Cody J. Dey

The effects of life history and sexual selection on male and female plumage colouration

Classical sexual selection theory provides a well-supported conceptual framework for understanding the evolution and signalling function of male ornaments. It predicts that males obtain greater fitness benefits than females through multiple mating because sperm are cheaper to produce than eggs. Sexual selection should therefore lead to the evolution of male-biased secondary sexual characters. However, females of many species are also highly ornamented. The view that this is due to a correlated genetic response to selection on males was widely accepted as an explanation for female ornamentation for over 100 years and current theoretical and empirical evidence suggests that genetic constraints can limit sex-specific trait evolution. Alternatively, female ornamentation can be the outcome of direct selection for signalling needs. Since few studies have explored interspecific patterns of both male and female elaboration, our understanding of the evolution of animal ornamentation remains incomplete, especially over broad taxonomic scales. Here we use a new method to quantify plumage colour of all ~6,000 species of passerine birds to determine the main evolutionary drivers of ornamental colouration in both sexes. We found that conspecific male and female colour elaboration are strongly correlated, suggesting that evolutionary changes in one sex are constrained by changes in the other sex. Both sexes are more ornamented in larger species and in species living in tropical environments. Ornamentation in females (but not males) is increased in cooperative breeders—species in which female–female competition for reproductive opportunities and other resources related to breeding may be high. Finally, strong sexual selection on males has antagonistic effects, causing an increase in male colouration but a considerably more pronounced reduction in female ornamentation. Our results indicate that although there may be genetic constraints to sexually independent colour evolution, both female and male ornamentation are strongly and often differentially related to morphological, social and life-history variables.

Manipulating the appearance of a badge of status causes changes in true badge expression

Signals of dominance and fighting ability (i.e. status signals) are found in a wide range of taxa and are used to settle disputes between competitive rivals. Most previous research has considered status-signal phenotype as an attribute of the individual; however, it is more likely that signal expression is an emergent property that also incorporates aspects of the social environment. Furthermore, because an individuals signal phenotype is likely to influence its social interactions, the relationships between status signals, social environment and individual quality are probably much more complex than previously appreciated. Here, we explore the dynamic relationship between social interactions and signal expression in a previously undescribed status signal, the frontal shield of the pukeko (Porphyrio porphyrio melanotus: Aves). We demonstrate that frontal shield size is a strong predictor of dominance status within social groups, even after controlling for potentially confounding variables. Then, we evaluate the relationship between social interactions and signal expression by testing whether manipulating apparent shield size influences (i) dominance interactions and (ii) future signal expression. By showing that decreasing apparent shield size causes both an increase in the amount of aggression received and a decrease in an individuals true shield size, we provide the first evidence of dynamic feedback between signal expression and social interactions. Our study provides important insight into the role of receiver-dependent (i.e. social) costs in maintaining signal honesty and demonstrates a unique approach to studying status signalling applicable to future studies on dynamic morphological signals.

The influence of supplemental feeding on survival, dispersal and competition in translocated Brown Teal, or Pateke (Anas chlorotis)

Abstract Supplemental feeding is widely used after the translocation of animals and is presumed to increase post-release survival or reproductive output. However, the results of empirical studies on supplemental feeding are equivocal and research is needed to determine the mechanisms by which supplemental feeding affects health and behaviour. Here, we studied the effect of supplemental feeding on the Brown Teal, or Pateke (Anas chlorotis), an endangered duck endemic to New Zealand, following four translocations of captive-bred individuals. Radio-telemetric monitoring showed no significant effect of supplemental feeding on post-release survival. Male birds dispersed further than females, and supplemental feeding decreased post-release dispersal. To reduce heterospecific competition at Brown Teal feeders, we also tested an exclusion device designed to prevent the main heterospecific competition, the Purple Swamphen (Pukeko in New Zealand, Porphyrio porphyrio melanotus), from accessing supplemental food. Although this device decreased the presence of Purple Swamphens at feeders, it also decreased use of feeders by Brown Teal. Ultimately, we concluded that supplemental feeding has value as a conservation tool for Brown Teal, particularly during releases in managed areas. Further studies on feeder design, as well as spatial and temporal patterns of use of feeders, are needed to maximise the positive effect of supplemental feeding on success of translocations.

Carotenoid‐based bill coloration functions as a social, not sexual, signal in songbirds (Aves: Passeriformes)

Many animals use coloration to communicate with other individuals. Although the signalling role of avian plumage colour is relatively well studied, there has been much less research on coloration in avian bare parts. However, bare parts could be highly informative signals as they can show rapid changes in coloration. We measured bill colour (a ubiquitous bare part) in over 1600 passerine species and tested whether interspecific variation in carotenoid‐based coloration is consistent with signalling to potential mates or signalling to potential rivals in a competitive context. Our results suggest that carotenoid bill coloration primarily evolved as a signal of dominance, as this type of coloration is more common in species that live in social groups in the nonbreeding season, and species that nest in colonies; two socio‐ecological conditions that promote frequent agonistic interactions with numerous and/or unfamiliar individuals. Additionally, our study suggests that carotenoid bill coloration is independent of the intensity of past sexual selection, as it is not related to either sexual dichromatism or sexual size dimorphism. These results pose a significant challenge to the conventional view that carotenoid‐based avian coloration has evolved as a developmentally costly, condition‐dependent sexual signal. We also suggest that bare part ornamentation may often signal different information than plumage ornaments.

