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Annual Report 2010

Characterization of Avian Hazards Following Chlorophacinone 

Use for Prairie Dog Control

Nimish B. Vyas, 

US Geological Survey, Patuxent Wildlife Research Center

Submitted to

US Fish and Wildlife Service Region 6

Kevin Kritz, Kimberly Dickerson, Joy Gober,

Scott Larson, Matt Schwarz
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Abstract: 

Black-tailed prairie dogs (BTPD), Cynomys ludovicianus, are a keystone species 

of the prairie. However many in the agricultural community consider them to be pests 

and BTPDs are subjected to large-scale private and governmental eradication campaigns. 

The US Environmental Agency's recent registration of the rodenticide Rozol® (active 

ingredient chlorophacinone) for BTPD control has raised concerns because of its

potential primary and secondary poisoning hazards to birds. I conducted a preliminary 

study to better understand avian exposure and effects following operational Rozol® 

applications to control BTPDs. My findings show that birds can be lethally exposed to 

Rozol®.

Background and Justification: 

Black-tailed prairie dog ecology: Black-tailed prairie dogs, (BTPD) Cynomys 

ludovicianus, currently occupy more than 970,000 ha of habitat in 10 western states. 

BTPDs are highly social, diurnal fossorial rodents and live in colonies that are divided 

into coteries. Colonies vary in size and may range from a fraction of a hectare to 

hundreds of hectares. Ecologically, BTPDs are a keystone species because they influence 

the prairie ecosystem’s structure, function, and composition. Unlike other species of 

prairie dogs, BTPDs remain active above ground throughout the year, making them a 

valuable food source for wintering raptors (Plumpton and Andersen 1997; Weber 2004;

Kotliar et al. 2006).

Black-tailed prairie dog control: Many in the agricultural community contend that BTPDs

damage crops, compete with livestock for forage, and their burrows drain fields of water 

and create a hazard to livestock and farm equipment (Derner et al. 2006; Lamb et al. 
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2006). Consequently BTPDs are considered a nuisance and are subjected to large-scale 

eradications, often times promoted and conducted by county, state, and federal agencies

(Zinn et al. 1999). Intentional BTPD eradication commonly involves placing poisons 

either in or outside the burrows (Forrest and Luchsinger 2006; Hoogland 2006). Until 

recently, zinc phosphide, aluminum phosphide, and carbon (for use in ignitable gas 

cartridges) had been the only active ingredients to have received full registration (Section 

3 registration) for BTPD control under the Federal Insecticide, Fungicide, and 

Rodenticide Act by the US Environmental Protection Agency (EPA). However, in the 

past few years, six states (Colorado, Kansas, Nebraska, Oklahoma, Texas, and Wyoming) 

were granted the authority by EPA, under the Federal Insecticide, Fungicide, and 

Rodenticide Act Section 24c, to issue Special Local Needs registrations for the 

rodenticide Rozol® (0.005 % active ingredient chlorophacinone, 2-[(p-chlorophenyl) 

phenylacetyl)] 1,3-indandione) to control BTPDs ( see for example, 

http://www.epa.gov/opprd001/24c/; http://www.colorado.gov/cs/Satellite/AgricultureMain/CDAG/1214812857241). Avian laboratory acute oral toxicity tests show technical 

grade chlorophacinone to be Moderately Toxic, however, on a subacute dietary basis 

chlorophacinone is categorized as Highly Toxic 

(http://www.epa.gov/oppsrrd1/REDs/2100red.pdf). Furthermore, EPA identified the risk 

of secondary poisoning from Rozol® as being greater than the risks from other 

rodenticides (Erikson and Urban 2004). In 2009, EPA granted full registration (Section 

3; EPA registration number 7173-286) to chlorophacinone for BTPD control and thereby 

expanded Rozol®'s availability to 10 states: Colorado, Kansas, Montana, Nebraska, New 

Mexico, North Dakota, Oklahoma, South Dakota, Texas, and Wyoming (attachment 1). 
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BTPDs occupy an estimated 971,000 ha in the US and Rozol® is becoming the 

rodenticide of choice for controlling BTPDs (http://www.fws.gov/mountainprairie/species/mammals/btprairiedog/; Mr. Fred Raish, supervisor, Yuma County Pest 

Control District [YCPCD], Colorado, personal communication).

In 1993, at EPA’s request, the US Fish and Wildlife Service (FWS) completed a 

Biological Opinion on 16 vertebrate control agents including chlorophacinone. However 

chlorophacinone was not registered for BTPD control at that time and therefore, its risk 

to non-target species at BTPD colonies was not reviewed. EPA subsequently noted in 

their 1998 Reregistration Eligibility Decision that additional consultation with FWS may 

be necessary if new uses are proposed. However, EPA allowed the new use of Rozol® 

for BTPD control without consultation with FWS despite documented non-target 

mortalities and letters stating ecological concern to EPA from state wildlife agencies, 

FWS, and non-governmental organizations. In consideration of the expanded use of 

Rozol® for BTPD control, Rozol®'s mode of action, wildlife incident reports, and the 

Rozol® label's requirements for preventing take of non-target wildlife, research is 

warranted to better quantify the hazards of chlorophacinone to avian species. Little is 

known about the hazards of chlorophacinone-contaminated BTPDs to free-ranging nontarget species. However, a few serendipitous non-target mammalian and avian carcass 

recoveries raise the possibility of the adverse effects of Rozol® on a larger scale (Scott 

Larson, Joy Gober, Kimberly Dickerson, FWS Region 6, personal communication; Stone 

et al. 2003). The paucity of incident data does not imply a lack of hazard, but highlights 

insufficient monitoring (Vyas 1999). Information on the effects of operational 
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applications of Rozol® to avian species is much needed by FWS Region 6 since it 

contains seven of the 10 states where Rozol® is now registered for BTPD control.

