Intraseasonal Reproductive Costs for the House Sparrow (Passer domesticus)

-House Sparrows (Passer domesticus) near Calgary, Alberta begin breeding in early spring and continue through to late summer. High productivity from previous broods is negatively correlated with fledgling production from second and third broods. Although fat reserves may limit the ability of females to raise young, there is no concomitant drop in clutch size or in the probability of renesting. Pairs that fledge many young in a year space fledgling production evenly over the breeding season but are most productive in midseason. The interval between fledging and the initiation of the next clutch increases with the number fledged. This delay, an indication of the physiological strain involved in rearing young, is greater for later broods and for females nesting in trees. Measures of reproductive effort (clutch size, number fledged, length of the nestling period) vary seasonally but give no indication of peaking for last broods. Thus, reproductive effort is not adjusted to parallel changes in the probability of surviving to the next breeding attempt. Received 3 November 1981, accepted 24 April 1982. A COMMON assumption underlying theoretical discussions of life-history phenomena is that trade-offs in life-history characteristics occur (Williams 1966, Charnov and Krebs 1974, Steams 1976, Snell and King 1977). For example, Pianka and Parker (1975) partition the reproductive value of an individual of age X in a stable nongrowing population into fecundity at age X and expected fecundity conditioned by survival to ages X + 1, X + 2, . . ., X + N. Clearly if survivorship is lowered by current reproduction, future fecundity is reduced. Many researchers of life-history phenomena in birds have investigated whether or not the modal clutch size is the most productive, as predicted by Lack (1947, 1966). If productivity is measured as the number of young fledged from a brood, then clutch sizes larger than modal appear to be optimal (Klomp 1970, von Haartman 1971, Jones and Ward 1976, Murphy 1978). If parental survivorship is reduced by the effort of raising a large brood, however, then the optimal clutch size might be lower than the most productive (Fisher 1930, Charnov and Krebs 1974). Studies testing the relationship between brood size and parental survivorship show equivocal results. Bryant (1979) found a reduction in the overwinter survivor1 Present address: Water Resources Branch, Ontario Ministry of the Environment 135 St. Clair Avenue W., Toronto, Ontario M4V 1P5, Canada. ship of double-brooded female House Martins (Delichon urbica), and male Pied Flycatchers (Ficedula hypoleuca) feeding many young were less likely to return to the study area than those feeding fewer young (Askenmo 1979). Lack (1966) and Kluyver (1970) present evidence suggesting an inverse relationship between brood size and parental survivorship. De Steven (1980), however, did not find a reduction in the survivorship of female Tree Swallows (Iridoprocne bicolor) as a result of brood enlargement. Smith (1981), after demonstrating a positive association between reproductive effort and survivorship, concluded that a tradeoff between fecundity and survivorship should not be assumed for passerine birds. The longevity and mobility of birds usually prevents a precise determination of mortality rates for individuals with a known reproductive history. If dispersers are a nonrandom sample of the population (Lowther 1979), then estimates of survivorship based on individuals staying at or returning to the study area will be biased. As a consequence, the effect of reproduction on survivorship is often estimated from physiological evidence. Weight losses for females through the breeding season suggest a physiological strain that may reduce survivorship (Newton 1966, Hussell 1972, Bryant 1979). For female House Sparrows (Passer domesticus), Pinowska (1979) has shown that levels of fat i fluence clutch size, while lean dry weight (an indicator of protein level) may determine the number of broods raised in a season. Norberg 25 The Auk 100: 25-32. January 1983 This content downloaded from 207.46.13.113 on Thu, 06 Oct 2016 04:21:08 UTC All use subject to http://about.jstor.org/terms 26 W. BRUCE MCGILLIVRAY [Auk, Vol. 100 (1981) presented the hypothesis that adult weight losses during breeding may be adaptive. This would be true if the reduced cost of collecting food for the nestlings increases fledgling numbers more than it reduces survivorship and future fecundity. Birds that raise more than one brood in a season may be expected to show differential reproductive effort and productivity through the breeding season. There is a higher probability of surviving from one brood to the next d-uring the breeding season than from the last brood of one season to the following spring. An increase in the level of