INTRASEASONAL REPRODUCTIVE COSTS FOR THE HOUSE SPARROW (PASSER DOMESTICUS) W. BRUCE McGILLIVRAY • Museumof NaturalHistory, University of Kansas,Lawrence, Kansas 66045 USA ABSTP,•CT.--HouseSparrows (Passer domesticus) near Calgary, Alberta begin breeding in earlyspring and continuethrough to late summer.High productivityfrom previousbroods is negativelycorrelated with fledglingproduction from secondand third broods.Although fat reservesmay limit the ability of femalesto raiseyoung, there is no concomitantdrop in clutchsize or in the probabilityof renesting.Pairs that fledgemany young in a yearspace fledgling productionevenly over the breeding seasonbut are most productivein mid- season.The interval between fledging and the initiation of the next clutchincreases with the numberfledged. This delay,an indicationof the physiologicalstrain involved in rearing young,is greaterfor laterbroods and for femalesnesting in trees.Measures of reproductive effort(dutch size, numberfledged, length of the nestlingperiod) vary seasonallybut give no indicationof peakingfor last broods.Thus, reproductiveeffort is not adjustedto parallel changesin the probability of surviving to the next breeding attempt. Received3 November 1981, accepted24 April 1982. A COMMONassumption underlying theoret- ship of double-broodedfemale House Martins ical discussionsof life-history phenomena is (Delichonurbica), and male Pied Flycatchers that trade-offsin life-history characteristicsoc- (Ficedulahypoleuca) feeding many young were cur (Williams 1966, Chamov and Krebs 1974, lesslikely to return to the study area than those Steams 1976, Snell and King 1977). For exam- feeding fewer young (Askenmo 1979). Lack ple, Pianka and Parker (1975)partition the re- (1966) and Kluyver (1970) present evidence productive value of an individual of age X in suggesting an inverse relationship between a stablenongrowing population into fecundity brood size and parental survivorship. De Ste- at age X and expected fecundity conditioned ven (1980), however, did not find a reduction by survival to ages X + 1, X + 2.... , X + in the survivorship of female Tree Swallows N. Clearly if survivorshipis lowered by current (Iridoprocnebicolor) as a result of brood en- reproduction,future fecundity is reduced. largement. Smith (1981), after demonstratinga Many researchersof life-historyphenomena positive associationbetween reproductive ef- in birds have investigated whether or not the fort and survivorship, concludedthat a trade- modal clutch size is the most productive, as off between fecundity and survivorshipshould predicted by Lack (1947, 1966). If productivity not be assumedfor passerinebirds. is measured as the number of young fledged The longevity and mobility of birds usually from a brood, then clutch sizes larger than prevents a precise determination of mortality modal appear to be optimal (Klomp 1970, von rates for individuals with a known reproduc- Haartman 1971,Jones and Ward 1976, Murphy tive history. If dispersers are a nonrandom 1978). If parental survivorship is reduced by sampleof the population (Lowther 1979),then the effort of raising a large brood, however, estimatesof survivorshipbased on individuals then the optimal clutch size might be lower staying at or returning to the study area will than the most productive (Fisher 1930, Char- be biased. As a consequence,the effect of re- nov and Krebs 1974). Studies testing the rela- production on survivorship is often estimated tionship between brood size and parental sur- from physiologicalevidence. Weight lossesfor vivorshipshow equivocal results. Bryant (1979) femalesthrough the breeding seasonsuggest a found a reduction in the overwinter survivor- physiologicalstrain that may reduce survivor- ship (Newton 1966,Hussell 1972, Bryant 1979). For femaleHouse Sparrows(Passer domesticus), Pinowska (1979) has shown that levels of fat • Present address: Water ResourcesBranch, On- influence clutchsize, while lean dry weight (an tario Ministry of the Environment 135 St. Clair Av- indicator of protein level) may determine the enue W., Toronto, Ontario M4V 1P5, Canada. number of broods raised in a season.Norberg 25 The Auk 100: 25-32. January 1983 26 w. BRUCEMcGxLLXVRA¾ [Auk, Vol. 100 (1981) presented the hypothesis that adult countedand numbered. Nestlings 5•5 days old were weight lossesduring breedingmay be adap- banded with a U.S. Fish and Wildlife Service alu- tive. This would be true if the reduced cost of minum leg band and color marked with leg bands collecting food for the nestlings increases after reachinga 20-g weight. Data analysis.