Objectives and Methods:

This study is a three-year project. The current annual report summarizes the 

observations, methods, and findings from three field visits that were conducted as part of 

a preliminary study during year 1 (2010) of the project. The overall objective of this 

project is to determine if Rozol® applications for BTPD control result in adverse effects 

to birds. In 2010, data were collected with respect to the following specific objectives 

listed in the Study Plan that was submitted to the FWS:

1. Determine the effects of chlorophacinone on the above-ground activity of BTPDs. 

2. Document the above-ground BTPD morbidity and mortality. 

3. Measure chlorophacinone residue concentrations in BTPDs.

4. Measure BTPDs’ susceptibility to predation.

5. Determine avian exposure and effects of chlorophacinone.

Field Visit 1:

This visit was conducted in early January 2010. The purpose of this visit was to 

observe and learn about BTPDs and the birds associated with BTPD colonies in winter. I 

observed BTPDs and birds at two protected BTPD colonies at The Nature Conservancy's

Fox Ranch, Kirk, Yuma County, Colorado and I was privileged to be invited by Mr. Fred 

Raish, supervisor, YCPCD, to visit three fields that had been treated with Rozol® for 

BTPD control in the past month. The three Rozol-treated fields were in proximity to 

each other and together they formed an almost contiguous area of approximately 65.5 ha

or 162 acres (Mr. Fred Raish, supervisor, YCPCD, personal communication). Because of 
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the high density of BTPD burrows, Rozol® application to this area occurred over 

approximately 3 weeks (Mr. Fred Raish, supervisor, YCPCD, personal communication). 

Observations. I observed BTPDs and birds at two BTPD colonies at Fox Ranch over 

two days. BTPDs were active above ground at both colonies. Large flocks of horned 

larks (Eremophila alpestris) were the only avian species seen feeding at the colonies. 

Two golden eagles (Aquila chrysaetos) and one red-tailed hawk (Buteo jamaicensis) were 

seen near but not at the colonies. By contrast, an hour at the Rozol® treated fields 

revealed six or seven ferruginous hawks (Buteo regalis) in the BTPD colonies (figure 1).

Large flocks of horned larks were also observed feeding in the Rozol®-treated fields. 

Walking through the fields revealed that Rozol® was visible in many BTPD burrow 

entrances (please see below section Field Visit 2: Results: Rozol® availability to wildlife 

and figure 5 for details). Rozol® is a green colored winter wheat based anticoagulant 

indandione rodenticide product (figure 2). EPA requires rodenticides, other than those 

used exclusively at agricultural sites, to be formulated with an indicator dye in order to 

help identify a human's or a household pet's exposure to the rodenticide (EPA 1998). For 

Rozol®, the indicator dye color is green and it can be used to determine exposure in 

BTPD and birds. I also frequently encountered bird droppings in burrow entrances. 

Based on the dropping size and shape and their abundance, I believe the droppings to be 

of horned larks. The majority of the droppings were grey-white in color (typical 

dropping coloration) but occasionally, green droppings were observed in the burrow 

entrances (figure 3). I did not see horned lark droppings in burrow entrances at the 

BTPD colonies at Fox Ranch. A larger green-colored dropping that, based on its size, 

shape, and texture and the common occurrence of the species in Yuma County, I believe
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the dropping to be from a ring-necked pheasant (Phasianus colchicus), was discovered in 

one of the treated fields. Additionally, grey-white droppings were seen on several BTPD 

entrance mounds and similar but green droppings were observed on some entrance 

mounds. Based on the droppings' size and consistency, I believe these droppings to be 

from a raptors such as the ferruginous hawks in the fields (figure 3). No BTPDs were 

observed at the Rozol®-treated fields due to the weather (BTPDs are not active above 

ground when it is overcast and very windy; Mr. Fred Raish, supervisor, YCPCD, 

personal communication).

Field Visit 2:

The actual preliminary field study was conducted during the second field visit 

(March-April, 2010).

Study Site

Work was conducted in a private pasture located along County Road CC, 

approximately 1.3 km (0.8 miles) south of Vernon, Yuma County, Colorado (attachment 

2). The BTPD colony on the pasture was scheduled to be poisoned with Rozol® during 

the 2009-2010 application season by the YCPCD as part of their on-going BTPD control 

program in Yuma County, Colorado. This site was identified as a potential study site and 

the landowner's permission for access onto the pasture was secured by the generous 

assistance of Mr. Fred Raish, supervisor, YCPCD. The BTPD colony in the pasture was 

divided into two locations. The main colony occupied approximately 17.3 ha (42.8 acres) 

of the pasture. The secondary part of the colony was composed of only a few BTPD 

burrows and was located approximately 0.6 km (0.4 miles)away from the primary colony. 
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I focused the research on the primary colony. Hereafter the term 'study site' refers to the 

primary and secondary colonies and the term 'colony' refers to the primary colony. 

Methods

Application: Rozol® was applied to the study site according to the label by certified 

pesticide applicators of the YCPCD. Based on YCPCD's calculations, approximately 231 

kg (510 lbs) of Rozol® were applied, treating 4080 holes. The southern half of the 

colony was treated on March 15, 2010 and the northern half of the colony and the 

secondary colony were treated on March 16, 2010. Within the colony, the area used by 

BTPDs contained low heavily grazed vegetation with patches of bare ground and was 

encircled by taller grasses and herbaceous vegetation. A barbed wire fence formed the 

perimeter along three sides of the colony (figure 4). Since it took two days to complete 

the Rozol® application (March 15 and 16, 2010,), it can take at least a week for 

mortality to occur after exposure, and BTPDs in the southern part of the northern half of 

the colony could easily venture into the southern half of the colony and become exposed

to Rozol® after the first application day, hereafter the two application days are combined 

as day 0 (day of application). Subsequent days of the study will be identified with 

respect to day 0. 

