Prepared in collaboration with the U.S. Fish and Wildlife Service

Model of Whooping Crane Energetics as Foundation 

for Development of a Method to Assess Potential Take 

during Migration

Open-File Report 2012–1156

U.S. Department of the Interior

U.S. Geological Survey

Model of Whooping Crane Energetics as 

Foundation for Development of a Method to 

Assess Potential Take during Migration

By Aaron T. Pearse and Sarena M. Selbo

Prepared in collaboration with the U.S. Fish and Wildlife Service

Open-File Report 2012–1156

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Pearse, A.T., and Selbo, S.M., 2012, Model of whooping crane energetics as foundation for development of a method 

to assess potential take during migration: U.S. Geological Survey Open-File Report 2012–1156, 13 p.
iii

Acknowledgments

This report is a product of a Quick Response Program project involving collaboration 

among Regions 2 and 6 of the U.S. Fish and Wildlife Service (USFWS) and the U.S. Geological Survey (USGS). Aaron Pearse also was supported by funds from the USGS Platte River 

Priority Ecosystem program and Northern Prairie Wildlife Research Center. 

U.S. Fish and Wildlife Service employees that provided helpful information or assistance 

during the inception and development of the energetic model included Amelia Orton-Palmer, 

Laila Lienesch, Jim Dubovsky, Dave Sharp, Tom Stehn, Martha Tacha, and Greg Watson. 

Amelia Orton-Palmer, Larry Igl, Joshua Stafford, and Elizabeth Masden provided valuable 

comments to earlier drafts.

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Energetic Model Basics................................................................................................................................2

Energy Expenditure...............................................................................................................................2

Energy Intake.........................................................................................................................................3

Constraints.............................................................................................................................................3

Derived Constraints..............................................................................................................................4

Energetic Model Inputs.................................................................................................................................4

Power of Flight.......................................................................................................................................4

Whooping Crane Mass ........................................................................................................................4

Flight Speed ...........................................................................................................................................4

Basal and Field Metabolic Rates .......................................................................................................4

Energy Content of Stored Lipids.........................................................................................................4

Empirical Estimates of Lipid Gain.......................................................................................................5

Potential Diet .........................................................................................................................................5

Energy Content of Foods......................................................................................................................5

Forage Efficiency ..................................................................................................................................5

Maximum Food Consumption .............................................................................................................6

Wet/Dry Weight Coefficients ..............................................................................................................6

Maximum Time per Foraging Bout and Bouts per Day...................................................................6

Potential Management and Regulatory Applications..............................................................................6

References Cited............................................................................................................................................7

Appendix........................................................................................................................................................11
iv

Tables

1. Potential food items that might be consumed by whooping cranes during 

migration ........................................................................................................................................5

2. True metabolizable energy of potential diet items for migrating whooping 

cranes.............................................................................................................................................6

3. Estimated extra days whooping cranes would need to replenish lipid 

reserves from hypothetical model runs using varying additional distances 

whooping cranes might fly for an entire migration season...................................................7

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Flow rate

meter per second (m/s) 3.281 foot per second (ft/s) 

kilometer per hour (km/h) 0.6214 mile per hour (mi/h)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Energy

joule (J) 0.738 foot-pound (ft·lb)

watt (W) 0.00134 horsepower (HP)
Model of Whooping Crane Energetics as Foundation for 

Development of a Method to Assess Potential Take during 

Migration

By Aaron T. Pearse and Sarena M. Selbo

Abstract

A whooping crane energetic model was developed as 

a component of a larger effort to ascertain potential take, as 

defined by the Endangered Species Act, of whooping cranes 

from proposed development of wind-energy infrastructure in 

the Great Plains of North America. The primary objectives 

of this energetic model were to (1) predict extra flight energy 

that whooping cranes may require to find suitable migration 

stopover sites if they are unable to use a primary site; and (2) 

express energy expended as additional time required to replenish lipid reserves used to fuel flight. The energetic model is 

based on three elements related to energy: expenditure of 

energy, intake of energy, and constraints to energy intake. 

The energetic model estimates each element and recognizes 

interactions among them. This framework will be most useful 

when integrated into a migration model that predicts incidence 

of avoidance of wind towers by whooping cranes and distances they might fly to find alternative stopover habitat. This 

report details work conducted in accordance with the U.S. 

