The Effect of Weather on Bioenergetics of Breeding American Woodcock Author(S): Dale L

Allen Press

The Effect of Weather on Bioenergetics of Breeding American Woodcock Author(s): Dale L. Rabe, Harold H. Prince and Erik D. Goodman Source: The Journal of Wildlife Management, Vol. 47, No. 3 (Jul., 1983), pp. 762-771 Published by: Wiley on behalf of the Wildlife Society Stable URL: http://www.jstor.org/stable/3808611 . Accessed: 20/03/2013 09:28

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This content downloaded from 35.8.11.2 on Wed, 20 Mar 2013 09:28:52 AM All use subject to JSTOR Terms and Conditions THE EFFECTOF WEATHERON BIOENERGETICS OF BREEDINGAMERICAN WOODCOCK'

DALEL. RABE,2Department of Fisheriesand Wildlife,Michigan State University,East Lansing,MI 48824 HAROLDH. PRINCE,Department of Fisheriesand Wildlife,Michigan State University,East Lansing,MI 48824 ERIKD. GOODMAN,Department of ElectricalEngineering and System Science, MichiganState University,East Lansing,MI 48824

Abstract: Simulation modeling was used to investigate the impact of weather on the bioenergetics of breeding and postbreeding American woodcock (Scolopax minor). Using air temperature and precipitation as inputs, the model calculates the daily energy requirements of an adult female woodcock and chicks along with the availabilty of their primary food, earthworms (Lumbricidae). Energetics were modeled from previous studies on woodcock and related species. Earthworm availability was modeled from field data collected in northern Michigan. Results suggest that the greatest potential for weather-related stress on woodcock occurs during the brood-rearing period, with nesting being the 2nd most critical time' When simulated earthworm availability during the brood period was compared with reproductive success data, it indicated that the impact of spring weather on earthworm availability is a significant factor affecting chick survival. J. WILDL.MANAGE. 47(3):762-771

Weather patterns are known to affect 1940, Glasgow 1958, Sheldon 1971), are woodcock behavior and extremes are a also affected by weather. Reproduction suspected cause of mortality. Studies have and growth rates have been shown to de- shown that cold spring temperaturescause cline outside an optimal temperature a decline in singing male activity (Duke range (Evans and Guild 1948, Satchell 1966) and that temperature and wind in 1955). More important to woodcock, how- combination affect the timing of spring ever, is the strong influence that soil mois- and fall migrations (Sheldon 1971, God- ture and temperature have on earthworm frey 1974, Coon 1977). However, little is activity and vertical distributionin the soil. known about the influence of weather on When conditions near the surface become mortality, largely because of the difficulty suboptimal, earthwormsrespond by either in finding woodcock and determining the migrating deeper in the soil or entering a cause of death. Mendall and Aldous (1943) state of aestivation (Guild 1948, Reynolds reported instances of nest losses and adult and Jordan 1975, Edwards and Lofty mortality on the breeding grounds follow- 1977) and become unavailable to feeding ing an extended period of inclement woodcock. weather, and Sheldon (1971) and Owen Our objective was to evaluate the direct (1977) believed adverse weather during and indirect effects that weather can have incubation and brood-rearing can cause on woodcock energetics and to identify significant chick mortality. periods of greatest potential impact. Be- Earthworms, which comprise 60-90% cause of the inherent difficulties in gath- of the woodcock diet (Aldous 1939, Sperry ering field data, simulation modeling was used to examine the relationship between energy requirements of woodcock and Michigan Agricultural Experiment Station Paper food 10251. availability. 2 Present address: School of Natural Resources, We thank D. Beaver and G. Dudderar, University of Michigan, Ann Arbor, MI 48109. Michigan State University, and C. Ben-

