INTERSEXUAL DIFFERENCES IN FOOD CONSUMPTION BY HAND-REARED GREAT-TAILED GRACKLE (QUISCALUS MEXICANUS) NESTLINGS

KEVIN L. TEATHER Departmentof Biology,Carleton University, Ottawa, Ontario KIS 5B6,Canada

ABSTRACT.--Imonitored food consumptionby hand-rearedGreat-tailed Grackle (Quiscalus mexicanus)nestlings to test the hypothesisthat malesand femalesconsume similar amounts of food but males direct more of their energy into charactersthat result in greater mass increase.Growth patternsof hand-rearednestlings reflected those of naturally reared nest- lings: maleswere heavier and had longer tarsi,and femaleshad longer 9th primarieson days 6-12 post-hatching.Males, on average,consumed 15.8% more food than femalesthroughout this period;the amountof food consumedper unit massincrease was similar for both sexes. The resultssuggest there may be substantialcost differences to parentsin rearing offspring of different sex. The nestling sex ratio for this and other sexually size-dimorphicspecies is generally close to one, and Fisher's (1930) theory that differential investment in sonsand daughtersshould result in a skewed sex ratio at the end of parental care is not supported. Received14 November1986, accepted21 March 1987.

FISHER(1930) suggestedthat the total invest- passerine(Dhondt 1970), fledgling sex ratios ment by parentsin rearing offspringof eachsex may actually be biased in favor of the larger should be equal. Subsequently, the relative sex. number of young of eachsex at the end of pa- Differencesin the costsof rearing malesand rental care should reflect the costsof rearing femalesin dimorphicspecies might be lessthan them. Becausethe costsof raising males and previously thought (Fiala 1981, Richter 1983, females in most speciesare probably similar, Bancroft 1984). This would be true if both sexes sexratios in mostpopulations are generallyclose require similar amountsof food despitesize dif- to one (Charnov 1982). In some species,how- ferences,or if parentssomehow compensate for ever, the young of one sexmay be significantly the demands of the larger sex at no extra cost largerthan the other.If individualsof the larger to themselvesdespite food-requirement differ- sexare more costlyto rear, then parentsshould ences.The first option might be true if the sexes equalize investment between the sexesby pro- differ in their allocationof energy to the dif- ducing greater numbers of the less expensive ferent componentsof growth and develop- sex. ment.Thus, while the largersex might put more Although Fisher'sideas have been confirmed energy into components that result in an in- mathematically (Kolman 1960; Verner 1965; creasein body mass,the smaller may direct sim- Leigh 1970;Charnov 1975, 1982), empirical evi- ilar amounts of energy into development that dence has been restrictedlargely to studiesof resultsin lessbody massincrease. On the other invertebrates(reviewed by Charnov 1982). In- hand, if food-requirement differencesare sig- vestigationsof altricialbirds in which nestlings nificant,parents might compensateby bringing exhibit sexual size dimorphism have been less larger, but not necessarilymore, food items to supportive of Fisher'spredictions. The sex ratio the larger sex. of nestlingsis generally closeto unity both at I hand-reared Great-tailed Grackle (Quiscalus hatching (Porter and Weinmeyer 1972,Newton mexicanus)nestlings from day 6 through day 12 1979, Fiala 1981, Bancroft 1983, Richter 1983, post-hatchingto testthe first hypothesis.Great- Weatherhead1983, Bortolotti 1984)and at fledg- tailed Gracklesare a highly dimorphic species ing (Willson 1966,Newton and Marquiss1979, in which adult males are approximately twice Patterson and Emlen 1980, Bancroft 1983; but asheavy asfemales (Selander 1958). I attempted seeFiala 1981).Indeed, in many dimorphic rap- to determine whether males require more food tors (Hickey 1942, Balfour and Cadbury 1979, than femalesof similarage, whether sexesdiffer Picozzi 1980, Collopy 1986) and at least one in their energyrequirements per unit mass,and

635 The Auk 104: 635-639. October 1987 636 KEVINL. TEATHER [Auk, Vol. 104

A- MALES B - FEMALES lOO

12

8o 13 13

6o

13 13 4o Fig. 2. Mass,tarsus, and 9th primaryshown as a percentageof normalgrowth for Great-tailedGrack- le nestlings.Normal growth was determinedfrom naturally reared nestlings,and sample sizes range from 5 to 35. Sample sizesfor hand-rearednestlings are as in Fig. 1.