Geological Survey and U.S. Fish and Wildlife Service Quick 

Response Program funded in fiscal year 2011 and will serve as 

a final report. 

Introduction

Generation of electricity using wind-powered turbines 

constitutes an expanding component of renewable energy 

development in the United States. The Great Plains of North 

America has vast potential for wind-powered electricity generation and represents valuable territory to the wind-energy 

industry (Elliott and others, 1987). Federally endangered 

whooping cranes (Grus americana) migrate through the Great 

Plains during fall and spring, traveling between their breeding 

grounds at Wood Buffalo National Park in Canada and wintering grounds at and surrounding Aransas and Matagorda Island 

National Wildlife Refuges along the Gulf Coast of Texas, USA 

(Allen, 1952). During migrations, whooping cranes stop at 

suitable sites for relatively short periods of time (generally less 

than 10 days) to rest and feed (Lewis, 1995). The presence of 

infrastructure associated with wind-energy development in or 

near stopover sites poses a potential risk to whooping cranes. 

Nineteen wind companies are working with the U.S. Fish and 

Wildlife Service (USFWS) to develop a Great Plains habitat 

conservation plan (HCP), under the Endangered Species Act 

(ESA). This plan outlines efforts to minimize and mitigate 

effects from potential take of whooping cranes. Under the 

ESA, take of wildlife is defined as actions that harass, harm, 

pursue, hunt, shoot, wound, kill, trap, capture, or collect 

endangered species and include attempts made to engage in 

any such conduct (Endangered Species Act, 1973). This effort 

will partly depend on a risk assessment of take of migrating 

whooping cranes and the effects of such take. The results of 

that assessment will inform appropriate siting and operations 

guidance to developers of wind-energy projects to avoid or 

minimize adverse impacts to whooping cranes. 

Experts have suggested whooping cranes prefer stopover 

habitat without vertical obstructions of sight (Armbruster, 

1990); thus, whooping cranes may avoid landing at otherwise 

suitable sites where wind turbines occur. The development of 

wind farms may, in some cases, negatively affect energy budgets of whooping cranes during migration, when seeking alternative stopover habitat increases energy demands. Additionally, presence of vertical obstructions such as wind towers may 

increase migration flight distances if birds avoid structures 

(Masden and others, 2009). The development of the HCP’s 

risk assessment model will partly rely on an understanding of 

whooping crane energy budgets during migration. To meet that 

purpose, funds associated with the Quick Response Program 

were provided to a team of biologists from U.S. Geological 

Survey (USGS) and the USFWS to develop a special-use 

energetic model of whooping crane migration, as described 

in this report. The primary objectives of this energetic model 

were to (1) predict extra flight energy that whooping cranes 

use in finding suitable stopover sites if they are unable to use 

a primary site; and (2) express energy expended as additional 

time required to replenish lipid reserves used to fuel flight. 

This additional time may cause delays in migration that could 
2 Model of Whooping Crane Energetics as Foundation for Development of a Method to Assess Potential Take

lead to whooping cranes arriving late to the breeding grounds, 

and ultimately sublethal impacts to fitness. If these effects 

were attributed to specific causes (for example, presence of 

a wind-energy facility), they could be considered take under 

the Endangered Species Act. Direct mortality because of wind 

structures (for example, collisions) will be addressed as part of 

the more comprehensive HCP.

Because this energetic model was developed to be a component of the larger HCP risk assessment effort, it may have 

limited use as a stand-alone product beyond offering insight 

into basic constraints whooping cranes must overcome during migration. The energetic model was developed under the 

assumption that certain predictions from a migration model 

for whooping cranes would be used as input parameters in the 

energetic model. These input parameters include incidence of 

avoiding wind turbines and distances whooping cranes might 

fly to circumvent them. Thus, this energetic model was not 

constructed to determine if whooping cranes avoided wind 

turbines or to determine distances they might fly to avoid towers and other structures. Integration with a migration model is 

necessary for expanded use (for example, Masden and others, 

2012). The energetic model does have flexibility to quantify 

extra energy expended as lipids used to sustain flight or an 

expectation of days needed to replenish used lipids; therefore, 

the energetic model could be used to estimate, over an entire 

migration, number of days whooping cranes were delayed 

because of foraging to replenish energy stores, a lipid deficit 

after reaching breeding grounds, or a combination of these 

potential impacts. Practitioners would need to supply predicted 

behavioral responses (for example, stay at a site and forage as 

compared to continued migration) to take advantage of this 

plasticity.

