Red Bird in a Brown Bag
Abstract
The House Finch is among the most mundane birds, so ubiquitous and familiar across the U.S. and Canada that it does not rate a glance from most bird enthusiasts. But males have carotenoid-based plumage coloration that varies markedly among individuals, making the House Finch a model species for studies of the function and evolution of colorful plumage. In more depth and detail than has been attempted for any species of bird, this book takes a tour of the hows and whys of ornamental plumage coloration. The book begins by reviewing the history of the study of colorful plumage, which began in earnest with the debates of Darwin and Wallace but which was largely forgotten by the middle of the 20th century. Documenting the extensive plumage variation among males both within and between populations of House Finches, the book explores the mechanisms behind plumage variation and looks at the fitness consequences of condition-dependent ornament display for both males and females. The book concludes by examining the processes by which carotenoid-based ornamental coloration may have evolved.
... House finches (Haemorhous mexicanus) are songbirds with sexually dichromatic plumage and elaborate male song, both of which are used by females of the species to assess mates [6,7]. Male plumage ranges from yellow to vibrant red, with females preferring red males [7], and males produce song with longer bouts that contain more unique syllables during the breeding season [8]. ...
... House finches (Haemorhous mexicanus) are songbirds with sexually dichromatic plumage and elaborate male song, both of which are used by females of the species to assess mates [6,7]. Male plumage ranges from yellow to vibrant red, with females preferring red males [7], and males produce song with longer bouts that contain more unique syllables during the breeding season [8]. The extreme variation in plumage colour and their highly social nature make house finches ideal to investigate how investment in a signal in one dimension ( plumage colour) affects males' investment in another (song) as a function of perceived competition for mates. ...
... In summer 2010 and 2011, house finches were captured around Davis, CA, USA and sexed by plumage [7]. Birds were held in aviaries at the University of California, Davis on naturally changing photoperiods. ...
Animals use multiple signals to attract mates, including elaborate song, brightly coloured ornaments and physical displays. Female birds often prefer both elaborate male song and intense carotenoid-based plumage coloration. This could lead less visually ornamented males to increase song production to maximize their attractiveness to females. We tested this possibility in the highly social and non-territorial house finch (Haemorhous mexicanus), in which females discriminate among males based on both song and on the intensity of red carotenoid-based plumage coloration. We manipulated male plumage coloration through carotenoid supplementation during moult, so that males were either red or yellow. Males were then housed under three social environments: (i) all red birds, (ii) all yellow birds or (iii) a mixture of red/yellow birds. We recorded song after presentation of a female. Red males produced more song than yellow males. But when yellow males were housed with red conspecifics, they produced more song relative to yellow males housed with equally unattractive yellow males. This study provides novel evidence that a male's plumage coloration and the plumage colour of his social competitors influence investment in song.
... The distinction between specialized and generalized traits provides a useful conceptual framework for many ecological and evolutionary studies (Amadon 1943;Futuyma and Moreno 1988;Bleiweiss 1990;Kelley and Farrell 1998;Johnson and Steiner 2000;Poisot et al. 2011;Dapporto and Dennis 2012;Vamosi et al. 2014). As usually understood, these terms refer to organismal adaptations in relation to extrinsic factors (Litsios et al. 2014;Wilson and Hayek 2015) as described by physical (Hailman 1977;Endler 1993;Endler and Théry 1996;Heindl and Winkler 2003;Seehausen et al. 2008), ecological (Nosil 2002;Håstad et al. 2005;Jablónski et al. 2006), and social (Baker and Parker 1979;Götmark 1994;Senar 1999;Slagsvold et al. 1995;Hill 2002;Seehausen et al. 2008) environments. In principal, the same generalist-specialist paradigm also can be applied to communication systems, which involve the exchange of information mediated by intrinsic factors relating to the signaling system of "senders" and the sensory systems of "receivers" through one or more signaling and sensory modalities. ...
... This paper tests the alignment ("tuning") patterns suggested by past comparisons within this more rigorous comparative framework. The results suggest specialized alignments of carotenoid-based plumage absorption with VS and UVS visual system-specific receptor sensitivities, which is sensible given the importance of plumage carotenoids to avian communication, health, and fitness (Hill 1996(Hill , 2002McGraw 2006). ...
