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Distribution and Abundance of Predators that Affect
Duck Production: Prairie Pothole Region

Predator Surveys

Record Sightings of Predators

The number of places in which a predator species was seen daily was recorded. (A place was defined as a 150-m diameter area, about equal to an average size farmstead). This, rather than the number of observed individuals, was selected as the abundance indicator to minimize influence of sightings of groups (e.g., flocks, litters at dens). Each day, each person working in a study area for >0.5 h independently recorded this information and the number of hours spent in the area. Moving predators were recorded only where first seen, and repetitive daily sightings of the same species in the same place were recorded only once by that person for that day.

We anticipated observers would have difficulty remembering exact numbers of places where they saw conspicuous common predator species (black-billed magpie, American crow, large hawks). We initially established, therefore, five categories for numbers of daily sightings of each species and species-group (none, 1-5, 6-10, 11-20, >20). In using observations recorded in this manner, we assigned each daily entry the midpoint value of the indicated category; the value of 21 was assigned to entries in the >20 category. The resulting data were inadequate to differentiate abundance among study areas where populations were low (most study areas). During the small unit management study, we requested that observers record daily the actual number of places where they saw predators. Large hawks were combined into a single category because of the difficulty of identifying hawks. However, observers competent in hawk identifications were instructed to list daily the relative abundance or number of individuals of each observed species.

Results for all observer-days were combined to calculate the average number of places where predators were seen per hour (observation rate). This index was especially useful for describing relative abundance of secretive species likely to be seen often when abundant (e.g., Franklin's ground squirrel, mink, weasels). The index was not useful for describing relative abundance of species unlikely to be seen, even when relatively numerous (most mammalian carnivores), or of low-density species likely to be seen repeatedly at conspicuous locations (e.g., great horned owls at nests and red foxes at dens near roads). For all species, however, the recorded sightings helped document species presence in study areas.

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