Avian parents and social insect colonies are victimized by interspecific brood parasites—cheats that procure costly care for their dependent offspring by leaving them in another species' nursery. ...Birds and insects defend themselves from attack by brood parasites; their defences in turn select counter‐strategies in the parasite, thus setting in motion antagonistic co‐evolution between the two parties. Despite their considerable taxonomic disparity, here we show striking parallels in the way that co‐evolution between brood parasites and their hosts proceeds in insects and birds. First, we identify five types of co‐evolutionary arms race from the empirical literature, which are common to both systems. These are: (a) directional co‐evolution of weaponry and armoury; (b) furtiveness in the parasite countered by strategies in the host to expose the parasite; (c) specialist parasites mimicking hosts who escape by diversifying their genetic signatures; (d) generalist parasites mimicking hosts who escape by favouring signatures that force specialization in the parasite; and (e) parasites using crypsis to evade recognition by hosts who then simplify their signatures to make the parasite more detectable. Arms races a and c are well characterized in the theoretical literature on co‐evolution, but the other types have received little or no formal theoretical attention. Empirical work suggests that hosts are doomed to lose arms races b and e to the parasite, in the sense that parasites typically evade host defences and successfully parasitize the nest. Nevertheless hosts may win when the co‐evolutionary trajectory follows arms race a, c or d. Next, we show that there are four common outcomes of the co‐evolutionary arms race for hosts. These are: (1) successful resistance; (2) the evolution of defence portfolios (or multiple lines of resistance); (3) acceptance of the parasite; and (4) tolerance of the parasite. The particular outcome is not determined by the type of preceding arms race but depends more on whether hosts or parasites control the co‐evolutionary trajectory: tolerance is an outcome that parasites inflict on hosts, whereas the other three outcomes are more dependent on properties intrinsic to the host species. Finally, our review highlights considerable interspecific variation in the complexity and depth of host defence portfolios. Whether this variation is adaptive or merely reflects evolutionary lag is unclear. We propose an adaptive explanation, which centres on the relative strength of two opposing processes: strategy‐facilitation, in which one line of host defence promotes the evolution of another form of resistance, and strategy‐blocking, in which one line of defence may relax selection on another so completely that it causes it to decay. We suggest that when strategy‐facilitation outweighs strategy‐blocking, hosts will possess complex defence portfolios and we identify selective conditions in which this is likely to be the case.
Birdsong generally functions to defend territories from same-sex competitors and to attract mates. Wild zebra finch males now are shown to sing prolifically outside the breeding season and without ...defending territories, suggesting potential social functions for birdsong beyond competition.
Historically, bird song has been regarded as a sex-specific signalling trait; males sing to attract females and females drive the evolution of signal exaggeration by preferring males with ever more ...complex songs. This view provides no functional role for female song. Historic geographical research biases generalized pronounced sex differences of phylogenetically derived northern temperate zone songbirds to all songbirds. However, we now know that female song is common and that both sexes probably sang in the ancestor of modern songbirds. This calls for research on adaptive explanations and mechanisms regulating female song, and a reassessment of questions and approaches to identify selection pressures driving song elaboration in both sexes and subsequent loss of female song in some clades. In this short review and perspective we highlight newly emerging questions and propose a research framework to investigate female song and song sex differences across species. We encourage experimental tests of mechanism, ontogeny, and function integrated with comparative evolutionary analyses. Moreover, we discuss the wider implications of female bird song research for our understanding of male and female communication roles.
ABSTRACT
Avian brood parasites lay their eggs in the nests of their hosts, which rear the parasite's progeny. The costs of parasitism have selected for the evolution of defence strategies in many ...host species. Most research has focused on resistance strategies, where hosts minimize the number of successful parasitism events using defences such as mobbing of adult brood parasites or rejection of parasite eggs. However, many hosts do not exhibit resistance. Here we explore why some hosts accept parasite eggs in their nests and how this is related to the virulence of the parasite. We also explore the extent to which acceptance of parasites can be explained by the evolution of tolerance; a strategy in which the host accepts the parasite but adjusts its life history or other traits to minimize the costs of parasitism. We review examples of tolerance in hosts of brood parasites (such as modifications to clutch size and multi‐broodedness), and utilize the literature on host–pathogen interactions and plant herbivory to analyse the prevalence of each type of defence (tolerance or resistance) and their evolution. We conclude that (i) the interactions between brood parasites and their hosts provide a highly tractable system for studying the evolution of tolerance, (ii) studies of host defences against brood parasites should investigate both resistance and tolerance, and (iii) tolerance and resistance can lead to contrasting evolutionary scenarios.
