Initially, the impact of increasing carer number on breeding success was assessed experimentally in cooperative breeders by measuring the consequences of removing nonbreeding helpers. In the first such study, Brown et al. This effect was driven by the effect of helpers on the number of further breeding attempts in a season, but whether or not carers also improved nesting success through provisioning was not clarified.
Indeed, only 3 previous studies have collected provisioning data during removal experiments. In this case, the reduced success following helper removal was mediated through a combination of reduced food provisioning and protection from nest predators. Using short-term 3—8h removals of helpers in cooperative groups of long-tailed tits Aegithalos caudatus , Hatchwell and Russell showed that helpers primarily allow breeders to reduce their provisioning rate rather than provide additional sustenance to the brood but see Hatchwell et al.
Finally, in chestnut-crowned babblers Pomatostomus ruficeps , we have shown that the provisioning rates of male carers are insensitive to temporary carer removals, and so the contributions of such carers to broods are fully additive Liebl et al. Although these studies suggest that helpers are beneficial, removal experiments have been criticized as a means to quantifying causal impacts of helpers Cockburn To avoid the potentially confounding impacts of removals, temporary manipulation of offspring numbers has been advocated as a more satisfactory test of the benefits of additional carers in cooperative breeders.
By temporarily translocating meerkat Suricata suricatta pups from groups, Clutton-Brock et al. Although not measured in the experiment, this effect presumably arose because each pup receives more food with increasing carer:pups ratios Clutton-Brock et al. Reciprocal brood size manipulations in superb fairy-wrens Malurus cyaneus , demonstrated that the rate at which nestlings received food was a function of carer:nestling ratios rather than group size per se Russell et al. Finally, in acorn woodpeckers Melanerpes formicivorus , experimentally enlarged broods were fed significantly more frequently than reduced broods, although feeding rates per nestling were greater in the latter Koenig and Walters These studies suggest that helpers are responsive to offspring demand and by extension presumably have a causal impact on offspring success.
Although the 2 experimental techniques outlined above are both used to test the causal impacts of helpers on brood and group success, whether or not the results derived from each are equivalent has not been tested. Removal experiments obviously offer a more direct means of quantifying the effects of carer numbers on offspring provisioning, but only if removing carers does not introduce confounding influences on provisioning Cockburn By contrast, although manipulations of brood size are suggested to circumvent possible issues arising from the removal of carers, they cannot clarify any potential confounding influences of territory quality and might introduce their own confounding influences if the begging signals to which carers respond to are determined by scramble competition in addition to immediate offspring nutritional requirements Royle et al.
In short, determining the influence of additional carers in cooperative breeding systems might require a multipronged experimental approach in which the potential confounding consequences of the multiple methods used are also tested. Here, we use both temporary carer removal and brood size enhancement experiments in the cooperatively breeding chestnut-crowned babbler to quantify the causal impact of carer number on offspring provisioning rates. First, we use carer removal experiments to test for causal effects of carer numbers on nestling provisioning rates and brood size enhancement experiments to test whether carers are responsive to changes in brood demand.
Second, we compare whether the 2 methods generate equivalent relationships between carer:offspring ratios and the average provisioning rate of each nestling. This comparison is facilitated by our equalizing of carer:nestling ratios in the 2 experiments. Third, we elucidate the mechanism underlying the differences found between the 2 experimental techniques; in particular, whether or not disruptive effects of carer removals or increased scramble competition induced by brood size enhancements are most likely to drive the differences.
Details of the arid zone climate and babbler socioecology are provided elsewhere Portelli et al. Briefly, chestnut-crowned babblers are a g endemic bird of inland regions of south-eastern Australia. Increased carer numbers are associated with reduced nestling starvation and increased probability of being multibrooded Russell et al. The principal provisioners are male fathers and nonbreeders related to the brood by at least second order ; breeding females contribute significantly less to provisioning overall and reduce their contributions as the number of carers increase Browning, Young, et al.
Prey items are always delivered singly, and provisioning rates explain the majority of estimates of biomass delivered to the brood Browning, Young, et al. All experiments were conducted within natural levels of variation Table 1. Summary of brood sizes, male carer numbers, carer:nestling ratios, and observation times for control and experimental days during the 2 experiments.
