Experimental Brood Size Research Articles

IS REPRODUCTION BY TREE SWALLOWS COST FREE?

Abstract We manipulated brood sizes of Tree Swallows (Tachycineta bicolor) in 1996 and 1997 to test for the existence of intra- and intergenerational costs of reproduction. Modal clutch size was six eggs, but experimental brood sizes ranged from two to nine young. Nestling starvation was higher in 1996 (and dependent on brood size) than in 1997, but in both years enlargement of brood size resulted in increased productivity. Nestling mass near fledging was negatively correlated with brood size, but tarsus length and wing chord were not. Food deliveries by parents increased steadily between broods of two to six young but then remained constant between broods of six to nine young. The loss of female mass between incubation and the end of the nestling period was positively related to the pair's total feeding effort, and female mass near fledging declined with increasing brood size. The latter decline disappeared, however, when broods of nine were omitted. Adult return rate (1996 to 1997) was highest among bir...

Haematozoan Infections in the Eurasian kestrel: Effects of Fluctuating Food Supply and Experimental Manipulation of Parental Effort

The influence of parental effort on susceptibility to parasitism was investigated experimentally in the Eurasian kestrel (Falco tinnunculus) in Finland. Parental effort was manipulated by either enlarging or reducing broods by 1-2 young, while unmanipulated broods served as controls. This was done during 3 breeding seasons, during which the densities of the major prey of kestrels (voles) were relatively low, high and intermediate, respectively. Two, taxon specific, methods were applied for determining the prevalence (per cent individuals infected) of extracellular Trypanosoma spp. and intracellular Haemoproteus spp. infection. Blood samples were taken from females during the incubation phase, and from both parents during the mid-nestling phase. Trypanosoma was more prevalent when food was less available. Furthermore, the increase in the prevalence of Trypanosoma among females was most pronounced during the poor vole year. In contrast, Haemoproteus infection seemed not closely related to annual supply of main prey. Manipulations of parental effort were related to parental infection, but the effect differed between genders. The prevalence of Trypanosoma in males (main provider of young) increased with experimental brood size, and there was an interaction between food supply and brood size manipulation so that the difference in prevalences between reduced and enlarged broods increased with decreasing food supply. Among females, no such an interaction was found. Instead, the effect of brood enlargement on Trypanosoma prevalence of females was apparent in the pear of relatively high vole densities only. Manipulation of brood size did not have clear effects on Haemoproteus infection in either gender. These results support the idea that increased parental effort may make hosts susceptible to haematozoan infection, and are the first to suggest that resource levels (food supply) can modify the susceptibility, indicating that the magnitude of reproductive costs can be attributed to Variation in environmental conditions.

Post-juvenile body moult in the Blue Tit Parus caeruleus: influence of age and nestling history

The influence of sex, age and nestling history (viz. brood size, hatching date and nestling condition) on the progress of post-juvenile body moult in the Blue Tit Parus caeruleus was investigated. The stage of body moult advanced with increasing age in both sexes, but was independent of hatching date, experimental brood size and nestling condition. The estimated age at the onset of post-juvenile moult was 23–46 days and the average estimated age at moult completion was 113 days. After controlling for age differences, females were on average more advanced in moult than males, although the magnitude of this sex effect differed in different study years. The results suggest that in juvenile Blue Tits, an individual's age and sex are more important determinants of the progress of the post-juvenile moult than nestling condition, brood size or hatching date.

Quality vs. quantity: energetic and nutritional trade‐offs in parental provisioning strategies