A model “reflects an abstraction of key features of a 

system into a simple set of ideas, words, or equations that 

represents the system” (Williams and others, 2002, page 

22). Models are approximations of truth that are intended to 

capture key features of systems, while excluding unnecessary detail; therefore, as with any model, this energetic model 

does not contain all of the complexities of individuals interacting with their environment. Output from a model is only 

as valid and reliable as the input parameters, assumptions, 

and constraints used to create it. Model developers should be 

diligent in documenting these components. Because relevant 

data specific to the whooping crane were lacking, parameters 

and relations developed from other species in the modeling 

process were used. Future research likely will provide more 

applicable parameter estimates, and practitioners may wish to 

modify input parameters as additional data become available. 

As models and simulations do not completely represent complex systems, their success can be measured by their ability to 

achieve an intended purpose—the objective(s) of the modeling 

effort. 

The remainder of this report comprises four sections. An 

initial section describes basic energetic model design, which 

comprises a description of how and why it was constructed 

and a partial list of limitations inherent in the current version. 

The following section includes description of input parameters 

used and full listing of citations. Assumptions used in development also are indicated. The next section provides examples 

of potential management and regulatory application of output. 

Finally, the energetic model is presented in a spreadsheet for 

ease of use, accessibility, and transparency for those interested 

in inspecting how it generates output (appendix A). Executing the energetic model with a programming language may be 

more useful when interfacing with a migration model. 

Energetic Model Basics

Energetics provide a useful framework to describe how 

organisms make a living in their environment. All organisms 

must acquire and expend energy to sustain life and conduct 

normal activities. This energetic model is based on three 

elements related to energy: expenditure of energy, intake of 

energy, and constraints to energy intake. The energetic model 

was developed to calculate each element, provide for interactions among elements, and find balance among them. This 

section provides explanation of how and why each element 

was selected and calculated, and how each fits into the overall 

energetic model to produce outputs. Additionally, this section 

provides a general description of methodology, whereas the 

following section includes detailed documentation of input 

parameters and citations.

Energy Expenditure

Whooping cranes use several methods of flight during 

migration, such as thermal spiraling coupled with gliding and 

forward-flapping flight (Kuyt, 1993). Flight powered by soaring and gliding, which was the most frequent method observed 

for migrating whooping cranes (Kuyt, 1993), requires greatly 

reduced energy compared to forward-flapping flight (Pennycuick, 1989). When thermal activity declined at dusk, 

whooping cranes used forward-flapping flight at relatively 

low altitudes to progress to a stopover site (Kuyt, 1993). This 

energetic model was developed based on the assumption that 

whooping cranes migrating diurnally may perceive wind 

turbines as obstructions at primary stopover sites and fly some 

extra distance (that is, a distance they would not have flown if 

the wind farm was not there) to find a suitable alternative stopover site. Thus, any energy used before avoidance behavior is 

considered part of their unaltered migration and may include 

soaring or flapping flight. 

To estimate extra energy used to avoid structures, an 

empirical method was used that determines power required 

for forward-flapping flight only, the method of flight expected 

of whooping cranes late in the day. This method relies on a 

derived equation from results of multiple studies of birds, 

wherein direct measurements of energy requirements for flight 

were measured (Norberg, 1996; Castro and Myers, 1988). 

