Michelle L. Hall

A review of hypotheses for the functions of avian duetting

Avian duets are striking for the remarkable precision with which duetting partners sometimes coordinate their songs. Duetting species are taxonomically diverse, and the form of their duets varies. The reasons some birds duet when most do not remains unclear despite numerous hypotheses for its function. I review work done so far on duetting, discuss evidence for and against hypotheses for its functions, and highlight approaches useful for future research. The four hypotheses that appear most promising are that individuals join their partners’ songs to form duets: (1) to avoid being usurped from a partnership, (2) to prevent their partner being usurped, (3) as a collaborative display in defence of some resource, or (4) to signal commitment to their partner. These hypotheses are not mutually exclusive, and duetting is likely to have multiple roles both within and among species. However, much basic research is still required. Characteristics of duets have rarely been quantified in detail, and information about variability among species in the precision of duetting is necessary, not only to test hypotheses about function, but also to define duetting more precisely. Quantifying the relative frequencies of alternative vocal strategies (for example, remaining silent when a partner sings versus joining in to form a duet) between species and in different contexts will help to determine why partners coordinate their songs to form duets. Furthermore, social systems and sex roles in duetting species are poorly understood, yet understanding these is critical to determining the functions of avian duetting.

Vocal performance influences male receiver response in the banded wren

In a variety of songbirds the production of trilled song elements is constrained by a performance tradeoff between how fast a bird can repeat trill units (trill rate) and the range of frequencies each unit can span (frequency bandwidth). High-performance trills serve as an assessment signal for females, but little is known about the signal value of vocal performance for male receivers. We investigated the relationship between trill rate and frequency bandwidth in banded wren (Thryothorus pleurostictus) songs. Trilled song elements showed the same performance tradeoff found in other passerines and individuals differed in performance of some trill types. We tested the hypothesis that males of this species assess each other based on trill performance with a two-speaker experiment, in which territory owners were presented with alternating renditions of the same song type manipulated to differ in trill rate. Subjects were significantly more likely to approach the faster trill stimulus first. However, subjects that received trill types closer to the performance limit spent less time close to the fast speaker. Our results show that male banded wrens discriminate and respond differently to songs based on their vocal performance. Thus, performance of physically challenging songs may be important in intra- as well as inter-sexual assessment.

Female song is widespread and ancestral in songbirds

Bird song has historically been considered an almost exclusively male trait, an observation fundamental to the formulation of Darwins 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.

Chapter 3 A Review of Vocal Duetting in Birds

Abstract Avian duetting is an unusual but taxonomically widespread phenomenon, occurring in over 400 species, representing 40% of bird families. Duets vary in form from loosely overlapping songs to highly coordinated duets where paired birds both adjust the timing and type of phrases they sing to fit those of their partner over the course of the duet. Duet coordination therefore signals how attentive an individual is to its partner, both to the partner and to other listeners. While some aspects of duetting are poorly understood, such as its ontogeny and causation (including hormonal and neural bases), it is clear that duetting serves multiple functions, including maintaining the year-round territories and partnerships that characterize many duetting species.

Visual mimicry of host nestlings by cuckoos

Coevolution between antagonistic species has produced instances of exquisite mimicry. Among brood-parasitic cuckoos, host defences have driven the evolution of mimetic eggs, but the evolutionary arms race was believed to be constrained from progressing to the chick stage, with cuckoo nestlings generally looking unlike host young. However, recent studies on bronze-cuckoos have confounded theoretical expectations by demonstrating cuckoo nestling rejection by hosts. Coevolutionary theory predicts reciprocal selection for visual mimicry of host young by cuckoos, although this has not been demonstrated previously. Here we show that, in the eyes of hosts, nestlings of three bronze-cuckoo species are striking visual mimics of the young of their morphologically diverse hosts, providing the first evidence that coevolution can select for visual mimicry of hosts in cuckoo chicks. Bronze-cuckoos resemble their own hosts more closely than other host species, but the accuracy of mimicry varies according to the diversity of hosts they exploit.

Increased Opportunities for Cuckoldry may be Why Dominant Male Fairy-Wrens Tolerate Helpers

The highest known rates of extra-pair fertilization (76%) occur in the superb fairy-wren (Malurus cyaneus), a bird that lives in both breeding pairs and in cooperative groups where 2-5 males assist a single female. Males living in groups are cuckolded more often than males in pairs, apparently because females can rely on helpers as an alternative source of care, and so do not need to allow their mate fertilizations. It is therefore unclear why dominant males tolerate helpers. Here we show that dominant males with helpers provide less parental care during the nestling period, and use this reduced workload to make extra-territorial forays which are used to court extra-group females. DNA fingerprinting suggests that this increased display rate provides them with an advantage in obtaining extra-group copulations. These data suggest that within-pair and extra-pair paternity will not always be positively correlated.

