Abstract: Elaborate solicitation displays are a common feature of interactions between care-givers and offspring. These displays are interpreted as the phenotypic expression of the conflict of interests between parents and offspring over parental investment. Offspring typically have siblings and thus do not exist in isolation. Therefore, they may adjust their begging in response to their siblings' begging, either competitively or cooperatively. Alternatively, begging may be independent of the begging efforts of siblings. Studies of avian begging have primarily focused on nestlings, where offspring are immobile and compete directly over the allocation of parental resources. We investigated the influence sibling begging had on individual fledgling begging in the cooperatively breeding pied babbler, Turdoides bicolor. Using experimental manipulations, we found that fledgling begging behaviour was negatively correlated with satiation and unrelated to the begging effort of siblings. Pied babbler care-givers were able to target increased provisioning to individuals with artificially increased demand while maintaining provisioning rates to the rest of the brood. Thus, fledglings were found to incur no provisioning costs or benefits from either increased or decreased begging by their siblings. We propose that the combination of targeted provisioning, flexible levels of provisioning and the dispersed nature of fledglings reduces the benefits of competitive or cooperative begging in this species.
Abstract:
Eavesdropping behaviour can increase the total amount of information available to an individual and therefore has the potential to provide substantial benefits. Recent research has suggested that some species are ‘information givers’, particularly social species with cooperative vigilance systems, and that these species may consequently affect community structure by influencing the behaviour and niche utilization of other species.
Here, using behavioural observations and playback experiments, we compared the behavioural change in a solitary species (the scimitarbill) and a social species (the pied babbler), to the presence and alarm calls of one another.
Our results revealed that scimitarbills underwent significant behavioural changes in the presence of social pied babblers: they reduced their vigilance rate by over 60%, increased their foraging efficiency and expanded their niche by moving into open habitat and excavating subterranean food items. In contrast, pied babblers – who have an effective intraspecific sentinel system – did not show significant behavioural changes to the presence or alarm calls of scimitarbills.
These results suggest that interspecific interceptive eavesdropping can provide significant benefits, influencing the behaviour and habitat utilization of eavesdropping species.
Abstract: One theory to explain the existence of conspicuous solicitation is that it is a way for young to ‘blackmail’ carers into provisioning them, by threatening their own destruction. Fledgling birds offer a unique opportunity to investigate the ‘blackmail theory’, as their mobility enables them to influence the predation risk they face. We investigated a novel solicitation behaviour in fledgling pied babblers (Turdoides bicolor), where fledglings use their location to influence provisioning rates. We show that fledglings face a trade-off: the ground is a much more profitable location in terms of provisioning rate from adult carers, but they are at greater risk from predators owing to their limited flying ability and slow response to alarm calls. Young babbler fledglings move to the ground when hungry, signalling their state, and this stimulates adults to increase their provisioning rates. Once satiated, fledglings return to the safety of cover. By experimentally increasing terrestrial predation risk, we found that adults increased their provisioning rate to terrestrial but not arboreal fledglings. Thus, by moving to a riskier location, fledglings revealed their need and were able to manipulate adults to achieve higher provisioning rates. These results provide support for the ‘blackmail theory’.
Abstract: Sentinel behaviour, where individuals take turns to watch for danger and give alarm calls to approaching predators, has been observed in a number of animal societies. However, the evolutionary causes of this behaviour remain unclear. There are two main, competing hypotheses regarding the evolution of sentinel behaviour. The first hypothesis is that it is a cooperative behaviour, where group members benefit from the detection of danger but share the workload of acting as a sentinel. The second is that it is a safe, selfish behaviour. Under the second hypothesis, once an individual is satiated, being a sentinel is safer because sentinels can detect threats more readily and can therefore escape from predators faster. We examined whether sentinels are safer than foragers in a wild, free-living cooperative bird (the pied babbler, Turdoides bicolor) with a well-described sentinel system. We found that sentinel behaviour was costly because (1) sentinels were targeted by predators more often, (2) they were further from cover than foragers, and (3) they took longer to reach the safety of cover following a predator alarm. These results suggest that individuals do not become sentinels because it is safer. This is the first study to demonstrate that sentinels are at greater risk of predator attack than foraging group members and suggests sentinel activity may have evolved as a form of cooperative behaviour.