... Corresponding degrees of freedom were then calculated as N (sample size) − 1 (the number of contrasts) − 1 (the number of contrasts except the basal contrast) − 1, as indicated (see Garland et al. 1991Garland et al. , 1993 d T-test of significant difference between the basal contrast and the sample of all remaining contrasts (see "Materials and Methods", and Garland et al. 1993) e One-tailed tests justified by expectations based on two earlier studies (Bleiweiss 2014(Bleiweiss , 2015. Critical value to determine conventional significance (P < 0.05) based on a t-distribution is provided for each test statistic and associated degrees of freedom f Complete = all specimens included in analysis; no outliers = outlier specimens identified in box-andwhisker plots (Fig. 3) (Hill 1996(Hill , 2002Rodriguez-Amaya 2001;McGraw 2006). Thus, parallel shifts in λR min among yellow and red carotenoids could arise if the same endogenous modifications were applied to different dietary precursors used by birds pertaining to each visual system. ...
Recent evidence that absorption maxima (λRmin) expressed by colorful plumage pigments align to diagnostic cone sensitivities of affiliated visual systems suggests that birds employ specialized signals in relation to their color vision. However, these studies compared different pigments and clades for the violet (porphyrins in non-passerines) and ultraviolet (carotenoids in passerines) sensitive system, which confounds chemistry and phylogeny with tuning patterns. To test whether signal alignments to violet (VS) and ultraviolet (UVS) systems transcend confounding factors, parallel analyses were conducted for a diversity of near-passerines, a group in which plumage carotenoids occur in taxa with either visual system. Conventional and phylogenetically informed analyses confirmed earlier findings: short wavelength absorbing (yellow carotenoid) pigments aligned λRmin with the violet-sensitive (V) cone of VS species but with the short wavelength-sensitive (S) cone of UVS species, whereas long wavelength-absorbing (red carotenoid) pigments aligned only with the S cone of VS species. More extensive variation among VS yellow carotenoids produced λRmin alignments to cone sensitivities that differed at shorter (peaks) versus longer (overlaps) wavelengths. Ancestral trait reconstructions indicated that signals evolved to match pre-existing VS systems, but did not resolve scenarios for UVS systems. Regardless of historical details, alignments expressed a higher-level pattern in which λRmin values were blue-shifted for yellow and red carotenoids in VS compared to UVS species, a pattern opposite that expressed by receptor sensitivities between systems. Thus, generalized functional designs attributed to avian color vision allow for specialized visual communication through the development of chromatic signals suited to each perceptual system.
... Once wrapping is complete, we cover each end with a 6 mm rubber tip 8 and the entire rod in 8 mm polyolefin heat shrink tubing 9 . Then, the two loose wires can be lightly stripped and soldered to 2 ft long leads 10 . These leads should be attached to the three position 1.5 mm connector 11 by crimping each wire to the 24-26 AWG terminals 12 and inserting them into the left-most and right-most positions. ...
... Due to high levels of dispersal (∼91.3% based on return rate), we banded an additional 47 house finches in the spring and summer of 2019 (immature: 63.8%; adult female: 8.5%; adult male: 27.7%). All birds were captured using perch traps 17 , and adult female and immature birds were distinguished based on the color of their secondary covert feathers [10]. ...
Radio-frequency identification (RFID) is an increasingly popular wireless technology that allows researchers to monitor wild bird populations from fixed locations in the field. Our lab has developed an RFID-equipped birdfeeder based on the Raspberry Pi Zero W, a low-cost single-board computer, that collects continuous visitation data from birds tagged with passive integrated transponder (PIT) tags. Each birdfeeder has a perch antenna connected to an RFID reader board on a Raspberry Pi powered by a portable battery. When a tagged bird lands on the perch to eat from the feeder, its unique code is stored with the date and time on the Raspberry Pi. These birdfeeders require only basic soldering and coding skills to assemble, and can be easily outfitted with additional hardware like video cameras and microphones. We outline the process of assembling the hardware and setting up the operating system for the birdfeeders. Then, we describe an example implementation of the birdfeeders to track house finches (Haemorhous mexicanus) on the campus of Queens College in New York City.
... All birds were captured using perch traps 18 . Immature birds were distinguished from adult females, which also lack the bright colours of adult males, by the colour of their secondary covert feathers (Hill 2002). The protocol for capture and banding was approved by the Institutional Animal Care and Use Committee of Queens College (Protocol #179), and conducted with a federal permit from the United States Department of the Interior (Permit #23708). ...