1. When faced with increased brood demand, parent birds provisioning young in the nest can make a variety of adjustments to their foraging and food allocation strategies. Logical extensions of classic optimal foraging theory predict increased provisioning effort to larger broods to be accompanied by changes in load size, foraging distance from the nest, as well as possible changes in the type and size of prey delivered. 2. We assessed such behavioural adjustments and their consequences in pairs of European starlings (Sturnus vulgaris) responding to a range of experimental brood sizes. Parents feeding larger broods increased their visit rates by spending less time in the nestbox and less time around the nestbox colony. High visit rates to larger broods were also associated with larger loads per visit and changes in the type of prey delivered to the nest. As a consequence, chicks in large and small broods received similar rates of food intake, but experienced differences in the nutritional quality of their food. Parents feeding larger brood sizes were able to increase their provisioning effort despite feeding in the same foraging sites, travelling at comparable flight speeds and maintaining similar body masses to parents feeding smaller broods. 3. Parental energetic expenditure, measured through doubly labelled water analyses, showed no effect of the brood size treatment. The greater proportion of indigestible material per gram of food delivered to the larger experimental brood sizes (i.e. soil from the guts of earthworms) was probably responsible for the fact that these chicks grew at slower rates and fledged at lower body masses, although we cannot rule out the possibility of lower growth rates due to higher energetic costs of sibling competition within larger broods. Lighter fledglings from large broods disappeared from the local area earlier in the summer, probably as a result of differential mortality rather than premature natal dispersal. 4. We discuss the adaptive significance of the provisioning trade‐off between quantity and quality of food items delivered by parents to the nest, with reference to natural variation in foraging conditions and brood demand.

ON THE COST OF REPRODUCTION IN LONG-LIVED BIRDS: THE INFLUENCE OF ENVIRONMENTAL VARIABILITY

Life history theory suggests that animals should balance their current investment in young against their chances to reproduce in the future. One fundamental prediction from the theory is that long-lived species should be restrictive in any increase of their current investment. It has been suggested that long-lived species, therefore, have evolved a fixed level of investment in young in order to maximize their own adult survival. However, recent experimental studies have shown that long-lived seabirds have a flexible reproductive performance and adjust their effort in raising young, both according to their own body condition and to the need of the chicks. In this study, we present a model of the optimal balance between reproductive effort and adult survival for long-lived birds breeding in a stochastic environment. During poor breeding conditions, maximum fitness is achieved either by not breeding at all, or by abandoning the brood. Beyond a certain threshold in breeding conditions, there is a steep increase in reproductive effort and an equally steep decrease in adult survival. The model is applied to two hypothetical long-lived seabirds differing in their potential fecundity. For the genotype with a potentially high fecundity, the model predicts a high threshold for breeding (i.e., breeding conditions need to be very good for the species to attempt breeding); above the threshold, the value of reproduction in terms of fitness is high. For the genotype with potentially low fecundity, the model predicts a low threshold for breeding, and the value of reproduction in terms of fitness is low. By increasing clutch size in the model, we examine the optimal response of the two genotypes to an experimental brood size manipulation. For both genotypes, the model predicts that the threshold for breeding is lower among controls than among enlarged broods, giving a range of possible outcomes of the experiment depending on breeding conditions. The few studies on brood enlargements in long-lived species carried out so far may support the predictions from the model.

Nestling cuckoos, Cuculus canorus, exploit hosts with begging calls that mimic a brood

Nestling cuckoos, Cuculus canorus , eject host eggs or young from the nest and are then raised alone by the hosts. Using reed warblers, Acrocephalus scirpaceus , as hosts, we investigated how the single cuckoo chick can command the same provisioning rate as a whole brood of host young. Large size alone is not sufficient to stimulate adequate provisioning because single blackbird, Turdus merula , or song thrush, T. philomelos , chicks of the same mass as a cuckoo were fed at a lower rate. Our experiments show that the key stimulus is the cuckoo chick9s rapid begging call (‘si, si, si, si ...’), which sounds remarkably like a whole brood of host chicks, and which it matched in calling rate. When single blackbird or song thrush chicks were accompanied by loudspeakers that broadcast either cuckoo begging calls or calls of a brood of reed warblers, the hosts increased their provisioning rate to that for a cuckoo chick. We suggest that the cuckoo needs vocal trickery to stimulate adequate care to compensate for the fact that it presents a visual stimulus of just one gape.

Costs of Reproduction in the Zebra Finch Taeniopygia guttata: Manipulation of Brood Size in the Laboratory

Brood size of Zebra Finches Taeniopygia guttata was manipulated in an attempt to identify a trade-off between current and subsequent reproduction in a laboratory situation with ad libitum food availability. The birds were able to raise a larger brood than the most frequent brood size under the same conditions. Initiation of the subsequent clutch was advanced after raising a small brood, and delayed after raising a large brood. The size of the subsequent clutch was not affected by the previous, experimental brood size. Thus, a cost of reproduction was observed in modification of the reproductive interval. Brood size also affected the prospects of the current, experimental brood. Both nestling survival and nestling weight at independence decreased with brood size. It has often been suggested that food availability limits reproduction in the field. This study shows that under unrestricted access to food, other factors restrain reproduction. Time allocation, energy expenditure and nutrient reserves of the parents are discussed as potential alternative constraints.

