animal matter than is suggested by the studies indicated above. Bartonek and 

 Hickey (1969a) reported that esophogeal contents of juveniles and adult females 

 in Manitoba were 87% and 92% animal matter, respectively. Adult males had con- 

 sumed only 2% animal matter. Their principal food was tubers of Potamogeton 

 (95%). 



Food habits in the southeast have been little studied, and we have found 

 but one detailed report on food habits in this area. Quay and Critcher (1965) 

 reported foods eaten by 62 Canvasbacks collected in Currituck Sound during win- 

 ters from 1947 to 1952. They found that the Canvasbacks had subsisted almost 

 entirely on vegetable matter, primarily the vegetative parts of pondweeds 

 ( Potamogeton spp. - 21.0% by volume) and their seeds (38.2%). The vegetative 

 parts and seeds of widgeongrass (12.3%) and southern naiad ( Najas guadalupensis 

 - 4.6%) were also important. No other identified plant material formed as much 

 as two percent of the diet. 



In both South Carolina and Georgia, banana waterlily ( Nymphaea mexicana ) 

 was a preferred food item (Cely 1980b). At Merritt Island NWR, Florida, where 

 this plant occurred but was unavailable to Canvasbacks, these ducks fed largely 

 on widgeongrass, muskgrass, raanateegrass ( Syringodium f iligormis ) , and inverte- 

 brates (Cely 1980b). McAtee reported that this plant was found in over 70% of 

 the Canvasbacks collected at Lake Surprise, Texas, but at Laguna Atascosa NWR, 

 where this plant did not occur, Canvasbacks fed primarily on widgeongrass (Cely 

 1980b). In Louisiana Canvasbacks eat acorns (Lowery 1931 i_n Palmer 1976b); 

 tubers of delta duck potato ( Sagittaria platyphylla ) were reported as an Impor- 

 tant food in the Mississippi Delta (McAtee 1917 i_n Bartonek and Hickey 1969a, 

 Palmer 1976b). 



Bartonek and Hickey (1969a), Palmer (1976b), and Bellrose (1976) give fur- 

 ther information on foods eaten in other portions of the range. 



IMPORTANT BIOLOGICAL PARAMETERS 



Egg Laying Throughout its range, the Canvasback begins nesting at about 

 the same time, late April to mid-May. Nest initiation peaks around 10-25 May 

 in the heart of the range (Bellrose 1976). 



Mean Clutch Size Redheads ( Aythya americana ) frequently parasitize nests 

 of Canvasbacks, which depresses the number of eggs laid by the host species. 

 In studies involving more than 500 nests, both parasitized and unparasitized, 

 the average clutch size for Canvasbacks was 7.9 eggs. In nests that are not 

 parasitized, the average is about 9.5 eggs with a range of about 7 to 12 (Bell- 

 rose 1976). 



Incubation Period Incubation takes 24-29 days (mean = 25)(Erickson 1948a). 



Hatching Success Nesting success varies dramatically from year to year and 

 place to place. In studies totalling 1,715 nests, 46.2% were successful in pro- 

 ducing young. Reports cited in Bellrose (1976) indicate that eggs may hatch in 

 as few as 2.7% of the nests in poor years. About 7.3 eggs hatch in the average 

 unparasitized nest, but only 6.0 hatch in parasitized ones (Bellrose 1976). 



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