Carer:nestling ratios include the breeding female, so do not reflect male carers divided by brood sizes. There was no difference on control days in nests used for carer number versus brood size manipulations in terms of brood sizes, numbers of male carers, or carer:offspring ratios. Additionally, carer:offspring ratios were comparable following carer removal and brood size enhancements.
Statistical comparisons are presented in Methods. Groups were captured in the morning and given the rest of the day to acclimate to their new group size; observations of provisioning behavior were conducted on the subsequent day details below. The breeding status of male carers was not known and polyandry is relatively common Nomano et al. Our inability to distinguish between such males is unlikely to influence our results because all those removed were known provisioners, and breeding and nonbreeding provisioners have equivalent provisioning rates across the range of unit sizes observed Browning, Young, et al.
For full details of numbers of male carers, brood sizes, and carer:offspring ratios before and after manipulation, see Table 1. All birds were released before sunset on the day following capture. Following release, birds are accepted back into their groups without sign of retribution Nomano et al. Because carer removals obviously generate reduced carer:offspring ratios, to facilitate comparison between the 2 methods, we used brood enlargement experiments to equalize the manipulated carer:offspring ratios across the 2 treatments Table 1.
Where possible, and typically, manipulations were conducted the day before observation so that, as with the helper removals, groups generally had several hours to acclimate to their new brood size before provisioning rates were monitored. The numbers of offspring added depended on initial brood size in both original and foster nests, so that enlarged broods never exceeded the maximum of 6 nestlings naturally found in this species and no nest was left without nestlings i. Nestlings were transported between nests by vehicle in a bird bag above hot water bottles when necessary ; chicks were never out of a nest for more than 45min usually ca.
All nestlings were returned to their original nests following the experiment, facilitated by banding all chicks prior to translocation. Provisioning rates were determined remotely using a passive integrated transponder PIT -tag system Browning, Young, et al. When PIT-tagged individuals pass through a camouflaged copper coil fitted to the entrance of their dome-shaped nests, the identity, along with date and time of all nest visits are recorded to an attached Trovan LID decoder.
For a number of reasons, the provisioning rate of the breeding female cannot be determined using PIT-tags alone Nomano et al. Excluding the provisioning rate of the breeding female from our analyses will not have a significant bearing on the results because evidence from nest cameras indicates that she contributes little overall to provisioning and carers are unresponsive to her provisioning levels Browning, Young, et al. This drawback aside, the key advantage of the PIT-tag system is that large amounts of provisioning data can be collected over long periods of time, which is particularly important in species such as babblers that, overall, have relatively low provisioning rates ca.
Provisioning rates were obtained for both a control and experimental day in each experiment. For groups involved in the removal experiment, control data were collected either the day before removals i. Thus, units were provided a day to recover from capture and acclimate to their new unit size before data collection.
For those involved in the brood size manipulations, control-day observations were conducted either the day before the addition of nestlings i.
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Furthermore, as control data were collected either immediately before or after experimental data, overall, broods were similarly aged on control versus experimental days. To generate a common currency between the 2 treatments, provisioning data were transformed to per capita provisioning rates i. Including observation time as a covariate had no impact in any model and so was not included in the analyses. We have found no evidence in previous studies to suggest that provisioning contributions by male carers are influenced by mechanisms of pay-to-stay Nomano et al.
As a consequence, we hypothesized brood begging intensity may be a significant mediator of carer provisioning rates. Furthermore, as chestnut-crowned babblers breed in dark, domed-nests and nestlings show no obvious color displays when begging, it is likely that acoustic signals represent the primary means of conveying hunger.
As such, we used measures of brood begging intensity to assess 2 key questions. First, we verified that begging intensity is sensitive to short-term variation in levels of hunger, and hence would be affected by the experiments conducted. To do so, we investigated the effects of per capita nestling provisioning rates over the previous hour and time since last feed on begging intensity.
Second, we then tested whether brood size or carer number additionally explained variation in begging intensity after controlling for these metrics of hunger. Begging behavior can be confounded by maternal effects Groothuis and Schwabl , clouding interpretation of any relationships among hunger, carer number, and brood size on begging intensity. In an attempt to break any links between prehatching maternal effects and posthatching begging patterns, we used split-design cross-fostering at hatching day 0 to generate broods comprising of 2—6 nestlings genetically deriving from different mothers.
