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Variation in Body Mass of Wild Canvasback and Redhead Ducklings

Results


Most mean differences in body mass within broods were around 35-50 g (Table 1). When expressed as a percentage of mean body mass, differences tended to be highest in Class IIA ducklings (6-7% in Canvasbacks, 9-11% in Redheads) and generally declined to 4-6% in Class IIC and older ducklings.

Table 1.  Mean differences in body mass (g) of Canvasback and Redhead ducklings within broods by age class.a
Sex Age class Number of ducklings Number of broods Difference in body mass (g)
nb mean of x SD Range
Canvasbacks:
F IIA 54 24 56 41.2 26.9 0-120
IIB 111 47 114 49.9 42.2 0-230
IIC 86 41 78 50.7 53.9 0-290
IIIA 9 5 10 74.2 46.8 10-170
IIIDc 16 9 12 45.8 40.2 0-120
M IIA 55 26 48 37.0 35.6 0-165
IIB 102 55 77 39.0 34.5 0-145
IIC 104 47 116 49.0 45.9 0-225
IIIA 14 8 7 49.3 34.0 10-95
IIID 22 8 21 63.2 86.8 0-325
F vs. M IIA 98 25 103 51.5 43.4 0-250
IIB 218 52 243 62.1 46.8 0-230
IIC 153 37 192 77.1 54.2 0-290
IIIA 60 6 24 90.0 64.1 5-245
IIID 13 7 43 107.1 62.4 0-395
Redheads:
F IIA 36 21 23 51.5 40.0 0-110
IIB 48 30 29 33.6 44.0 0-180
IIC 30 20 17 46.5 36.0 5-115
IIIA 6 4 2 37.5 31.8 15-60
IIID 6 4 2 52.5 46.0 20-85
M IIA 43 25 32 42.8 32.0 0-130
IIB 53 29 38 50.0 36.6 0-145
IID 19 13 7 35.0 48.8 10-145
IIIA - - - - - -
IIID 10 7 4 15.0 10.8 5-30
F vs. M IIA 61 16 67 46.9 37.5 0-135
IIB 78 17 76 53.0 58.0 0-315
IIC 32 11 36 57.5 31.5 0-120
IIIA 9 3 4 37.5 18.5 25-65
IIID 5 2 3 48.3 50.8 10-105
a Where broods contained ducklings of >1 age class, the data were summarized for the dominant age class of that sex.
b Number of all possible comparisons within brood.
c Ducklings in Class IIID could only be aged to Class III.

Assessment of factors affecting mean body mass in Canvasback ducklings was complicated by the lack of data in Class IIC and in 1976. By excluding age class IIC and 1976 in the first ANOVA, we were able to balance the design to assess effects of age class, sex, year, and season. Of the 4 factors, age class (F = 38.39; df = 1, 69; P = 0.001) and sex (F = 14.89; df = 1, 46; P = 0.0004) were significant. All other main effects (year and season) and interactions were not significant. Because season did not seem to account for variation in mass, we conducted a second ANOVA including Class IIC and 1976 and ignoring season. Effects of age class (F = 168.75; df = 2, 161; P = 0.0001), sex (F = 56.75; df = 1, 97; P = 0.0001), and sex-by-age class (F = 3.75; df = 2, 97; P = 0.0271) were significant. Within age classes, mean mass of male and female Canvasback ducklings did not differ in Class IIA but did differ in older age classes (Table 2).

Table 2.  Mean body mass (g) of Canvasback ducklings, expressed as LSMEANS, by age class and sex.
Age class Number of broods Number of ducklings Sex mean of xa SE
IIA 33 67 F 549.5A 14.81
31 62 M 562.2A 18.05
IIB 56 129 F 697.9A 10.86
62 113 M 739.1B 12.20
IIC 48 96 F 863.5A 10.39
52 114 M 930.0B 12.35
a Means within age class followed by a common letter are not significantly different (P > 0.05) using Fisher's protected LSD value.

