Variation in mating system among birds: Ecological basis revealed by hierarchical comparative analysis of mate desertion

Since most bird species are socially monogamous, variation among species in social mating systems is determined largely by variation in the frequency of mate desertion. Mate desertion is expected to occur when the benefits, in terms of additional reproductive opportunities, outweigh the costs, in terms of reduced reproductive success from the present brood. However, despite much research, the relative importance of costs and benefits in explaining mating system variation is not well understood. Here, we investigate this problem using a comparative method. We analyse changes in the frequency of male desertion at different phylogenetic levels. Differences between orders and families in the frequency of desertion are negatively associated with changes in the potential costs of desertion, but are not associated with changes in the potential benefits of desertion. Conversely, differences among genera and species in the frequency of desertion are positively associated with increases in the potential benefits of desertion, but not with changes in the potential costs of desertion. Hence, we suggest that mate desertion in birds originates through a combination of evolutionary predisposition and ecological facilitation. In particular, ancient changes in life-history strategy determine the costs of desertion and predispose certain lineages to polygamy, while contemporary changes in the distribution of resources determine the benefits of desertion and thereby the likelihood that polygamy will be viable within these lineages. Thus, monogamy can arise via two very different evolutionary pathways. Groups such as albatrosses (Procellariidae) are constrained to social monogamy by the high cost to desertion, irrespective of the potential benefits. However, in groups such as the accentors (Prunellidae), which are predisposed to desertion, monogamy occurs only when the benefits of desertion are very limited. These conclusions emphasise the additional power which a hierarchical approach contributes to the modern comparative method

Ancient ecological diversification explains life-history variation among living birds

Recent work suggests that life-history strategies lie along a precise line of equilibrium between mortality, fecundity and growth. However, it has proved remarkably difficult to find a convincing ecological explanation for why different organisms are at different positions along this line. This is surprising because life-history traits are generally considered to be closely connected to fitness. Here, we test four candidate variables which represent competing ecological explanations for the origin of avian life-history diversity: food type; foraging range; developmental mode; and nesting habit. First, we find that over 90% of the variation in key life-history traits occurs among lineages corresponding to the phylogenetic level of between families or above. This suggests that variation among living species is almost entirely due to events which occurred in the ancient evolutionary history of birds. Hence we use minor-axis regression models to quantify the direction and magnitude of ancient changes in 'reproductive effort'. We then test for ecological correlates of these ancient changes. Of the four ecological variables tested, only changes in nesting habit are significantly correlated with ancient changes in 'reproductive effort'. The adoption of safe nest sites among archaic birds is consistently associated with reduced 'reproductive effort'. We suggest that life-history variation among living birds is largely due to diversification in nesting habit which occurred over 40 Ma sp, between the Cretaceous and Eocene

Hot, dry and different: Australian lizard richness is unlike that of mammals, amphibians and birds

Aims: 1) To map species richness of Australian lizards and describe patterns of range size and species turnover that underlie them. 2) To assess the congruence in the species richness of lizards and other vertebrate groups. 3) To search for commonalities in the drivers of species richness in Australian vertebrates. Location: Australia Methods: We digitized lizard distribution data to generate gridded maps of species richness and β-diversity. Using similar maps for amphibians, mammals and birds we explored the relationship between species richness and temperature, actual evapotranspiration, elevation and local elevation range. We used spatial eigenvector filtering and geographically-weighted regression to explore geographical patterns and take spatial autocorrelation into account. We explored congruence between the species richness of vertebrate groups whilst controlling for environmental effects. Results: Lizard richness peaks in the central deserts (where β-diversity is low) and tropical northeast (where β-diversity is high). The intervening lowlands have low species richness and β-diversity. Generally, lizard richness is uncorrelated with that of other vertebrates but this low congruence is strongly spatially-structured. Environmental models for all groups also show strong spatial heterogeneity. Lizard richness is predicted by different environmental factors to other vertebrates, being highest in, dry and hot regions. Accounting for environmental drivers, lizard richness is weakly positively related to richness of other vertebrates both at global and local scales. Main Conclusions: Lizard species richness differs from that of other vertebrates. This difference is likely caused by differential responses to environmental gradients and different centres of diversification; there is little evidence for inter-taxon competition limiting lizard richness. Local variation in habitat diversity or evolutionary radiations may explain weak associations between taxa, after controlling for environmental variables. We strongly recommend that studies of variation in species richness examine and account for non-stationarity

Why do female migratory birds arrive later than males?

