Animals often gather information from other species by eavesdropping on signals intended for others. We review the extent, benefits, mechanisms, and ecological and evolutionary consequences of eavesdropping on other species' alarm calls. Eavesdropping has been shown experimentally in about 70 vertebrate species, and can entail closely or distantly related species. The benefits of eavesdropping include prompting immediate anti-predator responses, indirect enhancement of foraging or changed habitat use, and learning about predators. Eavesdropping on heterospecifics can provide more eyes looking for danger, complementary information to that from conspecifics, and potentially information at reduced cost. The response to heterospecific calls can be unlearned or learned. Unlearned responses occur when heterospecific calls have acoustic features similar to that used to recognize conspecific calls, or acoustic properties such as harsh sounds that prompt attention and may allow recognition or facilitate learning. Learning to recognize heterospecific alarm calls is probably essential to allow recognition of the diversity of alarm calls, but the evidence is largely indirect. The value of eavesdropping on different species is affected by problems of signal interception and the relevance of heterospecific alarm calls to the listener. These constraints on eavesdropping will affect how information flows among species and thus affect community function. Some species are 'keystone' information producers, while others largely seek information, and these differences probably affect the formation and function of mixed-species groups. Eavesdroppers might also integrate alarm calls from multiple species to extract relevant and reliable information. Eavesdropping appears to set the stage for the evolution of interspecific deception and communication, and potentially affects communication within species. Overall, we now know that eavesdropping on heterospecific alarm calls is an important source of information for many species across the globe, and there are ample opportunities for research on mechanisms, fitness consequences and implications for community function and signalling evolution.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. SUMMARY(1) This paper describes the survival of juvenile blackbirds according to their weight as 8-day-old nestlings. In a 4-year study of 748 individually colour-ringed nestlings, heavy nestlings were more likely to survive to breed in the study area than lighter nestlings.(2) Nestlings below 35 g never survived to fledge; those above 45 g showed a linear increase in the probability of survival to independence, up to heaviest nestlings in the sample; between 35 and 45 g there was a linear, but more rapid, increase in survival prospects with weight. The shape of the relationship between nestling weight and juvenile survival was similar in different years, through the season, and between brood sizes and age ranks within broods.(3) Differential mortality by nestling weight occurred from ringing to fledging, from fledging to 2 weeks after fledging (when the young are still fed by their parents and are easily located on territories), and from 2 weeks to 1 month after fledging (when the young are independent but can be difficult to locate). However, birds that survived this period apparently had a random chance of breeding.(4) These findings on juvenile survival suggest that nestling weight itself influences juvenile survival, not some correlate such as brood size.(5) A knowledge of the shape of the relationship between nestling weight and juvenile survival allows one to measure nestling 'quality' as the probability that a nestling will survive to breed. In practice, the probability of survival to shortly after nutritional independence might be an adequate estimate of relative probabilities of recruitment to the breeding population in many bird species.

Summary 1. The review aims to provide a simple conceptual framework on which to place recent studies of hatching asynchrony in altricial birds and to assess the evidence used in support of specific hypotheses. 2. Hatching asynchrony arises bsecause parents start incubation before laying is complete, but the precision of parental control is largely unknown. 3. Hypothesses concerning the functional significance of hatching asynchrony fall into four broad types. Hatching asynchrony might: (i) arise because of selection on the timing of events during the nesting period; (ii) facilitate the adaptive reduction in brood size; (iii) increase the energetic efficiency of raising the brood, or (iv) result from environmental or phylogenetic constraints. 4. The incubation pattern could function to minimize the losses of eggs, nestlings or adults to predators (or climatic sources of mortality), particularly in species which cannot actively defend their nest. The best evidence comes from comparative studies of hatching asynchrony. Early incubation might also be favoured if the food supply declines sharply through the breeding season, although the evidence is weak and indirect, or if there is a risk of brood parasitism. In species in which only the female incubates, early incubation could ‘force’ the male to invest more in the nestlings, but this idea remains to be tested. Males may be constrained by the risk of cuckoldry to delay incubation until laying is complete. 5. Hatching asynchrony could be adaptive by enabling the efficient reduction of brood size if food proves short after hatching (primarily because of a shortage of food in the environment or possibly because of a large proportion of ‘expensive’ nestlings in the brood in species which are sexually dimorphic). Observational evidence is often consistent with this hypothesis but few experimental studies provide adequate tests. Brood reduction could be adaptive in species (primarily eagles and pelecaniformes) which lay an extra egg to act as insurance against hatching failure, and again hatching asynchrony might facilitate brood reduction, although there are few experimental tests on such species. Hatching asynchrony might also enable sex ratio manipulation through selective brood reduction, although there is as yet no clear supportive evidence. 6. Ins species in which young have a marked peak in energy demand during the period of parental care, hatching asynchrony can reduce the peak demand of the brood, which might allow the parents to raise more healthy young. In many species such savings are likely to be small or absent. There is some behavioural evidence that hatching asynchrony can reduce fighting amongst nestlings and therefore lead to the more efficient use of energy by the brood. In general this effect seems small and the only energetic study found no difference in the energy requirements of synchronous and asynchronous broods. Other possible energetic advantages to hatching asynchrony have not been tested. 7. Environmental conditions during laying can influence ...

Most birds breed in pairs but at least 3% of passerine species are cooperative breeders, whereby more than two adults help to raise the young. The general rarity of cooperative breeding has led to the assumption that cooperative behaviour has evolved from the ancestral trait of pair breeding. However, it has been suggested that pair breeding may be the derived state in some taxa. The primary aim of this research was to test this suggestion using the genus Acanthiza, which contains examples of both cooperatively and pair breeding species. Mitochondrial DNA sequences were used to construct a phylogenetic hypothesis for the tribe containing Acanthiza, the Acanthizini. The breeding behaviour of the species sequenced was determined from records in the literature; where there were no such data the frequency of another social behaviour, flocking, was used as an indicator of breeding behaviour. The mapping of breeding systems onto the phylogeny led to the conclusion that cooperative breeding is the ancestral state in the Acanthizini, with pair breeding evolving twice in the genus Acanthiza. Models explaining the occurrence of cooperative breeding in terms of broad environmental factors or life history do not appear to be applicable to the genus Acanthiza. The pair breeding Acanthiza species cluster into two clades, suggesting some influence of phylogenetic history on the occurrence of the different breeding systems. Combining the results of this study with other data suggests the tendency to breed cooperatively could be ancestral in the superfamily Meliphagoidea.

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Nest predation is a key source of selection for birds that has attracted increasing attention from ornithologists. The inclusion of new concepts applicable to nest predation that stem from social information, eavesdropping or physiology has expanded our knowledge considerably. Recent methodological advancements now allow focus on all three players within nest predation interactions: adults, offspring and predators. Indeed, the study of nest predation now forms a vital part of avian research in several fields, including animal behaviour, population ecology, evolution and conservation biology. However, within nest predation research there are important aspects that require further development, such as the comparison between ecological and evolutionary antipredator responses, and the role of anthropogenic change. We hope this review of recent findings and the presentation of new research avenues will encourage researchers to study this important and interesting selective pressure, and ultimately will help us to better understand the biology of birds.