An empirical equation using only body mass was presented 
Energetic Model Basics 3

by Norberg (1996), which described 86 percent of variation 

in flight energy requirements (n = 33). Although a simple 

and useful relation, the heaviest bird included in the sample 

was slightly more than 1 kilogram (kg); thus, calculating the 

power of flight for a 7.2-kg whooping crane using this equation is tenuous. Therefore, using a measure of flight energy 

from birds closer to the size of a whooping crane would be 

recommended, if it becomes available. Flight energy initially 

is expressed in kiloJoules per second (kJ/s) and the energetic 

model converts this into energy consumed over a distance by 

incorporation of an estimate of flight speed. This estimate of 

energy use [kiloJoules per kilometer (kJ/km)] is used directly 

to determine extra energy required for flight. Three alternative empirical methods were presented by Castro and Myers 

(1988), using body mass and body mass with structural measurements to improve model fit. When using their best available method, which incorporated body mass and wing length, 

resulting flight energy requirements for a 7.2-kg bird with a 

wing length of 60 centimeters (cm) (Lewis, 1995) were nearly 

twice as much as that calculated using the mass only equation 

presented in Castro and Myers (1988) or the mass only equation from Norberg (1996). Therefore, it was suspected that 

available wing measurements did not properly represent wing 

length needed in the empirical flight energy equation. 

At the end of a migration day, whooping cranes are 

likely to use lipid stores to fuel flight because other sources 

of energy have already been expended; thus, the energetic 

model calculates lipids used to fuel extra flight by converting lipids to energy, assuming that lipid catabolism is not 100 

percent efficient. In the most simplistic version, lipids used 

to fuel extra flight must be replaced before further migration 

can occur. The energy required to synthesize this amount 

of lipids is calculated based on the energy content of lipids, 

taking into account that synthesis of tissue is not 100 percent 

efficient. Additionally, the energetic model can provide output 

for any conceivable “extra distance”, yet it may be reasonable 

to assume that whooping cranes would rarely fly greater than 

200 kilometers (km) after a day of migration, given that it may 

take them ~4.5 hours (hr) to cover 200 km at a flight speed of 

12 meters per second (m/s) (43 kilometers per hour (km/hr); 

Tacha and others, 1992).

An alternative method for determining power required 

for flight, which uses theoretical equations has been described 

by Pennycuick (1989). Masden and others (2010) used this 

method to calculate energetic costs of avoiding current and 

proposed wind farms for multiple species of seabirds. The 

method represents a detailed approach to calculating many 

aspects of flight performance and thus requires detailed input 

parameters, namely structural measurements of birds. Currently, measurements made with the exacting methods needed 

to run the model with reasonable accuracy are not available. 

Using estimates, the model predicts the cost of flight for a 

7.2-km whooping crane at 638 watts, or 29.8 kJ/km and also 

predicts an optimal flight speed of 21.4 m/s. Using these 

two model-derived values, the resultant energy requirement 

represents a 25 percent increase in energy expense compared 

with the current method. Yet, using a wingspan of less than 

20 percent difference from an estimated value (Lewis, 1995), 

estimated flight energy would be the same from each model. 

Because the flight program is sensitive to structural measurements, reliable measurements are needed before incorporating 

into the whooping crane energetic model. 

 Basal metabolic rate (BMR) is the measure of energy 

birds expend to maintain themselves. BMR can be estimated 

using a mathematical relation to body mass. Energy expenditure more than BMR is necessary to perform daily activities 

(for example, movement, thermoregulation); this energetic 

model incorporates a multiplier more than BMR to accommodate these functions. Overall energetic demand or field 

metabolic rate can be considered a fixed cost of living. Basal 

and field metabolic rates are known to vary with season, temperature, diet, and other factors (see Carey, 1996). These additional complexities were not included in the energetic model 

because it was not known how these parameters might affect 

metabolic rates of whooping cranes, leading to unknown error 

in calculations. The method used has been used in numerous 

situations, mainly for determining energetic needs of nonbreeding waterfowl in North America (for example, Reinecke 

and Loesch, 1996).

Energy Intake

Whooping cranes acquire energy by consuming food. 

Availability of various crops in the Great Plains and documented habitat use provide some information as to what 

types of foods whooping cranes may encounter and consume 

during migration. Minimal information exists as to what foods 

whooping cranes actually use or in what proportion foods 

are consumed. The energetic model allows for a mixed diet 

of 11 foods. Additional foods can be easily added. All foods 

have three basic parameters associated with them: (1) the 

true metabolizable energy of a food represents the energetic 

content of the food that birds can assimilate [kiloJoules per 

gram (kJ/g) dry mass]; (2) a factor that allows for conversion 

between wet and dry weights when necessary; and finally, (3) 

a rate of food consumption grams per minute (g/min) provides 

an estimate of how much energy birds can consume under a 

time constraint. Model practitioners can vary diet by changing 

percentages of dry mass consumed. The energetic model calculates weighted averages of energy content and consumption 

rate for use in the next portion of the energetic model.