reproductive effort would be expected as the probability of surviving to the next breeding attempt decreases. There is some evidence that high levels of reproductive effort lower the ability of females to invest in a subsequent brood. For Great Tits (Parus major), Kluyver (1963) found a lesser probability of a second brood if the first was large. Pinkowski (1977) noted a drop in the clutch size of second broods for Eastern Bluebirds (Sialia sialis) that raised a large first brood, and Smith and Roff (1980) found that an increase in the interbrood interval was associated with a large first brood for Song Sparrows (Melospiza melodia). In this study I look at the productivity of multibrooded House Sparrows through the breeding season and address two questions. First, is there variation in the productivity of successive broods within the breeding season associated with brood size in earlier broods (is there a demonstrable within-season cost associated with reproduction)? Second, do adults modify their reproductive effort in accordance with seasonal changes in the probability of renesting and surviving? METHODS AND MATERIALS Study site.-In 1977, the study areas were six farms near Conrich, Alberta, 5 km east of Calgary. For 1978, all data were collected at one of these farms, 8 km east of Calgary. At each of the farms, about 20 nest boxes were erected in fall 1974 (see Murphy 1978 for details). As well as nesting in boxes, the sparrows used buildings, machinery, and trees. At the farm studied both years, nests built in trees were monitored in addition to those in nest boxes. Data collection.-Nests were inspected at 3-5 day intervals from 2 May to 15 August 1977 and from 20 April to 13 August 1978. If weather conditions permitted, a 3-day interval was maintained. Eggs were counted and numbered. Nestlings 5-6 days old were banded with a U.S. Fish and Wildlife Service aluminum leg band and color marked with leg bands after reaching a 20-g weight. Data analysis.-Because the nests were not checked every day, exact clutch-initiation data and nestling ages were not known for all nests. For these cases, estimates were used. Nestling age when first found was estimated by comparing the weight of the largest nestling in a brood to average values obtained from broods of known age. Clutch-initiation date was estimated by assuming an average incubation period of 11 days and a laying rate of one egg per day (McGillivray 1978). This estimate was needed only for clutches begun before my arrival at the study area. In this study I investigated the productivity from multiple broods of a pair; hence, I made the general assumption that broods raised at a single nest through the breeding season were raised by the same pair. House Sparrows usually remain at the same site unless one of the pair dies (Summers-Smith 1963). To reduce the importance of this potential source of error, I imposed some residency conditions. If a tree nest was extensively modified following a nesting attempt, I assumed that a new pair was nesting. The average interval between successive clutch initiations for first through third clutches for each brood size was determined. Ninety five percent prediction intervals were calculated for each clutch-interval, brood-size combination. Nests where the interclutch interval fell outside these limits were excluded from further analyses. The interval between the departure of the last nestling and the initiation of the next clutch is called the interbrood interval. The date of fledgling departure was considered to be the midpoint of the interval between the last observation of the nestlings and the first nest check after departure. Fledging is a gradual process, so a departure date is only an approximate measure. Paired comparison t-tests were used to compare the mean interbrood interval of successive broods and different nest types. For the investigation of the effects of one brood on the next breeding attempt, data for the two years were pooled. Pooling requires trends to be consistent over the two years in order for the trends to be detected, and productivity for both years was similar except for the first broods. The strength of trends associated with brood size was measured by Pearson's product-moment correlation coefficient, r. Although significance levels were determined for r, paired variables were not tested for bivariate normality. Nonetheless, r is a useful indicator of trend. This analysis assumes that fledglings of equal weight contribute equally to parental fitness. This assumption may not be valid for a species with a long breeding season. Dyer et al. (1977) published the following survivorship equation for juvenile House Sparrows (from Summers-Smith 1963); Y = This content downloaded from 207.46.13.113 on Thu, 06 Oct 2016 04:21:08 UTC All use subject to http://about.jstor.org/terms January 1983] House Sparrow Reproductive Costs 27