1 In migratory birds males tend to arrive first on breeding grounds, except in sex-role reversed species. The two most common explanations are the rank advantage hypothesis, in which male–male competition for breeding sites drives stronger selection for early arrival in males than females, and the mate opportunity hypothesis, which relies on sexual selection, as early arrival improves prospects of mate acquisition more for males than for females. 2 To date, theoretical work has focused on selection for early arrival within a single sex, usually male. However, if fitness depends on territory quality, selection for early arrival should operate on both sexes. Here we use two independent modelling approaches to explore the evolution of protandry (male-first arrival) and protogyny (female-first arrival) under the rank advantage and mate opportunity hypotheses. 3 The rank advantage hypothesis, when operating alone, fails to produce consistent patterns of protandry, despite our assumption that males must occupy territories before females. This is because an individual of either sex benefits if it out-competes same-sex competitors. Rather than promoting protandry, the rank advantage mechanism can sometimes result in protogyny. Female–female competition is stronger than male–male competition early in the season, if females compete for a resource (territories occupied by males) that is initially less common than the resource of interest to males (unoccupied territories). 4 Our results support the mate opportunity hypothesis as an explanation of why protandry is the norm in migratory systems. Male-biased adult sex ratios and high levels of sperm competition (modelled as extra-pair young: EPY) both produce protandry as a result of sexual selection. Protogyny is only observed in our models with female-biased sex ratios and low EPY production. 5 We also show that the effects of sex ratio biases are much stronger than those of EPY production, explore the evidence for sex ratio biases and extra-pair paternity in migratory species and suggest future research directions

Global Patterns of Geographic Range Size in Birds

Large-scale patterns of spatial variation in species geographic range size are central to many fundamental questions in macroecology and conservation biology. However, the global nature of these patterns has remained contentious, since previous studies have been geographically restricted and/or based on small taxonomic groups. Here, using a database on the breeding distributions of birds, we report the first (to our knowledge) global maps of variation in species range sizes for an entire taxonomic class. We show that range area does not follow a simple latitudinal pattern. Instead, the smallest range areas are attained on islands, in mountainous areas, and largely in the southern hemisphere. In contrast, bird species richness peaks around the equator, and towards higher latitudes. Despite these profoundly different latitudinal patterns, spatially explicit models reveal a weak tendency for areas with high species richness to house species with significantly smaller median range area. Taken together, these results show that for birds many spatial patterns in range size described in geographically restricted analyses do not reflect global rules. It remains to be discovered whether global patterns in geographic range size are best interpreted in terms of geographical variation in species assemblage packing, or in the rates of speciation, extinction, and dispersal that ultimately underlie biodiversity

Topography, energy and the global distribution of bird species richness

A major goal of ecology is to determine the causes of the latitudinal gradient in global distribution of species richness. Current evidence points to either energy availability or habitat heterogeneity as the most likely environmental drivers in terrestrial systems, but their relative importance is controversial in the absence of analyses of global (rather than continental or regional) extent. Here we use data on the global distribution of extant continental and continental island bird species to test the explanatory power of energy availability and habitat heterogeneity while simultaneously addressing issues of spatial resolution, spatial autocorrelation, geometric constraints upon species' range dynamics, and the impact of human populations and historical glacial ice-cover. At the finest resolution (1 degrees), topographical variability and temperature are identified as the most important global predictors of avian species richness in multipredictor models. Topographical variability is most important in single-predictor models, followed by productive energy. Adjusting for null expectations based on geometric constraints on species richness improves overall model fit but has negligible impact on tests of environmental predictors. Conclusions concerning the relative importance of environmental predictors of species richness cannot be extrapolated from one biogeographic realm to others or the globe. Rather a global perspective confirms the primary importance of mountain ranges in high-energy areas