Analysis of factors affecting variance of body mass within Canvasback broods was inconclusive. Although the overall test of no difference was significant (F = 10.08; df = 34, 25; P = 0.0001), when the main effects (year, season, age class, and sex) were analyzed separately, no effects or interactions were significant (P > 0.35).

Variance within broods was less than that among broods for both Canvasback and Redhead ducklings, except for Class IIC Canvasback females and Class IIA Redhead males (Table 3). Variance was highest among broods for Class IIIA female and IIID male Canvasbacks and IIID female Redheads, but sample sizes were small.

Table 3.  Variance in body mass of Canvasback and Redhead ducklings among and within broods, by age class. Percent contribution to total variance is in parentheses.
Sex Age class Number of ducklings Number of broods Variance
Among broods (%) Within broods (%)
Canvasback
F IIA 54 21 3,670 (75) 1,212 (25)
IIB 111 36 6,432 (63) 3,784 (37)
IIC 86 28 2,285 (42) 3,166 (58)
IIIA 9 5 -90a (0) 3,738 (100)
IIID 16 9 6,111 (79) 1,588 (21)
M IIA 55 22 6,137 (82) 1,391 (18)
IIB 102 39 6,051 (63) 3,502 (37)
IIC 104 32 5,593 (59) 3,845 (41)
IIIA 14 8 3,693 (69) 1,655 (31)
IIID 22 10 794 (12) 5,953 (88)
Redhead
F IIA 36 18 3,912 (65) 2,115 (35)
IIB 48 24 3,496 (57) 2,590 (43)
IIC 30 18 2,668 (59) 1,825 (41)
IIIA 6 4 -- --
IIID 6 4 -661a (0) 1,906 (100)
M IIA 43 19 3,596 (47) 4,023 (53)
IIB 53 21 4,863 (60) 3,231 (40)
IIC 19 13 4,733 (71) 1,890 (29)
IIIA 3 3 -- --
IIID 10 7 3,194 (94) 189 (6)
a Variance component not different from 0.

We found no significant differences in body mass between single- or mixed-species broods for any age class, sex, or species (Tables 4 and 5).

Table 4.  Mean body mass (expressed as least squares means) of Canvasback ducklings by agea, sex, and brood type (mixed vs. pure species).
Sex Age class Mean body mass (g) P c
Pure brood (n) b Mixed brood (n) 
F IIA 516 (19/8) 584 (35/15) 0.056
IIB 694 (46/17) 705 (65/25) 0.646
IIC 890 (40/14) 852 (46/20) 0.133
IIIA 957 (6/2) 928 (3/3) 0.591
IIIDb 976 (7/4) 995 (9/5) 0.776
 
M IIA 516 (19/9) 576 (36/15) 0.155
IIB 739 (54/19) 739 (48/28) 0.996
IIC 922 (66/21) 919 (38/17) 0.939
IIIA 1,068 (4/3) 1,019 (10/5) 0.413
IIID 1,077 (10/4) 1,124 (12/6) 0.289
a Age classes as defined by Gollop and Marshall (1954).
b Sample size = (no. canvasback ducklings/no. broods).
c H0: µ pure =  µ mixed.
d Classified only as Class III.
Table 5.  Mean body mass (expressed as least squares means) of Redhead ducklings by agea, sex, and brood type (mixed vs. pure species).
Sex Age class Mean body mass (g) P c
Pure brood (n) b Mixed brood (n) b
F IIA 415 (13/6) 480 (23/15) 0.094
IIB 682 (22/8) 630 (26/22) 0.081
IIC 797 (6/2) 741 (24/18) 0.159
IIIA   867 (6/4)  
IIIDd 822 (3/2) 814 (3/2) 0.782
 
M IIA 461 (23/8) 489 (20/17) 0.441
IIB 708 (21/7) 660 (32/22) 0.229
IIC 836 (5/2) 784 (14/11) 0.309
IIIA   849  
IIID 885 (2/2) 915 (8/5) 0.606
a Age classes as defined by Gollop and Marshall (1954).
b Sample size = (no. redhead ducklings/no. broods)
c H0: µ pure =  µ mixed.
d Classified only as Class III.

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