The Role of Nutrient Reserves in Mallard Reproduction

University f Nebraska - Lincln DigitalCmmns@University f Nebraska - Lincln USGS Nrthern Prairie Wildlife Research Center Wildlife Damage Management, Internet Center fr 1-1981 The Rle f Nutrient Reserves in Mallard Reprductin Gary Krapu USGS Nrthern Prairie Wildlife Research Center, gkrapu@usgs.gv Fllw this and additinal wrks at: http://digitalcmmns.unl.edu/usgsnpwrc Part f the Other Internatinal and Area Studies Cmmns Krapu, Gary, "The Rle f Nutrient Reserves in Mallard Reprductin" (1981). USGS Nrthern Prairie Wildlife Research Center. 48. http://digitalcmmns.unl.edu/usgsnpwrc/48 This Article is brught t yu fr free and pen access by the Wildlife Damage Management, Internet Center fr at DigitalCmmns@University f Nebraska - Lincln. It has been accepted fr inclusin in USGS Nrthern Prairie Wildlife Research Center by an authrized administratr f DigitalCmmns@University f Nebraska - Lincln.

THE ROLE OF NUTRIENT RESERVES IN MALLARD REPRODUCTION GARY L. KRAPU U.S. Fish and Wildlife Service, Nrthern Prairie Wildlife Research Center, Jamestwn, Nrth Dakta 58401 USA ABSTP, ACT.--Mallard (Arias platyrhynchs) ppulatins breeding in temperate Nrth America btain a significant part f the energy and lipid requirements f reprductin at sites ccupied prir t arrival n the breeding grunds. Prtein fr egg frmatin, hwever, is btained principally frm the diet during the nesting perid. Bth sexes arrive heavy and fat in Nrth Dakta but experience substantial weight lss and lipid depletin during the nesting cycle. Weight lss is mst prnunced amng females and averages 25% frm prelaying t late incubatin. Bdy weights f bth sexes are psitively crrelated with carcass lipid cntent. The paired male draws upn lipids early in the nesting seasn when an activity center is being established and defended and when females are preparing t nest. The female's lipid reserves are utilized primarily during laying and early incubatin. The significance f lipid reserves diminishes as the nesting seasn prgresses, and females d nt acquire substantial lipid stres prir t renesting when the initial clutch is destryed. The magnitude f lipid reserves carried in female carcasses is psitively crrelated with clutch size frm mid-april t early June. Prtein transfer fr egg frmatin frm flight and leg muscle and bdy rgans can accunt fr nly a small part f the prtein requirement fr the clutch. By utilizing lipid reserves t meet energy requirements, the female can acquire sufficient prtein frm the diet t prduce a large initial clutch even when fds are relatively scarce, whereas the renesting female must rely entirely upn fd resurces available at the breeding site fr its nutrient and energy requirements. Received 3 January 1980, accepted 18 July 1980. REPRODUCTION by waterfwl requires a substantial allcatin f nutrients, especially lipids and prtein, t frm eggs. Egg and ylk size are prprtinately large in cmparisn t bdy size in Anatidae (Lack 1968), and the daily maximum cst f egg prductin has been estimated t be 52-70% f daily energy intake at cnstant bdy weight (King 1973). Nutrients fr reprductin must be either drawn frm bdy reserves r btained frm fd resurces at the time f breeding, r bth. Amng arctic-nesting geese that breed in envirnments where fd is scarce at the time f nesting, it has been shwn that stred nutrient reserves cntribute substantially t reprductin (Hansn 1962, Barry 1962, Ankney and Macinnes 1978, Raveling 1979). Althugh the Mallard is ne f the mst extensively studied waterfwl species in the wrld and many aspects f its bilgy are well knwn, nutrient dynamics during reprductin remain prly understd. This paper, based upn findings frm studies cnducted during 