Northern Prairie Wildlife Research Center
Domestic feed (livestock and pet food) or garbage (tinfoil, plastic, paper milk filters, and other items found in farm middens) occurred in 4% of scats overall (6% in spring and 2% in summer [Table 1]). We detected no effect of sex, season, year, or interactions on percentage occurrence of domestic feed or garbage in scats (P>0.50). Mean percentage occurrence of feed or garbage in scats overall was 4% (SE=9). We did not examine sources of these foods, but were informed by farm residents that skunks occasionally ate at food dishes placed for pets.
Plant foods, which included cereal grains, sunflower seeds, and fruits, occurred in 13% of scats overall (23% in spring and 6% in summer [Table 1]). Mean percentage occurrence of plant foods in scats was greater in spring (19%, SE=4) than in summer (3%, SE=7; F=5.11; 1,17 df; P=0.04), but other main effects or interactions were not significant (P>0.10).
Cereal grains were available near storage facilities on farms and in stubble fields throughout the study area during spring before tillage. There were no sunflower stubble fields on the study area in 1976 and 1977, but sunflower stubble fields were available in spring 1978. Some skunks consumed such large quantities of sunflower seeds that their scats were composed nearly entirely of sunflower seed hulls. We observed skunks foraging on sunflower seed heads, but did not quantify seed availability in stubble fields.
Animal foods, including vertebrates and invertebrates, occurred in 99% of scats overall (98% in spring and 99% in summer [Table 1]). We detected no effect of sex, season, year, or interactions on occurrence of animal foods in scats (P>0.45). Mean percentage occurrence of animal foods in scats overall was 98% (SE=2).
|Table 1. Percentage occurrence by year, seasona, and overall of food remains identified to lowest taxon in scats (n) collected annually from radiocollared adult striped skunks in east-central North Dakota, 15 April15 July 1976-78.|
|Food item||Percentage occurrence|
|Livestock, pet food||0||1||2||2||1||1|
|Small rodent (Rodentia)||40||18||44||38||33||35|
|Striped skunk (Mephitis mephitis)||1||4||1||3||2||2|
|Grasshopper, cricket (Orthoptera)||7||26||22||11||29||21|
|a Spring = 15 April31 May; summer = 1 June15 July.|
Vertebrates. Vertebrates that occurred in the greatest percentage of scats were birds (including eggs) and mammals, but scats also contained amphibians and reptiles (Table 1). Occurrence of vertebrates in scats differed among years (F=7.77; 2,53 df; P<0.01). Mean percentage occurrence of vertebrates in scats was less in 1977 (46%, SE=6) than in 1978 (74%, SE=4, t= ,3.92, 1 df, P<0.01), but we detected no difference between 1976 (67%, SE=10) and 1977 (t=1.79, 1 df, P=0.08) or between 1976 and 1978 (t=0.67; 1 df; P=0.50). Percentage occurrence of vertebrates in scats differed by season, depending on sex (F=5.58; 1,17 df; P=0.03; Table 2). Vertebrates occurred in a smaller percentage of scats of females than males in spring (t= 3.01, 1 df, P<0.01), but we did not detect a difference between females and males in summer (t=1.06, 1 df, P=0.31). Females in spring had less occurrence of vertebrates in scats than did females in summer (t= 3.28, 1 df, P<0.01), but we did not detect a difference in males between spring and summer (t=0.98, 1 df, P=0.34). Other main effects or interactions were not significant (P>0.46).
Occurrence of birds was affected by season (F=8.65, 1,17 df, P<0.01) and year (F=8.20, 2,53 df, P<0.01). Birds occurred less frequently in scats in spring (33%, SE=3) than in summer (51%, SE=5). Birds also occurred less frequently in scats in 1977 (24%, SE=6) than in either 1976 (52%, SE=10, t= 2.47, 1 df, P=0.02) or 1978 (51%, SE=4, t= 3.91, 1 df, P<0.01), but we detected no difference in occurrence of birds between 1976 and 1978 (t=0.06, 1 df, P=0.95). Other main effects or interactions were not significant (P>0.39). Among 691 total occurrences of birds in scats, eggs were encountered most frequently (Figure 1). Percentages of thin-shelled eggs of passerine birds and thick-shelled duck eggs were similar.