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Only nests without prior nestling mortality were used to avoid including the begging intensity of starving nestlings. Using internal nest cameras with audio recorders Browning, Young, et al. First, we used linear regression analyses to investigate the changes in overall brood provisioning rates between control and experimental days as a function of 1 the number of male carers removed and 2 the number of nestlings added.
When used with limited numbers of values in independent terms i. To determine the latter, we subsequently conducted permutation tests in which data were shuffled randomly among the independent values i. Doing so times allowed us to generate a null distribution of regression slopes against which the probability that our observed slope could arise by chance could be compared.
To test for a confounding effect of territory quality on responses following carer removal, we used a multiple regression analysis. In this case, natural logarithmic ln transformed per capita nestling provisioning rates on experimental days was fitted as the response term and control versus experimental carer numbers were fitted as explanatory variables. If nestling provisioning rates are determined by an autocorrelation between territory quality and numbers of male carers, we would expect a significant impact of control carer numbers in this analysis.
Second, to investigate whether the 2 experimental methods yielded equivalent results, we fitted ln-transformed per capita nestling provisioning rates on experimental days as the response term in a GLM, with experimental method helper removal vs. Third, to investigate whether differences between the experiments were driven by the carer removal or the brood enhancements, we compared the slopes of the relationships between carer:nestling ratios and per capita nestling provisioning rates on experimental versus control days in each experiment.
In this case, we conducted 2 residual maximum likelihood models REML one for each experiment , in which ln-transformed per capita nestling provisioning rates were fitted as response terms, treatment control vs.
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Finally, because we only found evidence to suggest that the brood size manipulation influenced per capita nestling provisioning rates, we investigated the effects of brood size on brood begging intensity. Here, we fitted whether or not carers encountered a maximally begging brood as a binary response term with logit link function in a generalized linear mixed model GLMM. The rate at which broods received food over the previous hour and the time between the previous and current feed were fitted as covariates to control for brood hunger levels, while carer number and brood size were fitted as fixed effects.
Breeding unit identity was fitted as a random term. On unmanipulated control days, broods received an average of As a consequence, when carers were removed, broods received an average of 8. Effects of manipulations on changes in brood provisioning rates. On experimental days, the temporary removal of 1—3 male carers a was associated with linear declines in brood provisioning rates, whereas the temporary addition of 1—3 nestlings b was associated with linear increases in brood provisioning rates.
Figures show raw data and best-fit functions. Groups used in the brood size manipulations provisioned an average of As a consequence, on experimental days, broods received an average of Surprisingly, the magnitude of this effect led each nestling to receive an extra 0. To elucidate whether territory quality contributed significantly to the apparently causal effects of carer numbers on brood provisioning rates, we investigated the relative impacts of control versus experimental numbers of male carers on per capita rates of nestling food acquisition on manipulation days.
These results suggest that any effect of territory quality on the number of male carers does not confound estimates of carer contributions in this system, corroborating a causal positive relationship between male carer number and rates of offspring food acquisition.
Despite equalizing carer:nestling ratios in the 2 experiments, equivalent relationships between carer:nestling ratios and per capita nestling provisioning rates were not observed. Put another way, each nestling received prey significantly more frequently with a carer:nestling ratio of that was generated by enlarging brood size e.
Comparing the outcome of the 2 approaches. For a given carer:nestling ratio, per capita nestling provisioning rates were higher following brood enhancement upper line than carer removal lower line. Figure shows ln-transformed raw data and best-fit logarithmic functions. The difference in provisioning rates arising from the 2 experiments could derive from disruptive effects of carer removals.
If carer removals were disruptive we would predict that 1 after controlling for variation in carer:nestling ratios, per capita nestling provisioning rates would be significantly reduced on removal days compared with control days and 2 the relationship between carer:nestling ratios and per capita nestling provisioning rates would be significantly flatter on manipulation days than control days. Neither was true Figure 3a. These findings suggest that carer removals do not confound carer effects in this system and so are unlikely to explain differences in the results obtained using the 2 experimental techniques.