--Becausethe nestswere not checked fledgling numbersmore than it reducessur- every day, exact clutch-initiation data and nestling vivorship and future fecundity. ages were not known for all nests. For these cases, Birds that raise more than one brood in a estimateswere used. Nestling age when first found seasonmay be expectedto show differential was estimatedby comparingthe weight of the largest reproductiveeffort and productivitythrough nestling in a brood to averagevalues obtained from the breedingseason. There is a higher proba- broods of known age. Clutch-initiation date was es- bility of survivingfrom one broodto the next timated by assuming an average incubation period ct-uringthe breedingseason than from the last of 11 days and a laying rate of one egg per day brood of one seasonto the following spring. (McGillivray 1978). This estimate was needed only An increasein the level of reproductiveeffort for clutchesbegun before my arrivalat the studyarea. would be expectedas the probability of sur- In this study I investigatedthe productivity from multiple broodsof a pair; hence,I made the general viving to the next breeding attempt decreases. assumptionthat broodsraised at a singlenest through There is someevidence that high levelsof re- the breeding seasonwere raised by the same pair. productiveeffort lower the ability of females House Sparrowsusually remain at the same site un- to invest in a subsequentbrood. For Great Tits less one of the pair dies (Summers-Smith 1963). To (Parusmajor), Kluyver (1963) found a lesser reducethe importanceof this potentialsource of error, probabilityof a secondbrood if the first was I imposed some residency conditions. If a tree nest large. Pinkowski(1977) noted a drop in the was extensively modified following a nesting at- clutch size of second broods for Eastern Blue- tempt, ! assumedthat a new pair was nesting. The birds(Sialia sialis) that raised a largefirst brood, average interval between successiveclutch initia- and Smith and Roff (1980) found that an in- tions for first through third clutchesfor each brood crease in the interbrood interval was associate0 size was determined. Ninety five percent prediction intervals were calculated for each clutch-interval, with a largefirst brood for SongSparrows (Me- brood-size combination. Nests where the interclutch lospizamelodia ). interval fell outside these limits were excluded from In this study I look at the productivityof further analyses. multibrooded House Sparrows through the The interval between the departureof the last nest- breeding seasonand address two questions. ling and the initiation of the next clutchis called the First, is there variation in the productivity of interbrood interval. The date of fledgling departure successivebroods within the breeding season was considered to be the midpoint of the interval associatedwith brood size in earlier broods (is between the last observationof the nestlingsand the there a demonstrable within-season cost as- first nest checkafter departure. Fledging is a gradual process,so a departure date is only an approximate sociatedwith reproduction)?Second, do adults measure. Paired comparison t-tests were used to modify their reproductiveeffort in accordance compare the mean interbrood interval of successive with seasonalchanges in the probabilityof re- broods and different nest types. nesting and surviving? For the investigationof the effectsof one brood on the next breeding attempt, data for the two years were pooled.Pooling requires trends to be consistent METHOVS ANt) MATERIALS over the two years in order for the trends to be de- Studysite.--In 1977,the studyareas were six farms tected, and productivity for both years was similar near Conrich, Alberta, 5 km eastof Calgary. For 1978, except for the first broods. The strength of trends all data were collected at one of these farms, 8 km associatedwith brood size was measured by Pear- east of Calgary. At each of the farms, about 20 nest son's product-momentcorrelation coefficient, r. Al- boxeswere erectedin fall 1974 (seeMurphy 1978 for though significancelevels were determined for r, details). As well as nesting in boxes, the sparrows paired variableswere not testedfor bivariate normal- used buildings, machinery, and trees. At the farm ity. Nonetheless, r is a useful indicator of trend. studied both years, nests built in trees were moni- This analysis assumesthat fledglings of equal tored in addition to those in nest boxes. weight contribute equally to parental fitness. This Data collection.--Nestswere inspectedat 3-5 day assumptionmay not be valid for a specieswith a intervalsfrom 2 May to 15 August 1977and from 20 long breeding season.Dyer et al. (1977) published April to 13 August 1978. If weather conditionsper- the following survivorship equation for juvenile mitted, a 3-day interval was maintained. Eggswere House Sparrows (from Summers-Smith 1963); Y = January1983] HouseSparrow Reproductive Costs 27 TABLE2. Interbroodinterval