| dc.description.abstracteng | In this study, I investigated the proximate mechanisms underlying an expression of the helping behaviour by examining feeding rate and frequency of helpers under varying food availability and predation risk in different breeding colonies (see chapter 2). Many hypotheses have developed to explain the underlying mechanisms of the cooperative breeding. However, it appears to have no single hypothesis that can explain it, instead several hypotheses can be applied for the evolution of cooperative breeding. The kin-selection, life-history and ecological constraints hypotheses are widely accepted, though none of them can explain the evolution of the cooperative breeding exclusively.
The nest predation hypothesis was underlined because the main cause of nest failure among cooperative breeding birds is nest predation (Brown 1987, Stacey and Ligon 1991), suggesting its potential selective factor for evolution of cooperative breeding (Poiani and Pagel 1997). Yet, not much empirical evidence has been found to support it. I could show that both low food availability and high predation risk increase the benefits of helping behaviour, thus they lead to expression of the helping behaviour. Our findings support the nest predation hypothesis that high nest predation pressure over ecological time favours the cooperative breeding (see Poiani and Pagel 1997). However, nest predation hypothesis cannot explain the evolution of cooperative breeding solely, it is one of the selective factors that work in conjunction with other selective factors such as ecological constraints and kin-selection.
Helpers at the nest have long been assumed to increase nestling survival through enhancing nestling's body condition. But in last two decades many studies have documented for some species that helpers at the nest appear to reduce nest predation (e.g. Austad and Rabenold 1985, Emlen and Wrege 1991, Mumme 1992, Schaub et al. 1992, Innes and Johnston 1996, Boland 1998, Valencia et al. 2003, Hatchwell 2004). The data on nestling body mass, nest attentiveness and egg-laying period along with observational records of helpers' active involvement in nest defense in this study showed that helpers improved nestling condition and reduced chick rearing time period at the nest, which both in turn reduce nest predation (see chapter 2). Therefore, helping behaviour appeared to be an effective anti-predator strategy since it reduces nest predation substantially (see chapter 2 and 3). Besides that, our findings revealed several other anti-predator strategies that azure-winged magpies employ under high predation risk. Greater concealment of the nest, proactive avoidance of the nest location from the predator nest along with involvement of helpers at the nest were the most effective anti-predator strategies in our study (see chapter 3).
Although, nest concealment is arguably the widespread anti-predator strategy not only for birds but for many other animal groups, most field studies found no evidence of which nest concealment reduced nest predation (reviewed by Borgmann and Conway 2015). Borgmann and Conway (2015) concluded that lack of evidence of nest-concealment hypothesis, i.e. birds prefer nest sites enclosed by dense foliage to reduce the likelihood of detection by predators and brood parasites (Martin et al. 1988, Martin and Li 1992), in large quantity of studies largely caused by morphological traits and methods used to measure concealment which in turn likely resulted in lack of support. Benefits through nest concealment may be traded-off by consequences associated with greater concealed nest site (Morosinotto et al. 2010, Borgmann and Conway 2015) or compensated by active defense in some bird species (Götmark et al. 1995, Cresswell 1997, Weidinger 2002, Remeš 2005, Fontaine and Martin 2006a). Thus, other anti-predator strategies may have cancelling effect on nest concealment, leading to lack of support of nest-concealment hypothesis. But my study with the comprehensive analyses of several factors which may affect nest predation showed strong support on nest-concealment hypothesis (see chapter 3).
Crows are the well-known nest predators in many avian species (Shields and Parnell 1986). Some studies revealed that the nest predation rate increased as nest distance of prey species decreased to the crow nest (Shields and Parnell 1986, Sullivan and Dinsmore 1990). In this study, I found that azure-winged magpies consistently tried to avoid spatially the crow nests both within and between breeding season and those of distant nests were preyed much less than nests closer to the crow nest (see chapter 3). Moreover, breeding pair experience appeared to reduce nest predation as well. So far, very little work exists regarding the effect of individual experience on nest predation. In this study, pair experience largely appeared to affect decision to build the nest closer to colony neighbours, which indeed reduced nest predation through joint nest defense of neighbouring birds (see chapter 3).
Individuals may respond by two opposing ways regarding nest density of both conspecifics and interspecifics in response to predator type. In case of evadable predators, collective nest defense of neighbours often increases the success of their common predators' eviction, thus, individuals are expected to nest closer to each other. Also other studies showed that nesting in high density resulted in decreased nest predation (Ritschel 1985, Picman et al. 1988), though much work is needed to understand what factors affect the decision whether to space out or space in their nests. The data on nest distance in this study indicated that azure-winged magpies prefer to nest closer to each other, probably to gain anti-predator benefits from neighbours (see chapter 4). Azure-winged magpies decreased distance between their nest in accordance with predation risk and nest closer to each other were more successful than nests further apart located (see chapter 4). It leads to nest clustering and independent sub-colony formations in terms of nest defense activities. Distinct clustering may have evolved as means of reducing predation to facilitate variety of collective defenses of neighbours and it can be regarded as proactive anti-predator strategy (Hamilton 1971, Lima 2009). Nest clustering may have several benefits: by reducing per individual vigilance and time spent for it (Birkhead 1977, Hoogland 1981, Brown 1987, Jungwirth et al. 2015), by increased detectability of predators (Hoogland and Sherman 1976, Brown and Brown 1987, Møller 1987), by decrease of the predator's incentive (Curio 1978, Hasson 1991) and by increase of the likelihood of successful defense (e.g. Perry and Andersen 2003). Very little studies found an empirical evidence for the predator deterrence hypothesis that predation promotes the nest clustering (Perry and Andersen 2003, Perry et al. 2008). Our findings that closer nests and later formed cluster nests experienced less predation support the predator deterrence hypothesis (see chapter 4).
Our findings demonstrated that azure-winged magpies have high cognitive capacity to assess predation risk in short time period. It was also shown that azure-winged magpies have a plasticity to execute complex and appropriate anti-predator strategies immediately after the nest predation events in accordance with changing predation risk and predator type (see chapter 3 and 4). The findings of gradual changes in nest concealment, nest distances, nest synchronization suggest that the adaptive behaviour of the azure-winged magpies inherits from generation to generation over long time period. | de |