Explaining the differences. This is supportive of scramble competition and suggests that carers will more commonly encounter maximal begging nestlings following brood size enhancement. Figures a and b show raw data and best-fit logarithmic functions, where c shows raw means for each brood and best-fit linear function. An alternative possibility is that differences are generated from the brood size manipulation experiment. In support, significant differences were found between per capita nestling provisioning rates on control versus experimental days in the brood size manipulation.
A likely mechanism for the relative increases in provisioning rates on experimental days of the brood size manipulation is scramble competition Royle et al.
These results verify that short-term changes in hunger even within an hour are sufficient to generate increased begging. These results support the hypothesis that begging intensity is influenced by competition among nest mates Royle et al. Our key aim was to test the effects of additional carers on a salient component of success in chestnut-crowned babblers. Provisioning rate is a key component of success in chestnut-crowned babblers: The rate of food acquisition is the primary determinant of prey biomass received by offspring Browning, Young, et al.
Using 2 independent experimental methods, we provide strong supporting evidence that having additional carers has a direct positive effect on offspring provisioning rates in a cooperative breeder. More specifically, the removal of carers caused reductions in rates of nestling food acquisition that could not easily be explained by territory quality or any potential disruptive effects of the experimental manipulation e.
Surprisingly, however, the 2 techniques did not generate quantitatively equivalent results: Each nestling was provisioned more often following brood size enhancement than following male carer removal, despite equal carer:nestling ratios in each. Finally, nestling begging intensity appears to be influenced by both hunger and brood size, suggesting that scramble competition might mediate the greater provisioning rates following brood enhancement.
Arguably the most significant problem with demonstrating and quantifying the causal effects of additional carers on parameters of success in cooperative breeders is variation in territory quality. This is because high-quality habitats can independently have both high breeding success and many carers Dickinson and Hatchwell Manipulations of brood size Clutton-Brock et al.
By contrast, carer removal experiments can be more illuminating in this regard because the confounding magnitude of territory quality versus the true magnitude of the carer effect will be borne in the effect sizes of the control versus manipulated numbers of carers on the response term. To illustrate, at the extremes, a sole effect of control carer number on the brood provisioning rates during experimental days would be supportive of a confounding effect of territory quality, whereas a sole effect of experimental carer numbers would be indicative of a causal carer effect.
We found support for the latter. This makes sense in chestnut-crowned babblers due to their plural breeding system Portelli et al. Despite the illustrated benefits, removing carers from groups might induce spurious changes to provisioning if, following removal, remaining carers have reduced foraging efficiency or partake in renewed disputes of dominance Cockburn We suggest that such an effect would flatten the relationship between carer:offspring ratio on experimental relative to control days. We found no evidence of this.
In addition, we found no compelling evidence to suggest that reduced provisioning following carer removal arose as a consequence of other potential confounds e. We found here that nestling begging intensity is sensitive to per capita nestling provisioning rates over the previous hour as well as time since last feed.
This sensitivity, combined with the fact that broods were subject to fewer carers for a day before observation, makes it highly unlikely that begging intensity did not rise following carer removal. Furthermore, it is unlikely that remaining carers compensated by delivering larger prey items, as there is no evidence for a relationship between carer number and the size of the single prey item delivered Browning, Young, et al.
Finally, we do not expect the breeding female would compensate for the provisioning care lost by removed individuals as she responds insufficiently to variation in helper number Browning, Young, et al. Thus, we found no obvious evidence to suggest that removing carers introduces significant confounds to the aims addressed. Although we suggest that manipulating carer number provides significant insight into causality and magnitude of the effect carers have on offspring, the results of the brood size manipulation were also illuminating.
Most notably, nestlings received prey at a significantly elevated rate following brood enlargement relative to their carer:nestling ratio. One possibility is that these differences are artificial and do not translate into differences in biomass delivered i.
He did not become involved with the movement for the Repeal of the Act of Union; true to his origins he remained a Catholic Unionist. He is best remembered for his ill-fated marriage to the beautiful Laetitia Bonaparte Wyse, niece of Napoleon Bonparte. In later life he became first British Ambassador to Greece.
This oil on canvas portrait of Sir Thomas was painted by his sister Harriet about the time of his marriage in Sir Thomas Wyse was the 19th century representative of the Wyse family of Waterford, a prominent Waterford family since the late Middle Ages.
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