In general, I walked east-west transects through the colony to search for evidence 

that may potentially create an exposure scenario to birds and to document the effects of 

Rozol®. Each search lasted 2-4 hrs and searches were conducted on 14 days over the 29 

day post application period (table 1). The transects were about 3m (10 ft) apart in short 

vegetation but were closer in taller, denser vegetation. Since the objective was to find 

evidence of poisoning and not to estimate the BTPD population, transect widths and rate 
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of walking the transects were not restricted by rigid methodology. In general, transects 

were walked in late afternoon following the Rozol® label. Information on the 

application, wildlife occurrence and behavior, signs of Rozol® exposure to wildlife, and 

adverse effects to wildlife were documented and appropriate samples were collected. 

Observations and sample collections were conducted for 14 days during the 29 day postapplication study period.

Observations: I recorded avian and non-target mammal use of the colony and noted 

evidence of Rozol® exposure in BTPDs and other wildlife in the colony. Signs of 

exposure included mortality, morbidity, discolored droppings, scavenging, possible blood 

stained soil, and a general change in the number of BTPDs and other wildlife following 

Rozol® application. I also noted if Rozol® was observed outside the burrow. In addition 

to making observations in the late afternoon, when my daily schedule allowed, I drove 

past the study site in late morning and early afternoon to observe avian predators and 

scavengers at the colony and BTPD activity. These observations were made while driving 

the car slowly along County Road CC in order to not frighten BTPDs in the pasture; 

BTPDs ran into their burrows if I stopped the car.

Sample collection and handling: Rozol® was collected from the applicator on day 0 for 

chlorophacinone analysis. Additionally, Rozol® that was still visible either on the ground 

surface or in the burrow entrances was collected on day 27 post-application for 

chlorophacinone analysis. These samples were collected by either a trowel or a long 

handled scooper. The collection was sifted through several different sized mesh to 

separate the Rozol® from the soil and litter. Dead and moribund BTPDs and non-target 

wildlife were collected for chlorophacinone residue analysis and for histopathology
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examination. I also used live animal box traps, baited with oats, Cheerios®, peanut 

butter, and commercially available timothy hay (green hay), coated with molasses, peanut

oil, and vegetable oil, either singly or in various combinations over the 29 day post 

application period but did not trap any BTPDs. Dark colored BTPD droppings were 

collected to determine the occurrence of blood since chlorophacinone is an anticoagulant 

rodenticide. Presence of blood will be determined with a phenophthalein presumptive 

blood test kit at the Patuxent Wildlife Research Center. All samples were stored in a 

freezer before shipping them to Patuxent Wildlife Research Center with the exception of 

a western meadowlark (Sturnella neglecta) which was placed in a refrigerator overnight 

and dissected the next day to harvest tissues for histopathology examination. Ten tissue 

samples from the skeletal muscle, heart, lung, kidney, spleen, brain, liver, and intestine 

were collected and placed in 10% neutral buffered formalin following protocol provided 

by Dr. Carol Meteyer, wildlife pathologist, USGS National Wildlife Health Center, 

Madison, Wisconsin. The tissues were shipped to Patuxent Wildlife Research Center, 

prepared into slides by American Histolabs Inc., Gaithersburg, Maryland, and sent to Dr. 

Meteyer for examination.

Results 

Rozol® availability to wildlife: The angle of the BTPD burrow entrance in relation to the

tunnel appears to influence the availability of Rozol® from the surface (figure 5). After 

application, Rozol® was visible from the surface in burrows with shallow-angled

entrances whereas no Rozol® could be seen from the surface in steep-angled burrow

entrances. Beginning day 2 post-application, an increasing number of shallow-angled 

burrows had Rozol® exposed into their entrances and onto the ground surface outside 
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their entrances by BTPDs performing burrow maintenance (figure 6). By day 27 postapplication, fewer burrows displayed Rozol® in their entrances and on the ground surface 

outside.

Avian species observed in the BTPD colony: 

In winter, BTPDs are a valuable food source for raptors and colonies of dead and dying 

BTPDs can attract flocks of raptors as documented in Field Visit 1 (Plumpton and 

Andersen 1997; Weber 2004; Kotliar et al. 2006; Mr. Carl Stogsdill, Stogsdill Ranch, 

personal communication). Field Visit 2 was conducted after the wintering raptor flocks 

had begun migrating north and no raptor flocks were seen at the colony. However, four 

species of raptors (turkey vulture, Cathartes aura, northern harrier, Circus cyaneus, redtailed hawk, and burrowing owl, Athene cunicularia) were observed at the colony. A pair 

of burrowing owls, established territory in the colony and eventually nested in a Rozol®

treated BTPD burrow. Additionally, a pair of great horned owls (Bubo virginianus) were 

detected on a tree close to the colony at dusk. Several resident and transient passerine 

species were observed using the colony for feeding and nesting. Western meadowlarks 

were seen in the taller vegetation in the colony during the early study period but later 

began foraging in the BTPD-grazed section of the colony. Several pairs of horned larks 

established territories in the BTPD-grazed part of the colony and often perched on BTPD 

mounds. A Say's phoebe (Sayornis saya) was observed for few days using BTPD 

mounds as platforms for darting after insects. Large flocks of chestnut collard longspurs 

(Calcarius ornatus) and horned larks used the colony as a migratory stopover site. 

