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Among vertebrates, particularly birds, evidence for facultative adjustment of brood sex ratios is inconsistent. Results from cooperatively breeding species are clearer, where sex-biased dispersal and helping behaviour may accentuate any differences in fitness payoffs to parents of raising sons versus daughters. We might expect similar payoff differences for cooperative helpers raising offspring of each sex, but this has rarely been tested. Bell miners, Manorina melanophrys, are known to adjust their primary brood sex ratio according to habitat productivity. Helpers of both sexes provision broods even when unrelated to offspring, which indicates the potential importance of direct fitness benefits (e.g. group augmentation) that should depend heavily on offspring sex. As only females disperse, we anticipated helpers would invest more heavily in male-biased broods, particularly if they were unrelated to broods and thus not gaining indirect benefits. Helpers did feed all-male broods at a greater rate, although, contrary to expectations, this was the result of higher levels of effort by related and not unrelated helpers, irrespective of helper sex. The subsequent interaction between relatedness and brood sex ratio was also an exclusively between-subjects effect, and not due to any within-subject facultative adjustment by individual helpers feeding at multiple nests. Furthermore, the acoustic characteristics of nestling begging calls showed no reliable sex differences; thus helpers probably lacked accurate information on nestling sex to make facultative adjustments according to brood sex ratio. Additional work examining these kinds of trade-offs in other cooperative species where sons and daughters yield differing payoffs to helpers is warranted. [Copyright &y& Elsevier]

Provisioning visits by helpers are normally assumed to confer positive fitness effects on nestlings, but few studies have actually examined the nutritional value of items helpers fed to offspring. In the cooperatively breeding bell miner, Manorina melanophrys, helpers deliver large proportions of ‘lerp’, a sugary secretion of psyllids (Hemiptera; Psyllidae). Although lerp is a major food type of adults, its nutritional value is uncertain, especially since nestlings in other nectarivorous species are usually fed only protein-rich arthropods. Helpers in this system are predominantly male and are often unrelated to broods they aid, suggesting helping might be a sexual display, with any nutritional benefits to nestlings being of secondary importance. Detailed observations revealed that the proportion of lerp delivered increased with nestling age, but that it did not differ between helpers and parents, or between related and unrelated helpers. Variation in delivered biomass (lerp+arthropods) had a positive effect on nestling condition, but variation in the proportion of prey constituted by lerp had no measurable effect on nestling mass or condition. Finally, the total amount of food, nestling body mass and condition were all positively related to the number of helpers provisioning broods. These results are consistent with helping in bell miners operating as cooperative investment in brood fitness. Given that lerp was as effective as invertebrate prey in promoting growth, these results also suggest a valid nutritional role for this easily procured food, presumably facilitated by the unusually early development of nestling digestive tracts to utilize these simple sugars. [Copyright &y& Elsevier]

Cooperative provisioning dynamics between members of a pair in biparental systems has received a lot of attention, both empirically and theoretically. The dynamics of provisioning in cooperatively breeding groups, however, remains poorly understood. Such groups often include unrelated helpers that may have very different provisioning rules from related helpers and parents, since they accrue different types of fitness benefits from helping. The bell miner, Manorina melanophrys, provides an ideal system in which to investigate cooperative provisioning, because substantial levels of care are provided by a number of unrelated helpers per nest. We experimentally increased brood demand via targeted begging playbacks during nest visits of either breeding males or unrelated male helpers. Both types of males used similar behavioural investment rules, significantly increasing food delivery rate during playback relative to control periods. Surprisingly, all other provisioners also increased their visit rates during playbacks, although to a lesser extent. This could only have been in response to the increase in visits by the target individuals, suggesting an additional indirect mechanism by which individuals in this cooperative setting assess brood demand. The resultant overall increase in food delivery during the playback periods caused nestlings to show the expected reduction in their own begging and an increase in body mass gain. It is therefore interesting how similar the evolved provisioning responses are for all types of group member, irrespective of whether such benefits of investment in the brood are derived via kin selection (fathers) or some future direct increase in fitness (helpers). [Copyright &y& Elsevier]

Research on cooperative breeding has begun to focus on direct fitness benefits gained by helpers, particularly when individuals are unrelated to those they assist. There has been considerable interest in helping possibly operating as a signal, either to show off individual quality to potential mates (‘social prestige’) or to ensure group membership (‘pay to stay’). However, empirical investigation of these phenomena remains sparse. Here we investigate the potential for signalling via provisioning behaviour in the bell miner, Manorina melanophrys, an obligate cooperative breeding species in which the predominantly male helpers are commonly unrelated to breeders. Aggression between birds was extremely rare, and there was little to indicate a pay to stay system. The presence versus absence of members of the breeding pair as a potential audience at the nest had little influence on helper behaviour (e.g. load size/composition, visit duration or frequency). Helpers did produce more individually distinctive vocalizations when in the presence of another helper or the breeding male, although presence of the breeding female (a likely target of male signals) surprisingly had no effect. There was also evidence that nest arrival times coincided somewhat. These results are probably best explained by the helpers and breeding males being involved in additional cooperative behaviours when away from the nest, such as mobbing. Overall, there does not appear to be any evidence that bell miner helpers use nestling provisioning to signal their quality and/or work rate to one another or to either member of the breeding pair. [Copyright &y& Elsevier]