Ian R. Hartley

SEXUAL DIMORPHISM IN BIRDS : WHY ARE THERE SO MANY DIFFERENT FORMS OF DIMORPHISM?

Variation in the extent of sexual dimorphism among bird species is traditionally attributed to differences in social mating system. However, there are many different forms of dimorphism among birds, and not all of them show an obvious correlation with social mating system. For example, recent work has shown that many highly polygamous species are, in fact, monomorphic, whereas many putatively monogamous species are dimorphic. In this paper we break up sexual dimorphism into subcomponents and then use comparative analyses to examine the pattern of covariation between these subcomponents and various aspects of sexual, social, and parental behaviour. Our first finding is that size dimorphism and plumage–colour dimorphism do not show the same pattern of covariation. Differences in size dimorphism are associated with variation in social mating system and sex differences in parental care, whereas differences in plumage–colour dimorphism are associated with variation in the frequency of extra–bond paternity. These results suggest that size dimorphism is associated with the sort of intrasexual competition described by traditional classifications of social mating system, whereas plumage–colour dimorphism is associated with cryptic female choice. However, when we break up plumage–colour dimorphism according to whether it is due to melanins, carotenoids or structural colours, we find that each category of plumage–colour dimorphism shows a different pattern of covariation. The correlation between overall plumage–colour dimorphism and the rate of extra–bond paternity is due to structural colours, whereas melanin–based dimorphism is associated with sex differences in parental care. The former result is particularly interesting given that new work suggests structural colours are associated with active sexual displays and the reflection of ultraviolet light.

Begging for control: when are offspring solicitation behaviours honest?

There is burgeoning interest in the idea that conspicuous begging displays, when parents are provisioning dependent young, advertise offspring need honestly to parents. Many empirical studies claim to support the theory of honest signalling of need, where parents control resource allocation. The evidence, however, also fits the predictions of recent models for the evolution of costly begging, where offspring control allocation. These models incorporate variation in offspring condition and show that the three main predictions of honest signalling models are also found with models of sibling scramble competition. Consequently, it is difficult to discriminate between the two different modelling approaches from their predictions, despite their having been the focus of much empirical work. In particular, the evidence indicates that the prediction that begging intensity signals offspring need honestly is strongly context dependent. Begging might be ‘honest’ only when the potential for conflict is low and food is not limiting.

Heterotrophic microbial communities use ancient carbon following glacial retreat.

When glaciers retreat they expose barren substrates that become colonized by organisms, beginning the process of primary succession. Recent studies reveal that heterotrophic microbial communities occur in newly exposed glacial substrates before autotrophic succession begins. This raises questions about how heterotrophic microbial communities function in the absence of carbon inputs from autotrophs. We measured patterns of soil organic matter development and changes in microbial community composition and carbon use along a 150-year chronosequence of a retreating glacier in the Austrian Alps. We found that soil microbial communities of recently deglaciated terrain differed markedly from those of later successional stages, being of lower biomass and higher abundance of bacteria relative to fungi. Moreover, we found that these initial microbial communities used ancient and recalcitrant carbon as an energy source, along with modern carbon. Only after more than 50 years of organic matter accumulation did the soil microbial community change to one supported primarily by modern carbon, most likely from recent plant production. Our findings suggest the existence of an initial stage of heterotrophic microbial community development that precedes autotrophic community assembly and is sustained, in part, by ancient carbon.

Bacterial pathogens in wild birds: a review of the frequency and effects of infection

