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Wildlife Monographs

Red Fox Predation on Breeding Ducks
in Midcontinent North America


Data were collected in 2 ways: (1) a questionnaire survey was conducted annually during spring and early summer, 1968-70, whereby we requested individuals who visited red fox rearing dens in the midcontinent area to record certain information about the dens they visited and to collect and submit waterfowl remains found at those dens and (2) data collections were made annually during spring and early summer, 1969-73, at fox dens in the 3-county intensive study area in eastern North Dakota by field crews working under our direction. Many aspects of fox biology associated with interpretations of duck remains found at the fox dens were discussed by Sargeant (1972) and Johnson and Sargeant (1977). Ducks found at dens reported in those studies were included in samples treated here. Numbers of dens visited in each sample area reflect investigator effort more than fox densities.

Midcontinent Questionnaire Survey

Packets containing postpaid individually numbered field forms and mailing envelopes (for submission of waterfowl remains) were sent to selected conservation agency employees and university students in Minnesota, North Dakota, and South Dakota who would likely encounter red fox rearing dens. Assistance was also solicited from a few conservation agency personnel and university students working in Iowa, Manitoba, Nebraska, and Wisconsin. Information requested for each den visited included den location, status of den use, relative abundance of waterfowl and waterfowl habitat within 3.2 km of den, and type of examination. Cooperators were asked to examine food remains found at each den, to record total number of ducks and geese they thought were represented by food remains found, and to send waterfowl remains to NPWRC. Cooperator estimates of waterfowl mortality were verified when possible by comparing numbers of ducks and geese reported to numbers represented by remains submitted for examination (some cooperators did not provide estimates or submit all waterfowl remains); estimates were corrected when appropriate. Duck species and sex determinations were with few exceptions based solely on food remains submitted for examination. We collected data from dens also and occasionally accompanied cooperators on visits to dens.

Ground Crew Visits to Dens in Intensive Study Area

The 6 detailed study townships in the 3-county intensive study area (Fig. 2) were systematically searched from aircraft during mid-May and early June each year, 1969-73, to locate fox rearing dens (Sargeant et al. 1975). In addition, all 6 townships were searched from aircraft in midApril 1969, and the 2 Barnes County townships were searched in early May 1970. Nearly all dens found during aerial searches, plus other dens brought to our attention by local residents, were visited by a ground crew. Additional dens found in April, May, June, and July during occasional aerial searches outside the townships, but within the intensive study area, also were visited. Many dens were partially or completely excavated by the ground crews but were restored in a manner that generally allowed reuse by foxes during subsequent years. Information recorded during each den visit included all data requested in the questionnaire survey. In addition, duck remains found in and around den entrances (surface remains) were kept separate from duck remains found underground through excavation (subsurface remains).

Duck Identification and Population Data

Ducks identified from body parts in food remains found at dens were considered to represent birds taken, but not necessarily killed, by foxes. However, we refer to all ducks found at dens as depredated birds. Duck remains were identified by comparison to reference material. Extra body parts for each species and sex were required before more than 1 duck of a species or sex was assigned to a den. Demographic data on duck populations were obtained from USFWS annual breeding pair surveys for census strata 43, 44, 46, and 48 (adjusted for visibility bias) from Stewart and Kantrud (1974), and from information recorded during visits to fox dens.

An index of duck availability to foxes in sample areas was calculated from subjective ratings for each of 3 descriptors recorded at time of den visit: (1) relative abundance of aquatic waterfowl habitat (none, little, moderate, or much) within 3.2 km of den, (2) distance (>3.2 km, 1.63.2 km, 0.8-1.6 km, or <0.8 km) from den to nearest wetland, and (3) relative abundance of ducks (none, few, moderate, or many) within 3.2 km of den. Each descriptor was assigned a rating of 0, 1, 2, or 3 with progressively higher ratings indicating greater availability of ducks to foxes. The 3 ratings for each den visited were then summed; the lowest score of 0 meant there were no wetland habitat and no ducks within 3.2 km of a den, whereas the highest score of 9 meant there was much wetland habitat within 3.2 km of the den, the den was within 0.8 km of a wetland, and many ducks were present within 3.2 km of the den. Scores were averaged to obtain the index value for each sample area.

Data on duck nesting chronologies were available from a duck nesting study conducted on a highway right-of-way in eastern North Dakota during 1976-77 (A. T. Klett, unpubl. data). In that study systematic weekly searches of 37 km of highway right-of-way were made each year from mid-April to early July, and numbers of nests found or known to be active each week were recorded by species.

