Great Horned Owl Site-specific Dietary Exposure to PCDF/Ds

in the Tittabawassee River Floodplain in central Michigan

Nadeau, M.W.1, Coefield, S.J.1, ZwiernikM.J.1, Fredricks, T.B.1, Seston, R.M.1, Tazelaar, D.T.1

, Plautz, S.C.1, Kay, D.P.2, Shotwell, M. S.2 and J.P. Giesy1,3

1. Zoology, Michigan State University, East Lansing, MI, USA. 2. ENTRIX, Inc. East Lansing, MI, USA. 3. Veterinary

Biomedical Sciences & Toxicology Centre, University of Saskatchewan, Saskatoon, SK, Canada.

Table 1. Mean (SD) avian TEQ concentrations (ng/kg ww) for GHO dietary components in the Tittabawassee River

floodplain and the estimated GHO potential daily dietary exposure (ADDpot).

UPSTREAM DOWNSTREAM

Species % of GHO diet (by biomass) GHO ADDpot (ng/kg/day) GHO ADDpot (ng/kg/day)

Muskrat 28.7 0.004 0.631

Eastern Cottontail 39.8 0.019 4.81

Common Raccoon* 11.12 0.005 1.35

Eastern Fox Squirrel 1.69 0.0001 0.001

Meadow Vole 5.73 0.001 0.0703

Northern Short-tailed Shrew 0.005 0.00004 0.006

Star-nosed Mole* 0.12 0.0009 0.151

Other herbivorous small mammals 1.6 0.002 0.176

Waterfowl* 6.96 0.001 0.189

Birds 4.14 0.047 0.761

Crayfish 0.05 0.00004 0.004

TOTAL GHO ADDpot (ng/kg/day) 0.081 8.142

*E. Cottontail TEQs used to estimate Raccoon, N. Short-tailed Shrew TEQs used to estimate Star-nosed Mole, Turkey (with skin on) TEQs used to estimate Waterfowl

GHO Prey Remain Analysis

RESULTS AND DISCUSSION

Prey remains and egested pellets were collected from 8 nests in the study area. Analysis of

the dietary items (n=297) shows a preference for meadow voles, muskrat, rabbits, and birds

in the GHO diet. Rabbits and muskrats contribute the majority of biomass consumed by the

owls (Figure 1) and as a result contribute the highest fraction of the GHO daily dietary

exposure to PCDF/Ds (table1). The two major congeners in the dietary items are 2,3,4,7,8-

PeCDF and 2,3,7,8-TCDF.

The 2006 site-specific GHO diet varies considerably from both the 2005 site-specific diet and

literature diets from the Michigan area (Figures 2 and 3). Determining the GHO site-specific

diet over multiple years allows researchers to catch trends or shifts in the dietary base that

could potentially alter GHO exposure to contaminants. In addition, a site-specific diet

eliminates levels of uncertainty that would result from utilizing a literature-based diet.

The GHO potential daily dietary exposure to PCDF/Ds (Table 1) was calculated using the daily

wildlife dose equation for dietary exposure (the GHOs normalized ingestion rate is

72g/kg/day).

m

ADDpot = Σ(Ck x FRk x NIRk)

k=1Analysis of the GHO ADDpot shows that GHO dietary exposure to PCDF/Ds is significantly

higher in the floodplain downstream of Midland, MI (8.142 ng/kg/day versus 0.081 ng/kg/day

upstream). However, the concentration of PCDF/Ds in the GHO diet is in the range of TCDD

concentrations considered safe for avian consumption (10-12 ng/kg ww) and would not be

expected to induce adverse effects. The downstream 2006 GHO ADDpot is less than the ADD

pot calculated based on prey collected in 2005 (Figure 4). The preponderance of muskrats in

the 2006 diet and their corresponding lower TEQ values account for this discrepancy.

CONCLUSIONS

• In 2006 the GHO potential average daily dietary dose of PCDF/D downstream of Midland, MI

was 8.1 ng TEQ WHO Avian/kg/day. This is ~ 100-fold greater than the ADDpot upstream of

Midland, but still falls within “safe” levels.

• The 2006 GHO prey base incorporated a substantial number of muskrats, which accounts

for the lower ADDpot in 2006 as compared to 2005.

• The 2006 GHO site-specific diet shifted to a more aquatic prey base

(crayfish, muskrats, waterfowl) than was seen in the 2005 site-specific diet. This shift

underscores the importance of collecting data over multiple years.

REFERENCES

• Charlebois, P.M., G.A. Lamberti. 1996. Invading Crayfish in a Michigan Stream: Direct and Indirect

Effects on Periphyton and Macroinvertebrates. JNABS 15(4):551-563.

• Craighead, J. J., F.C. Craighead, Jr.. 1956. Hawks, owls and wildlife. Stackpole Co., Harrisburg, PA.

• Eisler, R. 2000. Handbook of Chemical Risk Assessment, Vol 2. Lewis, Boca Raton, FL, USA.

