The Auk 118(1):176-190, 2001 VARIATION IN GROWTH OF NESTLING TREE SWALLOWS ACROSS MULTIPLE TEMPORAL AND SPATIAL SCALES JOHN 1v. MCCARTY• Departmentof Ecologyand Systematics, Cornell University, Ithaca, New York14853, USA ABSTRACT.--Differenceswithin a speciesin ratesof growth of nestlingscan be used as indicatorsof the quality of parental care,environmental conditions, and future successof offspring,whereas comparisons among different species may reflecta historyof different ecologicalconditions or life-historystrategies. The presesntstudy examinesthe patternsof variationin growth in nestlingTree Swallows(Tachycineta bicolor) from acrossthe species' range and comparesTree Swallowsto other species.Growth of Tree Swallowswas typical of other speciesin the family Hirundinidae.As a family, the Hirundinidaehave slower growth than typicalfor passerines.Growth rate of speciesof Hirundinidaewas not corre- lated with adult body massor averagebrood size. Contrary to predictions,species that are double-broodeddid not havehigher growth rates, but swallowspecies living at higherlat- itudes did have higher growth ratesthan tropical species.Substantial variation in growth rateswas observedamong populations of Tree Swallows,yet the amountof variationob- servedbetween breeding colonies only a few kilometersapart, or from the samecolony in differentyears, was as great as that seenin populationsseparated by hundredsof kilometers. Within a population,differences in growthamong years were correlatedwith temperature and food supply when nestlingswere being raised. No correlationbetween climate and growth was seenwhen comparingdifferent populations. Differences between populations werenot explainedby localhabitat, nor werelarge-scale geographic patterns evident. I used both experimentaland observationalevidence to evaluatethe implicationsof short-termre- ductionin growth for subsequentgrowth and survival.Nestlings were slowto recoverfrom evenvery shortperiods of delayedgrowth that occurearly in the nestlingphase. Return of nestlingswith experimentallyor naturallyinduced delayed growth was reduced, which sug- geststhat shortinterruptions in growthmay have long term effects on postfiedging survival, eventhough mass at fledgingis not affected.Received 9 August 1999, accepted 16 September 2000. NESTLINGS OF ALTRICIAL BIRDS exhibit sub- growth.In a broadercontext, those energy de- stantialvariation in growth ratesboth within mands,coupled with physiologicalconstraints and amongspecies. Variation within a species on growth,accentuate the trade-offsthat exist is oftenused as an indicatorof variabilityin pa- betweenmaximizing growth and attempting to rental care, environmental conditions, or nes- optimize other aspectsof life histories(Lack tling quality. Growth ratesof nestlingsinflu- 1968, Ricklefs 1984, Ricklefs and Starck 1998, encelength of time offspringare dependenton Starck and Ricklefs 1998b). their parents,their energy requirementsand Within a species,nestlings with below-aver- rate of food delivery required of the parents, age growthor size at fledginggenerally suffer and length of time they are exposedto nest fromreduced postfledging survival (Gebhardt- predators(Lack 1968, Bosque and Bosque1995, Henrich and Richner1998). Reducedgrowth Halupka 1998).Variation among species is typ- may also have a long-term effect on fitness, icallyseen as a resultof variationin life-history evenwhen it doesnot appearto resultin lower strategies.Altricial birds are amongthe fastest growing vertebrates(Case 1978), with most postfledgingsurvival, by decreasingthe ability smallpassetines attaining full adultmass with- to obtain a breeding territory or mate or by in 10 to 20 daysof hatching.High ratesof en- lowering subsequent fecundity (Gustafsson ergy intakeare necessaryto sustainthat rapid and Sutherland 1988, Richner 1992, Richnet et al. 1989,Lozano 1994). Although it is clearthat • Presentaddress: Department of Biology,Univer- growth is a good indicatorof future success, sity of Maryland, College Park, Maryland 20742, factors that determine variation in size and USA. E-mail: [email protected] mass are not as well understood. 176 January2001] Variationin NestlingGrowth 177 Previousstudies have generallynot differ- into the degreeof plasticitypossible on an evo- entiated between growth reductionsthat are lutionary time scale,whereas differences in life due to chronic food shortagesor that are history among related speciesmay indicate causedby poor parentalcare and thosecaused how growthresponds to changesin therelative by short-termfluctuations in foodsupply with- demandsplaced on developingnestlings. Sec- in a season.Several groups of birds with food ond, I examinedthe significanceof short-term suppliesthat are subjectto short-termfluctua- periodsof reducedfeeding on growth and the tions, suchas seabirds(Hawksley 1957, Dunn possiblelong-term effects of thosereductions. 