Table 5.4 Aggregate Percent Dry Mass and Percent Species Occurence of Foods Consumed by 12 Species of Waterfowl (579 birds) Collected During Fall on the Inner Bay of Long Point, 1992-1994.
Figure 5.9 Proportion of aquatic plant, agricultural grain, and animal matter in the diets of 12 Waterfowl species harvested in the Long Point Waterfowl Management Unit, 1991-1994.
Submergent plants are consumed by large numbers of invertebrates, fish, mammals and reptiles. However, this food source is probably most important for and readily exploited by waterfowl, particularly during migration (Crowder and Painter 1991, see Figure 5.9).
While some waterfowl species consume large quantities of agricultural grains and/or aquatic macroinvertebrates (primarily zebra mussels) , LPWWRF has determined that submerged aquatic macrophytes provide an essential food source for most waterfowl staging on Long Point during the fall (Figure 5.9, Appendix 1 to 12). They are presumably also essential for waterfowl during the spring at Long Point. Of 579 birds (12 species) sampled during the fall by LPWWRF, 40% of the food that had been consumed was aquatic plant matter. While over 50 species of plants were consumed, only 8 plant species represented over 1% of the combined dietary intake (aggregate percent dry mass of the diets). Therefore, while only a few species are consumed in substantial quantities, a wide array of aquatic plant species are eaten by waterfowl that stage on Long Point.
Proportion of Aquatic plant, agricultural grain, and animal matter in the diets of 12 waterfowl species collected on Long Point Bay, 1991-1994, based on the aggregate percent dry mass of dietary items. For sample sizes see appendices 1-12.
Figure 5.9 Proportion of aquatic plant, agricultural grain, and animal matter in the diets of 12 Waterfowl species harvested in the Long Point Waterfowl Management Unit, 1991-1994.
On an aggregate percent dry mass basis, wild celery was the most prominent submerged aquatic plant in the diets of ducks at Long Point (Table 5.4). However, while it was consumed by 67% of the species collected, it only represented a substantial portion only of the diet of Canvasbacks, Goldeneyes, and Redheads (Appendices 4, 6 and 11). Wild celery is particularly important for staging Canvasbacks, as the tubers and leaves of this plant constituted 75.7% percent of their diet. In fact, wild celery may be an even more important dietary item for waterfowl than these values suggest, as its tubers have a very high digestion rate, and may therefore have been under-represented in the dietary samples collected by LPWWRF.
Musk grass was the second most prominent plant in the diet of ducks collected at Long Point (Table 5.4). While it was consumed by 75% of the species collected , it was only consumed in substantial quantities by Redheads and American Wigeon. This is somewhat surprising given its high nutritive value and extensive distribution in the Bay (Table 5.2).
The increased availability of Naiads on the Inner Bay has probably been beneficial for waterfowl (Figure 5.4) While the two species constitute only 12.1% of the Bay's submerged plants, they are the third most commonly consumed aquatic plant, in terms of aggregate percent dry mass, and they were consumed by all 12 species of ducks that were sampled (Table 5.4). This plant was particularly prominent in the diet of American Wigeon (Appendix 1). Water weed, much like Naiads, has a limited, but apparently increasing distribution on the Inner Bay; it is also an important dietary item for waterfowl, particulary American Wigeon.
Not surprisingly, corn and oats were the most common dietary items on an aggregate percent dry mass basis. However, they were only consumed by 25% of the species collected. The high aggregate percent dry mass can be attributed to the fact that samples were collected from hunter shot birds, which were generally collected in the early morning and late afternoon. Waterfowl that consume agricultural grains generally make crepuscular foraging flights to gorge themselves on readily available waste grains, either in agricultural fields or waterfowl baiting areas, such as in the Long Point Waterfowl Management Unit. Therefore, species that tend to consume agricultural grains, such as Black Ducks, Green-winged Teal, and Wood Ducks, are often satiated with either corn or oats in the morning and evening when they tend to be shot by hunters (i.e. samples were all collected from hunter shot birds). Surprisingly, of 44 mallards sampled, none contained agricultural grains. In direct contrast to the consumption of large and readily available cereal grains, foraging aquatically for plant and animal matter generally incurs a higher search and handling effort (Rave and Baldassarre 1989; Petrie and Petrie 1998), causing birds to spend much larger portions of the day foraging. This high search and handling effort presumably makes it more difficult for birds to be totally engorged with food at any given time. Consequently, the dietary samples collected by LPWWRF are quite likely biased towards agricultural grains, and therefore, I suggest that submerged aquatic plants are much more important to waterfowl at Long Point than Table 5.4 would suggest. It is also important to note that while cereal grains have a high carbohydrate content, they lack some of the essential amino acids that can be obtained through a mixed diet of native aquatic macrophytes (Frederickson and Taylor 1982).
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