Abstract: Many cooperative bird species have an extended period of post-fledging care. Despite the fact that this period of care can last up to several months, it remains a relatively understudied stage of chick development. This period, when young are actively begging but highly mobile, provides an opportunity for young to maximise the amount of care they receive by selectively choosing particular adults to beg from. In pied babblers Turdoides bicolor (a cooperatively breeding passerine), fledglings closely follow foraging adults and beg for food regularly (a behavioural interaction termed social foraging). Using a combination of natural observations and experimental manipulations, we found that fledgling pied babblers preferentially socially forage with adult care-givers who have high foraging success, since this results in young receiving more food. By supplementally feeding adults to artificially increase their foraging success, we increased the proportion of time that fledglings chose to socially forage with them, confirming that fledglings are selectively choosing dyadic interactions with the best adult foragers. These results indicate that pied babbler fledglings are sensitive to and can respond to short-term changes in adult foraging success, enabling them to maximize their nutritional intake, a behavioural adjustment that has long-term benefits in this system.
Abstract:
Animals commonly steal food from other species, termed interspecific kleptoparasitism, but why animals engage in kleptoparasitism compared with alternate foraging tactics, and under what circumstances they do so, is not fully understood.
Determining what specific benefits animals gain from kleptoparasitism could provide valuable insight into its evolution. Here, we investigate the benefits of kleptoparasitism for a population of individually recognizable and free-living fork-tailed drongos (Dicrurus adsimilis) in the southern Kalahari Desert.
Drongos engaged in two foraging behaviours: self-foraging for small insects or following other species which they kleptoparasitized for larger terrestrial prey that they could not capture themselves. Kleptoparasitism consequently enabled drongos to exploit a new foraging niche.
Kleptoparasitism benefitted drongos most in the morning and on colder days because at these times pay-offs from kleptoparasitism remained stable, while those from self-foraging declined. However, drongos engaged in kleptoparasitism less than expected given the overall high (but more variable) pay-offs from this behaviour, suggesting that kleptoparasitism is a risky foraging tactic and may incur additional foraging costs compared with self-foraging.
This is the first study to comprehensively investigate the benefits of facultatively engaging in kleptoparasitism, demonstrating that animals may switch to kleptoparasitism to exploit a new foraging niche when pay-offs exceed those from alternate foraging behaviours.
Abstract: In many cooperatively breeding societies, only a few socially dominant individuals in a group breed, reproductive skew is high, and reproductive conflict is common. Surprisingly, the effects of this conflict on dominant reproductive success in vertebrate societies have rarely been investigated, especially in high-skew societies. We examine how subordinate female competition for breeding opportunities affects the reproductive success of dominant females in a monogamous cooperatively breeding bird, the Southern pied babbler (Turdoides bicolor). In this species, successful subordinate reproduction is very rare, despite the fact that groups commonly contain sexually mature female subordinates that could mate with unrelated group males. However, we show that subordinate females compete with dominant females to breed, and do so far more often than expected, based on the infrequency of their success. Attempts by subordinates to obtain a share of breeding impose significant costs on dominant females: chicks fledge from fewer nests, more nests are abandoned before incubation begins, and more eggs are lost. Dominant females appear to attempt to reduce these costs by aggressively suppressing potentially competitive subordinate females. This empirical evidence provides rare insight into the nature of the conflicts between females and the resultant costs to reproductive success in cooperatively breeding societies.
