Nest Predation on Black-Tailed Prairie Dog Colonies

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University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

USGS Staff -- Published Research US Geological Survey

2000

Bruce Baker U.S. Geological Survey

Thomas R. Stanley U.S. Geological Survey

Glenn Plumb U.S. Geological Survey

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Baker, Bruce; Stanley, Thomas R.; and Plumb, Glenn, "Nest Predation on Black-Tailed Prairie Dog Colonies" (2000). USGS Staff -- Published Research. 50. https://digitalcommons.unl.edu/usgsstaffpub/50

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. J. Wildl. Manage. 64(3):2000

NEST PREDATION ON BLACK-TAILED PRAIRIE DOG COLONIES

BRUCE W. BAKER,' U.S. Geological Survey, Midcontinent Ecological Science Center, 4512 McMurry Avenue, Fort Collins, CO 80525, USA THOMAS R. STANLEY, U.S. Geological Survey, Midcontinent Ecological Science Center, 4512 McMurry Avenue, Fort Collins, CO 80525, USA GLENN E. PLUMB,2 Badlands National Park, P.O. Box 6, Interior, SD 57750, USA

Abstract: Nest predation is the principal cause of mortaliv for many grassland birds. Predation rates may be higher on prairie dog colonies because they may have less available nesting cover and may increase predator abundance. We compared 14-day nest predation rates for 1,764 artificial nests on 102 black-tailed prairie dog (Cynovnys ludovicianus) colonies and their paired off-colony sites (similar habitat lacking prairie dogs) from 14 May to 26 June 1998 in South Dakota and Wyoming. Predation rates on colonies (66.2 t 2.2%; f t SE) were 29.5% higher than at off-colony sites (51.1 t 2.7%). Nesting cover on colonies was less dense and more uniform in structure and regression analysis showed differences in nest predation rates were correlated with estimates of mean nesting cover. Avian species associated with prairie dog colonies had smaller clutches and more broods/ year than species associated with off-colony sites, suggesting a mechanism that may help compensate for increased risk of nest failure. Factors that influence predator-prey dynamics (e.g., habitat fragmentation) or foraging success (e.g., insect availability) also may help explain higher risk of nest predation on prairie dog colonies. Our conclusions support others in recommending protection of large, intact prairie dog ecosystems. JOURNAL OF WILDLIFE MANAGEMENT 64(3):776-784

Key words: artificial nest, avian fitness, experimental nest, Cynomys leucurus, Cynomys ludovicianus, ground- nesting birds, habitat fragmentation, mixed-grass prairie, shrub-steppe, South Dakota, white-taled prairie dog, Wyoming.

Nest predation is the principal cause of nest form in structure, or predator abundance is loss for a great variety of birds (Ricklefs 1969, higher (Bowman and Harris 1980, Martin Martin 1993). Predation rates can be higher 1996). Prairie dogs (Cynomys spp.) may modify where nesting cover is less dense or more uni- structure and increase predator abundance on colonies; ecological functions ' E-mail: [email protected] that, in part, have been used to justify their sta- "resent address: Yellowstone Center for Resourc- es, PO. Box 168, Yellowstone National Park, WY tus a (Whicker and Detling 82190, USA. 1988, Miller et al. 1994, Kotliar et al. 1999). J. Wildl. Manage. 64(3):2000 NEST PREDATIONOX PRAIRIE DOG COLONIES Baker et a1 777