Family Planning in the Kestrel (Falco Tinnunculus): the Ultimate Control of Covariation of Laying Date and Clutch Size

Abstract The theory that individual birds maximize their fitness by the two major decisions in reproduction concerning date (when to start laying eggs) and clutch size (when to stop laying eggs) is empirically approached in the Kestrel by quantifying Fisher's Reproductive Value for both the clutch (Vc = c. Vo/2) and the parents (Vp). The reproductive value of an egg (Vo) was found to decrease monotonically with laying date (d) due to significant associations with d of the components So (probability for an egg to survive till fledging), S1 (probability to survive from fledging till age 1), S2 (survival age 1 till age 2), and P1 (probability of breeding at age 1). Vp declined negligibly with laying date, although there were significant associations between d (laying date) and N (probability of the nest to produce at least one fledging), Pr (probability of a repeat clutch following nest failure), and La (probability of local survival of the parents following breeding). In experiments where brood size at day 10 after hatching was increased or reduced, Vc increasing experimental brood size, while Vp simultaneously decreased. Total reproductive value (V = Vc + Vp) remained unaffected by the experiments. This result suggests that a rather broad range of clutch sizes maximizes total reproductive value, as far as detectable by the data. While the yield of kestrel hunting, and hence the number of young raisable with constant parental effort (and constant Vp), increased with the spring increase in vole population density, reproductive value of the clutches decreased. For any particular food situation (hunting yield) this leads to a unique combination of clutch size and laying date maximizing V. This could be worked out by calculating fitness contours for all combinations and for different yields (Fig. 12). The optimal solutions are on a declining slope, with smaller clutches associated with later dates. 59.4% of all clutches observed obeyed the maximization criteria. Furthermore, there was a reasonable, unbiased association between predicted laying dates and clutch sizes based on individual male hunting yields and observed dates and clutches as laid by the females. Qualitatively, any method predicts a seasonal decrease in the optimal clutch size when the environment improves while reproductive value declines with progressive date. Preliminary results from an experimental approach to test the assumption of a causal effect of date on Vo, using the release of juvenile kestrels reared in captivity under artificial light schedules, are presented.

THE CONSEQUENCES OF BROOD SIZE FOR BREEDING BLUE TITS. III. MEASURING THE COST OF REPRODUCTION: SURVIVAL, FUTURE FECUNDITY, AND DIFFERENTIAL DISPERSAL.

To determine how the cost of reproduction varies with brood size, a population of blue tits (Parus caeruleus) breeding in Wytham Wood, England, was manipulated over a three year period. Two hundred sixteen pairs were randomly assigned 3, 6, 9, 12, or 15 nestlings; nestlings were exchanged soon after hatching. Survival of adult females (as measured by the proportion recaptured in the following winter and/or spring) declined significantly with increasing brood size in two out of three years; there was significant year-to-year variation in the relationship of recapture rate to brood size. Mean female recapture rates (averaged over the three years) declined in a linear fashion (P < 0.01). There was no significant linear or curvilinear relationship between male-recapture rate and brood size in any of the three years nor was there a significant linear or curvilinear relationship for the data averaged over the three years. Nevertheless, recapture proportions for males differed significantly with respect to brood size (χ2 test, P < 0.05). The possibility that experimental brood size influences subsequent dispersal (and therefore biases measures of survival based on recapture rates to differing degrees) was examined by comparing distances moved by breeding adults from one year to the next. There was no relationship between brood size and dispersal distance within the study area for either sex, except that females given broods of three were significantly more likely to move more than 300 m than were those given broods of 6-15 young. Both males and females showed evidence of a cost with respect to future fecundity: as brood size increased, the number of surviving offspring produced in the following year decreased from 1.5-1.6 (for adults that had reared 3-6 young) to 0.4 (for those that had reared 15 young). The relationship of future reproductive success to experimental brood size did not differ among years or between the sexes. The number of eggs laid and number of young hatched in year n + 1 did not differ significantly with respect to brood size in year n; rather, differences in future fecundity reflected differences in the survival prospects of young reared in year n + 1.