1974-1977, describes the magnitude f lipid reserves carried t the breeding grunds, identifies their utilizatin patterns during the breeding cycle, and cnsiders the significance f lipid and ther nutrient reserves t clutch size and the life cycle f the species. METHODS Dispersed pairs f breeding Mallards frmed the sample fr weight and lipid measurements. These birds were captured r cllected in prairie pthle habitat within a radius f 160 km f Jamestwn, Nrth Dakta, frm spring arrival t terminatin f nesting. Bdy weights were btained fr 95 males and 151 females frm individuals either cllected fr nutrient analyses, captured by rcket netting fr telemetry research, r live-trapped at nest sites using a remte-cntrlled capture technique described by Shaiffer and Krapu (1978). Lipid and ther measurements were taken f 43 males and 71 females cllected 29 The Auk 98: 29-38. January 1981

30 GARY L. KRAPU [Auk, Vl. 98 during 1974-77. Stage f incubatin was determined fr 27 females by backdating frm hatching date; tw eggs were remved frm each clutch and placed in a mechanical incubatr n the same date a female was captured and weighed. Ttal bdy weight (wet) f cllected specimens was measured n a Pennsylvania scale t the nearest gram (g). Live-trapped specimens were weighed in the field using a Salter suspended scale calibrated t the nearest 5 g. Cllected males and females were plucked in the labratry in preparatin fr lipid extractin. The vary and viduct f each female were remved, weighed t the nearest 0.1 g n a triple-beam balance, and examined t determine reprductive status. Criteria used t classify reprductive status are as fllws: prelaying--frm arrival t vulatin, laying--frm vulatin f the first vum t vipsitin f the last egg f the clutch, and incubatin--female brding clutch. The prelaying stage includes tw phases, i.e. prenesting and rapid fllicular develpment. A prenesting status was assigned t dispersed pairs in early spring with vary weights <3.0 g and rapid fllicular develpment pre-vulating females with vary weights >3.0 g. Renesting females in rapid fllicular develpment were identified by the presence f a brd patch acquired during a previus nesting attempt. The gizzard, heart, liver, flight muscle (pectralis and supracracideus), and leg muscle were remved frm the carcasses f bth sexes and weighed separately wet t the nearest 0.1 g n a triple-beam balance. Fat adhering t the viscera was remved and included in the lipid analyses. Bdy cmpnents were stred in sealed plastic bags in airtight cntainers at -20øC. Preceding lipid extractin, samples were dried fr 6 h at a temperature f 105øC. Lipid extractin was perfrmed n hmgenized and dried samples by the Sxhlet prcess using petrleum ether and fllwing prcedures recmmended by Hrwitz (1975). The level f feeding activity by female Mallards during reprductin was measured using radi-telemetry and time-budget techniques as described by Dwyer et al. (1979). Clutch-size measurements are frm cmpleted clutches laid by wild females in suth central Nrth Dakta during 1974-1976. Clutch sizes were segregated and averaged fr each 10-day perid by date f nest initiatin; these perids were 11-20 April, 21-30 April, 1-10 May, 11-20 May, 21-30 May, and 31 May-9 June. Date f nest initiatin was determined by identifying stage f incubatin using the field candling technique described by Weller (1956) and backdating t date the first egg was laid. RESULTS Breeding phenlgy.