Diet of Canvasbacks During Breeding
Results
Canvasbacks consumed both plant and animal material throughout the breeding season (Table 1). The proportion of plant and animal foods consumed did not differ among reproductive periods in females (F = 0.58; 4 df, P = 0.678), nor between males and females (F = 0.64, 1 df, P = 0.427). The lack of significant differences is probably due in part to the great variability of diets among individuals. Fennelleaf pondweed and midge larvae were the predominant foods except during incubation in females, when snails were the major animal food. Overall, remnants of Gyraulus, Lymnaea, and Helisoma snails occurred in 23 females and 6 males.
Plant foods consisted primarily of tubers and shoots of fennelleaf pondweed. Tubers contributed 91% of the total pondweed volume when tubers and shoots were measured separately. Seeds occurred in about half of the samples, but the volume was relatively small (Table 1). Seeds from pondweed and Amaranthus comprised more than 60% of total seed volume.
| Table 1. Foods consumed by breeding canvasbacks in southwestern Manitoba, 1977-81. | ||||||||||||
| Food | Percent occurrence | Mean percent volume | ||||||||||
| Males | Females | Males | Females | |||||||||
| (14)b | Prea (11) |
RFG (8) |
Lay (17) |
Incub (5) |
Renest (7) |
(14) | Pre (11) |
RFG (8) |
Lay (17) |
Incub (5) |
Renest (7) |
|
| PLANT | 93 | 91 | 88 | 88 | 60 | 86 | 57.8 | 52.3 | 64.0 | 42.8 | 32.2 | 51.0 |
| Vegetation | 93 | 64 | 75 | 65 | 60 | 86 | 51.0 | 45.6 | 63.9 | 41.1 | 32.1 | 50.9 |
| Potamogeton pectinatus | 57 | 36 | 75 | 59 | 40 | 57 | 49.8 | 33.6 | 63.9 | 40.4 | 32.1 | 50.9 |
| Chara | 7 | 9 | 0 | 6 | 0 | 14 | trc | tr | 0 | 0.5 | 0 | tr |
| other | 21 | 36 | 0 | 6 | 20 | 29 | 1.2 | 12.0 | 0 | 0.2 | 0 | tr |
| Seeds | 43 | 55 | 38 | 65 | 20 | 57 | 6.8 | 6.8 | 0.2 | 1.8 | 0.1 | 0.1 |
| ANIMAL | 64 | 82 | 63 | 100 | 80 | 86 | 42.2 | 47.7 | 36.6 | 57.1 | 67.7 | 49.0 |
| Insecta | 64 | 64 | 63 | 100 | 80 | 86 | 40.7 | 46.3 | 35.9 | 48.4 | 11.0 | 41.4 |
| Odonata | 21 | 18 | 25 | 35 | 80 | 29 | 2.1 | 9.1 | 0.3 | 2.5 | 1.1 | 5.3 |
| Anisoptera | 0 | 9 | 0 | 6 | 40 | 14 | 0 | 0 | 0 | 1.4 | 0.2 | 5.3 |
| Zygoptera | 21 | 18 | 25 | 35 | 80 | 29 | 2.1 | 9.1 | 0.3 | 1.1 | 1.1 | 0.9 |
| Trichoptera | 14 | 18 | 25 | 53 | 80 | 14 | 1.4 | 0.2 | 11.9 | 9.9 | 4.2 | 0.7 |
| Chironomidae | 57 | 64 | 63 | 82 | 20 | 71 | 36.6 | 37.0 | 23.6 | 35.8 | 5.5 | 34.3 |
| Corixidae | 14 | 9 | 12 | 24 | 0 | 14 | tr | tr | tr | 0.2 | 0 | tr |
| Dytiscidae | 14 | 9 | 38 | 12 | 0 | 14 | 6.7 | tr | 0.1 | tr | 0 | 0.2 |
| Gastropoda | 21 | 18 | 12 | 47 | 80 | 14 | 1.5 | 0.1 | tr | 8.4 | 56.4 | 7.6 |
| Hirudinea | 0 | 9 | 0 | 12 | 20 | 0 | 0 | 1.2 | 0 | 0.4 | 0.4 | 0 |
| Other | 7 | 27 | 0 | 0 | 0 | 29 | tr | tr | 0 | 0 | 0 | tr |
| a Reproductive
categories: Pre = prenesting, RFG = rapid follicle growth, Lay = Laying;
Incub = incubation; and Renest = renesting. b sample size c tr = trace (<0.1%) |
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