Avian Predator Removal and Duck Nest Success in Central Alberta
D. EDWARD HOFMAN AND KEN J. LUNGLE
Alberta Forestry, Lands and Wildlife, Fish and Wildlife Division,
Third
Floor, 4901-48 Street, Red Deer, AB T4N 6M4, Canada; Alberta Forestry,
Lands and Wildlife, Fish and Wildlife Division, Seventh Floor,
6909-116 Street, Edmonton, AB T6H 4P2, Canada
Duck nest success studies have been conducted in several areas in Alberta in the 1980s (Greenwood 1984, 1985; Hofman 1986, 1987, 1988, 1989). These studies have shown that nest success rates (Mayfield) have declined considerably during recent years and indicated that a high proportion of nest loss is caused by predators.
The most recent nest success study focused on two 4-square-mile study areas in the parkland biome of central Alberta. Investigations were conducted on the two areas from 1 April to 31 July in each year from 1987 through 1989. Field activities during 1987 consisted of a normal nest success investigation; American crow and black-billed magpie removal was practiced on one of the study areas during 1988 and 1989. The objective of the study was to determine if nest success of ducks could be improved by the removal of crows and magpies, which were believed to be significant predators.
During 1988, crow and magpie removal commenced on 1 May and was continued through the nesting season. Predator removal was incomplete as there were always some crows and magpies present, but their numbers on the test area were greatly reduced from that on the control area. The results from this study suggested that the removal of crows and magpies may have improved duck nest success rates on the test area compared with the control, with apparent increases of 5.3% for all species, 5.7% for mallards, and 15.4% for blue-winged teal. These improvements, however, were not significant (P>0.05). Predator removal appeared to benefit later nesting species (blue-winged teal) to a greater extent than earlier nesting species (mallards). The average date of nest initiation for teal coincided with the period of greatest predator removal while the average date of nest initiation for mallards occurred prior to substantial removal. However, improvements in nest success may have also resulted from factors other than predator removal, such as improved habitat quality later in the season.
The results were further confounded since predation rates were not lower on the test area as opposed to that on the control area, even though avian predator removal was practiced.
In 1989, avian predator removal commenced earlier (1 April) in an attempt to determine if the nest success of earlier nesting species could be influenced by earlier predator reduction. As in 1988, predator removal was efficient and there was a substantial decrease in crow and magpie numbers on the test area. However, there was no significant corresponding increase in nest success rates on the test area. For all species of ducks, there was an apparent increase of 6.5% on the test area but the difference was not significant (P > 0.05) and may be due to other factors. In addition, nest predation rates were actually higher on the test area than on the control area when only hatched and depredated nests were compared.
This study demonstrated that crow and magpie removal alone was not sufficient to significantly improve duck nest success in central Alberta. Predator management programs designed to accomplish this objective should include management or removal of all predators if they are to be effective. In addition, nesting habitat must be of sufficient quantity and quality for the effect of predator management to be evident.
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