--widespread dispersal f Mallard pairs int prairie pthle habitat f suth-central and eastern Nrth Dakta typically begins during late March and early April. Timing f dispersal and establishment f activity centers differs between years in respnse t unstable and variable weather cnditins during late March and April. Significant nesting activity is underway by mid-april during years when weather cnditins are favrable, but when daytime maximum temperatures remain belw 10øC and snw cver is present, the nset f laying is delayed accrdingly (Krapu and Dty 1979). Laying was in prgress by 16 April, 26 April, 10 April, and 17 April during 1974-1977, respectively. Five mnitred nests hatched 25 days fllwing depsitin f the last egg f the clutch in the nest. First sightings f brds during 1974-1977 ccurred n 28 May, 1 June, 22 May, and 19 May. Bdy weight.--bth sexes are heavy at arrival n the breeding grunds but underg a marked decline in weight during the curse f the nesting seasn, with females experiencing a greater weight lss (Fig. 1). Average weight lsses amng paired males and females were 94 g and 217 g, respectively, frm mid-april t late May. Female bdy weight changed with reprductive stage (Table 1); a significant increase ccurred frm prelaying t laying (P < 0.01), fllwed by a highly significant decline during incubatin (P < 0.001). The 101-g increase in mean bdy weight f females frm prelaying t laying reflects the added weight f a mature r fully develped vary and viduct (Table 1) and the presence f an egg in the viduct f mst laying females. The female bdy weight curve during incubatin (Fig. 2)

January 1981] Nutrient Reserves and Mallard Reprductin 3 1 1300 - n=32 n=18 900-800 10 20 30 10 20 30 10 APRIL MAY JUNE Fig. 1. Mean bdy weights f breeding male and female Mallards in Nrth Dakta by 10-day intervals during the nesting seasn. reflects atrphy f reprductive rgans during the first week (Table 1), fllwed by a gradual leveling ff as lipid reserves are depleted. By late incubatin, females are highly emaciated; 11 live-trapped females weighed during the last 5 days f incubatin averaged 900.3 + 30.1 g (mean _+ SD), r 25% less than during prelaying. Reduced female feeding activity during incubatin prbably cntributes t weight lss. Observatins frm five radi-marked incubating females indicated that all regularly left the nest t frage during incubatin. Tw radi-marked females mnitred clsely during incubatin fed an estimated 1 h daily, whereas layers fed apprximately 8 h. The lss f bdy weight that ccurred between the laying and incubatin perids was statistically significant (P < 0.001) and due primarily t the diminished size f lipid reserves and reprductive rgans (Table 1). The general trend f decreasing bdy weight bserved during May amng sampled specimens (Fig. 1) reflects the rising prprtin f females either in an advanced stage f nesting (laying r incubatin) r having previusly made ne r mre unsuccessful nesting attempts. TABLE 1. Changes in bdy, tissue, and rgan weights and ttal lipids f females in relatin t breeding status. Sample sizes are in parentheses. Prelaying Laying Incubatin a Weight (g) (mean _+ SD) pb (mean _+ SD) P (mean -+ SD) Bdy 1,199.8 -+ 78.0 (19) ** 1,300.6 -+ 114.6 (10) *** 967.3 -+ 44.5 (3) Flight muscle" 65.1 -+ 5.6 (19) NS 65.1 -+ 5.5 (11) NS 58.3 -+ 1.9 (3) Leg muscle ½ 27.4-+ 3.1 (14) NS 27.2 -+ 5.4 (5) NS 24.8-+ 5.9 (3) Ttallipids 109.6-+ 33.7 (17) * 79.6-+ 37.2 (11) * 17.1 -+ 14.7 (3) Gizzard 33.7 -+ 6.9 (18) NS 28.6 -+ 4.3 (7) * 21.7 -+ 1.1 (3) Heart 13.4 -+ 2.3 (18) NS 14.2 -+ 2.7 (11) NS 11.1 -+ 1.3 (3) Liver 30.4-+ 6.4 (19) NS 31.3 -+ 5.7 (11) NS 27.1 -+ 4.6 (3) Ovary 6.3 -+ 8.7 (17) *** 31.9-+ 8.0 (8) *** 1.8 -+ 0.8 (3) Oviduct 13.5 -+ 9.5 (19) *** 32.1 -+ 5.0 (10) *** 7.2 -+ 1.3 (3) Females were cllected n day 6 f incubatin. bp = significance level f t-test between means in adjacent clumns. * = P < 0.05; ** = P < 0.01; *** = P < 0.001; NS indicares P > 0.05. ½ Lipid-free dry weight; ther measurements are n a wet weight basis.

