Action: Provide supplementary food through the establishment of food populations
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- One pre-1950 study in the USA found that waterfowl fed on specially-planted rye grass.
- Three studies from North America and Sweden found that attempts to support populations by establishing prey did not succeed. Whooping cranes Grus americana in the USA preferentially fed on scattered grains, over planted crops; attempts in Sweden to boost macroinvertebrate numbers were not successful and great horned owls Bubo virginianus in Canada did not respond to induced increases in prey populations.
Supplementary feeding can be an intensive and potentially expensive intervention. However, it may be possible to achieve the same results by planting crops or establishing prey populations.
Supporting evidence from individual studies
A replicated study between over the winters of 1946-1947 in wetlands in Alabama, USA (Givens & Atkeson 1952), found that waterfowl used rye grass Lolium multiflorum plantings as an alternate food source. Waterfowl used the rye grass plantings extensively, often in preference to other green browse, as it added a distinct microhabitat to mud flats exposed during winter draw-down of the reservoir. Rye grass was the only crop able to withstand periodic flooding and silting over the winter period and thus became a reliable food source for waterfowl. Experimental, small-scale plantings began in 1946. Large-scale plantings began in the fall of 1947. Higher mud flats are preferable as planting sites and the rate of seeding is 50 pounds per acre.
A small replicated study over the winters of 1964-1968 in 2 fenced experimental fields (39 ha each) within a coastal wildlife refuge in Texas, USA (Shields & Benham 1969), found that whooping cranes Grus Americana preferred supplemented grains and seeds to planted crops during periods of low food availability. On average, 164 and 100 whooping crane use-days were observed for planted crops from October-December and January-April respectively, whereas the average use-days over the same periods for spread grain were 390 and 524 (3607 use-days over the study period in total). Whooping cranes significantly preferred hegari Sorghum vulgare, corn and wheat. Whooping cranes preferentially fed along the tidal flats in good weather. However, the crops (especially wheat, corn, legumes, peanuts and peas) were extensively used by sandhill cranes Grus canadensis, snow geese Chen hyperboreanI and Canada geese Branta canadensis (181 000 goose and 233 000 crane use-days in total from 1964-1967).
A study in a small (1.4 ha) artificial lake in central Sweden between 1978 and 1980 (Andersson & Danell 1982) added wheat straw and hay to the lake in an attempt to boost macroinvertebrate biomass and wildfowl numbers. However, the number of wildfowl did not appear to be affected by the addition and there was little change in invertebrate biomass. The authors suggest that high populations of invertebrate predators were responsible for the lack of change.
A replicated, controlled study from 1989-1992 in 3 experimental blocks and 5 control blocks (all 1 km2) within a forest region in the Yukon, Canada (Rohner & Krebs 1998), found that artificially increasing the density of prey did not alter the territorial or social structure of great horned owls Bubo virginianus. Experimental owls on food-enriched territories did not show a difference in home-range size and patchiness of spatial use compared with control owls. However, the distances of owl locations to treatment blocks were significantly closer to experimental centre-points than expected by chance (on average, 0.6 km closer). At a larger scale, no owls vacated their territories to use experimental plots and no owls switched to a nomadic strategy. The authors speculate that territorial behaviour prevents large aggregations of predators at an intermediate spatial scale. Experimental blocks were provided with commercial rabbit chow added weekly all year; snowshoe hare Lepus americanus densities were 2.8-10.3 times higher than in control blocks.
- Givens L.S. & Atkeson T.Z. (1952) Use of Italian rye grass as a means of attracting waterfowl. The Journal of Wildlife Management, 16, 107-108
- Shields R.H. & Benham E.L. (1969) Farm Crops as Food Supplements for Whooping Cranes. The Journal of Wildlife Management, 33, 811-817
- Andersson A. & Danell K. (1982) Response of freshwater macroinvertebrates to addition of terrestrial plant litter. Journal of Applied Ecology, 19, 319-325
- Rohner C. & Krebs C.J. (1998) Response of Great Horned Owls to Experimental ‘Hot Spots’ of Snowshoe Hare Density. The Auk, 115, 694-705