ly petitioned the U.S. Fish and Wildlife Service to list the black-tailed prairie dog as a threatened species under the Endangered Species Act (K. Graber, T. France, and S. Miller, un- published report). Grassland and shrub-steppe birds, including species associated with prairie dogs, also have experienced regional population declines (Kotliar et al. 1999, Knopf 1996). Many of these species, such as the mountain plover (Charadeus montanus), homed lark (Eremo- phila alpestris), and McCown's longspur (Cal- carius mccownii), nest on the ground in prairie dog colonies where they may experience higher rates of nest predation (Baker et al. 1999). Nest predation rates may be higher on prairie dog colonies because nests may be easier for predators to locate and predators may be more abundant. Nests on colonies mav be easier to locate because vegetation typically is shorter and less dense, a result of grazing disturbance by prairie dogs (Clark 1977, Whicker and De- tling 1988). Prairie dogs clip vegetation both to consume and presumably to increase visibility for predator detection (King 1955). Clipped vegetation may create a mowed effect; thus, vegetation on colonies also may be more uniform in structure and contrast sharply with the surrounding habitat, especially in fragmented systems where colonies occupy a relatively small percentage of the landscape. Studies in fragmented tall-grass prairie and western shrub- Fig. 1. Range of the black-tailed prairie dog (Cynomys lu- steppe have shown higher rates of nest preda- dovicianus; Hall 1981) and complexes sampled for this study tion in smaller fragments, near edges, and (BC = Badlands-Conata, sampled 14 May-2 Jun, 1998; TB = Thunder Basin, sampled 6-26 Jun, 1998). Range of the white- where nesting cover is less dense (Burger et al. tailed prairie dog (C. leucurus) is shown for comparison;com- 1994, DeLong- et al. 1995, Yahner 1996). In ad- plexes were sampled by Baker et al. (1999; CB = Coyote dition, nest predators, such as coyotes (Canis Basin, sampled 27 May-18 Jun, 1997; MA = Moxa Arch, Sam- pled 18-29 Jun, 1997; and SB = Shirley Basin, sampled 30 latrans), badgers (Taxidea taxus), and deer mice Jun-16 Jul, 1997). Suitable habitat is fragmented or unoccu- (Peromyscus spp.), may be 3-5 times more pied in much of the depicted range for both prairie dog species, but especially for black-tailed. abundant on prairie dog colonies (O'Meilia et d. 1982, Agnew et d. 1986, Krueger 1986). Thus, if the risk of nest predation is higher Collectively, the 5 species of prairie dogs once where nesting cover is leis dense and more were widespread and abundant in west-central fragmented, and where predators are more North America (Fig. 1; Anderson et al. 1986). abundant, then nest predation rates should be However, prairie dogs have been treated as ag- higher on prairie dog colonies. ricultural pests since European settlement of Nest predation rates were 14% higher on col- the Great Plains. The range of the black-tailed onies than in similar sites lacking prairie dogs prairie dog (C, ludovicianus) has been reduced (hereafter, called off-colony sites) in a study of by as much as 98% due to habitat loss from 74 white-tailed prairie dog (C. leucurus) colo- agricultural and other development, poisoning nies in Colorado, Utah, and Wyoming (Baker et to eliminate forage competition with livestock, al. 1999). White-tailed prairie dogs tolerate tall- and sylvatic plague (Yersinia pestis; Marsh 1984, er vegetation" within colonies, occur at lower Miller et al. 1994). Prompted by continued de- densities, and occupy less fragmented land- clines, the National Wildlife Federation formal- scapes (public lands) than black-tailed prairie 778 NEST PREDATIOKON PRAIRIEDOG COLONIES Baker et al. J. Wildl. Manage. 64(3):2000

