THE AUK A QUARTERLY JOURNAL OF ORNITHOLOGY VOL. 104 APRIL 1987 NO. 2 EFFECTS OF NESTLING DIET ON GROWTH AND ADULT SIZE OF ZEBRA FINCHES (POEPHILA GUTTATA ) PETER T. BOAG Departmentof Biology,Queen's University, Kingston, Ontario K7L 3N6, Canada Al•STRACT.--Manipulationof the diet of Zebra Finch (Poephilaguttata) nestlings in the laboratoryshowed that a low-quality diet reducedgrowth ratesof nine externalmorpholog- ical characters,while a high-quality diet increasedgrowth rates.The growth of plumage characterswas least affectedby diet, while growth ratesof tarsusand masswere most af- fected. The treatments also produced differencesin the adult size of experimental birds, differencesnot evident in either their parentsor their own offspring.Diet quality had the strongestimpact on adult massand tarsuslength, while plumage and beak measurements were less affected. Analysis using principal componentsand characterratios showed that the shapeof experimentalbirds was affectedby the experimentaldiets, but to a minor extent comparedwith changesin overall size. Significantshape changes involved ratiosbetween fast- and slow-growingcharacters. The ratios of charactersthat grow at similar, slow rates (e.g. beak shape) were not affected by the diets. Environmental sourcesof morphological variation should not be neglectedin studiesof phenotypicvariation in birds. Received5 June 1986, accepted30 October1986. MORPHOLOGICAL differences between indi- fitness, and weather was seen in the nonran- vidual birds are often assignedfunctional sig- dom survival of House Sparrows collected by nificance, whether those individuals are of dif- Hiram Bumpus following a winter storm ferent species,different sexes,or different-size (O'Donald 1973, Fleischer and Johnston 1982). members of the same sex (Hamilton 1961, Se- Recently, investigators have tried to dem- lander 1966,Clark 1979,James 1982). In grani- onstrate a genetic basis for intraspecific mor- vores such as Darwin's finches (Geospizaspp.), phological variation in birds. Several studies morphologicaldifferences have been related to have shown that variation in morphological variation in feeding efficiency at all three of traits of ecologicalinterest is heritable (see re- these levels. Differences in feeding efficiency view by Boag and van Noordwijk 1986). Al- have in turn been correlated with variation in though such analyses are difficult to perform fitness due to spatial or temporal variation in and interpret, they have provided one of the seed availability (Boag and Grant 1981, Schlu- few means available to test the plausibility of ter and Grant 1984). Intraspecific morphologi- the hypothesisthat natural selectionis the pri- cal variation in several species,such as House mary determinant of the array of phenotypes Sparrows (Passerdomesticus; Johnston and Se- maintained within avian populations. lander 1971), Red-winged Blackbirds(Agelaius One common misunderstandingof heritabil- phoeniceus;Power 1969),Great Tits (Parusmajor; ity studies is that even if variation in a trait Hamilton 1961), and others (James 1970) has withintwo different populationsis highly her- been correlated with clinal variation in cli- itable, this does not guarantee that the differ- mate. A classicexample of the correlation be- ence in the characterbetween the populations tween intraspecific morphological variation, has a genetic basis.For instance,regressions of 155 The Auk 104: 155-166.April 1987 156 PETERJ. BOAG [Auk, Vol. 104 offspring bill size on parental bill size were I report a simple experimentwith laboratory- identical and significant in two years of data raised Zebra Finches (Poephilaguttata). I show collectedfor Geospizafortis (Boag 1983). This in- that qualitative differencesin the diets fed by dicated that the variation was highly heritable parentsto their young producesubstantial dif- in both years. However, y-intercepts in these ferences in the growth rates of several mor- sameregressions differed significantly(see Boag phological characters.When these young are 1983:fig. 1). Boag(1983) hypothesizedthat dif- followed until adulthood, the different nest- ferent population densities and territory sizes ling diets produceadult birds with significant- in the two yearsled to differencesin the quan- ly different morphologies. tity or quality of food fed to young, with the result that young grew to different adult sizes. Although it hasbeen known for sometime that METHODS nestling diets can alter growth rates (Bryant Adult Zebra Finches were selected at random from 1978), few authors have suggestedthat varia- an outbred laboratorycolony and mated to produce tion in growth rate contributes to adult mor- 14 pairs. Eachpair was placed in a 50 x 30 x 40 cm phologicalvariation in wild birds (but seeSmith