Analysing extinction risk in parrots using decision trees

Comparative analysis techniques have been successfully applied in a number of recent attempts to identify the species traits associated with a current threat of extinction although less often to predict which species may become threatened in the future. Although prediction of risk is obviously a priority, such analyses are undermined by the fact that there may be non-linear and non-additive relationships between the species traits used. A Decision Tree analysis can accommodate with such relationships and here it is used to explore factors affecting extinction risk in parrots. The results firstly verify that simple biological and biogeographical traits can separate threatened from non-threatened species. It is also possible to predict which species are likely to become threatened in the future. The utility of the method is not in testing evolutionary-based hypotheses to explain extinction risk, rather it is a simple and practical method of confirming and/or predicting levels of risk. For well known taxonomic groups it could be used to confirm current IUCN threat categories and identify which species should receive closest attention when the group is next reviewed. For poorly known groups it could be used to predict categories of threat for unclassified species from small groups of classified ones

Sex differences in the response to environmental cues regulating seasonal reproduction in birds

Although it is axiomatic that males and females differ in relation to many aspects of reproduction related to physiology, morphology and behaviour, relatively little is known about possible sex differences in the response to cues from the environment that control the timing of seasonal breeding. This review concerns the environmental regulation of seasonal reproduction in birds and how this process might differ between males and females. From an evolutionary perspective, the sexes can be expected to differ in the cues they use to time reproduction. Female reproductive fitness typically varies more as a function of fecundity selection, while male reproductive fitness varies more as a function sexual selection. Consequently, variation in the precision of the timing of egg laying is likely to have more serious fitness consequences for females than for males, while variation in the timing of recrudescence of the male testes and accompanying territory establishment and courtship are likely to have more serious fitness consequences for males. From the proximate perspective, sex differences in the control of reproduction could be regulated via the response to photoperiod or in the relative importance and action of supplementary factors (such as temperature, food supply, nesting sites and behavioural interactions) that adjust the timing of reproduction so that it is in step with local conditions. For example, there is clear evidence in several temperate zone avian species that females require both supplementary factors and long photoperiods in order for follicles to develop, while males can attain full gonadal size based on photoperiodic stimulation alone. The neuroendocrine basis of these sex differences is not well understood, though there are many candidate mechanisms in the brain as well as throughout the entire hypothalamo–pituitary–gonadal axis that might be important

Effect of Variation in Nestling Hunger Levels on the Begging Behaviour of Nestlings and the Provisioning Behaviour of Adult American Kestrels

Little is known about how variation in nestling begging intensity influences the behaviour of adult raptors and how responses of adult males and females to such variation might differ. Our objective was to manipulate the begging intensity of nestling American Kestrels (Falco sparverius) and examine the responses of adults. We studied 12 pairs of American Kestrels nesting in nest boxes from 1 March to 1 July 2014 at the Blue Grass Army Depot, Madison County, Kentucky. Nest boxes were modified with a separate compartment for a camcorder to record nestling behaviour, and a second camcorder was placed outside the nests to monitor adult behaviour. To manipulate nestling hunger levels, 12 to 26-day-old nestlings in six nests were deprived of food for 24 h and those in the other six nests were fed until satiated. At each nest, we alternated control (no treatment) and treatment (fed or food deprived) days over a 4 day period to minimise the possible effect of nestling age on adult and nestling behaviour. Nestling begging intensity differed among treatments, with nestlings in food-deprived nests begging with greater intensity after food deprivation and those in fed-treatment nests begging with less intensity after being fed. Adult male and female American Kestrels provisioned nestlings at similar rates, with both sexes feeding nestlings at higher rates after food deprivation and at lower rates after fed treatments. Thus, the begging behaviour of nestling American Kestrels varied with hunger level, and adult American Kestrels responded by adjusting provisioning rates. Although the response of adults to nestling begging suggests that natural selection might favour \u27dishonest\u27 begging to obtain more food, the potential costs of \u27dishonest\u27 begging, such as attracting predators, reduced immunocompetence, and loss of indirect fitness benefits if such begging negatively impacts siblings and parents, may outweigh any possible benefit