Table 1. Comparisons of type of disturbance to 1,764* ex- shortgrass/sagebrush-steppe of Wyoming, which perimental nests at 102 black-tailed prairie dog colonies and paired off-colony sites in South Dakota and Wyoming, 1998. allowed us to broaden our scope of inference and compare findings with a similar study of Tpe of disturbance On Off Total white-tailed prairie dogs (Fig. 1; Baker et al. None 405 503 908 1999). In adltion, each of these complexes Egg moved but intact 78 47 125 contained a minimum of 50 large active colo- Egg gone 126 39 185 nies. Egg crushed or broken 280 266 546 Total 889 875 1,764 Badlands-Conata contained portions of Bad- lands National Park and Buffalo Gap National *W'e omitted 3 nests that were flooded and 214 nests that we could not relocate Grasslands (Conata Basin) in South Dakota (Fig. 1). Common grasses were western wheatgrass (Agropyron smithii), buffalograss (Buchloe dogs, suggesting comparisons of nest predation dactyloides), blue grama (Bouteloua gracilis), rates on black-tailed prairie dog colonies and needleandthread (Stipa comata), and cheatgrass off-colony sites may show even greater lffer- (Bromus tectorum). Common forbs were fetid ences than for white-tailed prairie dogs (Baker marigold (Dyssodia papposa), scarlet globem- et al. 1999). allow (Sphaeralcea coccinea), and woolly plan- Our objectives in this study were to compare tain (Plantago patagonica). Pricklypear cactus predation rates of artificial nests placed on and (Opuntia polyacantha) was abundant. Homed off black-tailed prairie dog colonies and to sug- larks and western meadowlarks (Sturnella neg- gest factors that might explain observed differ- lect~)were common on prairie dog colonies; ences. Specifically, we determined (1)if 14-day nest predation rates were higher on prairie dog grasshopper sparrows (Ammodramus sauanna- colonies than at off-colony sites, (2) if lffer- rum) and western meadowlarks were common ences in nesting cover or prairie dog burrow at off-colony sites. Killdeer (Charadrius vocfer- density were good prelctors of lfferences in US),upland sandpipers (Bartramia longicauda), nest predation rates, and (3) if type of distur- and lark buntings (Calamospiza melanocoys) bance to nests lffered on and off colonies. also were present. Sites within the complex were grazed by bison (Bison bison) or domestic STUDY AREA cattle. Although sylvatic plague occurs in many Within the historical range of black-tailed black-tailed prairie dog populations, it has not prairie dogs, the more northern states of South been confirmed at Badlands-Conata. Recrea- Dakota, Montana, and Wyoming contain the tional prairie dog shooting was frequent in Con- majority of remaining populations. We selected ata Basin, but not permitted in Badlands. In our study colonies from 2 major complexes (an recent years, coyote populations have been re- aggregation of colonies, Biggins et al. 1993) in duced by aerial gunning and other means this region (Fig. 1). They represented the (which likely increased annual mortality relative mixed-grass prairie of South Dakota and the to previous recreational shooting) to decrease

Table 2. Comparisons of 14-day nest predation rates and habitat variables at 102 black-tailed prairie dog colonies and paired off-colony sites at the Badlands-Conata complex in South Dakota and the Thunder Basin complex in Wyoming, 1998.

Complex Badlands-Conata Thunder Basin (11~= 53) (n = 49) Site x SE r SE 14-day nest predation rates (%) On 67.9 3.2 64.4 3.0 Off 47.6 4.0 54.9 3.7 Nesting cover (cm) On 4.2 0.3 2.5 0.2 Off 7.2 0.5 8.1 0.5 SDb of nesting cover (cm) On 2.7 0.2 2.0 0.1 Off 3.3 0.1 5.9 0.4 Active burrowsha On 114.0 8.6 130.0 6.9 Off 0.7 0.5 0.2 0.1