Radio-frequency identification (RFID) is an increasingly popular wireless technology that allows researchers to monitor wild bird populations from fixed locations in the field. I have developed an RFID-equipped birdfeeder based on the Raspberry Pi Zero W, a low-cost single-board computer, that collects continuous visitation data from birds marked with passive integrated transponder (PIT) tags. Each birdfeeder has a perch antenna connected to an RFID-reader board on a Raspberry Pi powered by a portable battery. When a tagged bird lands on the perch to eat from the feeder, its unique code is stored with the date and time on the Raspberry Pi. These birdfeeders require only basic soldering and coding skills to assemble, and can easily be outfitted with additional hardware like video cameras and microphones. I outline the process of assembling the hardware and setting up the operating system for the birdfeeders. Then, I describe an example implementation of the birdfeeders to track House Finches Haemorhous mexicanus on the campus of Queens College in New York City.
... At some level, this is perfectly understandable: elucidating the mechanisms of sexual selection within species is an exciting challenge, and clearly difficult enough. Remarkable progress has been made, and we now have a detailed understanding of many classic model systems, including guppies (Houde, 1997), red deer (Clutton-Brock, Guinness & Albon, 1982), túngara frogs (Ryan, 1985), house finches (Hill, 2002), and crickets (Zuk & Simmons, 1997). However, broad-scale patterns in the evolution and distribution of sexually selected traits across the animal Tree of Life remain largely unexplored. ...
Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).
... Plumage ornamentation is a striking example of colour and pattern diversity in the animal kingdom and has attracted considerable research (Hill, 2002). Most studies have focused on sexual selection as the key mechanism to explain this diversity in ornamentation (Andersson, 1994;Dale et al., 2015). ...
The status signalling hypothesis aims to explain within-species variation in ornamentation by suggesting that some ornaments signal dominance status. Here, we use multilevel meta-analytic models to challenge the textbook example of this hypothesis, the black bib of male house sparrows (Passer domesticus). We conducted a systematic review, and obtained primary data from published and unpublished studies to test whether dominance rank is positively associated with bib size across studies. Contrary to previous studies, the overall effect size (i.e. meta-analytic mean) was small and uncertain. Furthermore, we found several biases in the literature that further question the support available for the status signalling hypothesis. We discuss several explanations including pleiotropic, population- and context-dependent effects. Our findings call for reconsidering this established textbook example in evolutionary and behavioural ecology, and should stimulate renewed interest in understanding within-species variation in ornamental traits.
... Since then, although the bacterial disease has stabilized to endemic levels, males in some populations remain redder than they were prior to the epidemic (Hill 2002). ...
Emerging infectious diseases are increasingly recognized as key threats to wildlife.
Batrachochytrium dendrobatidis (Bd), the causative agent of chytridiomycosis, has been
implicated in mass mortalities, population declines, and local and global extinctions of
many species of amphibians around the world. As such, it is currently the largest
infectious disease threat to biodiversity. Understanding the distribution and spatial
dynamics of Bd is crucial to predicting spread to new geographic areas, revealing the
history of infection, and developing appropriate management strategies. One of the most
striking features of Bd is the variability in outcome of infection that has been observed
within a species, among populations. By identifying and comparing differences in
variables that co-vary between populations exhibiting different infection characteristics,
we can start to disentangle the mechanisms allowing for parasite persistence and
proliferation. However, infection dynamics operate across nested levels of biological
organization: within-host processes underlie among-host processes within a population.
As such, this thesis works within the classical themes of spatial epidemiology to consider:
1) the distribution of Bd and the evidence for spatial heterogeneity in both the prevalence
and intensity of infection, and 2) the role of individual- and population-level traits in
defining infection outcome.
The research presented, identifies that Bd functions endemically within Rana pipiens
populations in Ontario. Outbreaks of chytridiomycosis are not observed, but infection
dynamics show significant interannual fluctuations related to stable geographic factors
and local climatic nuances experienced at particular host life history stages. However, Rana pipiens also display variation in resistance to the pathogen, mediated by
thermoregulation, dispersal behaviour, and phenotypic properties. Comparisons between
host populations show variation in skin-associated bacterial communities, which may
mediate susceptibility to chytridiomycosis. These bacterial communities are found to
vary across latitude and between sites experiencing different levels of anthropogenic
disturbance. Additionally, individual level traits, such as amphibian body temperature
and body size are reported to influence bacterial community. Hence, this research
highlights the importance of considering context-dependent individual- and populationlevel
environmental heterogeneity, when attempting to predict the infection risk of Bd.