Constraints

Animals are limited in how much energy they can intake 

in a set period of time. The energetic model provides for two 

basic ways in which to implement constraints. An integrated 

approach allows the model practitioner to supply a value 

for maximum energy intake, expressed as lipid acquisition 

per day. An estimate of lipid gain per day is not available 
4 Model of Whooping Crane Energetics as Foundation for Development of a Method to Assess Potential Take

specifically for whooping cranes; thus, a value based on estimates derived from other species must be used. 

The energetic model also includes calculations of 

potential lipid acquisition rates per day using two potential 

mechanisms. If food intake rate is high, animals may be 

constrained by the amount of food their digestive system can 

process in a day. This can be considered a food-based constraint and approximated by a maximum amount of food that 

can be consumed. If the food intake rate is relatively low, then 

animals may be limited by the amount of time they can put 

towards foraging in a day; this can be considered a time-based 

constraint represented by total minutes per foraging bout or 

day that birds can feed. Because the energetic model has great 

flexibility in users defining the animal’s diet, it is not known 

initially which factor will limit energy intake. The energetic 

model provides results based on both mechanisms.

Derived Constraints

After calculation of energetic demand and intake based 

on constraints, the energetic model makes predictions regarding the extra time needed to reconcile needs. Initially, the 

energetic model requires that a certain amount of food must 

be consumed to meet basic requirements (that is, field metabolic rate) and any intake above the base level can be used to 

replace lipids. Maximum energy that birds can intake relates 

to the two constraints set up above. This value is converted to 

lipids that can be synthesized per day and compared with the 

amount that the whooping crane must acquire based on extra 

flight distances. This results in the number of foraging bouts 

that the bird must initiate to meet demand and, assuming they 

must stay all day if required to initiate just one bout, how 

many extra days they must stay to meet demand. For example, 

if a bird must consume enough food to synthesize 20 grams 

(g) of lipids (extra energy demand) and they can acquire 10 g 

of lipids per foraging bout, then they must stay for two bouts, 

which can be completed in 1 day under the assumption that 

they feed twice daily. The energetic model provides output 

based on both constraints (food and time) and the larger of the 

two values can be interpreted as the overall output. 

Energetic Model Inputs

Power of Flight

The metabolic power required for flight was estimated 

using an equation derived from a combination of multiple 

datasets, wherein energy required for flight was derived using 

multiple techniques including respiration data from wind tunnel experiments, time-energy budget studies, and mass loss 

(Norberg, 1996).

Power of flight = 573M 0.813 (1)

where

power is measured in watts (J/s) and 

M is mass of a bird (kg). 

This equation does not include structural measurements 

of birds because accurate measurements do not exist for 

whooping cranes at this time. An alternative model has been 

described by Pennycuick (1989).

Whooping Crane Mass

Lewis (1995) reported captive males weighing 7.3 kg, 

captive females weighing 6.4 kg, and wild adults between 7.1 

and 7.9 kg. The energetic model initially was set at 7.2 kg, 

which represented the median between the two most extreme 

values. This value may be supplemented with masses of 

whooping cranes caught through trapping efforts in conjunction with the Global Positioning System (GPS) satellite marking project by the USGS, USFWS, and other research partners.

Flight Speed

For this report a value of 43 km/h was used, which 

represented a median value reported for sandhill cranes (Grus 

canadensis; 12 m/s) by Tacha and others (1992). Additional 

data may become available from the GPS satellite marking project. If using the Flight Program (Pennycuick, 1989), 

practitioners may use the minimum flight speed reported in the 

program results.

Basal and Field Metabolic Rates

The energetic model uses a calculation for basal metabolic rate generalized for all birds from Reynolds and Lee 

(1996): 

BMR = 1.311M 0.633 (2)

where 

M is a bird’s mass measured in grams and 

BMR is estimated in kilocalories per day and 

converted to kJ/day (1kcal = 4.187 kJ).

A multiplier of 3× BMR is used to account for the cost of free 

living and to derive a field metabolic rate (Prince, 1979).