Several unidentified sparrows mainly perched on the fence along the southern boundary 

of the colony and flew into the colony to feed. Other birds using the BTPD colony 
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included mourning doves (Zenaida macroura) and ring-necked pheasants that foraged in 

the colony and a pair of killdeer (Charadrius vociferus) that established their territory in 

the grazed portion of the colony. 

Signs of Rozol® exposure and adverse effects: Carcass searches over 11 days recovered 

two intact thirteen-lined ground squirrels (Spermophilus tridecemlineatus), nine intact 

BTPDs, seven scavenged BTPDs and one intact western meadowlark (table 1; figures 7, 

8, 9). Several BTPD remains (for example, feet, hide, head) were also found but were 

not collected. All carcasses were found above ground. All intact BTPD carcasses were 

within 3 m of their burrow entrances. The first mortality, a thirteen-lined ground squirrel 

was found on day 5 post-application on BTPD mound located close to taller vegetation. 

The second thirteen-lined squirrel was also collected from the taller vegetation in the 

colony on day 13 post-application. The first BTPD poisonings were observed on day 12 

post-application and included two intact carcasses, two scavenged carcasses, and 2 

moribund BTPDs (figure10). Many of the dead and moribund BTPDs had brownish-red 

coloration on their underside and under their tails (figure 11). A red-tailed hawk and a 

northern harrier were observed scavenging on BTPDs on day 12 post-application and 

turkey vultures were seen feeding on BTPD carcasses on days 23 and 24 post-application

(figure 12).

Most moribund BTPDs were seen close to their burrow entrances. The death of 

one moribund BTPD was documented on video. The BTPD was on its side with its legs 

moving in a cycling motion and displayed heavy breathing before dying. Signs of 

poisoning were observed up to the last day of the study, day 29 post-application. A total 

of eight moribund BTPDs were observed. On day 29 post application, one moribund 
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BTPD was captured in a net but died shortly afterwards (included in the nine intact 

BTPDS collected above).

One intact male western meadowlark carcass was found in the grazed section of 

the colony. No overt signs of trauma were apparent (e.g. wounds or loose feathers) and 

no signs of external bleeding were observed. The meadowlark was dissected to harvest 

tissues for histopathology examination and Rozol® residue analysis. Upon opening the 

carcass, the bird appeared to be healthy but the pectoral muscles displayed gross 

hemorrhaging (13). The gizzard and intestine were full; the gizzard contained insects 

parts and the intestine had a dark greenish liquid. The histopathology report documented 

abundant hemorrhage associated with the skeletal muscle and one focus of hemorrhage in 

the brain (Dr. Carol Meteyer, wildlife pathologist, USGS National Wildlife Health 

Center, Madison, Wisconsin, personal communication).

A green bird dropping, I believe it to be from a ring-necked pheasant based on the 

dropping's size and texture and the presence of ring-necked pheasants in the colony, was 

found on day 1 post-application. Another green bird dropping was encountered under the 

barbed wire fence on day 24 post-application. I believe this dropping to be from a 

western meadowlark based on its size and texture and the fact that meadowlarks were 

practically the only species that perched on that segment of the fence during Field Visit 2. 

A third green dropping of an unknown avian species (probably a passerine) was also 

found in the colony (figure 14). Other avian droppings appeared normal. Green and dark 

black colored BTPD droppings were observed beginning day 2 post-application and 

continued at least until day 29 post-application. The black droppings were mostly seen 

stuck to the inside wall of burrows near the entrances or on the burrow mounds at the 
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entrances (figure 15). Two BTPD burrow mounds were stained and the staining went

down into the tunnel entrance, possibly by blood (figure 16).

The BTPD population declined over the study period and on day 29 postapplication. Some of these BTPDs were appeared alert and active while others showed 

signs of poisoning. Moribund BTPDs were still observed above ground. No ground 

squirrels were seen in the colony by day 29 post-application.

Field Visit 3:

Transects as above were walked in June 2010 to locate affected animals and 

document breeding birds in the BTPD colony. Birds seen at the colony include: a pair of 

burrowing owls nesting in a Rozol® treated BTPD burrow, western kingbirds (Tyrannus 

verticalis), horned larks, lark buntings (Calamospiza melanocorys), western 

meadowlarks, lark sparrows (Chondestes grammacus), and mourning doves. One nest 

with nestlings of a ground nesting passerine species was found and one egg, possibly of 

killdeer, was encountered. The surviving BTPDs appeared active and alert and had pups 

but no ground squirrels were observed. In late April, some of the wheat seeds from the 

Rozol® bait had started germinating in burrow entrances. Sprouting seeds may be 

attractive to wildlife. Soil from a mound with possible blood was collected, along with 

burrowing owl castings and pellets and BTPD droppings to test for the presence of blood

as described above.

Discussion:

Implications of the findings

Though the work described above was conducted as a preliminary study, the 

results document avian exposure and effects following operational application of Rozol®
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for BTPD control. Rozol® is a green colored product winter wheat based anticoagulant 

indandione rodenticide product. Discolored (green) bird droppings imply that birds have 

been exposed to Rozol®. Green raptor droppings were found in Rozol® treated colonies 

during the field visit in January. The dead and dying BTPDs had attracted ferruginous 

hawks who in turn were exposed to Rozol® via secondary poisoning. A concentrated 

and debilitated (easy) food source is highly attractive and becomes a magnet for 

scavengers (see Vyas 1999 for review; see Vyas et al. 2003 for review; Chesser 1979).

No raptors were seen at BTPD colonies on Fox Ranch but a flock of ferruginous hawks 

had congregated at the Rozol® treated colony that was less than 32 km (20 miles) away. 