11 = number of colonies sdmpled h SD = standard demation. J. \I'ildl. Manage. 64(3):2000 NEST PREDATIOKOX PRAIRIEDOG COLONIES Baker et al. 779 risk of predation on reintroduced black-footed looked at colony boundaries to observe the type ferrets (Mustela nigripes). We sampled Bad- of habitat prairie dogs were colonizing and then lands-Conata 14 May-2 June 1998. searched nearby (typically 0.5-2.0 km) for sim- Thunder Basin National Grassland in eastern ilar sites lacking prairie dog colonies. If a suit- Wyoming was typical of shortgrasdshrub-steppe able off-colony site could not be located, the prairie dog range in much of Wyoming and colony was not sampled. Some off-colony sites Montana (Fig. 1). Common grasses were west- contained a few scattered prairie dog burrows. em wheatgrass, cheatgrass, blue grama, and needleandthread. Common forbs were scarlet Artificial Nest Data globemallow and woolly plantain. Big sagebrush \Ve placed artificial nests (3 = 9.8 nestdcol- (Artemisia tridentata), black greasewood (Sar- ony and 3 = 9.7 nestsloff-colony site for 1,981 cobatus vermiculatus), and fringed sagebrush placed nests) 100 m apart along a single transect (A. frigicla) were the dominant shrubs, and line in each of the 102 selected colonies and pricklypear was abundant. Homed larks and their paired off-colony sites, generally following McCown's longspurs were common on prairie the methods of Baker et al. (1999). Transects dog colonies; western meadowlarks, lark bun- began near the edge of a colony and continued tings, and Brewer's sparrows (Spizella breu~eri) through the center of prairie dog activity until were common at off-colony sites. Mountain plo- 900 m or the opposite side was reached (tran- vers, killdeer, and vesper sparrows (Pooecetes sect layout reflected colony shape and locations gramineus) also were present. The area was of active burrows). At off-colony sites, we tried grazed by domestic cattle; sylvatic plague was to mimic transect layout (direction, exposure) present, although most colonies we sampled on paired colonies. IVe estimated burrow den- had escaped recent outbreaks. Recreational sity by recordng the number of active prairie prairie dog shooting was frequent. We sampled dog burrows within 1.5 m of either side of the Thunder Basin 6-26 June, 1998. transect line (Biggins et al. 1993). Every 100 m METHODS along the line wehlaced a pin flag, packd 10 m perpendicular from the line, and located a suit- Experimental Design able nest site within a 2-m radus. We selected We sampled 102 black-tailed prairie dog col- nest locations using a search image that repre- onies (unit of replication) using 1,764 artificial sented likely nest sites for ground-nesting birds nests in a design that paired each colony with a typical of the area (Baicich and Harrison 1997). nearby control (off-colony site). We assumed This location was often at the base of or within differences in predation rates of artificial nests a shrub, grass, or pricklypear clump. Lie placed on and off colonies were proportional to scraped a shallow depression at each nest site dfferences in predation rates of natural nests and estimated nesting cover using a 1.5-m-tall (h4ajor and Kendal 1996). Nesting cover (visual Robe1 pole (marked in 2.5-cm increments) obscurity of vegetation) and burrow density placed at the center of the nest scrape (Robe1 were estimated on and off colonies to help ex- et al. 1970). We observed the pole from oppo- plain differences in nest predation rates. site directions (2 readingshest) from 1 m high Colonies at both complexes had recently and 4 m away, recording the highest interval been mapped by agency personnel via walking completely obscured by vegetation. This meth- or driving perimeters with a Global Positioning od blends vegetation height and density into 1 System or a combination of aerial photographs, measure and is often a good predictor of avail- topographic maps, and ground-truthing. From able nesting cover. After removing the pole, we their maps, we selected and sampled all colo- placed a roofing nail (head painted orange) in nies that were a minimum size of approximately each nest to ensure we had found the nest 400 X 800 m, had prairie dogs present or fresh scrape (not a similar natural depression) during scat (greenish-black in color) at burrows, and subsequent visits. In each nest, we placed a sin- had a suitable off-colony site. Paired colonies gle fresh Japanese quail (Coturnixjaponica) egg and off-colony sites typically had similar grazing on top of the nail. These eggs were cream-col- management (same grazing allotment contain- ored with dark specks, slightly smaller than ing either domestic cattle or bison), soil type, mountain plover or killdeer eggs, but larger and topography. To locate off-colony sites, we than homed lark or sparrow eggs (some small- 780 NEST PREDATIONON PRAIRIEDOG COLONIESBaker et al. J. Wildl. Manage. 