32 GARY L. KRAPU [Auk, Vl. 98 1100- Y= 1047.317-14.507 X + 0.3705 X 2 R2= 0.47 1000-900 - O 800-2 4 6 8 10 12 1 4 116 118 ;0 212 214 Fig. 2. DAY(S) INTO INCUBATION Bdy weight curve f female Mallards in relatin t stage f incubatin. The crrelatin cefficients f female and male bdy weights with ttal carcass lipids were highly significant (r = 0.60, 69 df, P < 0.001) and significant (r = 0.48, 41 df, P < 0.01), respectively. Lipid reserves.--bth sexes arrive n the breeding grunds with substantial lipid reserves. Lipid levels f 4 paired males and 6 paired females cllected during the 1-15 April perid averaged 85.7 +- 58.3 g and 108.9 _+ 36.5 g lipids, respectively. Lipid cntent amng pair members declined rapidly as the breeding seasn prgressed; lipids were largely exhausted by late May (Fig. 3). Lipid reserves f the male were utilized rapidly during the perid in which activity centers were being established, whereas female utilizatin f lipids tended t lag behind the male, althugh ultimately falling t a lwer level. The higher lipid levels f paired female 8 (Fig. 3) reflects that this bird was abut t begin laying, and lipid reserves remained largely intact. Lipid reserves change with reprductive status (Table 1). Seven prenesting females cllected shrtly after arrival carried lipid depts that averaged 109.2 +_ 42.5 g. Lipid cntent in female carcasses did nt decline significantly with grwing vary weight during the rapid fllicular develpment phase (Fig. 4A). Substantial lipid utilizatin, hwever, ccurred during the laying stage, and bdy lipid cntent was negatively crrelated with number f ruptured varian fllicles (Fig. 4B). Lss f lipids frm laying (an average f 4 eggs had been laid by females in the sample) t day 6 f incubatin averaged 63 g, r 57% f the lipid reserves present during prenesting (Table 1). Bdy weights and ttal lipid cntent f incubating females suggest that lipid reserves typically are exhausted by hatching date. Females rely upn lipids fr nly part f the energy requirements during incubatin. Tw radimarked individuals mnitred intensively thrughut incubatin averaged 2.6 h ff the nest daily f which an estimated 38% was spent feeding. Female lipid reserves and clutch size exhibited similar trends frm mid-april t early June (Table 2). The magnitude f lipid reserves in female carcasses was ps-

January 1981] Nutrient Reserves and Mallard Reprductin 33 P1 150 P8 140 PN 130 P3 120 110 P15 {0 L " 100 { 90 80 : 70 7 ) 50 P4 ip5( P 6RFDp9 P I 2 30 P16 20 L RFD RFD L L (R) (R) 10 L L 5 ' 10 ' 15 ' ; 0 25 ' 3' 5 ' 10 ' 15 ' 2'' 25 3' 5 ',' I 2 '; 5 30 ' APRIL MAY JUNE Fig. 3. Lipid reserves f individual Mallard pairs are cmpared by date and reprductive status. Females are dented by pen circles and males by slid circles. Reprductive categries are as fllws: prenesting (PN), rapid fllicular develpment (RFD), and laying (L). (R) dentes females knwn t be initiating renesting attempts. itively crrelated with clutch size when analyzed by 10-day interval during the nesting seasn (r = 0.98, 4 df, P < 0.01). Labile prtein reserves.--female Mallards apparently btain nly a small part f the prtein required t prduce a clutch and fr grwth f reprductive rgans frm stred reserves. Lipid-free dry weight f flight muscle did nt decline significantly during rapid fllicular develpment (Fig. 4C) r during laying (Fig. 4D), and mst change ccurred during incubatin. The lipid-free dry weight f leg muscle exhibited a pattern similar t flight muscle (Table 1). Sme additinal prtein may be available fr varian and viduct grwth frm ther bdy tissues. The gizzard is the mst prbable surce f significant prtein fr egg prductin frm amng bdy rgans; the weight f this rgan declined significantly (P < 0.05) frm laying t incubatin, while ther rgans exhibited nly minr weight lss (Table 1). Gizzards f males and females were f cmparable weight at arrival, but thse f paired females cllected frm late April t early July weighed significantly less than did thse f their mates, i.e. 36.1 -+ 4.2 g versus 28.5 -+ 7.4 g, respectively (P < 0.01). Why gizzards f females underg greater muscle atrphy during the nesting seasn is uncertain. It may be caused by a higher prprtin f animal matter in the diet, reduced feeding during incubatin, prtein mbilizatin fr egg frmatin, r a cmbinatin f these factrs. A decline in gizzard weight during nesting has been reprted previusly in female Lesser Snw Geese (Chen caerulescens) (Ankney 1977), Canada Geese (Raveling 1979), Cmmn Eider (S-