The day I visited the Rozol® treated colonies in January 2010, no BTPDs were seen 

above ground because of an overcast sky and strong winds. However, ferruginous hawks 

were seen soaring low over the BTPD colony, perched on the fields' perimeter barbed 

wire fence, and perched on the ground. These are typical hunting behaviors for 

ferruginous hawks on BTPD colonies (Wakeley 1978). These observation and reports 

from FWS reports show that ferruginous hawks are attracted to Rozol® treated colonies 

because of the availability of dead and moribund prey. The fact that these birds were 

present at a colony without above-ground BTPD activity on the day I visited and that 

they had not moved to another location indicates that the hawks have preyed on BTPDs 

at this location in the recent past and expect to encounter BTPDs in the near future. It is 

likely that the green stained raptor droppings are evidence of secondary poisoning to the 

hawks. My observations of a red-tailed hawk, a northern harrier and two turkey vultures 

feeding on BTPDs at the study colony during the second field visit also demonstrate the 

importance of a concentrated food source to birds. Typically, BTPDs either sounded 
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their alarm calls and ran to their burrows by the time I was approximately 15 m (50 ft) 

from them. However, the moribund BTPDs allowed me to approach them within 3 m 

before slowly raising themselves onto their legs and moving towards their burrow. My 

being able to get so close to the moribund BTPDs and netting one of them implies that 

they are easy prey (Hunt et al. 1992).

An unexpected finding is the exposure to songbird species. Traditional concerns 

about the risks of avian poisoning from Rozol® have focused on raptors becoming 

exposed via secondary poisoning (Erikson and Urban 2004) . However, my observations 

also show that other groups of birds are also exposed by ingesting Rozol® bait. Horned 

lark droppings were found in BTPD burrow entrances of Rozol®-treated colonies but not 

in burrow entrances of the BTPD colony at Fox Ranch. Horned larks are birds of open 

country (Beason 1995) and in general, they would not be expected to go into an enclosed 

area such as a burrow but the fact that their droppings were in burrow entrances in the 

Rozol® treated colonies implies that these birds were attracted into the burrow entrances 

by Rozol®. Their actual exposure to Rozol® is supported by the presence of green 

droppings found in the entrances. The western meadowlark mortality during Field Visit 

2 was also surprising. While his liver has not yet been analyzed to confirm for Rozol®

residue, the gross pectoral muscle hemorrhaging and the hemorrhage in the brain, lack of 

evidence of trauma, injuries, and illness, and a gastrointestinal track with food, suggest 

anticoagulant poisoning. Furthermore, western meadowlarks primarily utilized the 

barbed wire fence along the west boundary of colony for perching and singing and they 

were also the primary species which used the west fence. Birds flew from the fence into 

the grazed and ungrazed sections of the colony to feed. Based on these observations and 
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the size and texture of the dropping found under the west fence, I speculate that the green 

dropping was from a meadowlark. The green dropping was discovered during the carcass 

search the day before recovering the dead meadowlark. The western meadowlarks' diet

consists of grain during winter and early spring, and insects in last spring and winter 

(Davis and Lanyon, 2008). The upper gastrointestinal track of the meadowlark found 

during the study was replete with insect parts yet there is evidence of rodenticide 

exposure. This suggests that meadowlarks are attracted to Rozol®.

Adverse effects to BTPDs and non-target wildlife following operational 

application of Rozol® were first documented in this study on day 5 post-application and 

continued at least until my last day at the colony during Field Visit 2 (day 29 postapplication). The Rozol® label states that the treatment period is "October 1 and March 

15 of the following year, or before spring green-up of prairie grasses, whichever occurs 

later, when animals will most readily take the grain bait" (attachment 1). The reason

Rozol® is to be applied before green-up is to ensure that BTPDs feed on Rozol® instead 

of the vegetation. In the current study, Rozol® was applied on March 15 and March 16, 

2010 and green-up was expected later in the week (Mr. Fred Raish, supervisor, YCPCD, 

personal communication; figure 17). Despite competition from fresh vegetation over the 

course of Field Visit 2, adverse effects to BTPDs and non-target wildlife were observed 

in this study.

Documentation of adverse effects to birds depends on how readily the effect can 

be detected. An observer is more likely to detect an adverse effect when discovering a 

mortality event than when coming across a sublethally poisoned bird with no overt signs 

of stress. Additionally, the definitive nature of mortality requires no speculation about 
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the consequences of the effect to the birds. Therefore mortality is the most commonly 

reported adverse effect for birds.

A major obstacle to gathering evidence of avian poisonings following Rozol®

applications is that dead birds are difficult to find in the field. On a landscape level, 

avian mortality may be overlooked because of the vastness of the areas that are treated 

with Rozol® and the spacio-temporal mosaic of the applications. The size of the areas

involved renders it impossible to monitor all locations subjected to Rozol® applications . 

The situation is further complicated by the fact that Rozol® use occurs primarily on 

private properties, making the sites inaccessible for regular monitoring. On the local 

scale, a mortality even can also go undetected. The initial discovery of a carcass and the 

outcome of a subsequent search are moderated by the scavenger and predator 

populations, animal behavior, carcass morphology, habitat, vegetative structure, searcher 

experience, and search intensity. Rozol®'s mode of action also is a critical variable for 

successful recovery of a carcass because 1) Rozol® is not an acute toxin so bird 

mortalities would not be confined to treated locations, 2) Rozol® is an anticoagulant that 

requires multiple feedings and death can occur 1-3 weeks after application, 3) birds are 

mobile and can die away from the treated location. An avian carcass found away from 

the treated site may be discounted as resulting from Rozol® use (see Vyas 1999 for 

review).

