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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		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	a	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.

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).