64(3):2000 mouthed predators are unable to break quail turbed nests on a colony and the off-colony site. eggs). Estimators for pi, ci,and their variances were We typically placed equal numbers of nests derived using standard maximum-likelihood on a colony and its paired off-colony site on the methods (Larsen and Mam 1986), and for a giv-- same day, which ensured comparable exposure en colony x site combination, are equivalent to to predation across sites. After an average ex- the estimators in Johnson (1979; Baker et al. posure time of 9.8 days (range = 1-14 days), 1999). Because closed-form solutions for these nests were checked to determine if they had estimators do not exist, we used Newton's meth- been Isturbed. When logistical constraints per- od to solve for the parameters (Swokowski mitted, we left eggs in the nest for an average 1988). We computed 14-day nest predation of an additional 6.6 days (range = 4-10 days), rates as 1 - piI4 for colonies and 1 - cil"or and again checked them for disturbance. We off-colonv sites. We selected 14 davs to mimic recorded nest condition each time it was the incubation period typical of grassland and checked and used 4 categories to describe type shrub-steppe birds. Variances for 14-day pre- of nest hsturbance: none, egg moved but intact, dation rates were derived using the delta meth- egg gone, and egg crushed or broken. We did od (Seber 1982:7-9). not attempt to identify specific nest predators Tests for differences (d) between colonies based on appearance of eggshell fragments, be- and off-colony sites for 14-day artificial nest lieving this technique leads to misidentification predation rates, nesting cover, and burrow den- problems (Baker 1978, Hernandez et al. 1997, sity were constructed using paired t-tests, where Marini and Me10 1998, Sargeant et al. 1998). d = yo, - yofi and y is the variable of interest. We were unable to relocate 214 nests during Specifically, for the 14-day predation rate of ar- the study, primarily because cattle, ranchers, or tificial nests we tested Ha: 2 > 0, for nesting prairie dog shooters had pulled our flags. We cover we tested H,: 2 < 0, for standard devia- also omitted 3 flooded nests from the analysis. tion of nesting cover we tested Ha: 2 < 0, and for the number of active prairie dog burrows/ Statistical Procedures ha we tested Ha: 2 > 0. For all tests the null We estimated 14-day nest predation rates for hypothesis was Ha: 2 = 0. One-tailed tests were artificial nests by modeling the success or fail- used because hypotheses were specified a ure of a nest as an independent Bernoulli trial priori. Differences in nesting cover, standard with parameter n(t), where a(t) is the proba- deviation of nesting cover, and burrow density bility a nest survives an interval t days in length were used in a multiple regression analysis to (t may vary among nests). If we assume the dai- determine whether these variables were good ly survival probability of a nest is constant over predictors of hfferences in 14-day predation t, we can replace n(t) with pt where p is the rates of artificial nests between colonies and off- daily survival probability of the nest, giving us colony sites. Akaike's Information Criterion the probability distribution (AIC; Akaike 1973) was used to select the regression model best supported by the data. RESULTS where, y = 1 if the nest survives (i.e., it "suc- ceeds") and y = 0 if the nest is depredated (i.e., We summarized disturbance type for 1,764 it "fails"). Assuming that for the i-th colony (i artificial nests and found eggs undisturbed (un- moved and intact) at 908 nests and dsturbed at = 1, . . . , 102) daily survival probabilities on colonies (pi) and off-colony sites (ci) are ho- 856 nests (Table 1).More nests failed on colo- mogeneous (i.e., colony x site combinations are nies than off-colony sites for all 3 types of &s- homogeneous), the likelihood function for the turbance; a total of 54% of 889 nests failed on observed data is proportional to colonies compared to 43% of 875 nests at off- colony sites. Comparisons showed 14-day nest predation rates were 29.5% higher (tlo, = -4.6, P < 0.001) on prairie dog colonies (66.2 + 2.2%; f where, for given i and t, s and s ' are the number 2 SE) than at off-colony sites (51.1 + 2.7%). of successful nests on a colony and the off-col- This pattern was similar at both complexes, but ony site, and f and f' are the number of dis- differences in predation rates were greater in J. Wildl. Manage. 