Many mortalities that are discovered are not reported because of the public’s

ignorance that the wildlife mortality should be reported and to whom it should be 

reported, fear of prosecution, camaraderie, procrastination, and apathy. When a mortality 

event is reported to the correct authorities, an investigation to determine the cause of 
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death is not always immediately possible by the authorities due to distance, weather, 

terrain, private property issues, and other on-going activities. Furthermore, because of 

limited resources, only one of two searchers may be able to respond and yet be required 

to search large areas, thereby limiting search intensity. An increasing interval between 

the onset of mortality and recovery of evidence reduces the chances of salvaging 

carcasses that would be suitable for laboratory analysis to determine the cause of death 

because of scavenging, decomposition, and chemical degradation (see Vyas 1999 for 

review).

Mortality reports confirm the hazards from the operational use of Rozol® to 

birds, however the magnitude of the effects is not known. In general, the percent of 

carcasses that are detected, reported, and confirmed for the cause of death is small but it 

provides invaluable information on the hazards to birds. These carcasses, On the 

landscape level, the carcasses that have been confirmed for the cause of death strongly 

imply that operational Rozol® applications result in avian mortalities and on a local 

level, the evidence state that the actual number of avian losses per mortality event 

typically exceeds the number of carcasses recovered. I believe, in the current study, the 

recovery of only a single avian carcass to be an artifact primarily of breaks in the days I 

conducted carcass searches during the 29 days post-application of the study. In light of 

the obstacles in finding carcasses, it would not be unreasonable to assume that more birds 

may have died from Rozol® application in the study colony.

Relating the findings to the pesticide label
20

The pesticide label is considered the law for directing the pesticide's use. The 

findings reported here raise concerns regarding the adequacy of the Rozol® label to 

ensure safety to birds (attachment 1).

1. The label states: "Apply...bait at least 6 inches down active prairie dog 

burrows. Do not apply bait on or above ground level. Make sure no bait is left on the 

soil surface at the time of application. Applicator must retrieve and dispose of any bait 

that is spilled above ground or placed less than 6 inches down the burrow entrance."

First, the angle of the BTPD burrow entrance in relation to the tunnel appears to 

influence the availability of Rozol® from the surface. During Field Visits 1 and 2, I 

noticed that, though applied 6" down the tunnel, Rozol® was visible from the surface in 

burrows with shallow-angled entrances. This may be an artifact of the application 

equipment used to place Rozol® in the burrow. The bend in the tunnel may inhibit the 

application equipment from reaching deeper into the burrow (figure 5). Consequently

birds are able to reach the Rozol®. 

Second, as part of their burrow maintenance, BTPDs kick the Rozol® out of 

burrows with shallow-angled entrances onto the ground surface. The fact that Rozol®

was placed on a burrow entrance ramp may allow the BTPDs to kick it out while 

maintaining their burrows (figure 5). Rozol® was seen on the ground surface beside the 

burrow entrances and in the burrow entrances during Field Visits 1, and 2. Rozol® on 

the ground surface as a result of continuous burrow maintenance and not misapplication, 

was observed beginning day 2 post-application through day 29 post-application during 

Field Visit 2. The label's requirement that Rozol® be placed in the burrow to minimize 

surface availability is undermined by BTPD behavior.
21

2. The label states: "The applicator must return to the [application] site within 5 to 

10 days after bait application to collect and properly dispose of any bait or dead or dying 

prairie dogs that may have come to the surface. The applicator must also return to the site 

14 to 21 days after bait application to collect and properly dispose of any additional bait 

or dead or dying prairie dogs that may have come to the surface. Bury carcasses on site 

in holes dug at least 18 inches deep or in inactive burrows... to avoid non-target animal 

scavenging. Burial includes covering and packing the hole or burrow with soil".

First, the label requires the applicator to return to the application site and collect 

and dispose bait or dead or dying prairie dogs that may have come to the surface in order 

to minimize secondary poisoning of scavengers. The label, however, does not specify 

how many times the applicator must return to the site within 5 to 10 days and during 14 

to 21 days post-application. In the current study, dead and moribund BTPDs were 

documented on each day that a carcass search was conducted starting on day 5 postapplication. Therefore if the applicator does not visit the application site daily for at least 

one month post-application and remove carcasses and the exposed Rozol®, any

protection provided to birds by the Rozol® label is considerably diminished

Second, the label does not specify how the search for spilled and surfaced Rozol®

and dead and moribund BTPDs must be conducted in order to ensure that the label 

requirements are satisfied. Currently, the label requirements could be fulfilled by 

scanning the area with binoculars from the field perimeter or by conducting the search 

while driving a vehicle through the colony. With either method or with similar methods, 

the applicator will not be able to satisfy the intent of the label requirement (Vyas 1999). 
22

To satisfy the label, searchers need to walk at a moderate pace in a systematic method to 

ensure coverage of the entire area.

Third, the label does not instruct the applicator as to what must be done with dead 

or moribund non-target wildlife at the treated site. These poisoned animals are also a 

hazard to predators and scavengers. Additionally, the label provides no information for 

reporting non-target mortalities to state and federal wildlife authorities.

Fourth, the label recommends disposing of the carcasses by burying them in holes 

and in inactive burrows. However, scavengers such as the North American badgers

(Taxidea taxus) have a highly developed sense of smell and are powerful diggers and 

would be expected to reach the BTPD carcass. Additionally, BTPD burrows may have 

more than one entrance so plugging one entrance will not exclude fossorial scavengers 

from reaching buried BTPD carcasses.

3. The label states: "Collect and bury carcasses in late afternoon near sundown, to 

reduce the potential of scavenging animals finding carcasses."

First, I observed raptors feeding on BTPDs in late morning, and in early and late 

afternoon. The above label statement may lessen the availability of poisoned prey for 

nocturnal animals, but it does not necessarily reduce the chances of secondary poisoning 

for diurnal avian predators and scavengers.