Sinton, Texas.Groups of 5-6-day-old nestlingswere taken from nestson the eveningsof 25 May (n = 9), 1 June (n = 10), and 15 June (n = 12) and placed in pairs in natural neststhat had been brought into the laboratory.Nests were kept outsidein the shadedur- ing the day exceptduring extremeheat or rain. Dur- ing the eveningnestlings were returnedto the lab- o oratory,where the temperaturewas maintainedat 22- 26øC.Moist towels and a heating pad were placed over the nests to maintain humidity. Nestlingswere fed a homogeneousmixture of cat- fish chow, wild-bird starter, egg, gelatin, vitamins, molasses,and water (after Lanyon and Lanyon 1969) throughcalibrated syringes. Nestlings were fed every 10-20 min from 0630 to 2030 until they stoppedgap- ing. Thus, nestlingsdictated how much food they received. Fecal sacs were collected from individuals after each feeding to calculatedigestive efficiency. 6 7 6 9 10 11 12 Nestlingswere weighed before feeding each morn- ing andagain at 2030.Tarsus and 9th primarylengths DAYS POST-HATCHING (mm) were recordedeach evening. All birds in the first group and birds of uncertain Fig. 1. Differences in mass,tarsus, and 9th pri- sexin groups2 and 3 were sexedby dissectionat the mary (+1 $D) in hand-reared male (0) and female (¸) end of the 5-6 daysin captivity. Birdswhose sexwas Great-tailedGrackle nestlings. Sample sizes (given at obviousby their growth patternswere returned to the top) are the same for the three variables.As- natural nests. terisksindicate significantdifferences at P < 0.05 (t- tests). RESULTS

whether sexes convert similar proportions of Males were significantlyheavier and had sig- food into increasing body mass. nificantly longer tarsi than females on days 6- 11 (Fig. 1). Females,however, had significantly longer 9th primaries than males on days 6-9 MATERIALS AND METHODS (Fig. 1). Although these resultsare similar to The study was conductedbetween 25 May and 22 differences between the sexes in natural nests June 1986 at the Welder Wildlife Foundation near (Teather unpubl. data), the growth of all three October1987] FoodConsumption byGreat-tailed Grackles 637

TABLE1. Relationship between amount of food con- sumed and mass increase for males and females. MALES Values represent the ratio [dry massof food con- sumed (g) - dry mass of feces (g)/mass in- .EMA,ES creaseper day (g) and are given ñ 1 SD. Probability values were calculated from t-tests on arcsine trans- formations.Sample sizesare as in Fig. 1.

Days post- hatch- ing Males Females P 6 1.63 + 0.62 2.63 + 1.57 0.236 7 1.61 + 1.21 1.82 + 1.39 0.688 8 1.30 _+ 0.42 1.14 + 0.17 0.201 9 1.13 + 0.47 1.09 + 0.24 0.782 10 1.14 + 0.18 1.32 + 0.21 0.030 11 1.59 _+ 1.27 1.43 + 0.47 0.731 DAYS POST- HATCHING 12 0.94 ñ 0.16 1.44 0.236