64(3):2000 NEST PREDATIO~oh PRAIRIE DOG COLONIES Baker et ul. 781 the mixed-grass prairie complex at Badlands- sistent with a similar study of white-tailed prai- Conata (z = 20.3 + 4.7%; tS2 = -4.3, P < rie dogs in Wyoming, Utah, and Colorado (Fig. 0.001) than in the sagebrush-steppe complex at l), where nest predation rates averaged 14% Thunder Basin (2 = 9.5 + 4.4%; t48 = -2.2, P higher on colonies than at off-colony sites (Bak- = 0.018; Table 2). Nesting cover estimates were er et al. 1999). Thus, nest predation was con- significantly lower (tlol = -11.7, P < 0.001) on sistently higher on colonies for both black-tailed colonies (3.4 2 0.2 cm) than at off-colony sites and white-tailed prairie dogs at 5 major com- (7.6 + 0.4 cm). Standard deviation of nesting plexes (combined data set of 3,208 artificial cover also was significantly lower (tlol = -7.4, nests at 176 colonies). White-tailed prairie dog P < 0.001) on colonies (2.4 + 0.1 cm) than at colonies also had less nesting cover than off- off-colony sites (4.5 + 0.3 cm), suggesting nest- colony sites (Robe1 cover; Baker et al. 1999). ing cover on colonies was more uniform in Re-analysis of white-tailed prairie dog data from structure than at off-colony sites. Density of ac- Baker et al. (1999) also showed standard devi- tive prairie dog burrows was significantly higher ation of nesting cover was significantly less (tR1 (tlol = 21.8, P < 0.001) on colonies (121.7 + = 4.7, P < 0.001) on colonies (2.4 + 0.3%) than 5.6 burrows/ha) than at off-colony sites (0.4 + at off-colony sites (4.9 + 0.5%). Thus, in both 0.3 burrowha). Patterns were similar at both black-tailed and white-tailed prairie dog habitat, complexes, nesting cover on colonies was less nesting cover was more uniform in structure dense and more uniform in structure than at where nest predation rates were higher, provid- off-colony sites; burrow density was greater on ing empirical evidence in support of Martin's colonies (Table 2). (1996) general prediction that uniform nesting To test the influence of habitat variables on cover can increase nest predation rates. nest predation, we regressed the difference in In addtion to reducing available nesting cov- nest predation rates on and off colonies against er, prairie dog colonies may attract nest preda- the differences in mean nesting cover, standard tors by creating favorable habitat or by serving deviation of nesting cover, and burrow density as an abundant source of prey. At Badlands Na- The best model, as selected by AIC (AIC = tional Park, deer mice and northern grasshop- -225.972), included mean nesting cover (P = per mice (Onychomys leucogaster) were 3 and 0.045) and standard deviation of nesting cover 4 times more abundant on colonies than at off- (P = 0.111); burrow density was not included. colony sites (Agnew et al. 1986). Indeed, predation by mice and other small-mouthed pred- DISCUSSION ators may even have been under-represented in our study because some species or individuals Patterns of Nest Predation of mice are unable to break quail eggs (Marini Nest predation rates may be higher on colo- and Melo 1998). Our field observations did not nies (29.5% higher in this study) if nests are suggest that prairie dogs are nest predators easier for predators to locate. Grazing distur- (Baker et al. 1999), but as prey themselves, bance by prairie dogs typically reduces height prairie dogs may attract predators that prey on and canopy cover of vegetation (Whicker and nests. Predators may be attracted to colonies Detling 1988), and higher nest predation rates specifically to search for nests (primary preda- are typically correlated with decreased nesting tion) or may find nests incidentally (secondary cover (29 of 36 studies; Martin 1993); our re- predation) as they hunt prairie dogs, small sults support this pattern. We found significant- mammals, or insects. For example, Vickery et ly less nesting cover on prairie dog colonies and al. (1992) demonstrated that striped skunks cover was a significant predictor of nest preda- (Mephitis mephitis) incidentally located and tion rates. In addition, nest predation can be depredated grassland bird nests while searching higher where cover is more uniform in structure for invertebrates. Thus, nest predation rates on because predators that develop a more narrow colonies may be higher both because the abun- search image are rewarded with higher success dance of predators is higher and because there (Bowman and Harris 1980, Martin 1996). In is less available nesting cover suggesting that our study, standard deviation of nesting cover the interaction of predator abundance and nest- was significantly lower on colonies and was at ing cover may be an interesting topic for further least weakly correlated with differences in nest study. predation rates. Results of this study are con- These patterns of higher nest predation that 782 NEST PREDATIONOX PRAIRIE DOG COLONIESBaker et al. J. Wildl. Manage. 