34 GARY L. KRAPU [Auk, Vl. 98 160-140- 120- A ß Y =131.78-12.61 X r: -0.67 ß P<O.05 B 100-80- O 60-40- 20- Y r 124.90-0.76 X r:-0.43 P>O.O$ 80- c D - 70- _(2 60- LLI ua 50- u) 40-30- 20- Y :66.73-0.14 x r = -0.26 P>O.05 Y :64.48 + 0.35 x r: +0.13 P>O.05 5 ' 1;015 [ ; 0 2]53 [ 0 3[5 I 2 3 4 5 6 7 8 OVARY WEIGHT (g) NO. RUPTURED FOLLICLES (RAPID FOLLICULAR DEVELOPMENT) (LAYING) Fig. 4. Ttal carcass lipid cntent and lipid-free dry weight f flight muscle f female Mallards are pltted in relatin t vary weight (A,C) and number f vulated fllicles (B,D) during rapid fllicular develpment and laying, respectively. materia mllissima) (Krschgen 1977), Black Duck (Anas rubripes) (Reinecke 1977), and Wd Duck (Aix spnsa) (Drbney 1977). The prtein cntent f the gizzard increases linearly with wet weight in the Cmmn Eider (Krschgen 1977). Applying his findings t Mallards, I fund that apprximately 4 g f gizzard prtein is expended during the prenesting t incubatin interval and may be available fr egg frmatin. A wild Mallard clutch laid during 11-20 April averages 11 eggs (Table 2) and cntains apprximately 69 g f prtein TABLE 2. Clutch size (mean-+ SD) and carcass lipid reserves f female Mallards are cmpared during six 10-day intervals frm 11 April-9 June. Sample sizes are in parentheses. Time interval Trait 1 2 3 4 5 6 Clutch 11.1 _+ 0.9 10.4 _+ 1.7 9.1 _+ 1.2 9.4 -+ 2.0 8.6 -+ 1.3 7.8 -+ 1.1 size (35) (67) (66) (35) (25) (13) Ttal 115.5 _+ 42.1 96.0 -+ 38.1 57.9 -+ 33.3 55.4 -+ 27.0 20.7 -+ 13.7 12.0 -+ 4.8 lipids a (6) (22) (10) (13) (6) (6) a Measurements are expressed in g.

January 1981] Nutrient Reserves and Mallard Reprductin 35 TABLE 3. A cmparisn f bdy, tissue, certain rgan weights, and ttal carcass lipids f female Mallards in the rapid fllicular develpment phase f initial and renesting attempts. Nesting attempt Initial Renest Weight _+ SD n -+ SD n t P Bdy 1,217.8 -+ 79.4 a 11 1,065.4 -+ 54.6 7 4.43 <0.001 Flight muscle b 65.3 -+ 5.6 11 60.0 -+ 1.9 7 2.35 <0.05 Ttal lipids 116.4 -+ 18.9 10 29.9 -+ 17.4 7 9.59 <0.001 Gizzard 32.5 -+ 6.3 10 21.8 -+ 1.1 2 2.31 <0.05 Heart 14.3 -+ 2.1 11 12.8 -+ 1.9 7 1.51 >0.10 Liver 32.4 -+ 7.4 11 31.8 -+ 6.8 7 0.17 >0.10 All weight measurements are expressed in g. These data are expressed as lipid-free dry weight; ther measurements are wet weight values. (Krapu unpubl. data). If it is assumed that sme labile prtein is drawn frm the bdy musculature and gizzard t supply egg prtein, a reasnable assumptin based n the findings in ther species (Jnes and Ward 1976, Ankney and Macinnes 1978, Raveling 1979), then a majr part f the prtein needs f female Mallards must still be met frm fd surces available at the time f breeding. Status f nutrient reserves during renesting.--bdy, tissue, and certain rgan weights f females changed markedly frm initial t later nesting attempts. Bdy weight lss between initial and renesting attempts was highly significant (P < 0.001) and averaged 152 g (Table 3); apprximately 57% f the decrease came frm the depletin f lipid reserves. Lipid cntent in carcasses f females abut t renest averaged 74% less than amng females initiating the first nest; this change was highly significant (P < 0.001). These data indicate that females d nt stre substantial amunts f lipids prir t renesting but rather that energy requirements are acquired largely frm the diet at the time the clutch is prduced. Lipid reserves f the male are als depleted by the time f renesting (Fig. 3). Lipid-free dry weight f flight muscle averaged 5.3 g less amng renesting females, prbably reflecting weight lss assciated with incubatin f the previus clutch. Gizzard weight was reduced by 33% amng renesters, but the sample size was small (Table 3); neither the heart nr liver weight changed significantly (P > 0.10). DISCUSSION Bdy-weight trends f male and female Mallards