Animal populations can exhibit dramatic variation in individual fitness, and microbiota are emerging as a potentially understudied factor influencing host health. Bacterial diversity and community structure of the gut microbiome are associated with many aspects of fitness in animals, but relatively little is known about the generality of these relationships in wild populations and non‐mammalian taxa. We studied the northern cardinal Cardinalis cardinalis , a member of a taxon that is ecologically important but underrepresented in microbiome research: songbirds. To test for relationships between the microbiota and host fitness, we sampled the cloacal microbiomes of wild cardinals and measured body condition index, assessed coloration of sexual ornaments (beak and plumage), and collected blood to estimate the glucocorticoid response to stress. Both alpha and beta bacterial diversity were related to individual variation in body condition and several sexual ornaments, but not glucocorticoid concentrations. Our results from a free‐living songbird population add to a growing body of research linking avian host fitness to internal bacterial community characteristics. This study sets the stage for manipulative experiments to determine how challenges to fitness and microbiomes may upset these relationships.
There are many environmental, life-history, and physiological predictors of disease in animals, but we need more studies that examine multiple factors simultaneously to better understand comparative risks of infection in wildlife. The House Finch (Haemorhous mexicanus) is a North American songbird that has served as a model ecological system for assessing avian disease predictors and responses. Among its various diseases is coccidiosis, an intestinal parasitic infection that is more common in urban finches and in males with drab plumage. Here, we examined additional nutritional–physiological predictors of coccidiosis presence and severity in House Finches, with an emphasis on plasma concentrations of lipid-soluble vitamins (retinol, or vitamin A; tocopherol, or vitamin E) as well as blood glucose and ketones. We found that plasma retinol concentration significantly predicted the presence of coccidiosis; coccidia-infected finches had lower levels of retinol than those without coccidiosis. We also found an effect of the sex*retinol level interaction on coccidiosis severity; retinol levels were lower in more-infected females, but no such pattern was found in males. Overall, we demonstrate a unique link between retinol and intestinal disease status in a wild bird species; to date, such a relationship had only been uncovered in domestic chickens. Mechanistically it is possible that, due to the antioxidant and immunosupportive properties of retinol, animals deficient in retinol are more susceptible to coccidial infections, or that, as in poultry, infection directly lowers retinol levels; experimental manipulations are needed to disentangle these possibilities and to understand why the negative relationship between retinol and coccidiosis severity exists in females but not males.
Flamingos (Phoenicopteriformes) are relatively well studied species of bird, both in the wild and in captive environments. Arguably, the two most famous anecdotes surrounding flamingo behaviour concerns their time spent on one leg (what is this related to?) and time spent preening (is it more than other species of waterbird?). We conducted an observational study at WWT Slimbridge Wetland Centre on five species of flamingos to test whether empirical evidence supported these anecdotes. We coupled this observational study with a global meta-analysis of published data on waterbird preening behaviour to allow a comparison of flamingos with other large waterbird taxa (Anseriformes, Ciconiiformes, Gruiformes, and Pelecaniformes). Analysis using zero-inflated generalised linear models indicated that flamingo unipedal behaviour was more common inside housing and on water. Preening effort varied markedly between flamingo species, but we detected no influence of any of the weather variables that were tested. Furthermore, our meta-analysis of waterbird behaviour indicated an interaction between latitude and season, whereby birds spent more time preening at higher latitudes during the non-breeding season, but the opposite trend was detected during the breeding season. Contrary to the popular perception, we found no evidence that flamingos spend more time preening than other waterbirds; indeed, our analysis indicated that, among the five waterbird orders assessed, Pelecaniformes preened the most. However, waterbird preening effort was higher for captive populations (across all species) relative to their free-living counterparts. Our study provides new insight into long-held perceptions of the behaviour of flamingos and other waterbirds.
Mate choice is a critical decision, requiring time and energy to assess potential partners' genetic quality. Consequently, in many species females have evolved the ability to utilize social information by copying the mate choices of others, usually based on visual cues. However, chemical cues offer advantages, such as not requiring active observation of copulations. Using Drosophila melanogaster, we provide the first demonstration of chemical mate choice copying. Student females selected the same male genotype that a teacher female mated at higher frequently than expected by chance. Chemical mate choice copying requires sensing both male and female cues, which might indicate that that male genotype has been chosen by other females. Our work suggests that females, in the presence of mated females, increase choosiness at the virgin stage, elevating sexual selection on male traits. This study provides novel evidence that exploiting social information is more prevalent in flies than previously assumed.