762 J. Wildl. Manage. 47(3):1983

This content downloaded from 35.8.11.2 on Wed, 20 Mar 2013 09:28:52 AM All use subject to JSTOR Terms and Conditions WOODCOCK BIOENERGETICS * Rabe et al. 763 nett, Michigan Department of Natural metabolic rate (BMR) and costs of ther- Resources, for reviewing earlier drafts of moregulation. Because there has been no this manuscript. We also thank J. Tautin, laboratory measurement of woodcock me- U.S. Fish and Wildlife Service, for sup- tabolism, a BMR estimate of 21.8 kcal/ plying detailed harvest reports for Mich- day was computed from the Aschoff and igan. This work was supported by the Ac- Pohl (1970) equation for non-passerines celerated Research Program for Migratory using an average female weight of 190 g Shore and Upland Game Birds, U.S. Fish (Owen and Krohn 1973). The cost of ther- and Wildlife Service contract 14-16-0008- moregulation was estimated using the 2092, and the Michigan State University general inverse relationship between tem- Agricultural Experiment Station. perature and metabolism discussed by and Farner THE MODEL King (1961), Kendeigh (1969), King (1974), and Ricklefs (1974). For The simulation program (Rabe 1981) temperatures below the thermoneutral was written in FORTRAN IV computer zone (using 10 C as the lower critical tem- language. The model computes daily en- perature), daily BMR was increased 0.12 ergy requirements of an adult female kcal/g/C. Thermoregulatory adjustments woodcock from the time of arrival on due to acclimation at lower temperatures northern breeding grounds to fall migra- have been shown to be small relative to tion and her brood from hatch until dis- total energy requirements in Anatidae bandment, along with the biomass of (Owen and Reinecke 1979) and were not earthworms potentially available to the included. Thermoregulatory costs for birds. Timing of events in the model fol- temperatures above the thermoneutral lows the chronology of breeding and post- zone were not considered important be- breeding activities for the northern Lower cause woodcock generally spend diurnal Peninsula of Michigan. In this part of the periods under a forest canopy. breeding range, woodcock generally be- Total energy cost of activity was ex- gin arriving during mid-March and the pressed as the sum of products for the peak hatch occurs during early May (G. amount of time spent in each activity A. Ammann, pers. commun.). In the mod- (resting, walking, feeding, and flying) el we assumed that adult woodcock do not multiplied by the energy cost for that ac- molt until after the brood has been raised. tivity (expressed as a multiple of BMR). Inputs to the model are daily average Estimates of activity and associated ener- temperature and precipitation. Weather gy costs are given in Table 1. data (1965-80) were obtained from the Nesting energetics included both the National Weather Service at Houghton cost of ovarian development and the cost Lake, Michigan. of producing eggs. The daily reproductive tissue cost estimate of 3.0 kcal was based Hen Energetics on the assumption that the rate of recru- Daily energy requirements of the hen descence is spread equally over a 10-day were divided into 4 components: mainte- period, and that the total energy content nance, activity, nesting, and molting. of the tissue is 30 kcal. Total tissue cost These components were assumed to be in- was based on a mature organ weight of 8 dependent and total energy requirements g (D. L. Rabe, unpubl. data), an energy were obtained by summation. density of 1.9 kcal/g, and a production Maintenance costs included both basal efficiency of 50% (Brody 1945). Cost es-

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Table1. Percentageof timewoodcock hen and chicks spend resting, walking, feeding, and flying during different periods. The basalmetabolic rate conversion factor for converting activity to caloricequivalent is included.