The importance of wild birds as potential vectors of disease has received recent renewed empirical interest, especially regarding human health. Understanding the spread of bacterial pathogens in wild birds may serve as a useful model for examining the spread of other disease organisms, both amongst birds, and from birds to other taxa. Information regarding the normal gastrointestinal bacterial flora is limited for the majority of wild bird species, with the few well‐studied examples concentrating on bacteria that are zoonotic and/or relate to avian species of commercial interest. However, most studies are limited by small sample sizes, the frequent absence of longitudinal data, and the constraints of using selective techniques to isolate specific pathogens. The pathogenic genera found in the gut are often those suspected to exist in the birds’ habitat, and although correlations are made between bacterial pathogens in the avian gut and those found in their foraging grounds, little is known about the effect of the pathogen on the host, unless the causative organism is lethal. In this review, we provide an overview of the main bacterial pathogens isolated from birds (with particular emphasis on enteropathogenic bacteria) which have the potential to cause disease in both birds and humans, whilst drawing attention to the limitations of traditional detection methods and possible study biases. We consider factors likely to affect the susceptibility of birds to bacterial pathogens, including environmental exposure and heterogeneities within the host population, and present probable avenues of disease transmission amongst birds and from birds to other animal taxa. Our primary aim is to identify gaps in current knowledge and to propose areas for future study.

Female Song Attracts Males in the Alpine Accentor Prunella collaris

In the Pyrenees, alpine accentors bred in polygynandrous groups of up to four males and four females. Complex songs were produced by females as well as males. Females sang only during their fertile period, and they sang at a greater rate when alone than when they were accompanied by one or more mateguarding males. Female songs elicited approaches from males, but not from other females, and females often solicited copulations as soon as the male arrived. Playbacks showed that males respond differently to, and hence distinguish between, male and female songs. This study provides the first evidence that female song in birds attracts males and we suggest that it has evolved in alpine accentors because both sexes compete intensively for mates in the polygynandrous groups. Female song may advertise quality as well as receptivity for mating because older females, which laid larger clutches, sang more complex songs. We show that female song not only attracts mates but may also provide females with a mechanism for sharing mating access among several males, so females obtain maximum paternal care for their offspring.

The polygynandrous mating system of the alpine accentor, Prunella collaris. II. Multiple paternity and parental effort

Parentage was determined by DNA fingerprinting for 110 young from 38 broods. There was no intraspecific brood parasitism and no young were fathered by males from outside the polygynandrous group. Nineteen (50%) broods were fathered by one male (15 by alpha, four by beta), 17 by two males (14 alpha and beta, two alpha and gamma, one beta and gamma) and two by three males (alpha, beta and gamma). A males paternity share of a brood increased with his share of the matings, measured as the proportion of time he gained exclusive access to the female. This relationship was the same for alpha and beta males, which suggests that their copulations were of equal potency. Alpha males gained a larger share of the paternity by guarding fertile females and their overall paternity within the group tended to increase with female nesting asynchrony, although not significantly so, and to decrease with more competing subordinate males. Males were more likely to help feed nestlings if they gained a greater share of the matings with the mother. There was no difference between alpha and beta males in the relationship between the probability of helping and mating share. Given a choice between two synchronous broods, males preferred to help where their mating share was greatest. When alpha and beta males helped at the same nest their share of the feeds reflected their share of matings, not their dominance rank. When only one male helped at a nest, alpha males decreased their amount of help with decreased mating share, whereas beta males did not. This difference may arise because alpha males have greater chances of mating with other females in the group, so opportunity costs of helping are greater for them. How these different helping responses influence female preference for alpha versus beta males is discussed.

Sibling competition and the evolution of growth rates in birds

Variation among bird species in growth rates is traditionally attributed to differences in energy availability and developmental mode. However, the extent and form of competition among siblings for limited food resources may also be an important determinant. Kin–selection–based models of intrabrood competition suggest that nestling growth rates should be highest in those species in which siblings are likely to be less genetically related to one another (half–sibs rather than full–sibs). We test this novel prediction using the frequency of multiple paternity as an index of average sib relatedness within broods. As predicted, we find a significant positive association between the rate of multiple paternity within broods and nestling growth rates. Furthermore, this holds true when we control for the effects of variation in other factors that may be associated with variation in growth rate, such as body size, brood size, mating system and the form of parental care. We suggest, therefore, that variation in growth rate among bird species is not simply dependent on proximate ecological and developmental factors but is also strongly influenced by interactions, over an evolutionary time–scale, among kin.