Tolerance of female co-breeders in joint-laying pukeko: the role of egg recognition and peace incentives

Joint laying, where multiple females contribute eggs to a single nest and provide parental care, is a rare breeding system in the avian world. Currently, little is understood about the fitness consequences that joint laying imposes on all members of a cooperatively breeding group, and this is necessary for understanding the evolution of this unusual breeding system. Here, we combine descriptive and experimental studies to understand the costs of joint laying in the pukeko, Porphyrio porphyrio melanotus. Our study shows when total clutch size is large (as a result of two females contributing eggs to a joint clutch), a lower percentage of eggs hatch. As a result, joint-laying females were reproductively compromised relative to females that nested singly. Given this apparent cost of joint laying, it is surprising that females tolerate the eggs of co-breeding females. Thus, in a follow-up study, we tested whether female pukeko restrain from ovicide to avoid the risk of retaliation by the co-breeding female (i.e. the peace incentive hypothesis). Females did not retaliate or cease parental care in response to experimental removal of their eggs from joint nests, and thus, we found no evidence to support the peace incentive hypothesis. However, we suggest that the risk of nest desertion, most likely initiated by male partners, may be an indirect cost preventing the evolution of ovicide. This threat of nest abandonment could force primary laying females to tolerate eggs laid by secondary females.

Direct benefits and evolutionary transitions to complex societies

The selective forces that drive the evolution of cooperation have been intensely debated. Evolutionary transitions to cooperative breeding, a complex form of cooperation, have been hypothesized to be linked to low degrees of promiscuity, which increases intragroup relatedness and the indirect (that is, kin selected) benefits of helping. However, ecological factors also promote cooperative breeding, and may be more important than relatedness in some contexts. Identifying the key evolutionary drivers of cooperative breeding therefore requires an integrated assessment of these hypotheses. Here we show, using a phylogenetic framework that explicitly evaluates mating behaviours and ecological factors, that evolutionary transitions to cooperative breeding in cichlid fishes were not associated with social monogamy. Instead, group living, biparental care and diet type directly favoured the evolution of cooperative breeding. Our results suggest that cichlid fishes exhibit an alternative path to the evolution of complex societies compared to other previously studied vertebrates, and these transitions are driven primarily by direct fitness benefits.The selective forces that drive the evolution of cooperation have been intensely debated. Evolutionary transitions to cooperative breeding, a complex form of cooperation, have been hypothesized to be linked to low degrees of promiscuity, which increases intragroup relatedness and the indirect (that is, kin selected) benefits of helping. However, ecological factors also promote cooperative breeding, and may be more important than relatedness in some contexts. Identifying the key evolutionary drivers of cooperative breeding therefore requires an integrated assessment of these hypotheses. Here we show, using a phylogenetic framework that explicitly evaluates mating behaviours and ecological factors, that evolutionary transitions to cooperative breeding in cichlid fishes were not associated with social monogamy. Instead, group living, biparental care and diet type directly favoured the evolution of cooperative breeding. Our results suggest that cichlid fishes exhibit an alternative path to the evolution of complex societies compared to other previously studied vertebrates, and these transitions are driven primarily by direct fitness benefits.

Investigating the influence of social dominance on survival during a pukeko cull

Lethal control of wildlife is commonly used by conservation practitioners for population control. In some areas of New Zealand, changes in land-use and management have led to large increases in pukeko (Porphyrio melanotus melanotus) range and numbers. This native rail is sometimes considered a pest species, as they are known to uproot vegetation including tree seedlings, grass and crops. Here, we provide the first data on mortality during a lethal control operation that aimed to reduce pukeko population size at Tawharanui Regional Park in the North Island of New Zealand. We combined mortality records with individual measurements and colour banding re-sighting data to determine whether sex or dominance influenced survival. We found that frontal shield size (a strong proxy for social dominance) did not significantly influence the probability of being culled. There was also no significant difference in the probability of being culled between sexes. Our study provides important insights into mortality in a native species during lethal control, which could influence population recovery and social dynamics.

Climate change ecology: Hot under the collar

A 34-year study of collared flycatchers demonstrates that males are evolving to be less ornamented in response to rising temperatures.

Plumage coloration in passerine birds

Plumage colour scores (N=5831 species) and predictor variables (N = 2471 species) in passerine birds. This data set includes plumage scores for males and females as calculated from RGB (red, green, blue) values measured from plates in the Handbook of the Birds of the World (see text of article for details). Additionally we include the values of the 5 key predictor variables: 1) body size (first phylogenetic principle component (PPC1) of body mass and wing length), 2) tropical life history (PPC1 of latitude, seasonality and clutch size), 3) sexual selection (PPC1 of mating system, sexual size dimorphism and paternal care), 4) cooperative breeding (no = 0 and yes = 1) and 5) migration (none = 0, partial = 0.5 and complete = 1). All predictor variables were scaled to a mean = 0 and standard deviation = 1. These variables were used in the multiple predictor phylogenetic generalized least squares model, Markov Chain Monte Carlo generalized linear mixed models and phylogenetic controlled d separation path analysis (Table 1b, Figure 4 and Figure 5 of the article respectively). Scientific names, English names and TipLabels correspond to the phylogenetic trees provided by www.birdtree.org (Jetz, W., Thomas, G., Joy, J., Hartmann, K., & Mooers, A. 2012. The global diversity of birds in space and time. Nature, 491(7424), 444-448).