Energy Content of Stored Lipids

A value of 39.8 kJ/g was used as energy content of stored 

lipids (Kendeigh and others, 1977). Lipids are not biosynthesized with 100 percent efficiency; thus, 75 percent metabolic 

efficiency was used (Weathers, 1996). Finally, catabolism 

of lipids also is not completely efficient. A value of 95-percent efficiency was used, because Stryer (1995) reported 

that the yield from oxidation of fatty acids is about 9 kcal/g 

(37.7 kJ/g).
Energetic Model Inputs 5

Empirical Estimates of Lipid Gain

Numerous estimates of average lipid gain with time have 

been reported for waterfowl and larger waterbirds. Krapu 

and others (1985) found that lesser sandhill cranes gained 

13–16 g of lipids per day during spring staging in the central 

Platte River Valley and that greater sandhill cranes gained 

25 grams per day (g/d) (G. L. Krapu, unpub. data). Greater 

white-fronted geese (Anser albifrons) acquired lipids at a rate 

of 14 g/d, while staging in the Rainwater Basin of Nebraska 

(Krapu and others, 1995). Spring-staging snow geese (Chen 

caerulescens) gained 17.5 g of lipids per day in Saskatchewan, 

Canada. Finally, Raveling (1979) reported Canada geese 

(Branta canadensis) gained 16–22 g/d. Of all of these reports, 

greater sandhill cranes are most closely related to whooping 

cranes phylogenetically and structurally; therefore, lipid gain 

for greater sandhill cranes was used.

Potential Diet

Potential food items were determined from feeding studies and habitat use patterns of whooping cranes (table 1 and 

references therein). Potential staging sites that may contain 

these foods should be viewed in the context of current agriculture practices. Substantial changes in food resources and 

availability have occurred across the Great Plains in the past 

50 years, with indications of continued changes in the future 

(for example, Krapu and others, 2004, Pearse and others, 

2010).

Energy Content of Foods

Food items have various energetic contents and animals 

are able to extract different amounts of energy from them. 

True metabolizable energy content of foods that whooping 

cranes might consume during migration are reported in table 2.

Forage Efficiency

A value of 0.7 g/min was used as a rate that whooping 

cranes could acquire food once they find a foraging patch with 

available food. This value is based on observations of common 

cranes (Grus grus) wintering in Spain feeding on cereal crops 

(Alonso and Alonso, 1992). Forage rates for corn, soybeans, 

and field peas are likely higher because they are larger seeds; 

150 percent higher forage rate than for cereals was used 

(assumption without supporting citation). Forage rate for 

protein and invertebrates is likely much less given the distribution of these resources and that the rate is based on a dry mass 

basis. Forage rate was reduced by 80 percent (assumption 

without supporting citation). These values are without strong 

empirical support and are important factors in determining the 

amount of lipids that birds can acquire per day from the perspective of the time constraint. Using a 100 percent corn diet 

as an example, decreasing foraging rate of corn by 25 percent 

(0.79 g/min), decreased estimated lipid gain per day from 

107 to 59 g (45 percent decrease). Decreasing corn foraging 

rate 50 percent (0.53 g/min), lead to a 90 percent decrease in 

estimated lipid gain per day. Additional research is needed to 

better understand diet choice and forage efficiency of migrating whooping cranes. Finally, these values can be manipulated 

to represent relatively more or less rich patches of forage that 

might be available based on different scenarios.

Table 1. Potential food items that might be consumed by whooping cranes during migration.

[ND, North Dakota; SD, South Dakota; NE, Nebraska; KS, Kansas; OK, Oklahoma; TX,Texas]

Potential item Source(s) Potential staging sites1

Corn (Zea mays) Shields and Benham, 1969; Howe, 1989; Lingle and others, 1991 ND, SD, NE, KS, OK, TX

Soybean (Glycine max) Lingle and others, 1991 ND, SD, NE, KS, OK, TX

Wheat grain (Triticum aestivum) Shields and Benham, 1969; Howe, 1989; Johns and others, 1997 All

Wheat shoots Uhler and Locke, 1970 All

Peas (Pisum sativum) Shields and Benham, 1969 SD, ND

Sorghum grain (Sorghum bicolor) Shields and Benham, 1969; Howe, 1989; Lingle and others, 1991 KS, OK

Chufa tuber (Cyperus esculentus) Uhler and Locke, 1970; Howe, 1989 All

Barnyard grasses (Echinochloa spp.) Howe, 1989 All

Smartweed seeds (Polygonum spp.) Howe, 1989 All

Protein Allen, 1952 All

Invertebrates Uhler and Locke, 1970 All

1

Represents probable locations where food items might be found.
6 Model of Whooping Crane Energetics as Foundation for Development of a Method to Assess Potential Take

Table 2. True metabolizable energy of potential diet items for 

migrating whooping cranes.