Hena Chicks

Activity Nesting Incubation Brooding Postbrooding Preflight Postflight BMR multipleb Resting 75 83 56 67 57 56 1.3 Walking 16 10 9 13 9 9 2.0 Feeding 18 5 34 16 34 34 2.0 Flying 2 2 1 3 0 1 15.0

a Data from Wenstrom (1973). b Based on studies by Prange and Schmidt-Nielsen (1970), King (1974), and Prince (1979). timates for the eggs are based on a 4-egg limited. To use data from other studies, clutch, laid at a rate of 0.8 egg/day (Shel- growth rates of woodcock chicks were ex- don 1971). The rate of egg development pressed as a percentage of adult weight. was assumed to follow a bell-shaped curve This conversion was made using an aver- (King 1973), and the total energy content age hatch weight of 16 g and a growth of an egg (58 kcal) is based on an average rate of 5.5 g/day (Dwyer et al. 1982) for weight of 17 g (G. A. Ammann, pers. com- the 30-day period modeled. To simplify mun.) and an energy density of 1.7 kcal/ the calculations, an average adult wood- g, again assuming a production efficiency cock weight of 175 g was used rather than of 50%. including separate calculations for each Estimates for the cost of molting were sex. based on the energy content of the feath- Estimates of maintenance energy re- ers. A total cost of 88 kcal was estimated quirements for chicks were made by com- using a total feather weight of 8 g, an en- puting a BMR value (Aschoff and Pohl ergy density for protein of 5.5 kcal/g, and 1970) using the chick's weight and mul- 50% efficiency of biosynthesis. Daily en- tiplying by a metabolic conversion factor ergy costs were calculated by apportion- to correct for the relatively high metabol- ing the total cost over 120 days according ic rates of precocial chicks during early to the intensity of molt (Owen and Krohn stages of development (Ricklefs 1974). 1973). Insufficient information was avail- Costs for thermoregulation were calculat- able to evaluate thermal conductive losses ed from the estimate of BMR using the during molt, but it was assumedto be small same temperaturerelationship used for the because woodcock molt during summer. hen. The cost of was esti- Chick energetic growth Energetics mated from gross animal weight and its Major energy requirements for chicks corresponding energy density, assuming a during the brood-rearing period are 50% efficiency for biosynthesis. Gross en- maintenance, activity, and growth. Al- ergy density of chicks changes during though juveniles undergo a partial molt, growth (Ricklefs 1974). Estimates for it does not begin until after broods dis- woodcock chicks were derived from data band. Total energy requirements were on dunlin (Calidris alpina) chicks (Norton computed by summation in the same 1970), which showed a linear increase of manner as for the hen. 1.5-1.8 kcal/g from hatch to maturity. Energetics data for woodcock chicks are Total energy content of the chick was

J. Wildl. Manage. 47(3):1983

This content downloaded from 35.8.11.2 on Wed, 20 Mar 2013 09:28:52 AM All use subject to JSTOR Terms and Conditions WOODCOCK BIOENERGETICS * Rabe et al. 765 computed by multiplying the weight of the chick by the appropriate energy density, and daily cost for growth by subtract- ing the total energy content on the previous from that of the current Ho to day day. 0 do Energy requirements for chick activity were computed in the same manner as for L.. the adult hen. Likewise, time allocations z (Table 1) for the chicks were assumed to 0J 15 30 45 60 be the same as for the hen during the O SOILMOISTURE (%) brood-rearing period. The only exception 0U) was the 1st 14 days after hatch, when flight 0 activity for chicks was set at zero. -j LiJ Earthworms 0,' 0 The biomass of earthworms available to k o 0 woodcock was modeled as a function of So the earthworms' vertical distribution in the soil. Because the model uses air temperature and precipitation data as inputs, it was first to at least a necessary develop 0 10 20 40 crude model for soil conditions as a func- 30 SOIL TEMPERATURE(C) tion of atmospheric weather. and Fig. 1. Scaling factors used to model soil moisturelosses Soil moisture temperature dynam- from percolation,evaporation, and transpiration. ics are affected by a number of factors including soil depth, type, and compac- tion, vegetative cover, slope, and air-tem- loss due to evaporation and transpiration perature differential (Hillel 1971, Baver are related to temperature, the combined et al. 1972). In general, soil temperature effect of these components was modeled exhibits a lag response to changes in air as a function of soil temperature using a temperature and was modeled according- scaling factor (Fig. 1). ly. Moisture additions were computed us- Earthworm availability was modeled as ing a simple linear relationship, where a function of soil moisture and tempera- each 1.0 cm of rainfall was equated to a ture conditions. A scaled distribution 3.5% increase in soil moisture. function (Fig. 2) for temperature effects Percolation, evaporation, and transpi- was derived from studies on lethal tem- ration were considered the major causes peratures (Grant 1955, Edwards and Lofty of water loss from soil. As the moisture 1977) and earthworm growth rates (Guild content of soil decreases, the bond be- 1948, Satchell 1955). The functional re- tween water and soil particles becomes lationship for earthworm response to stronger, making further water loss more moisture (Fig. 2) was developed from difficult (Foth and Turk 1972). For this studies by Olson (1928), El-Duweini and reason, the rate of percolative water loss Ghabbour (1965), and Gerard (1967). Us- was modeled as a function of soil moisture ing these scaling factors, availability was content (Fig. 1). Because rates of water computed as a percentage of the maxi-