Synopsis
The cost of supporting traits that increase mating opportunities and maximize the production of quality offspring is paid in energy. This currency of reproduction is enabled by bioenergetic adaptations that underlie the flexible changes in energy utilization that occur with reproduction. This review considers the traits that contribute to variation in the capacity of an organ to produce ATP. Further, it synthesizes findings from studies that have evaluated bioenergetic adaptations to the production of sexually selected traits and performance during reproduction and the role of change in mitochondrial respiratory performance in the tradeoff between reproduction and longevity. Cumulatively, these works provide evidence that in selecting for redder males, female finches will likely mate with a male with high mitochondrial respiratory performance and, potentially, a higher probability of mitonuclear compatibility. Females from diverse taxa allocate more to reproduction when the respiratory performance of mitochondria or density of the inner mitochondrial membrane in the liver or skeletal muscle is higher. Finally, reproduction does not appear to have persistent negative effects on mitochondrial respiratory performance, countering a role for mitochondria in the trade-off between reproduction and longevity. I close by noting that adaptations that improve mitochondrial respiratory performance appear vital for optimizing reproductive fitness.
In many species of animals, red carotenoid-based coloration is produced by metabolizing yellow dietary pigments, and this red ornamentation can be an honest signal of individual quality. However, the physiological basis for associations between organism function and the metabolism of red ornamental carotenoids from yellow dietary carotenoids remains uncertain. A recent hypothesis posits that carotenoid metabolism depends on mitochondrial performance, with diminished red coloration resulting from altered mitochondrial aerobic respiration. To test for an association between mitochondrial respiration and red carotenoids, we held wild-caught, molting male house finches in either small bird cages or large flight cages to create environmental challenges during the period when red ornamental coloration is produced. We predicted that small cages would present a less favorable environment than large flight cages and that captivity itself would decrease both mitochondrial performance and the abundance of red carotenoids compared to free-living birds. We found that captive-held birds circulated fewer red carotenoids, showed increased mitochondrial respiratory rates, and had lower complex II respiratory control ratios—a metric associated with mitochondrial efficiency—compared to free-living birds, though we did not detect a difference in the effects of small cages versus large cages. Among captive individuals, the birds that circulated the highest concentrations of red carotenoids had the highest mitochondrial respiratory control ratio for complex II substrate. These data support the hypothesis that the metabolism of red carotenoid pigments is linked to mitochondrial aerobic respiration in the house finch, but the mechanisms for this association remain to be established.
In many nuclear families, dependent offspring receive unequal shares of parental investment. Initial overproduction can be adaptive from the perspective of parents, but parents must be able to identify appropriate candidates for favourite status. We studied early nestling development in house sparrows (Passer domesticus), a species in which the loss of some brood members is common, testing the prediction that body mass and carotenoid-rich flange colours are important to nestling success. There was substantial variation in both traits within broods, even only 1 day after hatching. Nestlings low in the within-brood mass hierarchy gained more mass if they displayed more carotenoid-rich flanges than broodmates. Position in the colour hierarchy did not, however, predict mass gain for individuals that were heavier than their broodmates. Nestlings that were heavier or had more carotenoid-rich mouths were also less likely to be the victim of brood reduction. Our results suggest that house sparrow parents use both nestling body size and mouth colour when making allocation decisions. Understanding both how and when offspring traits and parental preferences function is key to understanding how selective pressures act on offspring–parent communication.
Avian plumage coloration could be altered by external factors like dust accumulation, but the effect of environmental conditions on such post-moult colour changes are poorly studied. Here, we investigated how exposure to the atmosphere in habitats of differing anthropogenic pressures modifies UV/blue structural plumage coloration — a sexually selected trait in eastern bluebirds Sialia sialis. We collected feather samples from live birds and distributed them in three types of habitats: (i) urban roads with low traffic, (ii) urban roads with high traffic, and (iii) rural roads with low traffic. After 4 weeks, the brightness of feathers decreased in all types of habitats-resulting in less-ornamented coloration. Changes in UV and blue chroma, however, varied with habitat type; chroma decreased in urban areas (becoming less ornamented), but increased in rural locations (becoming more ornamented). According to physiological models of avian colour vision, however, these changes in coloration tended to be below perceptual thresholds. Our results underscore the importance of how human activity can influence plumage colour of individual birds during the period between moult cycles which could have an impact on avian visual signalling. The study opens venues for further studies focusing on how the optical properties of specific airborne particles can influence coloration of wild-living animals.