64(3):2000 we observed for prairie dogs in North America age. Predator control may have exacerbated the may occur in ecologically similar species of oth- effects of fragmentation by altering predator er regions. In Central Argentina, the plains viz- composition and abundance. In particular, erad- cacha (Lagostomus maximus, family Chinchilli- ication of wolves (Canis lupus) from the Great dae) is a large, colonial, burrowing rodent of Plains may have increased abundance of smaller semiarid scrub and grasslands. Like prairie more effective nest predators. In a test of the dogs, vizcachas reduce height and cover of mesopredator release hypothesis, Rogers and grasses and shrubs, increase forb cover, provide Caro (1998) found that songbird nest success habitat for associated species (e.g., burrowing was increased by the presence of coyotes, which owls, Athene cunicularia, nest in vizcacha bur- apparently had suppressed populations of rac- rows), and may attract nest predators (e.g., fe- coons (Procyon lotor), a more effective nest lids; Branch et al. 1996; L. C. Branch, Univer- predator. In fragmented systems, mesopredator sity of Florida, personal communication). In release may combine with fragmentation effects Australia, the burrowing bettong (Bettongia le- to further increase localized nest predation sueur, family Potoroidae) is a mid-sized, colonial rates (Crooks and Soule 1999). Thus, fragmen- (warrens) burrowing marsupial, that once was tation and predator control may have altered widespread and abundant throughout the arid predator-prey dynamics on colonies and possi- and semiarid mainland. Like prairie dogs, bet- bly increased nest predation rates after bird tongs suffered severe population declines fol- species evolved associations with prairie dogs. lowing European settlement; isolated popula- Finally, bird species associated with prairie tions survive only on offshore islands (Baker and dogs may compensate for greater risk of nest Noble 1999). Before populations declined, bet- predation by reducing the importance of in&- tongs likely altered vegetation structure (e.g., vidual nests. In a comparison of nest predation reduced brush encroachment) and provided a and fecundity for birds in various nesting guilds locally abundant food source for predators (No- (e.g., excavating, ground-nesting, shrub-nest- ble 1997). Thus, because the risk of nest pre- ing), Martin (1995) found that species with dation is higher on prairie dog colonies in North higher nest predation rates had smaller clutches America, we predict that similar patterns may and more brooddyear. Comparisons with our occur on vizcacha colonies in Argentina and data suggest this same pattern may apply to bettong colonies in Australia. birds that nest on prairie dog colonies; mean clutch size was lower and mean number of Compensation for Higher Nest Predation brooddyear was higher for species associated Why would birds nest on prairie dog colonies with prairie dog colonies than with off-colony if the risk of nest predation is greater than at sites for both black-tailed (clutch size 3.0 on, off-colony sites? As speculation, we offer several 4.4 off; broods 2.5 on, 1.6 off) and white-tailed hypotheses that may operate independently or (clutch size 2.7 on, 3.4 off; broods 2.8 on, 1.8 in combination. For some species, the cost of off) prairie dogs (Fig. 1; B. W. Baker, unpub- increased nest predation may be offset by in- lished data). For this comparison, estimates of creased foraging success. Insects important to mean clutch size and number of broods/year foraging birds may be more numerous and eas- were obtained from the literature and associa- ier to catch on colonies, especially at critical tions were based on density estimates obtained times during the nesting cycle (Olson 1985, in 1996 and 1997 at the same 5 complexes sam- Baker et al. 1999). pled in this study and Baker et al. (1999). Prai- Habitat fragmentation following European rie dog associates were killdeer, mountain plo- settlement may have increased nest predation ver, mourning dove (Zenaida macroura), rates on black-tailed prairie dog colonies. Both homed lark, and McCown's longspur; off-colony complexes we sampled have been fragmented associates were grasshopper sparrow, vesper by poisoning, sylvatic plague, or other causes. sparrow, sage sparrow (Amphispiza belli), clay- As fragmented colonies become smaller and colored sparrow (Spizella pallida), Brewer's more isolated within the landscape they may sparrow, lark bunting, and western meadowlark. function as predator patches (Baker et al. 1999). Thus, comparisons from both black-tailed and These unique features may attract predators white-tailed prairie dog ecosystems support from the surrounding habitat matrix and pro- Martin's (1995) predction that species nesting vide a more narrow and successful search im- where predation rates are higher have smaller J. Wildl. Manage. 