Reproductive success and survival of swallows (Hirundo rustica) : effects of age and body condition

Abstract

This thesis presents the results of a study on the Swallow Hirundo rustica carried out in Central Scotland between April 1986 and August 1989. Their social behaviour and life history are described. Adults were found to be markedly faithful to both their mate and site. Notable differences between results observed here and with other similar studies were the apparent lack of any sexually selected infanticide or intra-specific nest parasitism. These results were attributed to differences in colony size. Intra- and inter-sexual variation in adult body size was measured and the presence of any age-related trends identified. Older birds had significantly bigger wing and outertail lengths but skeletal measures and inner tail-length did not vary in size. Swallows were found to be sexually size dimorphic for several parameters and these findings were discussed in relation to three hypotheses. Variation in reproductive performance between years and individuals was described. Clutch size and number of young fledged was lower for second- than first-broods but even after controlling for this, breeding performance still declined seasonally. Possible mechanisms associated with this common finding were explored. The number of broods attempted in a season made an important contribution to seasonal reproductive performance. Double brooded Swallows: (i) bred earlier, (ii) were older and (iii) were more successful during their first brood (cf. single-brooded). Since any measure of seasonal performance is likely to be an incomplete measure of fitness, attention was also given to understanding what factors affected adult and juvenile survival. Offspring which hatched earliest and from first broods were most likely to be recruited. There was no evidence to support a positive association between fecundity and parental survival in Swallows studied here, however. The role of individual characteristics in shaping reproductive performance was examined. Body size was only weakly associated, whereas parental age was strongly correlated with breeding success; yearlings laid later, had smaller clutch sizes and fledged fewer young during a season. Although females which were monitored over two successive seasons laid earlier in their second season they did not differ significantly for any other parameter compared. Data from other studies were reviewed and possible hypotheses to explain age-related trends were considered. It was concluded that the improved performance of older Swallows was related at least in part to individual differences and selective mortality. In an attempt to manipulate reproductive effort brood sizes were experimentally altered by adding (Enlarged) or removing (Reduced) one, two or three nestlings shortly after hatch. Un-manipulated broods served as Controls. The size of the first brood reared affected the probability that a second clutch would be laid as well as the timing (IBI) and, (iii) success (but not size) of the second clutch. The effect of manipulation on the IBI and occurrence of second brood was asymmetrical. Temporal variation, however, could not explain differences in future fecundity between first brood treatment categories. Early desertion in relation to clutch or brood reduction was discussed in relation to the "Concorde Fallacy". Although most pairs were able to rear additional young, nestling quality was adversely affected. Juvenile survival was related to brood size such that parents which reared Control broods were most likely to produce recruits. Manipulation of brood size also had an effect on adult survival but the effect differed between sexes and broods. The clearest and most significant result was that Swallows which reared Reduced broods (first or second) were more likely to survive (cf. Control or Enlarged broods). These findings were not attributed to differential dispersal of adults. A review of the literature indicated that this was the first study to publish results on the possible effects of manipulation of second broods for parental survival. The pattern of adult body mass during the nesting cycle was described. Males and females reached a minimum mass when the nestlings were aged between Days 9and 16 (NP II) and Days 17-23 (NP III) respectively. Only during these two stages were males heavier than females. Possible implications associated with a decrease in mass while feeding nestlings (cf. incubation) were discussed. Analyses of a sample of adult carcasses enabled body condition to be determined precisely. Quantitative methods of assessing the condition of live birds in the field were developed and validated against carcass analysis results. Muscle thickness as measured by a portable ultra-sound device and body mass were both considered to give reliable estimates of condition. A number of predictions following from the assumption that parental condition was related to current and future fecundity or overwinter survival were tested. There appeared to be no significant relationship between condition while feeding first brood nestlings and, a) the IBI, or b) occurrence of second broods. This applied to parents rearing both natural and experimental broods. There was some evidence to suggest that the condition of parents after the brood had fledged might be of greater importance. More data are necessary to confirm this finding, however. Female condition at any stage in the nesting cycle (lst or 2nd brood) was not related to overwinter survival. Data for males, however, supported the prediction such that birds in poorer condition during NP II were less likely to survive. Possible reasons for differences between the sexes were explored. One suggestion proposed was that females were better able to regulate their effort to maximise fitness and and so males were possibly "victims" of their partners variability. The possibility that single- and double-brooded species may vary in their allocation of resources was considered and there was some evidence to support this suggestion for Swallows observed here.