Occurrence of mammals in scats differed by year (F=6.83, 2,53 df, P<0.01). Mammals occurred less frequently in scats in 1977 (23%, SE=5) than in 1978 (48%, SE=4, t= 3.69, 1 df, P<0.01), but we detected no difference in occurrence of mammals between 1976 (38%, SE=9) and 1977 (t=1.37, 1 df, P=0.17) or between 1976 and 1978 (t= 0.96, 1 df, P=0.34). Among 498 total occurrences of mammals in scats, small rodents were encountered most frequently, especially voles (Microtinae) and mice (Cricetinae) (Figure 1). The category "Other" mammal includes insectivores (Soricidae), rabbits (Leporidae), skunks, and livestock. Skunk remains were mostly hair, but included 1 scat with remains of a juvenile skunk.
Among 31 total occurrences of amphibians in scats, 26 (84%) were salamanders. Based on our reference collection of skeletal parts obtained locally, we believe they were Ambystoma tigrinum. All reptiles were snakes (Colubridae).
|Table 2. Percentage occurrencea (% / SE) of remains of vertebrate foods in scats collected during spring (15 April31 May) or summer (1 June15 July) from female and male radiocollared adult striped skunks (n) in east-central North Dakota, 1976-78.|
|Season||Sex||n||% / SE|
|Spring||Female||19||51 / 5 Ab|
|Spring||Male||24||69 / 4 B|
|Summer||Female||27||70 / 3 B|
|Summer||Male||12||58 / 10 AB|
as least squares means.
b Means followed by the same letter do not differ (P>0.05)
Invertebrates. Invertebrates that occurred in the greatest percentage of scats were insects, but scats also contained crayfish (Decapoda), snails (Gastropoda), millipedes (Diplopoda), and spiders (Araneida) (Table 1). We detected no effect of sex, season, year, or interactions on occurrence of invertebrates in scats (P>0.16). Mean percentage occurrence of invertebrates in scats overall was 93% (SE=5). Beetles (Coleoptera), moths (Lepidoptera), and grasshoppers and crickets (Orthoptera) were the most common insect taxa. We detected no effect of sex, season, year, or interactions on occurrence of insects in scats (P>0.15).
Among 2,698 total occurrences of insects in scats, ground beetles (Carabidae), noctuid moths (Noctuidae), scarab beetles (Scarabaeidae), and carrion beetles (Silphidae) were encountered most frequently (Figure 1). The category "Other" insect included mostly bees (Apidae), ants (Formicidae), and flies (Diptera). Flies were primarily Syrphidae and Calliphoridae. Insects were mostly adult life forms, except noctuid moths and flies that were nearly all larval stages, and scarab beetles that were 27% larval stages. Grasshoppers appeared to be adults, but probably included nymphs as well.
|Figure 1. Number (n) of occurrences and proportions (%) of specific food items or taxa in 1,248 scats collected from radiocollared striped skunks in east-central North Dakota, 15 April15 July 1976-1978. Thin eggshell is ≤0.16 mm. Thick eggshell is >0.16 mm. Small bird is ≤blackbird (Icteridae) size. Large bird is >blackbird size. Totals may not sum to 100% due to rounding error.|
We documented 1 instance of a female skunk depredating a red-winged blackbird (Agelaius phoeniceus) nest in 1977 and 21 instances of skunks (8 females and 4 males) depredating duck nests (5 in 1976, 2 in 1977, and 14 in 1978), including 4 nests that we were monitoring. At 8 duck nests, the entire clutch (5-13 eggs) was eaten during the first feeding bout, but skunks did not always eat an entire clutch of duck eggs at a single feeding. At 13 duck nests from which skunks departed on their own accord while being observed (not frightened away), intact eggs (2-5) were left from 5 original clutches (7-11); however, all remaining eggs were consumed within 1-2 days, presumably by skunks. Of 90 duck nests monitored during the study, 68 were depredated, but evidence was insufficient to identify the predator responsible for their destruction, except for the 4 where we detected skunks. One monitored gadwall (Anas strepera) nest was discovered at hatching by a male skunk, but was not depredated.
We found bird carrion at 4 foraging sites of 3 skunks in 1978. A female and male skunk both fed on the carcass of an adult male mallard, but 8 days apart. Two females fed on separate carcasses of American coots (Fulica americana); 1 of these females also fed on the carcass of a red-winged blackbird.
We discovered only 1 foraging site with mammal remains, where a Franklin's ground squirrel (Spermophilus franklinii) was consumed by a female skunk. We did not determine if the squirrel was killed by the skunk or scavenged.
We did not find amphibian remains at any foraging sites. We once observed a skunk eating a garter snake (Thamnophis sp.), which it immediately regurgitated.