1975,Konarzewski and Taylor 1989)and aerial Althoughthe importanceof chronicfood short- insectivores (Koskimies 1950, Lack and Lack ages is well known, relatively little attention 1951,Bryant 1978,Wrege and Emlen 1991),are has been paid to short-termfluctuations in re- observedto undergo periods of interrupted sourcesand their effecton subsequentaspects growth and developmentunder adversecon- of an individual's biology. I present observa- ditions,resuming normal growth once condi- tional and experimental evidence to examine tions improve. The long-term effectsof those the importance of short-term reductions in temporary growth reductions have seldom growth. been explored,but if starvationinterferes with critical developmental stages, permanent METHODS changescould result. Previousstudies have found that nestlings subject to short-term Tree Swallowsbreeding in nestboxes were studied shortagesof food eventuallyattain full body at the CornellUniversity Experimental Ponds Facil- ity (42ø30'N76ø27'W), near Ithaca, New York. This mass(Wiggins 1990b, Negro et al. 1994),mak- facility consistsof two breeding sites located ap- ing it unclear whether such reductions in proximately 2 km apart. Unit One supported ap- growthhave a long-termeffect on postfledging proximately 55 to 75 pairs of breeding Tree Swal- survival if survival dependssolely on body lows, and Unit Two had between 10 and 23 pairs. size. Insect abundancewas measureddaily using suction In contrastto the view that intraspecificvar- trapsrunning during daylight hours, and high tem- iation is due to effects of the environment, var- peraturewas recordeddaily. Thesesites and meth- iation among speciesis often viewed as the ods for sampling insects are describedin detail in adaptive outcomeof different selectionpres- McCarty and Winkler (1999a, b). All values are re- ported as means + SE. sures.Predation rates, food availability,num- Nestlingsof all ageswere weighedto the nearest ber of breedingattempts per year, and level of 0.1 g during the 1990-1993breeding seasons using competitionamong siblings are all thoughtto either Pesolaspring scales or a portableOhaus elec- influenceinterspecific differences in growth tronic balance.In addition, nestlingswere measured (Lack1968, Bosque and Bosque1995, Halupka on day 10 only in 1989,and thosedata are included 1998).Even though the sourcesof intraspecific in analysesof return rates. Swallows breeding at and interspecificvariation in growth are ulti- thosesites are monitoredclosely for the exactdate of mately the same,the two types of variation hatchingto determinenestling age. All nestlingages have seldombeen consideredtogether. are given as hatch day equal to nestling day 1. Lengthsof flattenedand straightenedwing chord The presentstudy employed two approaches (hereafter"wing length"), the 9th (outermost)pri- to addressthe questionof the ecologicalsig- mary, and 6th (outermost)rectrix featherwere mea- nificanceof variation in growth rates of Tree suredto the nearest0.5 mm usinga ruler with a wing Swallows (Tachycinetabicolor) and variation and featherstop. Length of the manuswas calculated amongthe speciesin the Hirundinidae.First, I from difference between wing and 9th primary describedboth the inter- and intraspecificvar- length. Length of the tarsometatarsus(henceforth iation in growth. Variation in growth among "tarsus") was measuredto the nearest0.1 mm using differentspecies and amongdifferent groups of dial calipers. Nestling Tree Swallows typically Tree Swallows was comparedto factors that fledge on day 21; disturbing nestlingsafter day 15 may causepremature fledging, so sample sizes for might contributeto variation,such as climate, older nestlings are small and come primarily from life history (clutchsize and numberof broods nestlingsremoved for other studies. per season),habitat, food supply, and geo- Growth curves were fitted to mass data and graphiclocation. The absolutedegree of varia- growth rate constantswere calculatedfor nestlings tion among related speciesprovides insight from Ithaca in eachyear and at both breedingloca- 178 JOHNP. MCCARTY [Auk, Vol. 118 tions. Nestlingsat Ithaca were measuredat more metric Kendall rank correlationor Mann-Whitney than one age,but not every day, providing a mixed tests (Conover 1980). longitudinalsample (Ricklefs 1983). Logistic growth Growth of speciesin the Hirundinidae was com- curves are suitable for Tree Swallows (Zach and Ma- paredto otherspecies of passerines.Ricklefs (1968a) yoh 1982)and curveswere fitted to meanmasses for providesgrowth constantsand ratio of asymptotic eachpopulation using an iterative,least-squares pro- massto adult massfor