breeding in Nrth Dakta cmpare favrably with spring weights reprted by Flk et al. (1966) in Czechslvakia. Females in bth areas are heavy at the nset f the nesting seasn and experience a marked decline in bdy weight between April and June. In the case f males, sme decline in bdy weight was evident at bth lcatins frm April t May. In Czechslvakia, a marked rise in bdy weight f males ccurred between May and June. My data are scant fr the late May-June perid but suggest a similar trend. At bth sites, lipid utilizatin presumably is respnsible fr a majr part f the bserved declines in bdy weight. Lipid reserves carried t the breeding grunds cntribute substantially t the needs f Mallard pairs during reprductin. Lipid depts reduce the fd intake necessary t satisfy male energy requirements, thus permitting increased attentiveness t the female during a perid f intensive fraging t meet nutrient requirements fr egg frmatin. Using time-budget analyses, Dwyer et al. (1979) reprted that

36 G^R¾ L. KR Pu [Auk, Vl. 98 male and female Mallards spent 20 and 55% f the daylight hurs, respectively, fraging during the laying perid. The energy requirement f a male is particularly high during the establishment f an activity center early in the nesting perid when fd resurces are being partitined amng pairs. Flight is the mst energy-intensive activity (Owen and Reinecke 1979), and the frequency f pursuit flights by males is highest during the prenesting perid, with the duratin f flights increasing slightly during laying (Titman 1973). Pursuit flights and ther aggressive behavirs assciated with defense f the pair's activity center prbably accunt fr the utilizatin f a significant part f the male's lipid reserves during late April and early May, particularly when high ppulatin densities lead t intense intraspecific cmpetitin. Male attentiveness t the female during the perid f egg frmatin increases the amunt f time she can actively frage and lwers the prbability f her falling victim t a predatr; bth factrs enhance recruitment. Findings presented in this paper suggesthat a majr part f the lipid reserves f the female are utilized during the laying stage, presumably t frm ylk in develping fllicles and as an energy surce fr securing prtein-rich fds. On a daily energy-intake basis early in the nesting seasn, the energy cst fr egg frmatin in free-living ppulatins f Mallards is less than the maximum rate f 52-70% estimated by King (1973) fr females at cnstant weight because f the substantial cntributin f stred lipids. The initiatin f nesting by Mallards appears t be timed t cincide with access t adequate prtein-rich fd resurces t meet needs fr egg frmatin. Fd resurces apparently must reach a certain threshld level n the breeding grunds befre nesting will ccur given the need fr significant intake f dietary prtein. My data suggest that mbilizatin f labile prtein reserves frm the sarcplasm f flight muscle, as described fr the Red-billed Quelea (Quelea quelea) by Kendall et al. (1973) and Jnes and Ward (1976), r frm ther tissues can accunt fr nly part f the prtein required by female Mallards t frm the initial clutch f eggs. Prtein utilized in egg frmatin by Mallards apparently is btained principally frm the diet. It has been shwn that female Mallards frage principally n invertebrates during the laying perid (Swansn et al. 1979). Female Pintails and Wd Ducks similarly btain prtein needs fr egg frmatin primarily thrugh dietary intake rather than endgenus reserves (Krapu 1974, Drbney 1977). An adaptive advantage f endgenus lipid reserves in early spring is that females can frage fr scarce aquatic invertebrates f high prtein cntent fr lng perids, even thugh searching fr these fds is energetically inefficient in cmparisn t fraging n waste grains available n agricultural lands. Female Mallards experimentally cnfrnted with lw prtein plant diets f the type nw available n agricultural lands in early spring cntinued t lay, but at reduced rates, and prduced eggs with lwer hatchability rates than did females feeding n a prtein-rich diet (Krapu 1979). This relatinship implies that access t animal matter during the nesting seasn favrs higher recruitment rates bth by increasing the prbability f a female cntinuing t renest until successful and by prducing mre ffspring when successful because f a larger clutch size and higher hatch rate. Invertebrate intake by females prvides a mre balanced ratin f calcium and the essential amin acids that frm egg cnstituents (Krapu and Swansn 1975). When cmpared by 10-day interval, the psitive crrelatin between the magnitude f female lipid reserves and the number f eggs laid by females suggests that clutch size is mderated by the magnitude f stred lipids. A psitive crrelatin

