the nests were initiated by 6 June. We found no significant effect (F = 0.43; 2, 2 df; P = 0.70) of hen age on either the start or the length of the nesting season.
|Fig. 3. Nest initiation period of resident mallard hens, showing the 10, 50, and 90% quantiles on the Koenig and Woodworth study areas, North Dakota, 1978-81.|
Hammond and Johnson (1984) defined the central span of nesting to be the interval from the date when 10% of observed nestings began to the date when 90% began. The central span of nesting from mallards on the Koenig study area was 42 days.
The nest initiation period of resident gadwall hens varied little among years. The first 10% of the nests were initiated within 2 days of 20 May except during 1979 when nesting was delayed 6 days (Fig. 4). Age of gadwall hens was significantly (F = 27.0; 2, 6 df; P = 0.001) related to nest initiation period (Fig. 5). The first 10% of nests were initiated by 15 May for ATY hens, by 21 May for ASY hens, and by 29 May for SY hens. Older hens also nested over a longer period. The central span of nesting averaged 30 days for all hens but varied from 24 days for SY hens to 40 days for ATY hens. The sample size of resident blue-winged teal hens was too small to examine arrival and nesting parameters.
|Fig. 4. Nest initiation period of resident mallard hens, showing the 10, 50, and 90% quantiles on the Koenig and Woodworth study areas, North Dakota, 1978-81.|
|Fig. 5. Nest initiation period related to the age of resident gadwall hens on the Koenig and Woodworth study areas, North Dakota, 1976-81.|