Effects of Brood-Size Manipulations on Parental Investment, Breeding Success, and Reproductive Endocrinology of House Sparrows

-Brood sizes of House Sparrows (Passer domesticus) were altered experimentally by adding or subtracting 2 nestlings. Unaltered broods served as controls, and experimental brood sizes were within the normal range found under natural conditions. Feeding rates of both parents increased with brood size, and although nestling mass decreased with brood size, most pairs were able to fledge the extra young added to their broods. Males rearing larger broods invested less in nest-site defense and mate-guarding activities, and females rearing larger broods took longer to initiate subsequent broods and produced smaller subsequent clutches. However, the productivity of the subsequent broods did not decrease. Adult survivorship was not affected by the brood manipulations. In males, circulating levels of dihydrotestosterone increased significantly with brood size. Levels of other hormones, including luteinizing hormone (LH), testosterone, estradiol-17#, and corticosterone (B), were not related to brood size in either sex, although in females LH and B titers tended to increase with brood size. Males feeding larger broods tended to have less body fat, but otherwise there was no relationship between brood size and body condition. These results suggest that adults tending larger broods were not unduly stressed by their extra efforts, at least when feeding nestlings. However, the increased interbrood interval and decreased subsequent clutch size associated with rearing larger broods may have resulted either from the increased energetic and nutrient demand on females after the young fledged or simply from the extra time required to rear the additional fledglings to independence. Received 10 July 1986, accepted 1 March 1987. IMPLICIT in many discussions of life-history phenomena is the assumption that organisms trade off two or more conflicting activities in a manner that maximizes their overall reproductive output (e.g. Williams 1966, Charnov and Krebs 1974). For example, most iteroparous organisms are assumed to forgo maximum annual productivity to increase subsequent survivorship, thereby maximizing lifetime fecundity (e.g. Kluyver 1963, Williams 1966, Charnov and Krebs 1974). Moreover, even within a given breeding season, other trade-offs may occur (e.g. McGillivray 1983). For males, Trivers (1972) proposed a fundamental trade-off between competing with other males to fertilize the ova of females and providing parental care to the offspring they produce. Ecological restrictions on these two options have been used to explain mating systems (e.g. Emlen and Oring 1977). We have suggested that endocrine and associ1 Present address: Department of Zoology, NJ-15, University of Washington, Seattle, Washington 98195 USA. ated behavioral changes of monogamous male House Sparrows (Passer domesticus) during the breeding season reflect a compromise between these two options (Hegner and Wingfield 1986a). In our study area, a given pair of House Sparrows may attempt from 3 to 5 broods within a nesting season. In adults, circulating levels of reproductive hormones vary considerably during different stages of nesting. Plasma concentrations of luteinizing hormone (LH), androgens, and estrogens are maximum during the egg-laying stage of each brood, decline rapidly during incubation, remain low after the young hatch, and rise again as the young approach fledging. In females, rising levels of LH and estradiol (E2) following fledging are indications of physiological preparation for laying the next clutch (Hegner and Wingfield 1986b). In males, levels of LH and testosterone rise rapidly to maximum concentrations when nestlings reach 9-10 days of age, well before the young fledge (typically when 14-15 days old). This reflects a transition between a state of high investment in parental care, when testosterone levels are 470 The Auk 104: 470-480. July 1987 This content downloaded from 157.55.39.4 on Fri, 09 Sep 2016 04:28:40 UTC All use subject to http://about.jstor.org/terms Julv 19871 Brood Manipulations in House Sparrows 471 low, to one of high investment in male-male competition, when testosterone levels are high (Hegner and Wingfield 1986a). We examined the physiological and reproductive effects of altering the demand for parental care on free-living House Sparrows. For some pairs we increased the demand for parental care by adding two young to the nest, for others we decreased that demand by removing two young from the nest, and for a third group the number of nestlings was unchanged. We subsequently determined the effects of these manipulations on investment in parental care, investment by males in male-male competition, circulating levels of reproductive hormones, body condition, breeding success in the experimental and subsequent broods, and adult survival to the next year.