Fig. 3. Averageamount of food consumedper day (ñ1 SD) by male and female nestlingsfrom day 6 to day 12 post-hatching.Asterisks indicate significance body masswas best described by a linear rela- level (* 0.1 > P > 0.05, ** 0.05 > P > 0.01, *** P < tionship (food consumed= 7.26 + 0.44.mass; 0.01; t-tests).Sample sizes are as in Fig. 1. r = 0.790, P < 0.001). Subsequently,because males were larger, they consumedmore food variables was depressed in hand-reared nest- than females of similar ages throughout the lings (Fig. 2). study period (Fig. 3). The average amount of The relationshipbetween body massand en- food consumedby males between days 5 and ergy requirements generally follows a power 12 was 272.5 g, which was 15.8%more than the function (reviewed by Power 1983). I found, amount consumedby females(235.4 g). During however, that the amount of food consumed vs. the sameperiod malesweighed 30.0%more than females.Therefore, although males consumed more food than females, the difference was not directly proportional to the difference in body size. Females, in fact, consumed slightly more food than males of equal mass(Fig. 4). To determine whether males and females dif- t't 50 fered in the amount of food consumedper unit massgained eachday, I calculatedthe ratio [dry massof food consumed(g) - dry massof feces

U.I (g)]/massgained (g). No consistentdifferences were found between values obtained for males and females over the study period (Table 1). Z