Linkages between soil biota, nitrogen availability, and plant nitrogen uptake in a mountain ecosystem in the Scottish Highlands

This study examined the seasonal partitioning of nitrogen (N) between plants and soil microorganisms on a mountain plateaux (Carex bigelowii—Racomitrium lanuginosum heath) in the Scottish Highlands. Specifically, we determined whether there is a temporal relationship between the abundance of dominant soil fauna, soil nitrogen availability, and the partitioning of N between microbes and plants. We found that soil microorganisms and plant tissue of Carex contain similar portions of the total N pool, but that this N partitioning varies greatly over the growing season. At the onset of plant growth in May, the microbial N pool was at its lowest (1.8% of total N), and available N was sequestered by the severely N-limited microbial biomass. At this time, plant N requirement appears to have been met by the use of internal reserves of N in roots. Significant net mineralisation of N was not detected until microbial demands for N had been satisfied in June. Peak rates of N mineralisation corresponded to maximal plant biomass and shoot N content of Carex in July, whereas microbial sequestration of N was found to be most intense in late season when plant demands had subsided after senescence. Although microbial biomass was lowest in early season, we find no convincing evidence to support the notion that microbial competition for N is limited at this time. The patterns of N partitioning that we describe are likely to have implications for the retention of N in these high mountain ecosystems.

Parental investment and family dynamics: interactions between theory and empirical tests

The pattern of parental investment (PI) seen in nature is a product of the simultaneous resolution of conflicts of interest between the members of a family. How these conflicts are resolved depends upon the mating system, the genetic mechanism, on whether extra PI affects current or future offspring, and the behavioural mechanisms underlying supply and demand of PI. Until recently very little empirical work has been done to underpin these key determinants of conflict resolution. This review examines recent empirical progress in understanding both (1) how conflict is resolved and (2) its evolutionary consequences. How offspring demand interacts with parental supply of resources determines how conflict is resolved. Two extremes are: passive parental choice of competing offspring, relating to offspring control of resource allocation, and active parental choice relating to parental control. Although most previous empirical work has tended to conclude or assume that parents primarily control resource allocation decisions, recent studies explicitly examining predictions from theoretical analyses have shown that offspring control of resource allocation is more important than previously realised. The amount of PI supplied at resolution depends not on who controls food allocation, however, but on the nature of the supply and demand mechanisms. These have yet to be established experimentally, but a recent regression model illustrates how this could be achieved in the field. Determination of the effect of supply on demand (ESD) and the effect of demand on supply (EDS) mechanisms is critical to parent–offspring conflict theory, which has not been adequately tested empirically. There is an underlying, and until recently untested, assumption of models of intrafamilial conflict that there is genetic variation for both offspring demand and parental supply behaviours, so that the behaviours can coevolve. Recent studies on great tits, burrower bugs and mice all found evidence for genetic variation in supply and demand behaviours, but the predicted negative correlation between genes expressed in mothers and their offspring (i.e. parent–offspring coevolution), was found only for burrower bugs. The lack of a negative relationship for great tits and mice may have been a consequence of antagonistic coevolution between the sexes (sexual conflict). These studies illustrate the importance of the underlying genetics and mating system in determining conflict resolution, and point to the need for new models (especially of interbrood competition) taking differences in the genetics and the co-evolution of the ESD and EDS mechanisms into account. We also discuss the importance of the comparative approach in determining evolutionary consequences of conflicts, and use the recent work on growth costs of begging to illustrate the difficulties of measuring costs of conflict in an evolutionary currency. The recent growth in empirical work on conflicts in families illustrates an increasing, and increasingly productive, integration between theoreticians and empiricists.

The energetic costs of nest building in birds

Parental investment in reproduction is generally limited by the availability of food and so avian life-history research has generally focused on the brood rearing phase, when food requirements are greatest. Only relatively recently has the focus extended to the incubation phase and, even more recently, to the nest-building phase where there is now observational, comparative and experimental evidence that avian nest building is an energetically and temporally expensive activity. This review emphasises that nest construction is energetically expensive and we suggest that such costs should be incorporated into avian life-history research.