The Auk 118(1):191-210, 2001 COMPARISON OF THE REPRODUCTIVE BIOLOGY OF TWO NEOTROPICAL WRENS IN AN UNPREDICTABLE ENVIRONMENT IN NORTHEASTERN COLOMBIA JORGEA. AHUMADA • Departmentof Ecologyand EvolutionaryBiology, Princeton University, Princeton, New Jersey08544-1003, USA ABSTRACT.--Buff-breasted(Thryothorus leucotis) and Rufous-and-white(T. rufalbus) wrens living in a dry forestin northeastColombia (Parque Nacional Natural Tayrona)are faced with a large year-to-yearuncertainty in the arrival time of the rainy season,as well as the amountof rain falling in the first six monthsof the year.Those factors are thoughtto be importantcues used by thosespecies in theirreproductive decisions. In thisstudy, I gathered data on severalreproductive parameters (clutch size, nestingsuccess, timing of breeding, renestingattempts) for both speciesduring two yearsof contrastingrainfall patterns.I col- lectedinformation on the foragingbehavior of bothspecies to identifytheir main food and to studyhow rainfall affectsthe dynamicsof thoseresources. Buff-breasted Wrens fed most- ly in the understory,gleaning arthropods from upper and lowerleaf surfaces,dry branches, and aerial litter. Numbersof arthropodsin thosemicrohabitats depend strongly on the amountof rainfall;understory arthropod levels are low duringthe dry seasonand increase with the arrival of the rains.Buff-breasted Wrens timed their reproductionwith the arrival of the rains in both years,delaying the onsetof breedingsignificantly and continuingto breedduring the dry year(1994). Rufous-and-white Wrens spent a largeproportion of their time feedingon arthropodsin the leaf litter Number of arthropodsin the litter variedlittle betweendry and wet periods.Therefore, Rufous-and-white Wrens had a moreconstant food environmentdespite large differences in rainfallwithin andbetween the yearsof the study. Thatspecies started breeding earlier in the dry seasonand extended its breeding longer than Buff-breastedWrens. My observationssuggest that the evolutionof the reproductivestrat- egiesin thosespecies was mostly through the changeof behavioralparameters rather than physiologicalreproductive parameters such as changesin clutchsize, egg size,or number of broods.Received 14 June1999, accepted16 September2000. MANY BIRDS LIVE in environments that are similar body size (Martin 1987, Paladino 1989). somewhatunpredictable as to when resources That imposesconstraints on the life-history are availablefor growth, maintenance,and re- characteristicsthat birds canevolve (Walsberg production.When should birds breed in an en- 1983).Thus, when the arrival time of food nec- vironmentthat is unpredictablein its favora- essaryfor reproductionis unpredictable,birds bility for reproduction?The very existenceof a are constrainedto breedwhenever pulses of re- distinctiveperiod of the year when mostindi- sources are above a certain threshold. This viduals of a bird speciesbreed--a breeding translatesinto a "tracking"strategy in which season--suggeststhat most of the time, the individuals shouldbe able to detectand pro- amountof foodavailable to themis justenough cess information on the amount of food avail- to sustain their basic metabolic needs. Because able to them and time their reproduction of their high turnoverrate of energyper unit of accordingly. body weightand their inabilityto allocatelarge Few studieshave specificallyexamined ef- amountsof energyto shortand long-termstor- fectsof the duration and intensity of resource age and growth,birds are more dependenton pulseson timing and durationof the breeding food for breeding than other endothermsof seasonof birds. That generally requires de- tailed knowledgeof the temporaldynamics of the resourcesthat birds requirefor reproduc- t Presentaddress: Laboratorio de Ecologfade Pob- laciones,Departamento de Biologla,Universidad Jav- tion aswell aslong-term data on breeding phe- eriana,Bogota, Colombia and Departmentof Botany, nology.One of the best data setsavailable for Universityof Georgia,Athens, Georgia 30602, USA. temperatezones comes from a long-termstudy E-mail:[email protected] in the Hubbard-Brook forest (Holmes et al. 191 192 JORGEA. AHUMADA [Auk, Vol. 118 1986). Most speciesin that forest depend on in birds--providing a natural setting to com- pulses of lepidopteran larvae for successful pare how differencesin their respectivefood breeding. Between outbreaks,birds usually dynamics affectsonset and duration of repro- havea lower reproductiveoutput and success, duction.In this study,I gatheredinformation suggestingthat mostspecies depend on brief on morphology,foraging behavior, and repro- but intensepulses of food whichconstrain the ductionof thosespecies and measuredvaria- time of breeding. tion in abundance of their main food: arthro- Suchlong-term