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

Predator Surveys


Counting American Crows and Black-billed Magpies Along Road Transects

For this survey, an investigator drove the 16-km center-road of each study area or nearby road if no center-road was available. The investigator stopped at the midpoint of each quarter section and for 1 min counted all American crows and black-billed magpies seen within 0.20 km (survey plot) of the vehicle in each of the 2 adjacent quarter sections (one on each side of road). The investigator also recorded presence or absence of each species in each quarter section as detected by sight or sound while at each stop and while driving between stops. Birds observed departing a survey plot or quarter section were counted only at the site where first seen. The survey period was 2 h after sunrise to 2 h before sunset, but most counts were made during 1000-1500 h. Counts were not conducted during inclement weather or when winds exceeded 28 km/h. The sampling scheme for study areas in Canada consisted of three counts (on different days) made during each of four sample periods (early May-mid-May, late May-early June, mid-June-late June, and early July) for a total of 12 counts for each study area. Nine counts during the first three sample periods were scheduled for the Central Flyway study areas in the United States. The numbers of counts in individual study areas varied somewhat (Appendix Table 4).

For our purposes, the most useful index from this survey was average percentage of quarter sections in which each species was detected (seen or heard) because distribution and abundance of breeding pairs were better reflected by this parameter than by numbers of detected individuals. The average number of detected American crows and black-billed magpies was unduly influenced by sightings of grouped birds (110 American crows in one instance) that presumably included many transitory individuals. Data from all counts for a study area were averaged because analyses revealed no differences in counts among survey periods.


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