Bird song has historically been considered an almost exclusively male trait, an observation fundamental to the formulation of Darwin's theory of sexual selection. Like other male ornaments, song is ...used by male songbirds to attract females and compete with rivals. Thus, bird song has become a textbook example of the power of sexual selection to lead to extreme neurological and behavioural sex differences. Here we present an extensive survey and ancestral state reconstruction of female song across songbirds showing that female song is present in 71% of surveyed species including 32 families, and that females sang in the common ancestor of modern songbirds. Our results reverse classical assumptions about the evolution of song and sex differences in birds. The challenge now is to identify whether sexual selection alone or broader processes, such as social or natural selection, best explain the evolution of elaborate traits in both sexes.
The interactions between avian interspecific brood parasites and their hosts provide tractable and informative systems for investigating coevolution. Generally, these investigations have emphasized ...the egg and chick stages of the coevolutionary arms race; however, recent studies demonstrate that coevolution operates at all stages of the host nesting cycle and emphasize the importance of reciprocal adaptations prior to deposition of the parasite egg in the host nest: the ‘frontline’ of the arms race. Here we review the diversity of adaptations at the frontline and its implications for our understanding of brood parasite–host relationships. Coevolution at the frontline can fundamentally shape the life histories, morphologies, physiologies and behaviours of both brood parasites and their hosts, and influences the trajectories and outcomes of their subsequent coevolutionary interactions. We advocate the incorporation of frontline interactions in empirical and theoretical investigations of brood parasite–host arms races to provide a more holistic understanding of the coevolutionary processes in these systems.
► We review frontline adaptations in avian brood parasite–host relationships. ► Frontline adaptations operate prior to parasite egg deposition in host nests. ► Like adaptations at the other host nesting stages they result from coevolution. ► They may also influence evolutionary trajectories of arms races. ► Thus, they are essential for a holistic understanding of parasite–host arms races.
Genetic data can be highly informative for answering questions relevant to practical conservation efforts, but remain one of the most neglected aspects of species recovery plans. Framing genetic ...questions with reference to practical and tractable conservation objectives can help bypass this limitation of the application of genetics in conservation. Using a single-nucleotide polymorphism dataset from reduced-representation sequencing (DArTSeq), we conducted a genetic assessment of remnant populations of the endangered forty-spotted pardalote (Pardalotus quadragintus), a songbird endemic to Tasmania, Australia. Our objectives were to inform strategies for the conservation of genetic diversity in the species and estimate effective population sizes and patterns of inter-population movement to identify management units relevant to population conservation and habitat restoration. We show population genetic structure and identify two small populations on mainland Tasmania as 'satellites' of larger Bruny Island populations connected by migration. Our data identify management units for conservation objectives relating to genetic diversity and habitat restoration. Although our results do not indicate the immediate need to genetically manage populations, the small effective population sizes we estimated for some populations indicate that they are vulnerable to genetic drift, highlighting the urgent need to implement habitat restoration to increase population size and to conduct genetic monitoring. We discuss how our genetic assessment can be used to inform management interventions for the forty-spotted pardalote and show that by assessing contemporary genetic aspects, valuable information for conservation planning and decision-making can be produced to guide actions that account for genetic diversity and increase chances of recovery in species of conservation concern.
The interactions between avian obligate interspecific brood parasites and their hosts provide tractable systems for studying coevolutionary processes in nature. This review highlights recent advances ...in understanding coevolution in these systems. First, we discuss the evolution and phylogenetic history of avian brood parasitism. Next, we examine coevolved adaptations and counteradaptations in brood parasites and hosts at all stages of the host nesting cycle: those that precede laying of the parasitic egg and those at the egg, chick, and fledgling stages. We then consider the factors that affect the evolution of offense and defense portfolios (the suites of adaptations and counteradaptations across the nesting cycle), and the outcomes of coevolutionary interactions between brood parasites and hosts. Ongoing efforts to document the diversity of host defenses and parasite offenses will facilitate understanding of coevolutionary processes and the ecological and evolutionary consequences of species interactions in the natural world.
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