[kJ/g; kiloJoule per gram]

Potential item kJ/g

(Animal model) Source

Corn 16.3 (Canada goose) Petrie and others, 1998

Soybean 14.9 (Canada goose) Petrie and others, 1998

Wheat grain 16.1 (Canada goose) Joyner and others, 1987

Wheat shoots 10.1 (Canada goose) Petrie and others, 1998

Field peas 12.3–15.62 (poultry) Castell and others, 1996

Sorghum grain 15.7 (Canada goose) Petrie and others, 1998

Chufa tuber 16.9 (Canada goose) Petrie and others, 1998

Wild millet 13.8 (Canada goose) Petrie and others, 1998

Smartweed 

seeds

6.7 (Canada goose) Petrie and others, 1998

Protein 17.1 (generic bird) Bell, 1990

Invertebrates 17.2 (generic bird) Bell, 1990

Maximum Food Consumption

The energetic model estimates maximum food consumption based on an allometric equation that was used in Amano 

and others (2004) for an optimal foraging model for geese. 

The intercept of the equation is based on brant geese (Branta 

bernicla) weighing 1.5 kg and able to consume 150 g fresh 

weight of food (Prop and Deerenberg, 1991). The exponent in 

the equation was based on Ricklefs (1996).

Wet/Dry Weight Coefficients

These coefficients were needed to convert food consumed 

from wet to dry mass. Values of 1.1 for seeds (Amano and others 2004), 5.2 for vegetation (Amano and others, 2004), and 

2.3 for invertebrates (that is, based on the assumption that they 

are 60-80 percent water; Chapman, 1998) were used.

Maximum Time per Foraging Bout and Bouts per 

Day

During late April, there are 12 plus hours of daylight, 

depending on latitude (vernal equinox); thus, the energetic 

model was developed assuming birds could spend up to 

10 hours per day foraging. This is an assumption with no supporting citation. Spring-staging sandhill cranes fed in croplands during mid-morning and late afternoon (U.S. Fish and 

Wildlife Services, 1981); thus, it was assumed that migrating 

whooping cranes might have two foraging bouts per day.

Potential Management and Regulatory 

Applications

Under the ESA, take of wildlife is defined as actions that 

harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, 

or collect endangered species and include attempts made to 

engage in any such conduct (Endangered Species Act, 1973). 

Harm is further defined as an act which actually kills or injures 

wildlife. Such acts may include substantial habitat modification or degradation where such action actually kills or injures 

wildlife by substantially impairing essential behavioral patterns, including breeding, feeding, or sheltering. Incidental 

take is take that results from, but is not the purpose of, an 

otherwise lawful activity (that is, wind-energy project). Incidental take of an endangered species is a prohibited action that 

must be exempted or permitted through a regulatory process 

(Endangered Species Act, 1973).

To provide an indication of how this energetic model 

could be used to determine effects to migration chronology, 

leading to incidental take, hypothetical scenarios are presented 

where whooping cranes are assumed to travel extra distances 

of 10–200 km during their entire migration. Total flight distance during migration is estimated to be 4,000 km; therefore, 

these hypothetical distances reflect 0.3–5 percent increases in 

overall flight distance. For these scenarios, three basic diets 

were used, which reflect different landscapes that whooping 

cranes might encounter during migration. A corn-dominated 

diet consisting of 70 percent corn, 10 percent chufa tuber, 

10 percent wild millet, 5 percent smartweed, and 5 percent 

protein might be found in corn-growing regions within the 

U.S. portion of the migration corridor. A wheat-dominated diet 

consisting of 60 percent wheat seeds, 20 percent field peas, 

10 percent wild millet, 5 percent chufa tubers, and 5 percent 

protein represents portions of the corridor without substantial 

corn agriculture. Finally, a diet consisting entirely of wetland 

foods (that is, 35 percent chufa, 30 percent millet, 25 percent 

smartweed, 10 percent protein) was included, which could be 

available at any stopover site. 

