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Interpreting Evidence of Depredation of Duck Nests
in the Prairie Pothole Region

Part II: Evidence of Depredation by Predator Species

Methods Used to Obtain Original Data

In Part II, we describe responses to nests, affect on hens and eggs, and evidence of depredation at or near duck nests by the principal predators of duck nests in the Prairie Pothole Region. Accounts are based on literature (Appendix A) and on verification data gathered intermittently during 1972-92. A comparative summary of the relative importance of certain evidence as indicators of depredation by each of the principal predators is in Appendix B, Table 2.

Methods Used to Obtain Original Data

We obtained verification data by (1) offering duck eggs to predators in 2-m2 cages or 18.6-m2 pens, (2) offering duck eggs in artificial nests to predators in a 0.1-ha compound or a 4.1-ha enclosure, (3) photographing depredations of duck eggs in artificial nests by free-ranging predators with time-lapse cameras set to photograph at 1530 s intervals (Sargeant et al. 1987), (4) observing free-ranging predators depredate loose eggs placed in unfenced natural sites, (5) opportunistically monitoring depredations of duck eggs in natural nests by radio-equipped free-ranging predators, or recording data from natural duck nests at which free-ranging predators were observed, (6) examining nest depredation records (prototypes of record in Appendix D, Fig. 1) for natural duck nests destroyed by predators for information about the depredations, (7) examining natural duck nests destroyed by predators in a fenced 25-ha exclosure that protected duck nests from most mammalian predators, and (8) examining eggshells from natural duck nests assumed destroyed by a predator species because of unique evidence at the nest.

The cages, pens, and compound were at Jamestown, North Dakota. The enclosure, which included a small pond and elevated observation booth, was at Woodworth, North Dakota (Sargeant and Eberhardt 1975, Rudzinski et al. 1982). Work involving time-lapse cameras, observing and monitoring free-ranging predators, and obtaining records of destroyed natural duck nests was conducted at numerous locations in Canada and the United States. The exclosure was at Hitterdal, Minnesota.

Only duck eggs (usually unincubated mallard eggs) were offered to predators. Artificial nests (except those offered to red foxes during 1972-73) were made of material salvaged from natural duck nests. Some artificial nests were used repeatedly; all eggshells and shell fragments were removed from them before additional eggs were placed in the nests. Quantification of each depredation variable was estimated from visual examination of evidence.

Verification data obtained for each predator species ranged from general observations to detailed records of specific predator behaviors. Data were written or verbal descriptions, or coded entries on a nest depredation record (prototype of record in Appendix D, Fig. 1). Amount of information gathered increased as the value of variables for describing patterns of depredation became evident. Thus, sample sizes for variables varied within and among predator species. Results are reported as specifically as possible or warranted; means are unweighted averages. Rounding accounts for minor differences in some percentages in text and their corresponding values in tables.

Coyotes and Red Foxes

Two pairs of coyotes and 7 pairs of red foxes were studied in the enclosure. All were captured in spring as juveniles and paired in summer or autumn with a nonsibling. Each pair was held separately until placed in the enclosure at ≥l year old; their staple ground meat and natural prey diets included ducks and duck eggs. One of each pair was marked with dye for visual identification. Responses of the animals to artificial nests were observed from the elevated booth.

Coyotes were studied in 1983. Study of each pair lasted about 20 days, beginning with 3 days of acclimation to the enclosure. Then, for 16 days, coyotes were offered 4 artificial nests in late afternoon of alternate days. Two of 4 nests contained a physically restrained live mallard to simulate incubation. Coyotes were observed during evenings of days when nests were offered. Responses by coyotes were described verbally on cassettes; pertinent locations (e.g., prey cache sites) were plotted on maps. Investigators visited nests in the morning after eggs were offered to record evidence of depredations.

Six pairs of red foxes were studied in the enclosure during 1972-73. Each pair was offered 21 artificial nests and a restrained live mallard, and 3 artificial nests with no duck. Study sequence for each pair began with 4 days of acclimation to the enclosure. Acclimation was followed by 3, 4-day periods when 1-3 nests were offered daily in late afternoon. Each period was separated from the next period by 1 day. After approximately a 10-day recess, 3 nests were simultaneously offered.

Red foxes were observed during evenings of days when nests were offered and again the next morning. Verbal descriptions of responses to nests were recorded on cassettes; pertinent locations (e.g., prey cache sites) were plotted on maps. After each observation, the investigator visited each nest to record evidence of depredation.

Another pair of red foxes was studied in the enclosure in 1985. The foxes destroyed 11 artificial and 1 natural nest during a 6-day period. Depredation data for each nest were recorded the day after the nest was offered.

Verification data were also obtained in 1984 for 2 free-ranging red foxes, each of which destroyed an artificial nest monitored with time-lapse photography.