Temporal coordination signals coalition quality.

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Radical loss of an extreme extra-pair mating system

BackgroundMating outside the pair-bond is surprisingly common in socially monogamous birds, but rates of extra-pair paternity (EPP) vary widely between species. Although differences in life-history and contemporary ecological factors may explain some interspecific variation, evolutionary forces driving extra-pair (EP) mating remain largely obscure. Also, since there is a large phylogenetic component to the frequency of EPP, evolutionary inertia may contribute substantially to observed EP mating patterns. However, the relative importance of plasticity and phylogenetic constraints on the incidence of EP mating remains largely unknown.ResultsWe here demonstrate very low levels of EPP (4.4% of offspring) in the purple-crowned fairy-wren Malurus coronatus, a member of the genus with the highest known levels of EPP in birds. In addition, we show absence of the suite of distinctive behavioral and morphological adaptations associated with EP mating that characterize other fairy-wrens. Phylogenetic parsimony implies that these characteristics were lost in one speciation event. Nonetheless, many life-history and breeding parameters that are hypothesized to drive interspecific variation in EPP are not different in the purple-crowned fairy-wren compared to its promiscuous congeners.ConclusionSuch radical loss of an extreme EP mating system with all associated adaptations from a lineage of biologically very similar species indicates that evolutionary inertia does not necessarily constrain interspecific variation in EPP. Moreover, if apparently minor interspecific differences regularly cause large differences in EPP, this may be one reason why the evolution of EP mating is still poorly understood.

Multiple benefits of cooperative breeding in purple-crowned fairy-wrens: a consequence of fidelity?

1. Kin selection is one of the mechanisms that can explain apparent altruism by subordinate individuals in cooperatively breeding species, if subordinates boost the production of kin. We compared productivity and breeder survival in pairs with and without subordinates in a genetically monogamous cooperatively breeding bird, the purple-crowned fairy-wren Malurus coronatus. 2. Additive effects of subordinate help increased productivity. Total feeding rates to the nest were increased by two or more subordinates, and fledgling production was greater in larger groups. Not all subordinates contributed to nestling feeding, and the effect of group size was greater when non-contributors were excluded from analyses, suggesting that increased fledgling production was a direct result of help. 3. Compensatory effects of subordinate help improved breeder survival. Assisted breeders reduced their workload by 20-30%, irrespective of the number of helpers. Although re-nesting intervals were not affected by group size, reduced breeder feeding rates resulted in improved survival and breeders in larger groups survived better. 4. Subordinates and nestlings are usually progeny of the breeding pair in this species, and benefits of cooperative breeding are very different from three congeners with extremely high levels of extra-group paternity (EGP). In these Malurus, fledgling production and survival of male breeders are not enhanced in larger groups. This is consistent with the expectation that kin-selected benefits vary with relatedness, and thus levels of EGP. 5. We tested whether benefits of cooperative breeding in 37 avian species varied with levels of extra-group mating. Both direct and phylogenetically controlled comparisons showed that improvement of (male) breeder survival and enhanced productivity are more likely when fidelity is higher, as predicted when investment of subordinates correlates with relatedness to offspring. This pattern highlights the importance of considering the genetic mating system for understanding the evolution of cooperative breeding.

Brood Parasitism and the Evolution of Cooperative Breeding in Birds

Power in Numbers Avian brood parasites target particular bird species to raise their offspring, sometimes at great cost to the foster family. Feeney et al. (p. 1506; see the Perspective by Spottiswoode) analyzed the global distribution of brood parasitism and found a correlation with the occurrence of cooperative breeders across multiple taxa. For example, Australian fairy wrens breed both singly and in cooperative groups, but the group breeders are better able to resist parasites than lone pairs, indicating that the prevalence of cooperative breeding may be a response to brood parasites. The skewed global distribution of cooperatively breeding birds may result from their coevolution with brood parasites. [Also see Perspective by Spottiswoode] The global distribution of cooperatively breeding birds is highly uneven, with hotspots inAustralasia and sub-Saharan Africa. The ecological drivers of this distribution remain enigmatic yet could yield insights into the evolution and persistence of cooperative breeding. We report that the global distributions of avian obligate brood parasites and cooperatively breeding passerines are tightly correlated and that the uneven phylogenetic distribution of cooperative breeding is associated with the uneven targeting of hosts by brood parasites. With a long-term field study, we show that brood parasites can acquire superior care for their young by targeting cooperative breeders. Conversely, host defenses against brood parasites are strengthened by helpers at the nest. Reciprocally selected interactions between brood parasites and cooperative breeders may therefore explain the close association between these two breeding systems.