Durability of Duck Remains at Dens

Carryover of duck remains at dens from 1 year to the next was of concern because red foxes often use the same dens during successive years (Pils and Martin 1978:11). Durability of duck remains at dens from 1 year to the next was assessed by examining remains at some dens during the same or successive years and by placing commonly found duck remains at 3 simulated dens. Simulated dens were at NPWRC on a south facing hillside, a gravel pit bank, and a river bank. These sites represented diverse soil, moisture, and sunlight conditions. Three wire baskets (2.5-cm mesh), each containing a duck head and a small and large duck wing, were placed at each simulated den on 27 April 1970. At each den a basket was placed on the soil surface immediately outside the den entrance, another on the soil surface inside the den, and a third a few centimeters under soil inside the den. The conditions of the duck remains were examined on 3 May 1971. Based on these examinations, we discarded all prey remains found at dens that we suspected were from previous years.

Calculation of Predation Rate Index

We used the average number of ducks found on the surface and in den entrances (surface ducks) per den visited as an index of numbers of ducks taken per fox family. Johnson and Sargeant (1977) used this predation rate index to estimate numbers of mallards taken by foxes in North Dakota. In calculating the index we included data from all active and recently used dens, including dens at which no food remains were found. Searches for dens were conducted throughout the approximate 3-month fox denning season, but few dens were found during the first month after whelping when pups remained underground (Sargeant 1972). Thus, our predation rate indices are based largely on prey remains found during the latter two-thirds of the denning season. Nevertheless, we believe they reflect fox use of ducks during the entire denning season because little food is present at early season dens when pups are too young to consume much prey (Sargeant 1972,1978).

Interpretation of Data

Accurate counts of numbers of ducks brought to dens were difficult to obtain from remains found at dens because foxes dismember ducks after capture (Sargeant and Eberhardt 1975), consume nearly all body parts (Sargeant 1978), and scatter remains about dens. Remains often were limited to a few feathers. Our chances of counting all ducks were good if only 1 duck was present and if a thorough search that included den excavation was conducted. However, if several ducks were represented, especially if they were the same species and sex, our counts were often low because too few body parts were found to identify all individuals. Searches of dens by our ground crews tended to be more thorough than searches made by cooperators.

Our assessments of percentage species composition of ducks found at dens are subject to bias because remains of all species were not equally identifiable. Dabbling ducks were nearly always distinguished from diving ducks, but about 11% of the ducks found at dens were not identified to species. About one-half of the unidentified dabbling ducks were known to be blue-winged teals or green-winged teals by the small size of body parts, especially primaries. In contrast, northern shovelers were easily distinguished from other species by the bright white rachis of their primaries. Remains of northern pintails were easier to identify than remains of mallards and gadwalls. Northern pintails have a light primary rachis and dark underwing lining whereas both mallards and gadwalls have a dark primary rachis and white underwing lining. Bias for other ducks, although unknown, is inconsequential because relatively few ducks of other species were found.

We had greater difficulty ascertaining sex than species of ducks found at dens because more remains were often needed. Sex identifications for all species favored drakes because drake plumage is more distinctive than hen plumage. Sex could not be determined for 7% of the northern pintails, 10% of the American wigeons, 11% of the gadwalls, 17% of the blue-winged teals, 22% of the mallards, 25% of the green-winged teals, and 34% of the northern shovelers. Sex of diving ducks was much more difficult to ascertain than sex of dabbling ducks because of plumage similarities, but few diving ducks were found at dens.

Latitude related differences in synchronies of the fox denning and duck nesting seasons also affected results. The denning season of red foxes in the midcontinent area lasts 10-15 weeks (Sargeant 1972) and is progressively delayed with increases in latitude (Storm et al. 1976:19). Sargeant et al. (1981) found the mean whelping date for red foxes in Iowa and South Dakota was about 15 March and in North Dakota about 31 March; presumably whelping was earlier in Nebraska and later in Canada. Because of changing den visibility factors related to pup age (Sargeant et al. 1975), most of the South Dakota dens were visited from mid-April to mid-June whereas most of the North Dakota dens were visited from early May to early July (Fig. 5). Duck nesting in the midcontinent area usually begins in mid-April and is completed by mid-July but, unlike fox whelping, is only slightly delayed in most species with increases in latitude (Bellrose 1976:215,240,272). The differences in timing and amount of overlap of the denning and nesting seasons likely resulted in progressively less representation of early nesting ducks and progressively greater representation of late nesting ducks at dens as latitude increased.

gif -- Visits to Red Fox Rearing Dens

Fig. 5. Temporal distribution of visits to red fox rearing dens in South Dakota (405 dens) and North Dakota (739 dens).

The most important cumulative effects of all biases known to us are that they likely resulted in underestimation of both the number of ducks taken by foxes and the percentage of hens taken by foxes. The effects of these biases should be considered in interpretation of our findings and in comparisons of our findings with those of other studies.
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