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--- OBSERVED SIMULATED O0 - SA I-D C,4 A \ < '

Q) I I-- w

(I) 0-4 SOIL MOISTURE(%) Eo

LL .J cci* 0: 6 0 P IN

0 10 20 30 40 0 SOIL (C) C; TEMPERAOISTURE I- an APR MAY JUN JUL AUG

Fig. 3. Simulatedand observed soil moisture(%) and earthwormabundance (g/m2) data for Aprilthrough August 1976.

20 30 40 0 10 extraction technique (Reynolds 1977). Soil SOILTEMPERATURE (C) moisture determinations were made using Fig. 2. Scaling factors used to model the relationshipbe- a gravimetric method (percent moisture tween soil moistureand temperature,and the availabilityof Both moisture and earthwormsto woodcock. by weight). earthworm samples were averaged among the 3 sam- ple sites for comparison with simulation results. The maximum biomass of earth- mum biomass of earthworms in a partic- ular area. worms recorded for any sampling period was 108 g/m2 in June. FieldObservations of EarthwormAbundance RESULTS Soil moisture and earthworm biomass Testingthe EarthwormModel data were collected weekly at 3 locations Comparisons of simulated and actual in Missaukee County, Michigan between soil moisture conditions (Fig. 3) indicate 10 April and 17 August 1976. The collec- that the model reasonablytracked ob- tion sites were about 25 km from the Na- servedfield conditions.The largestdevia- tional Weather Bureau Station at Hough- tions from observedsoil moisturecondi- ton Lake. These data were used to test the tions were a 5% underestimate in late May fit of the earthworm simulation submodel. and a 6%overestimate in the last sampling All 3 sites were in aspen (Populus spp.)- period in August. Overall, the model re- dominated forest communities that were spondedwell to increasesand declinesin considered good woodcock habitat. Soil moisture conditions. types ranged from loam to sandy loam. Earthwormabundance in the uppersoil Earthworm abundance was sampled in 2 strata fluctuatedconsiderably over rela- 0.25-m2 plots at each site using a Formalin tively short periods (Fig. 3) and followed