Raccoons and Striped Skunks

We obtained verification data for 5 raccoons (4 wild-caught, 1 free-ranging) and 21 striped skunks (4 wild-caught, 17 free-ranging). In 1986, 3 wild-caught raccoons were individually studied for 2-13 days in the compound and 1 wild-caught raccoon was studied for 10 days in the enclosure. Raccoons were first confined individually in pens, where they were fed duck eggs to obtain data on location of openings in eggshells. They were then placed in the compound or enclosure, where, during several days, each was offered 1-3 artificial nests. Nests that were incompletely depredated during a day were left undisturbed and rechecked 1-5 days later. No new nests were offered during the interval. Data analyzed were collected during final visits to nests, 1-5 days after each nest was offered. The free-ranging raccoon was monitored via time-lapse photography in 1991, while it depredated an artificial nest.

Four skunks were individuals studied in the compound during 1986; 15 skunks, radio-equipped and freeranging, were discovered depredating natural duck nests during a study of striped skunk ecology during 1976-78 (Sargeant et al. 1982). The remaining 2 were unmarked, free-ranging individuals encountered depredating natural nests, 1 each in 1983 and 1990.

Radio-equipped skunks were discovered depredating natural nests when we walked to sites after we suspected the skunks were feeding on duck eggs. We avoided disturbing skunks and returned later to examine nest sites. Principal data recorded were number of depredated eggs and the appearance of eggshells. Depredation data for the 2 other free-ranging skunks were recorded after they departed nests.

Skunks placed in the compound were each studied for 10-20 days. Each was offered an artificial nest on most days. Depredation data were recorded 1 day after each nest was offered.

American Badgers

We obtained verification data for American badgers primarily by examining our file of 4,233 nest depredation records for natural duck nests destroyed by predators in the Prairie Pothole Region during 1984-92. Each nest was found before it was destroyed, and it was monitored to determine fate (Klett et al. 1986). We restricted our examination to nests with ≥6 eggs upon last visit by an investigator.

We assumed nests were destroyed by badgers if ≥1 egg was cached at the nest and covered by ≥2 cm of soil and/or debris. We believe badgers are unique among predators in the Prairie Pothole Region in caching eggs this way. In addition to selected file data, we obtained eggshells from co-workers during 1986-92. These eggshells were from 17 natural nests, with each nest assumed to have been destroyed by badgers, based on criteria described above. Eggshells were examined for types and locations of openings.

Minks and Weasels

Verification data for minks were limited largely to appearance of loose eggs depredated by 3 wild-caught minks confined individually in cages in 1979. Several eggs were simultaneously offered to each mink, and eggshells were collected daily. We also obtained data during 1988-91 from natural nests in the exclosure, assumed destroyed by free-ranging minks. The exclosure was largely free of predators, except mink (determined by observations, trapping, and monitoring track plots), which could pass through 5-cm diameter openings of the mesh fence.

We did not study weasels, but we obtained shells of duck eggs depredated by captive wild-caught ermines and long-tailed weasels from J. P. Fleskes (Iowa Coop. Fish Wildl. Res. Unit, Iowa State Univ., Ames).

Franklin's Ground Squirrels

Verification data for Franklin's ground squirrels came primarily from artificial duck nests depredated by free-ranging individuals during 1983-84. Nests were placed where each was likely to be encountered by different individuals and were monitored with time-lapse cameras. Nests usually were offered in early morning and removed that evening. Nests left overnight were covered with a hood to prevent depredation while cameras were inoperative. Depredation data were recorded during interim and final visits to nests. A nest with ≥1 depredated eggs was classified as depredated by Franklin's ground squirrels if only that species was photographed at the nest. Additional data about treatment of eggs and/or appearances of eggshells were obtained during 1983-84 and 1992 by observing free-ranging Franklin's ground squirrels depredate loose eggs.

Black-billed Magpies, American Crows, and Gulls

Verification data for black-billed magpies, American crows, and gulls were obtained by using time-lapse photography of depredations of artificial duck nests by free-ranging individuals (all species), and by observing free-ranging individuals depredate loose duck eggs (black-billed magpies, crows). Procedures for recording data and determining predator species responsible for depredations were the same as for Franklin's ground squirrels.

Black-billed magpies and crows were studied during 1983- 85. Each species was offered artificial nests at locations where nests were likely to be discovered by different individuals. Nests usually were ≤ 100 m from an occupied nest of the target species; each nest was monitored for 1-4 days. Eggs removed by predators between photo-frames were assumed taken by photographed species. Nests known to have been visited by ≥ 2 predator species were excluded from analyses.

Free-ranging black-billed magpies were watched depredating loose duck eggs at 1 site in 1992. Eggs and eggshells were collected after the magpies left the site.

Gulls were studied during 1985 and 1991 by offering artificial nests at 3 locations where ring-billed gulls and/or California gulls were numerous. Each location was <3 km from a different nesting colony of ring-billed gulls and California gulls. Nests were placed to be conspicuous to aerial predators. Each nest was monitored 1 day. Responses by gulls to loose duck eggs were watched at 2 landfills, 1 site in each of 1985 and 1992. These observations were done to gather data on appearance of eggshells, and to determine how gulls opened and carried duck eggs. Eggshells were collected after depredations ended.

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