January 1981] Nutrient Reserves and Mallard Reprductin 37 between ptential clutch size and nutrient reserves available at the nset f laying has been reprted fr the female Lesser Snw Gse (Ankney and Macinnes 1978). In this species, feeding declines t a lw level during laying (Ankney 1977), in cntrast t the Mallard, which frages actively during the laying perid (Dwyer et al. 1979). Clutch size and laying date in the Mallard presumably are under significant genetic cntrl, as suggested by Batt and Prince (1979), with lipid reserves being an adaptive mechanism that allws the female t acquire adequate nutrient resurces t prduce a clutch f ptimum size. The decline in clutch size that ccurs as the nesting seasn prgresses appears t be related t the exhaustin f lipid reserves. Ricklefs (1974) indicated that when energy deprivatin t the female des ccur, the size and quality f the eggs are nt altered appreciably, nly the number. Age influences the magnitude f lipid reserves carried by Mallards during the nesting seasn. This difference may result because inexperienced yearlings are less efficient at capturing invertebrate prey, which leads t mre rapid lipid utilizatin. Subtle differences in physical cnditin, timing f nesting, and clutch size that have been detected between yearling and adult female Mallards suggest that adults can better cpe with unfavrable envirnmental cnditins existing at the breeding site. Cmpared t yearlings, adult wild female Mallards tend t carry mre lipids, t lay larger clutches, and t initiate laying earlier (Krapu and Dty 1979). Stred lipids appear t mderate clutch size thrugh their influence n female capacity t secure prtein needs. When lipid reserves are expended, the energy csts assciated with female fraging effrts t secure prtein are n lnger met frm fat depts, which increases the energy requirement that must be met frm the diet during the perid f egg frmatin and presumably intensifies the impact f prtein n clutch size. Drbney (1977), wrking with Wd Ducks, cncluded that the availability f prtein was mst likely t limit clutch size. His cnclusin was based upn the facts that prtein requirements were high, the fraging effrt needed t acquire prtein was extensive, and prtein is stred in nly limited quantities. In the case f the Mallard, prtein intake presumably limits clutch size, with lipid reserves prviding the energy resurces necessary fr the female t acquire the invertebrate fds and prtein needs fr the initial clutch under all but the mst severe envirnmental cnditins. Clutches laid by renesting females, hwever, are largely the prduct f the fd-resurce base available n the breeding grunds. Therefre, while mst females lay a clutch during mst years, the level f renesting activity can be expected t vary widely amng years in respnse t the status f fd resurces available in wetlands at the breeding site. ACKNOWLEDGMENTS I thank D. G. Jrde and S. Stewart fr assistance in prcessing the carcasses f cllected specimens, C. W. Shaiffer fr supprt in live-trapping birds, L. M. Cwardin and D. S. Gilmer fr making available data n bdy weights, G. A. Swansn fr prviding additinal specimens fr carcass lipid analysis, V. A. Admaitis and J. A. Shesmith fr cnducting lipid analyses, and D. H. Jhnsn fr aid in statistical analyses. I am grateful t C. D. Ankney, R. D. Drbney, W. R. Gfrth, D. H. Jhnsn, D. G. Raveling, K. J. Reinecke, and M. W. Weller fr critically reviewing earlier drafts f this manuscript. LITERATURE CITED ANKNEY, C. D. 1977. Feeding and digestive rgan size in breeding Lesser Snw Geese. Auk 94: 275-282.