64(3):2000 NEST PREDATIONON PR.AIRIEDOG COLONIES Baker et a/. 783 clutches and more broods. Clearly, factors other through the Department of the Interior re- than association with prairie dogs may explain search needs process and was funded by the observed fecundity differences (e.g., difference U.S. Geological Survey, Fort Collins, Colorado. in body size), but the possibility of an evolu- tionary pattern developed from higher preda- LITERATURE CITED tion pressures is interesting and warrants fur- AGNEK.\v, D. \li. URESK,AND R. h.1. HANSEN.1986. ther investigation. Flora and fauna associated with prairie dog colonies and adjacent ungrazed mixed-grass prairie MANAGEMENT IMPLICATIONS in western South Dakota. Journal of Range Man- Fie believe the higher rates of nest predation agement 39:135-139. AKAIKE,H. 1973. Information theon as an extension on prairie dog colonies should not be viewed as of the maximum likelihood principle. Internation- a license to continue the widespread eradcation al Synposium on Information Theory 2:267-281. of prairie dog populations in North America. ANDERSON;E., S. C. FORREST,T. \v. CLARK,.4ND L, Rather, declines in prairie dog populations have RICHARDSON.1986. Paleobioloa; biogeography, likely contributed to parallel declines in popu- and systematics of the black-footed ferret, ,2lus- teln nigripes (Audubon and Bachman), 1851. lations of closely associated species, such as the Great Basin Naturalist Memoirs 8:ll-62. mountain plover (Knopf 1996, Kotliar et al. BAICICH,P. J., AND C. J. 0.HARRISOX. 1997, A guide 1999). If, as we speculate, fragmentation of to the nests, eggs, and nestlings of North ;\mer- large prairie dog complexes may contribute to ican birds. Second edition. Academic Press, San higher nest predation rates on colonies, then Diego, California, USA. BAKER,B. W. 1978. Ecological factors affecting wild agency and other land managers should focus turkey nest predation on South Texas rangelands. their efforts on the coordinated protection of Proceedings of the Annual Conference of the large intact prairie dog ecosystems rather than Soiltheastern Association of Fish and Wldlife piecemeal protection of isolated colonies. This Agencies 32:126136. recommendation is consistent with black-footed AND J. C. NOBLE.1999. People, vermin and ferret reintroduction guidelines, which incre- loss of biodiversity: prairie dogs in North America and burrowing bettongs in Australia. Pages 647- mentally value complexes by their relative area 648 in Proceedings YI International Rangeland (Biggins et al. 1993). Also, the protection of Congress, To\vnsville, Australia. large, intact complexes, especially those on pub- T. R. ST.IXLEY,AND J. A. SEDGLVICK.1999. lic lands, is the primary focus of recommenda- Predation of artificial ground nests on white- tions developed by State Wildlife Agencies in tailed prairie dog colonies. Journal of fvildlife hlanagement 63:270-277. response to the 1998 petition to list the black- BIGGINS,D. E., B. J. MILLER,L. R. HASEBURY,B. tailed prairie dog as a threatened species (\%'. OAKLEAF,A. H. FARMER,AND R. CRETE.1993. Van Pelt, Arizona Game and Fish, personal A technique for evaluating black-footed ferret communication). habitat. Pages 73-88 in J. L. Oldemeyer, D. E. Biggins, and B. J. hliller, editors. Proceedings of ACKNOWLEDGMENTS the s)-mposium on the management of prairie dog complexes for the reintroduction of the black- Fie are most grateful to our dedicated field footed ferret. U.S. Fish and \Vildlife Senice Bi- research assistants M. D. Cogswell, S. A. Meh- ological Report 13. ta, A. C. Rinker, J. H. Tasoff, J. D. Tjornehoj, BOLV\IAN,G. B., AND L. D. HARRIS.1980. Effect of and C. M. Wingert. Lie thank B. P. Bessken and spatial heterogeneity on ground-nest depreda- tion. Journal of if'ildlife Management 44:806- W. M. Perry for providmg maps of prairie dog 813. colonies and technical support at Badlands- BR.INCH,L. C., D. \'ILLARREAL, .4ND J. L. HIERRO, Conata and T. Byer for similar support at Thun- AND K. M. PORTIER.1996. Effects of local ex- der Basin. J. A. Sedwck initially suggested the tinction of the plains vizcacha (Lagostomnus mas- idea of evaluating nest predation on prairie dog im)lus) on vegetation patterns in semi-arid scrub. Oecologia 106:389-399. colonies and S. 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