Overall energy content of each diet differed based on 

their relative composition. The corn-dominated diet had an 

average estimated energy content of 13.5 kJ/g and foraging 

efficiency rate of 14.5 kiloJoules per minute (kJ/min). The 

wheat-dominated diet had similar energy content (13.4 kJ/g) 

but lower foraging efficiency of 11.4 kJ/min. The diet consisting of wetland foods had the lowest average energy content 

and foraging efficiency rate (11.1 kJ/g; 8.4 kJ/min). Accordingly, the energetic model predicted whooping cranes would 

be delayed the shortest amount of time if they consumed the 

most efficient diet dominated by corn, and would require the 

longest time when consuming exclusively a wetland-based 

diet (table 3).

It is important to note that this energetic model should not 

be used exclusively to define or rank stopover sites, because 

it only incorporates foraging resources that might be available at a potential stopover site. Minimally, a stopover site 
References Cited 7

Table 3. Estimated extra days whooping cranes would need 

to replenish lipid reserves from hypothetical model runs using 

varying additional distances whooping cranes might fly for an 

entire migration season.

[km, kilometer]

Extra 

distance 

(km)

Corn-dominated 

diet

Wheat-dominated 

diet

Wetland-foods 

diet

10 1 1 1

50 1 1 4

100 1 2 8

200 2 4 16

must include a safe roost site. Specific characteristics of roost 

sites have been developed and include wetlands with areas of 

shallow water, with minimal visual obstructions, situated in 

areas of relatively low disturbance, and in a matrix of potential 

foraging habitats (Armbruster, 1990). Indeed, croplands in isolation would not likely be considered suitable stopover habitat.

Based on these hypothetical scenarios, take of whooping cranes may occur through harm if whooping cranes avoid 

large structures (for example, group of wind turbines) and, in 

essence, lose suitable stopover habitat. Adverse effects may 

occur if stopover sites are avoided and whooping cranes fly 

extra distances to find suitable habitat without large structures. 

Flying longer distances requires additional energy expenditure 

and would equate to an extra day or days to replenish energy 

reserves (table 3). Extra days foraging and replacing depleted 

energy reserves may result in delays in reaching whooping 

crane breeding grounds. This delay may equate to harm to 

whooping cranes if that delay translates into reduced breeding 

success (that is, less time to hatch and fledge young). Additionally, whooping cranes may continue migration and arrive 

on the breeding grounds with a lipid deficit. In this instance, 

they may either spend extra days building required reserves 

to nest or their reproductive effort could be reduced (for 

example, clutch size, egg mass). Thus, incidental take may 

occur through decreased reproductive output through multiple 

mechanisms related indirectly to loss and avoidance of otherwise suitable migratory habitat.

The energetic model described within lays out a foundation for ESA practitioners and project proponents to more fully 

understand and evaluate potential risk associated with windenergy projects on the landscape. The use of this energetic 

model with others being developed may assist in estimating 

potential incidental take to the whooping crane and, more 

importantly, may help guide discussions about conservation 

measures that can be implemented across the landscape to 

avoid, minimize, and mitigate potential impacts to whooping 

cranes.

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Appendix

The following figures are screenshots of a spreadsheet program used to implement the energetic model described in this report. Certain input parameters such as “extra distance traveled” 

(cell B14) and “% diet (dry mass)” (cells C24–C34) were populated with values for illustration 

purposes only. 
12 Model of Whooping Crane Energetics as Foundation for Development of a Method to Assess Potential Take

Appendix 1. Screen Shots of Energetic Model Showing Calculations
Appendix 1. Screen Shots of Energetic Model Showing Calculations 13
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For more information concerning this publication, contact:

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8711 37th Street Southeast

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(701) 253–5553

Or visit the Northern Prairie Wildlife Research Center Web site at:

http://npwrc.usgs.gov/

Pearse and Selbo—Model of Whooping Crane Energetics as Foundation for Development of a Method to Assess Potential Take—Open-File Report 2012–1156