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HEN 0 *

NESTING

MOLT CE WODCC 0 o 8 o O0• " ACTIVITY 0o C- o w MAINTENANCE JAN FEB MARAPR MAY JUN JUL AUG SEP wa Fig. 5. Relationship between simulated earthworm availabil- S0 CHICK ity (g/m2)and energy requirementsfor woodcock (hen and 4 chicks combined). w 0 GROWTH z w Sr ACTIVITY a of 105 04 peak approximately kcal/day. MAINTENANCE Maintenance accounts for about 30% of the annual and activ- JAN FEB MARAPR MAY JUN JUL AUG SEP energy requirement ity uses nearly 60% of the total. In con- Fig. 4. Energypartitioning for a hen and chick woodcock. trast, nesting and molt account for a relatively small portion of total energy needs, although egg-laying does cause a 50% in- much the same pattern of change as soil crease in energy requirements for 3-4 moisture. Simulated abundances corre- days. At its peak, molt adds only 8% to sponded well to observed values. During total daily requirements. Thermoregula- April-June the model tended to underes- tion adds approximately 12% to daily en- timate actual earthworm abundance; ergy requirements when the hen first ar- however, during the summer months it rives on the breeding grounds. Cost of accurately tracked major changes in avail- thermoregulation steadily declines until ability. early May, when it is no longer a factor. As with the Woodcock adult, energy requirements Energetics for activity of chicks are nearly twice that To examine the potential impact of of maintenance (Fig. 4). Growth, on the weather on the energy requirements of average, accounts for approximately 20% woodcock (both hen and chicks) and of daily energy requirements. Energy re- earthworm availability, the model was run quirements of chicks increase rapidly the using long-term daily averages of temper- 1st few days after hatch and are nearly ature and precipitation. In this way, it was equal to those of the hen after 4-5 days. possible to eliminate annual variability and Based on an average hatch date of 5 May, examine food-energy relationships under the model indicates that woodcock chicks average weather conditions. are not subject to additional thermoregu- Based on an arrival date of 20 March, latory costs under average weather con- a partitioning of energy requirements (Fig. ditions. 4) indicates that total daily energy requirements for the hen are relatively con- Weather-relatedEnergy Stress stant at about 60 kcal/day, except during To identify periods of potential energy the nesting period, when they increase to stress, the combined energy requirements

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at least 1,100 wings, and in most instances, Y - -0.41 +O.14x 7 on more than 2,000 wings. Results of a - a•, r 0.-O regression analysis of these data (Fig. 6) show a significant linear relationship (P < fun)770 k 7 r = between food Ss 9 5 0.05, 0.53) availability 87 and chick survival. Data fit the line best 767 74 in years of low earthworm availability and 71 0 77 tend to be more variable in years of abun- 66..69 dant food. 40 45 50 55 60 65 EARTHWORMAVAILABILITY (g/m2) DISCUSSION Fig. 6. Regression analysis for average simulated earth- Weather conditions on a wormavailability (g/m2) duringthe broodperiod (20 Apr-1 Jun) year-to-year and reproductivesuccess ratios (chicks per adult hen in the basis may vary considerably. Because fall for 1965-80. harvest) Michigan, woodcock are not able to anticipate ab- normal weather conditions, their breeding strategy is probably based on average of the hen and brood (4 chicks) were su- weather conditions. Results of this simu- perimposed on a plot of earthworm avail- lation study suggest that much of ob- ability (Fig. 5). Because hen and chicks served woodcock breeding behavior is a forage in close proximity, particularly result of bioenergetic constraints. during the 1st 2-3 weeks, their exploita- The model indicates that the arrival of tion of food resources for this period is woodcock on northern breeding grounds most appropriately considered on a com- coincides closely with increased earth- bined basis. This comparison suggests that worm activity near the soil surface. If the greatest potential energy stress is like- woodcock were to arrive much earlier, not ly to occur during the brood-rearing pe- only is it likely that food would be less riod. There is also evidence to indicate that abundant, but thermoregulatory costs nesting can be stressfulon the hen because would be much greater as well. Prebreed- of reduced food availability during early ing weights (Owen and Krohn 1973) in- spring. Even though food abundance de- dicate that females arrive on the breeding clines during the summer, earthworms grounds with enough energy reserves to appear to be sufficiently numerous rela- endure short periods of inclement weath- tive to the energy needs of woodcock. er or food shortage;however, nesting fail- If earthworm availability during the ure and adult mortality have been attrib- brood period is a limiting factor, greater uted to extended periods of cold weather juvenile mortality would be expected in (Mendall and Aldous 1943). years when earthworms are scarce. This Conversely, if woodcock delay their hypothesis was tested by comparing av- migration to avoid inclement spring erage earthworm abundance during the weather, they may significantly decrease brood period (20 Apr-1 Jun) with repro- their chances for successfully raising a ductive success data (expressed as the brood. Nests are typically initiated by the number of chicks per adult hen in the fall end of March and eggs hatch in early May harvest) for Michigan. Reproductive suc- when earthworms are normally most cess data were obtained through the U.S. abundant near the soil surface. If nesting Fish and Wildlife Service annual wing is delayed appreciably, there is a greater survey, with each year's estimate based on likelihood that chicks will be confronted