breeding cage, provided with a wicker nest cup, and Zach 1979; James 1982, 1983; Boag 1983; shreddedburlap nestingmaterial, cuttlebone,an oys- Alatalo and Lundberg 1986). Laboratory data ter shell-grit mixture, and water. Cages were main- on birds (Allee and Lutherman 1940, Johnson tained indoors on a 14L:10D lighting schedule at 1971) and mammals (Harrison 1959, Lister and constanttemperature and humidity, visually but not McCance 1965) suggest that diet in early life acousticallyisolated from each other. Cageswere as- can affect adult size. signedat randomto one of three experimentalgroups, This possibility is supported by elegant ex- which were treated identically exceptfor the quality of food available from the end of incubation until periments on Red-winged Blackbirds (James the young were approximately14 weeksold. Parents 1983). Reciprocal transplants of Red-wings be- were removed when young were about 35 days of tween either end of two separate geographic age. Breeding under all treatments commencedsi- clines in body size and shape showed that the multaneously. transferredyoung grew to resembletheir foster Families in the "standard-quality diet" treatment parents more than their biological parents. groupwere fed finch seedmix (red and white millet, Jamesconcluded that Red-wing clines in mor- with canary, rape, and flax seed) and standardnest- phology had a large nongenetic component, ling food mixture (see Table 1) ad libitum.The seed and that there probably existed complex co- mix contained about 14% crude protein, while the variation between genetic and environmental nestling mix had about 24% protein, basedon Kjel- causesof individual differences in morpholo- dahl analyses(Skoog and West 1969). In addition, thesefamilies received minced, hard-boiled hen's egg gy. with added multivitamin-mineral powder (Perve- The heritability studies discussedabove, as line•, 8 in 1 ProductsInc., about 1 g/100 g of egg) well as the transplantexperiments (James 1983), oncea week (about 1 g wet massegg mixture for each signal a growing need to considerthe origins adult or young in cage). Three days after the egg of phenotypic variation in avian populations. ration, each cageof birds receivedapproximately 5 g Such awarenesscame early to ecologistswork- wet massof fresh lettuce or spinach.The four stan- ing with other taxa. For instance,the morpho- dard-dietpairs each produced one brood,with a total logical plasticity of plants routinely forcedbot- of 6 male and 5 female offspring surviving to 1 yr of aniststo dissectecotypic variation in their study age or more. populations before invoking adaptive argu- Families in the "high-quality diet" group were given the standardseed mixture, and a high-protein ments concerning individual differences.This nestling mixture (approximately38% protein; seeTa- approachwas epitomized by the classicecotype ble 1) ad libitum.They receivedabout 1 g per bird of transfers conducted by Clausen et al. (1940). the egg mixture every day and 5 g of fresh greens Until recently, most avian ecologistshave as- every other day. Five high-diet pairs eachproduced sumedimplicitly that birds have essentiallyde- one brood, for a total of 10 male and 4 female off- terminate growth, and that phenotypicdiffer- spring at 1 yr of age. Familiesin the "low-quality diet" group were giv- encesbetween adults accuratelyreflect genetic en the standardseed mixture, and a low-protein nest- differences, which are amenable to evolution ling mixture (about 7% protein; see Table 1) ad libi- by •,atural selection. tum. From the end of incubation on, they received April1987] ZebraFinch Morphology 157 about 5 g of greens once a week, but no egg. Five TABLE1. Composition(% dry mass)of nestlingmix- low-diet pairsproduced one broodeach, with 8 male turesused in experimentaldiets. and 8 female offspringat 1 yr. All three nestling food mixtures had roughly equal energy content [about Diet quality 3,000kcal/kg or 12.6 x 106J/kg, basedon published Start- valuesfor componentingredients; metabolizable en- Low dard High ergy content (Fisher 1972) was not measured]. Rolled oats 34.0 Nine external morphologicalcharacters were mea- Pablum(baby cereal) 24.0 suredon the original parentsbefore the experiment, Wheat germ 10.0 on the experimentalbirds as they grew (oncea day Skim milk powder 20.0 12.0 from hatch until day 14, every 2 days until day 40, Gelatinpowder 10.0 7.6 once a week until day 98, and at 1 yr of age), and on Vitamin-mineralpowder a 1.0 1.0 1.0 the fully grown offspring later produced by the ex- Alfalfa, dried 1.0 1.0 1.0 perimental birds. Offspring of experimental birds Dry dog food 6.0 20.0 were produced by mating individuals with nonsibs Tryptophan 0.1 Cystine 0.1 from the sametreatment group. Methionine 0.2 The measurementstaken