We sampled 53 sites in 1977 where skunks (45 females and 13 males) foraged on insects and 27 similar sites in 1978 (34 females and 11 males; Table 3). Twenty-two of these sites were occupied simultaneously by >1 animal. We observed skunks foraging on insects at some sites and also found small holes dug (digs) in litter or sod and overturned cow dung (pads).
|Cow pad with exposed scarab larvae.||Noctuid larvae from foraging site in wet meadow.|
We commonly found noctuid moth larvae, including Crymodes devastator and Euxoa auxiliaris, at foraging sites during both spring and summer. Eighty-one percent of sites with noctuid larvae were in wet meadows, often characterized by foxtail barley (Hordeum jubatum), prairie cordgrass (Spartina pectinata), and rushes (Juncus spp). Larvae were usually under 2-3 cm of decaying litter. Often, hundreds of digs were present in meadows where skunks had foraged for several nights. At one place where a skunk left a visible trail in wet vegetation while foraging, we counted digs at 5 randomly located 0.25-m² sites along the trail and found a mean density of 27.2 digs/m² (SE=3.4). Nearby, at 8 randomly located, undisturbed 0.25-m² sites in litter sampled to a depth of 5 cm, we found a mean density of 26.0 noctuid larvae/m² (SE=2.8). Overall in 1977, the density of noctuid larvae in undisturbed litter at 27 foraging sites averaged about 12/m², with a maximum of 66/m² in summer (Table 3). We found few sites where skunks foraged on noctuid larvae in summer of 1978.
We found scarab beetles, mostly larvae, at upland foraging sites in pastures (Table 3). During spring and summer, sod in upland sites contained Phyllophaga spp. and Diplotaxis spp., including P. rugosa, P. anxia, P. crenulata, and D. tristis. Density of larvae averaged about 22/m², with a maximum of 70/m² in spring 1977. Foraging for phytophagous scarabs was characterized by numerous digs about 5 cm deep in sod, sometimes concentrated in patches of buckbrush (Symphoricarpos occidentalis). During summer, skunks also foraged on scarab larvae (Aphodius spp. and Bothynus spp., including A. fimetarius, B. gibbosus, and B. relictus) in cow pads in upland sites. Density of these scarabs averaged about 42 larvae/sampled pad, with a maximum of 96/pad in 1977. Skunks tipped over or tore apart cow pads to obtain coprophagous scarabs, but frequently numerous larvae remained in pads after foraging. We did not sample free-flying scarabs, but adults often were abundant on the ground surface during periods of emergence.
We found grasshopper adults and nymphs at foraging sites during summer, primarily Melanoplus bivittatus, but including Scudderia spp., Acridinae, and in 1978, a few Tettigoniidae (Table 3). Densities of grasshoppers at foraging sites along roads, trails, and drainages averaged 17/m³, with a maximum of 111/m³ in 1977. Greatest percentage of foraging sites with grasshoppers was in summer of 1978. We observed skunks using a walk-pounce method of catching grasshoppers.
We occasionally found ground beetle adults and larvae at foraging sites, but densities seldom exceeded a few adults or larvae/m² (Table 3). They were mostly Harpalus spp., Calosoma spp., and Agonum spp., including H. pensylvanicus, C. calidum, as well as Amara carinata, and Pterostichus lucublandus.
|Table 3. Number (n) of sites in east-central North Dakota where foraging on insects by radiocollared striped skunks was detecteda, frequency (%) of those sites were specific insect taxa were detected and sampledb, and mean number and range of organisms at those sites during spring (15 April31 May) or summer (1 June15 July).|
|Spring (21)||Summer (32)||Spring (6)||Summer (21)|
|9.5||1/m² (0)||12.5||1/m² (<1-1/m²)||33.3||7/m² (3-11/m²)||4.8||8/m² (0)|
|42.9||12/m² (1-27/m²)||56.3||12/m² (<1-66/m²)||50.0||2/m² (1-3/m²)||4.8||8/m² (0)|
|57.1||22/m² (6-70/m²)||6.3||6/m² (1-10/m²)||33.3||7/m² (2-11/m²)||9.5||14/m² (12-16/m²)|
|0||18.8||39/padc (10-96/pad)||0||19.0||45/pad (20-64/pad)|
|0||18.8||23/m³ (<1-111/m³)||0||66.7||10/m³ (2-26/m³)|
|0||3.1||20/m² (0)||16.7||2/m² (0)||0|
Detected by telemetry monitoring and observation.
b Sampled by multiple soil excavations (0.25 m² diameter area × ~5 cm deep) or net sweeps (0.30 m² diameter net × sweep distance).
c Cow pad.