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This content downloaded from 35.8.11.2 on Wed, 20 Mar 2013 09:28:52 AM All use subject to JSTOR Terms and Conditions WOODCOCK BIOENERGETICS * Rabe et al. 769 with a declining food supply at a time ability during late April and May directly when their energy requirements are in- influences chick survival. Even though creasing. Three-egg clutches laid by late- there is a significant fit to a linear model, nesting birds (Sheldon 1971) may be an the pattern of data points suggests that adaptation for compensating for de- food tends to limit the maximum potential creased earthworm numbers in summer. for reproductive success, but that other By reducing clutch size from 4 to 3, the mortality factors (e.g., predation, disease, total energy demand of the family unit etc.) may prevent woodcock from reach- becomes proportionally less and the chance ing that level in years when food is abun- for survival of individual chicks is en- dant. Such an explanation would account hanced. Because chicks have few energy for the relatively low production ratios in reserves to draw upon, a consistent food 1965, 1969, and 1975; however, there is supply is more critical at this time. no readily apparent explanation for the Although Rabe (1979) reported an in- unusually high success ratio in 1973. stance of a possible 2nd nesting attempt, MANAGEMENT limited earthworm supplies during sum- IMPLICATIONS mer may be the reason that woodcock The popularity of woodcock as a game generally do not raise more than a single species has increased greatly in recent brood per year. Theoretically, there is years. In 1975 an estimated 1.5 million enough time for a 2nd brood to reach adult birds were harvested nationwide, provid- size before fall migration, but chances of ing between 2.5 and 3.0 million user-days success are likely to be reduced because of recreational hunting (Artmann 1977). of an unreliable food supply. This represents a 79% increase in harvest The model indicates that earthworm from a decade earlier. Indications are that availability increases from summer to fall, this trend will continue. which agrees with the results of woodcock With greater demand, there is need for food-habits studies. The percentage of in- more accurate population data to ensure sects consumed was highest (38%) in Au- that the species is not over-harvested. Be- gust (Sperry 1940). Beetles were the most cause of the seclusive nature of the bird, abundant food of birds collected during however, reliable population estimates are summer in Massachusetts (Sheldon 1971). difficult to obtain. Currently, the only an- In contrast, earthworms comprised 86% of nual population estimate is a spring census the diet in Nova Scotia (Pettingill 1939) of singing males. This is inadequate for and Maine (Aldous 1939) during October. estimating fall populations because it does Woodcock apparently supplement their not account for variability in reproductive diet with insects and other foods during success or mortality from spring to fall. the summer when earthworms are less Although field surveys designed to di- abundant. Although spring food habits and rectly measure fall populations would be food availability have not been studied time-consuming and expensive, simula- thoroughly, we believe that woodcock are tion models such as ours potentially could most dependent on earthworms during the provide a relatively inexpensive means of spring because insects eaten during the improving fall population estimates when summer are not yet available. used in conjunction with the spring sur- The relationship between earthworm veys. Because the model uses weather data availability and woodcock reproductive to predict chick mortality, it could easily success (Fig. 6) implies that food avail- be modified to monitor reproductive suc-

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J. Wildl. Manage. 47(3):1983

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J. Wildl. Manage. 47(3):1983

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