study information is largely pods that occur on live plant tissuein the unavailablefor Neotropicalbirds. Thereis ev- understoryand in leaf litter. The two yearsof idencethat suggests that some Neotropical spe- my study were dramatically different in ciesexhibit a distinctbreeding season (see re- amount of rainfall and in the date of initiation view in Poulin et al. 1992), but examinationof of the rainy season.This offeredme a unique communitypatterns shows that other species opportunityto studythe reproductive strategy breedall year (Skutch1950, Miller 1963,Snow of bothspecies when faced with environmental and Snow 1963, 1964; Gradwohl and Green- variation in food supply. It also allowed me to berg 1990).Do the latter speciesrely on re- gain new insightinto someof the proximate sourcelevels that are more constantthrough mechanisms and ultimate causes that underlie time or that occur in long resourcepulses? the life historyof thosespecies. What types of food resourcesshow this dy- namicbehavior? Can speciesthat rely on these METHODS resourceschoose to breedin periodswhen nest STUDY AREA predationand othercauses of nestloss are low? Thereis partial evidencethat suggestingthat This studywas carried out in ParqueNacional Nat- Neotropicalbirds that utilize foods that are ural Tayrona(henceforth "Tayrona") located on the constantthrough time breed year round (Miller northeastCaribbean coast of Colombia(Fig. 1). The and Miller 1968).Good candidatesfor thosere- park consistsof a strip of land of 15,000ha of dry sourcesare arthropods in bark and rotten forestand scrubreaching from the coastalplains to wood (Pierpont1986), arthropods in leaf litter the SierraNevada of SantaMarta, and rising from (Levingsand Windsor1990, Poulin et al. 1992), sealevel to about400 m. This region of the country somecommunities of shrubunderstory arthro- receivesmoderate amounts of rainfall on a yearlyba- sis(<1,500 mm) and is highly seasonalas a resultof pods (Young 1994), or resourcesassociated the seasonalmovement of the thermal equatorwith with human-made habitats. In contrast, most two dry seasons(December to Marchand July to Au- shrubunderstory arthropods and fruits vary in gust) and two wet seasons(September to November abundanceseasonally (Janzen 1973a, b, 1980; and April to June) (Fig. 2A). Annual averagetem- Wolda 1978a,b, 1980,1990; Levings and Wind- peratureis around25øC with daily temperaturesos- sor 1990, Poulin et al. 1992, Blake and Loiselle cillating between 20 to 31øC (taken from Instituto 1992, Heideman 1989, Hilty 1980, Kinnaird Nacionalde Hidrologia,Meteorologia y Adecuaci6n 1992,Levey 1988,Loiselle and Blake 1991,Van de Tierras--HIMAT). That climaticpattern is influ- Schaiket al. 1993). encedby the presenceof the SierraNevada of Santa Marta which can affectthe extentof rainfall by con- Thryothorusleucotis and T. rufalbusare two densationof moisture along its northeastslope speciesof insectivorouswren, (Buff-breasted (Herrmann 1970). This has created an east-west and Rufous-and-whitewren, respectively)liv- moisturegradient along the park that determinesthe ing sympatricallyin the northeasterndry for- structureof the vegetation(Herrmann 1970, Hernan- estsof Colombia.They constitutea goodsys- dez-Camachoand Rodriguez-Guerrero1972). De- temfor studinghow temporal variation in food spite that generalannual pattern in rainfall season- abundanceaffects timing and intensity of re- ality, the amount of rain in a given year varies productionbecause (1) the dry forestthey live dramaticallyespecially in the first half of the year in has highly unpredictablerainfall not only (Fig. 2B).An analysisusing Colwell's index (Colwell 1974, Beissingerand Gibbs 1993) showsthat pre- fromyear-to-year, but alsowithin a year,which dictabilityof rainfall for Tayronais low [0.239]when might influenceinsect and arthropodabun- comparedto climaticstations within the sameregion dancein their habitat,and (2) the speciesdiffer but outside of the influence of the Sierra Nevada de in feedingheight and slightlyin body size-- Santa Marta (Ahumada 1995). I installed a rain factors that are known to influence food choice gauge and a maximum-minimum thermometerin January2001] Reproductionoftwo Thryothorus wrens 193 Sierra Nevada 5 km Venezuela Caribbean Sea N Colombia Los Naranjos Per6 Santa Mart Parque Nacional Natural Tw Road Rio/ Piedras Sierra Nevada de Santa Marta FIG.1. Geographicallocation of the study site in northeastColombia. the park guard stationlocated at Tayronaand data (Desmoncusorthacanthos), and shrubs.Several species were collecteddaily from August 1992to June1993. of birds inhabit the secondaryforest at LosNaranjos The two years(1993 and 1994)in whichreproductive and mostof themare common.Among the dominant information of the wrens