• Zwiernik, M. J., K. D. Strause, D. P. Kay, A. L. Blankenship, and J. P. Giesy. (2007) „Site-Specific

Assessments of Environmental Risk and Natural Resource Damage Based on Great Horned

Owls‟, Human and Ecological Risk Assessment, 13:5, 966-985.

• U.S. Environmental Protection Agency (USEPA). 1993. Wildlife Exposure Factors Handbook. Office

of Research and Development, Washington D.C. EPA/600/R-93/187a&b.

2006 mean upstream PCDF/D concentrations

in GHO dietary components

2006 mean downstream PCDF/D concentrations

in GHO dietary components

0

5

10

15

20

25

Av

ian

TE

Qs

(ng

/kg

)0

500

1000

1500

2000

2500

3000

3500

4000

4500

Av

ian

TE

Qs

(ng

/kg

)

Muskrat

Eastern

CottontailCommon

RaccoonEastern Fox

SquirrelMeadow Vole

Northern Short-

tailed ShrewStar-nosed

MoleDeer/White-

footed MouseWaterfowl

Birds

Crayfish

ACKNOWLEDGEMENTS

This project would not have been possible

without the Michigan State University Museum

vertebrate collections that aided in identifying

prey remains.

Funding was provided through an unrestricted

grant from The Dow Chemical Company to

Michigan State University.

ABSTRACT

Great horned owls (Bubo virginianus) in the Tittabawassee River floodplain in the vicinity of

Midland, Michigan, USA, are exposed to elevated levels of polychlorinated dibenzofurans

(PCDFs) and polychlorinated dibenzo-p-dioxins (PCDDs) found in the floodplain soils. The

site-specific dietary exposure of the great horned owl (GHO) was determined by analyzing

prey remains found in the nest and egested pellets collected from the base of nest and

roosting trees. In addition, small mammals, rabbits, muskrats, and passerine birds were

collected from the floodplain upstream and downstream of Midland and analyzed for PCDF/D

concentrations. The average daily intake (ADI) of PCDF/D was predicted by use of a

weighted average dietary concentration expressed as 2,3,7,8 tetrachlorodibenzo-p-dioxin

equivalents (TEQ). In 2005 the GHO ADI of PCDF/Ds downstream of Midland was 1.1x101ng

TEQs/kg/day. Monitoring the GHO dietary composition over multiple years allows

determination shifts in prey base that may impact the GHO exposure to contaminants.

INTRODUCTION

Concentrations of polychlorinated dibenzo-p-dioxins and furans (PCDD/DF) in floodplain

soils and sediments of the Tittabawassee River have prompted an examination of their

potential impact on the terrestrial ecosystem.

As top predators great horned owls (GHO) have the potential to be exposed to high

contamination levels via bioaccumulation. By taking advantage of the owls‟ practice of

egesting the indigestible fur and bones of its prey, researchers can construct the GHO diet

on a site-specific basis. This data can then be used to determine the GHO dietary exposure

to contaminants.

Here we present data from a study designed to evaluate the GHO site-specific dietary

exposure to PCDD/DF in identified prey items.

METHODS AND MATERIALS

• Dietary items were collected from GHO nests in the study area and identified to the lowest

taxonomic level possible.

• Prey items were collected adjacent to identified GHO nests. Small mammals were sampled

at two locations upstream and six locations downstream of Midland, MI. Rabbits were

sampled at two locations upstream and three locations downstream of Midland, MI.

• After homogenization prey items were analyzed for PCDF/D concentrations.

• Chemical extraction followed EPA method 3540C and 3541.

• Congener-specific PCDF/D analysis was conducted with GC/high resolution MS following

EPA method 8290.

• Results are corrected based on recoveries and non-detect congeners = ½ detection limit.

• Great horned owl food intake rate is based on literature values (Craighead and

Craighead, 1956).

• Great horned owl average potential daily dose estimated based on avian-specific World

Health Organization (WHOAvian) TCDD equivalency factors.

MuskratMeadow Vole

Eastern Cottontail Rabbit

0.045

11.329

0.081

8.142

0

2

4

6

8

10

12

AD

Dp

ot

(n

g/k

g/d

ay

) 2005 2006

Upstream Downstream

Figure 4. Year-to-Year comparison of Total GHO ADDpot (ng/kg/day)

in upstream and downstream nest sites

Figure 1. 2006 Tittabawassee River GHO

Dietary Composition by Biomass

29%

40%

11%

2%

6%

1%

7%4%

Figure 2. 2005 Tittabawassee River GHO

Dietary Composition by Biomass (Coefield 2006)

83%

7%

2% 8%

Figure 3. GHO Dietary Composition

by Biomass (Strause 2005)

29.74

35.73

19.84

0.11

0.94

22.64

Muskrat

Eastern Cottontail

Common Raccoon

Eastern Fox

SquirrelMeadow Vole

Northern Short-

tailed ShrewStar-nosed Mole

Other Herbivorous

Small MammalsWaterfowl

Birds

Crayfish

Tittabawassee River site-specific GHO diet Literature-based GHO diet

National Food Safety and Toxicology Center

Prey Remains