38 GARY L. KRAPU [Auk, Vl. 98 --, & C. D. MACINNES. 1978ß Nutrient reserves and reprductive perfrmance f female Lesser Snw Geese. Auk 95: 459-471. BARRY, T. W. 1962. Effect f late seasns n Atlantic Brant reprductin. J. Wildl. Mgmt. 26: 19-26. BATT, B. D. J., & H. H. PRINCEß 1979. Laying dates, clutch size and egg weight f captive Mallards. Cndr 81: 35-41. DROBNEY, R. D. 1977. The feeding eclgy, nutritin, and reprductive bienergetics f Wd Ducks. Unpublished Ph.D. dissertatin, Clumbia, Missuri, Univ. Missuri. DWYER, T. J., G. L. KRAPU, & D. M. JANKE. 1979. Use f prairie pthle habitat by breeding Mallards. J. Wildl. Mgmt. 43: 526-531. FOLK, C., K. HUDEC, & J. TOUFAR. 1966. The weight f the Mallard, Arias platyrhynchs and its changes in the curse f the year. Zlgicke Listy 15: 249-260. HANSON, H. C. 1962. The dynamics f cnditin factrs in Canada Geese and their relatin t seasnal stresses. Arctic Inst. Nrth Amer. Tech. Pap. N. 12. HORWlTZ, W. 1975. Official methds f analysis, 12th ed. Washingtn, D.C., Assc. Off. Anal. Chem. JONES, P. J., & P. WARD. 1976. The level f reserve prtein as the prximate factr cntrlling the timing f breeding and clutch size in the Red-billed Quelea (Quelea quelea). Ibis 118: 547-574. KENDALL, m.d., P. WARD, & $. BACCHUS. 1973. A prtein reserve in the pectralis majr flight muscle f Quelea quelea. Ibis 115: 600-601. KING, J. R. 1973. Energetics f reprductin in birds. Pp. 78-107 in Breeding bilgy f birds (D. S. Farner, Ed.). Washingtn, D.C., Natl. Acad. Sci. KORSCHGEN, C. m. 1977. Breeding stress f female eiders in Maine. J. Wildl. Mgmt. 41: 360-373. KRAPU, G. L. 1974. Feeding eclgy f Pintail hens during reprductin. Auk 91: 278-290. ß 1979. Nutritin f female dabbling ducks during reprductin. Pp. 59-70 in Waterfwl and wetlands--an integrated review (T. A. Bkhur, Ed.). Prc. Symp. Nrth Central Sectin, The Wildl. Sc., & H. A. DOTY. 1979. Age-related aspects f Mallard reprductin. Wildfwl 30: 35-39., & G. m. SWANSON. 1975. Sme nutritinal aspects f reprductin in prairie nesting Pintails. J. Wildl. Mgmt. 39: 156-162. LACK, D. 1968. Eclgical adaptatins fr breeding in birds. Lndn, Methuen. OWEN, R. B., JR., & K. J. REINECKE. 1979. Bienergetics f breeding dabbling ducks. Pp. 71-93 in Waterfwl and wetlands--an integrated review (T. A. Bkhur, Ed.). Prc. Symp. Nrth Central Sectin, The Wildl. Sc. RAVELING, D. G. 1979. The annual cycle f bdy cmpsitin f Canada Geese with special reference t cntrl f reprductin. Auk 96: 234-252. REINECKE, K.J. 1977. The imprtance f freshwater invertebrates and female energy reserves fr Black Ducks breeding in Maine. Unpublished Ph.D. dissertatin, Orn, Maine, Univ. Maine. RICKLEFS, R. F. 1974. Energetics f reprductin in birds. Pp. 152-292 in Avian energetics (R. A. Paynter, Jr., Ed.). Cambridge, Massachusetts, Publ. Nuttall Ornithl. Club N. 15. $HAIFFER, C. W., & G. L. KRAPU. 1978. A remte cntrlled system fr capturing nesting waterfwl. J. Wildl. Mgmt. 42: 668-669. SWANSON, G. A., G. L. KRAPU, & J. R. $ERIE. 1979. Fds f laying female dabbling ducks n the breeding grunds. Pp. 47-57 in Waterfwl and wetlands--an integrated review (T. A. Bkhur, Ed.). Prc. Symp. Nrth Central Sectin, The Wildl. Sc. TITMAN, R. D. 1973. The rle f the pursuit flight in the breeding bilgy f the Mallard. Unpublished Ph.D. dissertatin, Frederictn, New Brunswick, Univ. New Brunswick. WELLER, m. W. 1956. A simple field candler fr waterfwl eggs. J. Wildl. Mgmt. 20: 111-113.