DISCUSSION 0 3o

U. Male Great-tailed Grackle nestlings con- sumedmore food than females of equal age and therefore required a greaterabsolute amount of food to be reared to fledging. The difference 45 55 65 75 65 95 in food consumption over the 5-6-day study period was probably a minimum estimate for MASS (G) two reasons.First, the growth of hand-reared Fig. 4. Food consumed per day as a function of maleswas depressedmore than that of females, body mass. Regressionsfor males and females are and, thus, differences in food requirements of significantly different (F = 3.30, P = 0.04). nestlings reared in natural nests are probably 638 KEVINL. TEATHER [Auk, Vol. 104 larger. Second, becausethe disparity in size be- Although male Great-tailed Grackles require tween the sexes continues to increase while the more food than females, the sex ratio remains nestlingsare under parental care(approximate- near one throughout the nestling period (Se- ly another 2 weeks after fledging), food con- lander 1960, 1961), an observation consistent sumption differences probably continue to in- with studies of other dimorphic altricial nest- crease. These results agree with those of Fiala lings (see references in introductory para- and Congdon (1983), who showedthat the gross graphs).These results appear to contradictFish- energy intake of male Red-winged Blackbirds er's (1930) prediction that there should be an (Agelaiusphoeniceus), during days 0-10, was ap- overproduction of the lessexpensive sex during proximately 1.3 times that of females. In con- the period of parental care.There are three pos- trast, male and female European Sparrowhawk sible explanations for this. First, higher food (Accipiternisus) nestlings, despite large size dif- consumptionby the larger sex may not be ac- ferences, consume similar amounts of food companied by increased coststo parents. This (Newton 1978), and male and female Golden would be true if parents met the demands of Eagle(Aquila chrysaetos) nestlings do not differ larger nestlingsby feeding them larger, and not in food consumption or energy metabolized necessarilymore, food items. This might occur (Collopy 1986). if some food items were too large to be fed to Although males consumemore food than fe- the smaller sex or if parents opportunistically males, requirements may not be proportional captured food items of varying size and dis- to their size. My resultssuggest that at constant tributed theseto offspringaccording to nestling mass females actually consume slightly more size. food than males. This differs from male Red- Second,selection favoring overproduction of winged Blackbirds, which consume slightly the smaller sex might be offset by selection fa- more food than females of similar mass (Fiala voring overproductionof the larger sex. Fish- 1981). Unfortunately, nestlingsin both studies er'sprediction that the lessexpensive sex should suffered from reduced growth rates so the re- be producedin greaternumbers is basedon the lationshipbetween massand food consumption assumptionthat the mortality rate of both sexes remains unclear. is similar while under parental care.Fisher also An alternative is that dimorphic nestlings predicted, however, that if mortality rates of might channel energy into different characters. sonsand daughtersdiffered, the sexratio should Thus, while energy of the larger nestling is di- be initially biasedin favor of the sexthat suffers rectedto structuresthat result in increasedbody the greatest mortality. Indeed, there is some mass,the smaller sexmay divert energy to phys- evidence, at least from passerines,that the larg- iological charactersthat enable it to compete er sexexperiences higher nestling mortality rates with its larger brood mates. This may explain under certain conditions (seebelow). Mortality why feather development is more advanced in ratesof males and femalesalso may differ dur- females than in males in Yellow-headed Black- ing the period between fledging and indepen- birds (Xanthocephalusxanthocephalus) and Boat- dence, although I know of no data that address tailed Grackles (Quiscalusmajor) (Richter 1983, this question. Bancroft 1984). Data from my study do not sup- Last, although there may be substantialcost port this interpretation. Despite more advanced differencesin rearing sonsand daughters,ini- feather developmentin females,the massgained tial sex ratios are a function of random segre- per unit food consumed was similar for both gation of sex chromosomesand not subjectto sexes.Although it might be argued that the parentalcontrol. If this is the case,parents would variability between nestlingsmasked potential be expectedto adjusttheir food provisionto the differences,it is more likely that feather growth overall energy demands of the brood. Because is not more costlyto femalesthan males.Female energy demandswould be higher in nestscon- feather development, although initiated earlier taining a higher proportion of the larger sex, (Teatherunpubl. data),proceeds at a rate similar productionof the largersex should suffer under to males.Thus, on a daily basisboth sexesprob- stressedor food-limiting conditions. Examina- ably direct similar amountsof energy to feather tion of the few populations with skewed sex development. Any added coststo the female ratios at fledging support this interpretation. In should occur only during the initial stagesof nearly all cases, the fledgling sex ratio was feather growth. skewed toward the smaller sex, and this was a October1987] FoodConsumption byGreat-tailed Grackles 639 result of higher mortality of the larger sex un- selection and social behavior (R. D. Alexander der experimentally or naturally stressedcon- and D. W. Tinkleß Eds.). New Yorkß Chiron Press. ditions(Howe 1977,Cronmiller and Thompson ß 8r J. D. CONGDON. 1983. Energetic conse- 1981, Fiala 1981, Bancroft 1983, Roskaft and quences of sexual size dimorphism in nestling Slagsvoid 1985; but see Dhondt 1970). Red-winged Blackbirds.Ecology 64: 642-647. FISHERßR.A. 1930. The genetical theory of natural To evaluateprecisely the importanceof adap- selection. Oxfordß Clarendon Press. tive sex-ratio manipulation in altricial birds, HICKEYßJ.J. 1942. Easternpopulations of the Duck further data are required on energy require- Hawk. Auk 59: 176-204. mentsand mortality ratesof eachsex through- HOWEßH. F. 1977. Sex ratio adjustmentin the Com- out the dependent period, how food is distrib- mon Grackle. Science 198: 744-746. uted to nestlings,and sex ratios of offspring KOLMAN, W. 1960. The mechanism of natural selec- after periods of food abundance and food stress. tion for the sex ratio. Amer. Natur. 94: 373-377. LANYON,W. E., & V. H. LANYON. 1969. A technique