Abstract: Fork-tailed drongos, Dicrurus adsimilis, are facultative kleptoparasites of many bird and mammal species. They use complex alarm call strategies, a combination of both true and false alarm calls, to procure food items from their hosts. The evolution of this interspecific interaction may have been catalysed by a selective benefit for drongos that were able to procure a wider diversity of food resources through kleptoparasitism. We tested the hypothesis that drongos, which principally hawk their prey from a perch, employ kleptoparasitism to exploit a terrestrial foraging niche for which they do not possess the appropriate morphological adaptations, as indicated by bill morphology. We analysed the bill characteristics of the terrestrially foraging pied babbler, Turdoides bicolor, a primary host for the fork-tailed drongo, and both drongo and pied babbler museum specimens to establish potential links between differences in functional morphology and foraging ecology. We also observed the foraging behaviour of babblers and drongos in the Kalahari Desert. Drongo bills were significantly shorter, thicker and less curved than those of pied babblers, and large energy-rich subterranean prey items, such as insect larvae, were not procured by drongos while self-foraging. Furthermore, drongos preferentially attempted to kleptoparasitize prey types that they did not capture while self-foraging. We suggest that the calorific and nutrient content of a larger and more diverse set of prey, particularly buried larvae, which drongos did not excavate on their own, establishes a selection pressure for kleptoparasitism and may have contributed to the complex behavioural interactions between drongos and their hosts.
Abstract: For cooperative breeding to be a strategy under positive selection, individuals should accrue some benefit, or at least not suffer a loss, compared to alternative breeding strategies. Cooperative breeding is often associated with habitat saturation (where the opportunities for pairs to establish new territories are limited), and such species are, thus, assumed to have lower per capita reproductive success than their non-cooperative counterparts. To test whether different breeding strategies affect reproductive success and progression through each breeding stage, we compared cooperative versus non-cooperative breeding strategies across a range of small to medium-sized birds in semi-arid open and grassland habitats in southern Africa. We found that in general, cooperative breeders fledged their young significantly earlier and raised more broods per season than non-cooperative species of similar body mass. Within phylogenetic families, we found cooperative species had shorter nestling periods, and that the duration of the nestling period tended to decline as cooperative group size increased. We then examined in detail two species sharing the same habitat and similar foraging niches, but with different breeding strategies: a cooperative breeder and a biparental breeder. In each species, we followed the fate of breeding attempts from incubation until nutritional independence of young. We found that the cooperative breeder fledged young earlier and invested in overlapping broods more often, resulting in lower nest predation and multiple clutches successfully raised to independence per season. This resulted in more offspring surviving per adult. These results suggest that, due to changes in breeding behavior as a consequence of helper presence (earlier fledging, overlapping broods), cooperative breeding could provide equivalent reproductive success to non-cooperative breeding strategies.
Abstract:
Recent mass mortalities of bats, birds and even humans highlight the substantial threats that rising global temperatures pose for endotherms. Although less dramatic, sublethal fitness costs of high temperatures may be considerable and result in changing population demographics. Endothermic animals exposed to high environmental temperatures can adjust their behaviour (e.g. reducing activity) or physiology (e.g. elevating rates of evaporative water loss) to maintain body temperatures within tolerable limits. The fitness consequences of these adjustments, in terms of the ability to balance water and energy budgets and therefore maintain body condition, are poorly known. We investigated the effects of daily maximum temperature on foraging and thermoregulatory behaviour as well as maintenance of body condition in a wild, habituated population of Southern Pied Babblers Turdoides bicolor. These birds inhabit a hot, arid area of southern Africa where they commonly experience environmental temperatures exceeding optimal body temperatures. Repeated measurements of individual behaviour and body mass were taken across days varying in maximum air temperature. Contrary to expectations, foraging effort was unaffected by daily maximum temperature. Foraging efficiency, however, was lower on hotter days and this was reflected in a drop in body mass on hotter days. When maximum air temperatures exceeded 35.5 °C, individuals no longer gained sufficient weight to counter typical overnight weight loss. This reduction in foraging efficiency is likely driven, in part, by a trade-off with the need to engage in heat-dissipation behaviours. When we controlled for temperature, individuals that actively dissipated heat while continuing to forage experienced a dramatic decrease in their foraging efficiency. This study demonstrates the value of investigations of temperature-dependent behaviour in the context of impacts on body condition, and suggests that increasingly high temperatures will have negative implications for the fitness of these arid-zone birds.