Second, because mortality from Rozol® may occur over several weeks, BTPDs 

who have been exposed to Rozol® and are still active above ground, remain a source for 

secondary poisoning to diurnal avian predators. While the label tries to reduce hazard to 

non-target wildlife from dead and dying BTPDs and from Rozol® that has surfaced, the 

label does not address reduction of hazard created by contaminated yet active BTPDs. 
23

The current label requirements for Rozol® regarding BTPD carcass removal by 

the applicator are laborious but still fall short of mitigating for the risks from Rozol® to 

federally protect avian species under the auspices of the Migratory Bird Treaty Act. At a 

recent meeting between the Director of the Division of Registration of the Office of 

Pesticide Programs of the EPA and Rozol® applicators in North Dakota, it was brought 

to light by the applicators that the carcass removal requirements as stated on the current 

label are, in addition to being laborious, unrealistic and therefore label non-compliance is 

commonplace (Mr. Scott Larson, FWS Region 6, personal communication). I am aware 

of similar label non-compliance in two other states. Dead and dying animals can be 

quickly removed by predators and scavengers (Vyas 1999; Vyas et al. 2003). If the label 

requirements for returning to the application site for removing BTPD carcasses are 

interpreted by applicators as requiring only one visit within 5 to 10 days and during 14 to 

21 days post-application, the interval between application and carcass collection will be

too great. Consequently, many BTPD and other carcasses will be missed during 

searches, thereby reducing the perception of the hazard but not the actual poisonings. 

Since the label requirements for carcass removal address only BTPDs, applicators 

returning to the application site would confine their search to the treated site. Since it 

may take an animal 1-3 weeks to die following a Rozol® application, a highly mobile 

non-target species such as birds can die away from the BTPD colony and thereby be 

missed in any effort to document Rozol®'s hazards to birds.

The EPA finalizes and approves the pesticide label that will be placed on the 

product. The function of a pesticide label is not just to provide information on 

application rates, times, and methods but also to provide this information to minimize 
24

environmental harm. Based on the findings of this study thus far, it would not be 

unreasonable to state that the current Rozol® label is inadequate in providing proper 

safety to federally protected birds.

Acknowledgements

I thank the US FWS Region 6 for their support in conducting this study especially Kevin 

Kritz, Kimberly Dickerson, Joy Gober, Scott Larson, Matt Schwarz, and Larry Gamble. I 

also thank Fred Raish and the staff of the Yuma County Pest Control District for their 

assistance. I am grateful to William Burnidge and Chris Pague of The Nature 

Conservancy for their assistance. I express my gratitude to Dr. Carol Meteyer, USGS, for 

helping me with the tissue collection protocol and conducting the histopathology 

examination of tissues, and Josh Melby, Colorado Division of Wildlife, for guidance and 

for providing BTPD carcasses. I also sincerely acknowledge Nathan Andrews, Laura 

Quakenbush of the Colorado Division of Agriculture, the staff of the PWRC, especially 

John B. French, Barnett Rattner, Gary Heinz, and Kinard Boone for their help and 

guidance.
25

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Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the 

Birds of North America Online: 

http://bna.birds.cornell.edu/bna/species/195doi:10.2173/bna.195

Chesser, RK. 1979. Opportunistic feeding on man-killed prey by ferruginous hawks. The 

Wilson Bulletin. 91:330-331.

Davis SK and Lanyon WE. 2008. Western Meadowlark (Sturnella neglecta). The Birds 

of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; 

Retrieved from the Birds of North America Online: 

http://bna.birds.cornell.edu/bna/species/104doi:10.2173/bna.104

Derner JD, Detling JK, Antolin MF. 2006. Are livestock weight gains affected by blacktailed prairie dogs? Frontiers in Ecology and the Environment 4: 459-464.

EPA. 1998. Rodenticide Cluster. Reregistration Eligibility Decision. Environmental 

Protection Agency. Washington, DC. Available at:

http://www.epa.gov/oppsrrd1/REDs/2100red.pdf

Erikson W and Urban D. 2004. Potential risks of nine rodenticides to birds and nontarget 

mammals: a comparative approach, United States Environmental Protection 

Agency.

http://www.regulations.gov/search/Regs/home.html#documentDetail?R=0900006

4800b7862

Forrest SE and Luchsinger JC. 2006. Past and current chemical control of prairie dogs. in

Saving North America's Western Grasslands, Hoogland J. Ed. Island Press, 

Washington, DC, pp. 115-128

Hoogland JL. 2006. Why have so many prairie dogs disappeared? in Saving North 

America's Western Grasslands, Hoogland J. Ed. Island Press, Washington, DC, 

pp. 89-93. 

Hunt KA, Bird DM, Mineau P, and Shutt L. 1992. Selective predation of 

organophosphorus-exposed prey by American kestrels. Animal Behaviour.

43:971-976.

Kotliar NB, Miller MJ, Reading RP, Clark TW. 2006. The prairie dog as a keystone 

species. in Saving North America's Western Grasslands, Hoogland J. Ed. Island 

Press, Washington, DC, pp 53-64.

Lamb BL, Reading RP, Andelt WF. 2006 Attitudes and perceptions about prairie dogs. 

in Saving North America's Western Grasslands, Hoogland J. Ed. Island Press, 

Washington, DC, pp 108-114.
26

Plumpton DL and Andersen DE. 1997. Habitat use and time budgeting by wintering 

ferruginous hawks. The Condor. 99:888-893.

Vyas NB. 1999. Factors influencing the estimation of pesticide-related wildlife mortality. 