ACKNOWLEDGMENTS for rearing passerinebirds from the egg. Living Bird 8: 81-93. I am grateful to Dr. JamesTeer and the Rob and LEIGH,E.G. 1970. Sexratio and differential mortality BessieWelder Wildlife Foundationfor providingfa- between the sexes. Amer. Natur. 104: 205-210. cilitiesto conductthis project. Yvette Halpin put much NEWTON,I. 1978. Feedingand developmentof Spar- timeßcareß and patienceinto raising nestlings.Dr. rowhawk Accipiternisus nestlings. J. Zool. Lon- Patrick Weatherhead offered encouragement,sup- don 184: 465-487. portßand criticismthroughout the study. I alsothank 1979. Population ecologyof raptors. Ver- Drs. Andy HornßMarty LeonardßEgbert Leighß and millionß South Dakotaß Buteo Books. Kent Fiala for critically reading the manuscriptand ß& M. MARQUISS.1979. Sex ratio among nest- providing useful suggestions.The Natural Sciences lings of the EuropeanSparrowhawk. Amer. Nat- and Engineering ResearchCouncil of Canada sup- ur. 113: 309-315. portedthis researchthrough an operatinggrant to P. PATTERSON,C. B., & J. M. EMLEN. 1980. Variation in Weatherheadand a postgraduatescholarship to me. nestling sex ratios in the Yellow-headed Black- bird. Amer. Natur. 115: 743-747.

LITERATURE CITED PICOZZI,N. 1980. Foodßgrowthß survival and sex ratio of nestling Hen HarriersßCircus c. cyaneus BALFOURßE., 8r C. J. CADBURY.1979. Polygyny,spac- in Orkney. Ornis Scandinavica11: 1-11. ing and sexratio amongHen Harriers Circuscy- PORTERßR. D., 8r S. N. WEINMEYER.1972. Reproduc- aneusin Orkneyß Scotland. Ornis Scandinavica tive patterns in captive American Kestrels(Spar- 10: 133-141. rowhawks). Condor 74: 46-53. BANCROFTßG.T. 1983. Reproductivetactics of the POWER,R.H. 1983. Ecologicalimplications of body sexually dimorphic Boat-tailed Grackle (Aves). size.Cambridgeß Englandß Cambridge Univ. Press. Ph.D. dissertationßTampa, Univ. South Florida. RICHTER,W. 1983. Balancedsex ratios in dimorphic 1984. Growth and sexualdimorphism of the altricial birds: the contribution of sex specific Boat-tailed Grackle. Condor 86: 423-432. growth dynamics.Amer. Natur. 121: 158-171. BORTOLOTTI,G.T. 1984. Evolutionof the growth rate ROSKAFT,E., & T. SLAGSVOLD. 1985. Differential mor- and nestling sex ratio in Bald Eagles(Haliaeetus tality of male and female offspring in experi- leucocephalus).Ph.D. dissertationßTorontoß Univ mentally manipulated broods of the Rook. J. Toronto. Anim. Ecol. 54: 261-266. CHARNOVßE. L. 1975. Sex ratio selectionin an age- SELANDER,R. K. 1958. Age determination and molt structuredpopulation. Evolution 29: 366-368. in the Boat-tailed Grackle. Condor 60: 353-376. 1982. The theoryof sexallocation. Monogr. 1960. Sex ratio of nestlingsand clutch size Pop. Biol. No. 18. in the Boat-tailed Grackle. Condor 62: 34-44. COLLOPY,M.W. 1986. Foodconsumption and growth 1961. Supplemental data on the sex ratio in energeticsof nestling Golden Eagles.Wilson Bull. nestling Boat-tailed Grackles. Condor 63: 504. 98: 445-458. VERNER,J. 1965. Selection for the sex ratio. Amer. CRONMILLER,J. R., 8r C. F. THOMPSON. 1981. Sex ratio Natur. 99: 419-421. adjustmentin malnourishedRed-winged Black- WEATHERHEAD,P. J. 1983. Secondary sex ratio ad- bird broods. J. Field Ornithol. 52: 65-67. justment in Red-winged Blackbirds (Agelaius DHONDT,A.A. 1970. The sexratio of nestling Great phoeniceus).Behav. Ecol. Sociobiol. 12: 57-61. Tits. Bird Study 17: 282-286. WILLSONßM. F. 1966. Breeding ecologyof the Yel- FIALA,K.L. 1981. Reproductivecost and the sexratio low-headedBlackbird. Ecol. Mongr. 36: 51-77. in Red-wingedBlackbirds. Pp. 198-214in Natural