Journal of Toxicology and Industrial Health. 15:186-191.

Vyas NB, Spann JW, Albers E, and Patterson D. 2003. Pesticide-laced predator baits: 

considerations for prosecution and sentencing. Environmental Lawyer. 9: 589-

608. 

Wakeley JS. 1978. Hunting methods and factors affecting their use by ferruginous hawks.

Condor. 80:327-333.

Weber DA. 2004. Winter raptor use of prairie dog towns in Denver, Colorado vicinity. in 

Proceedings of the 4th International Symposium on Urban Wildlife Conservation

Symposium. Shaw WW, Harris LK, Vandruff, LE. Eds. May 1-5, 1999. Tuscon, 

Arizona. pp.195-199.

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27

Table 1. Timeline of observations and sample collection following Rozol® application.

Day 0

- Rozol® 

application. 

- Rozol® 

collected.

Day 1

- Green colored 

gamebird dropping 

found.

Day 2

- Rozol® being kicked out of 

burrows.

- Green colored BTPD droppings 

found.

Day 3

- No searches 

conducted due 

to weather.

Day 4

-Nothing 

remarkable 

observed.

Day 5

- Green and black colored 

BTPD droppings.

- 1 dead intact thirteen-lined 

ground squirrel collected.

Day 6- Day 11

- No searches 

conducted due to 

weather.

Day 12

- 1 red-tailed hawk 

and 1 northern harrier 

seen feeding on dead 

BTPDs.

- 2 intact dead 

BTPDs collected.

- 2 scavenged BTPDs 

collected.

- 2 moribund BTPDs 

observed.

Day 13

- 1 red-tailed hawk and 1 

northern harrier overhead, hazed 

by my presence. 

- 1 intact dead thirteen-lined 

ground squirrel collected.

- 2 intact dead BTPDs collected.

- 1 scavenged BTPD collected.

- 1 moribund BTPD observed. 

- Possible blood stains found at 

burrow entrance. 

- Several BTPD carcass 

fragments observed.

Day 14- Day 22

- No searches 

conducted: trap 

acclimation 

period.

Day 23

-1 turkey 

vulture 

observed 

scavenging on 

a BTPD.

- No searches 

conducted: trap 

setting.

Day 24

- Green colored meadowlark 

dropping found under west 

fence.

-2 turkey vultures observed 

scavenging on BTPD.

- 2 intact dead BTPDs 

collected.

- 2 scavenged BTPDs 

collected.

- 1 moribund BTPD 

observed.

Day 25

- 1 intact dead 

western 

meadowlark 

collected. 

- 1 scavenged 

BTPD collected.

Day 26

- 1intact dead BTPD 

collected.

- 1 scavenged BTPD 

collected.

- 1 moribund BTPD 

observed.

- Green colored 

passerine dropping 

observed.

Day 27

- 1intact dead BTPD collected.

- Rozol® collected.

- BTPD droppings collected.

- Soil from burrow entrance with 

possible blood stains collected.

Day 28

- No searches 

conducted due 

to weather.

Day 29

- 1 moribund 

BTPD 

collected.

- 2 moribund 

BTPDs 

observed.
28

Attachment 1. Rozol® label.
29

Attachment 2. Preliminary study site location.
30

Figure 1. One of the ferruginous hawks over a Rozol® treated BTPD colony

during Field Visit 1. No BTPDs were above ground because of the weather.

Figure 2. Rozol®.
31

(a) (b) (c)

Figure 3. Bird droppings at a Rozol®-treated colony: (a) normal colored horned lark 

droppings in burrow entrance, (b) few green colored horned lark droppings along with 

normal horned lark droppings in burrow entrance. Rozol® is visible from the surface in 

images (a) and (b). (c) green colored raptor dropping. 

Figure 4. Study colony. Grazed by BTPD (foreground) and un-grazed habitats. 
32

(a) (b)

(c)

Figure 5. Possible influence of BTPD burrow entrances in relation to their tunnels, 

Rozol® placement and Rozol® availability to birds: (a) burrow with steep-angled 

entrance (b) burrow with a shallow-angled entrance. Green arrows and shape depict 

Rozol® placement and Rozol® location after BTPDs kick it out of the burrows. (c) the 

equipment used during Rozol® application at the study colony included a rigid pole with 

an attached cup. 
33

Figure 6. Examples of Rozol® in burrow entrances and above ground.

 
34

 

 

 

Figure 7. Thirteen-lined ground squirrels (top two photos) and some of the BTPD 

mortalities recovered intact. All intact BTPD carcasses were found near burrows. 
35

 

 

 Figure 8. Some of the scavenged BTPDs found during carcass searches. The first photo 

shows a BTPD that was snagged on the fence as northern harrier, who was feeding on the 

BTPD at the study colony, flew off with the BTPD but the prey's weight forced her to fly 

low and the BTPD did not clear the fence.
36

 Figure 9. Western meadowlark carcass detected in the study colony.

 Figure 10. One of the moribund BTPDs 

from the study colony.

Figure 11. Many of the dead and 

moribund BTPDs had brownish-red 

coloration on their underside and under 

their tails. Similar discoloration

in Figure 10.
37

Figure 12. A turkey vultures scavenging a BTPD on 

day 23 post-application.

Figure 13.Western meadowlark recovered from the study

colony displaying hemorrhaging on the pectoral muslces.
38

 Figure 14. Two of the green (Rozol®-exposure) avian droppings

found in the study colony.

Figure 15. Dark black BTPD droppings were seen starting day 5

post-application and continued to be found at least until day 29 postapplication.
39

Figure 16. BTPD mound stained possibly

by blood.

Figure 17. View of the vegetation in the study 

colony on day 0. Green-up expected later in 

the week.