Significance statement
Plumage coloration plays an important role in avian visual communication. In addition to moult-based change, coloration may change between moult cycles due to exposure to external factors like, for example, dust accumulation. Ecological factors modifying a passive plumage change, however, remains poorly studied. We collected samples of structurally coloured feather from a species wherein plumage colour is sexually selected and exposed them to outdoor conditions. We demonstrate that UV/blue chroma of coloration changes differently in urban and rural habitats. It increased in rural and decreased in urban areas. On the other hand, brightness decreased in both habitat types. Using physiological models of avian colour vision, however, we found that these colour changes were likely not perceptible to birds. Our study provides the first evidence that changes in coloration of fully grown feathers may be habitat dependent.
Urbanisation is accelerating across the globe, transforming landscapes, presenting organisms with novel challenges, shaping phenotypes and impacting fitness. Urban individuals are claimed to have duller carotenoid‐based colouration, compared to their non‐urban counterparts, the so‐called ‘urban dullness’ phenomenon. However, at the intraspecific level, this generalisation is surprisingly inconsistent and often based on comparisons of single urban/non‐urban populations or studies from a limited geographical area.
Here, we combine correlational, experimental and meta‐analytical data on a common songbird, the great tit Parus major, to investigate carotenoid‐based plumage colouration in urban and forest populations across Europe.
We find that, as predicted, urban individuals are paler than forest individuals, although there are large population‐specific differences in the magnitude of the urban‐forest contrast in colouration. Using one focal region (Malmö, Sweden), we reveal population‐specific processes behind plumage colouration differences, which are unlikely to be the result of genetic or early‐life conditions, but instead a consequence of environmental factors acting after fledging.
Finally, our meta‐analysis indicates that the urban dullness phenomenon is well established in the literature, for great tits, with consistent changes in carotenoid‐based plumage traits, particularly carotenoid chroma, in response to anthropogenic disturbances.
Overall, our results provide evidence for uniformity in the ‘urban dullness’ phenomenon but also highlight that the magnitude of the effect on colouration depends on local urban characteristics. Future long‐term replicated studies, covering a wider range of species and feeding guilds, will be essential to further our understanding of the eco‐evolutionary implications of this phenomenon.
Even as numerous studies have documented that the red and yellow coloration resulting from the deposition of carotenoids serves as an honest signal of condition, the evolution of condition dependency is contentious. The resource trade-off hypothesis proposes that condition-dependent honest signalling relies on a trade-off of resources between ornamental display and body maintenance. By this model, condition dependency can evolve through selection for a re-allocation of resources to promote ornament expression. By contrast, the index hypothesis proposes that selection focuses mate choice on carotenoid coloration that is inherently condition dependent because production of such coloration is inexorably tied to vital cellular processes. These hypotheses for the origins of condition dependency make strongly contrasting and testable predictions about ornamental traits. To assess these two models, we review the mechanisms of production of carotenoids, patterns of condition dependency involving different classes of carotenoids, and patterns of behavioural responses to carotenoid coloration. We review evidence that traits can be condition dependent without the influence of sexual selection and that novel traits can show condition-dependent expression as soon as they appear in a population, without the possibility of sexual selection. We conclude by highlighting new opportunities for studying condition-dependent signalling made possible by genetic manipulation and expression of ornamental traits in synthetic biological systems.
Young animals often solicit care from their parents using behaviors and morphologies collectively termed begging. Many nestling altricial birds add to their familiar postures and vocalizations by displaying colorful mouthparts, hypothesized to enhance their visual conspicuousness and/or be favored by choosy parents. We evaluated these two hypotheses with experimental manipulations of the flange tissue of house sparrow (Passer domesticus) nestlings. We allowed parents to interact with nestlings that appeared to have either carotenoid-rich or carotenoid-poor flanges, and then with nestlings that appeared similar in carotenoid-richness but differed in either brightness (total light reflected) or ultraviolet (UV) richness. These three features of flange color comprise much of the variation observed both within and among species. None of our three manipulations shaped parents’ ability to transfer prey efficiently to nestlings, a proxy for how visually detectable nestlings were. Parents preferentially allocated prey to nestlings that appeared carotenoid-rich, but did not bias allocation when we manipulated brightness or UV reflectance. These patterns suggest that nestlings displaying carotenoid-rich flanges benefit from parental favoritism, but that variation in brightness and UV reflectance do not independently shape parental care, at least within the range a house sparrow parent might encounter in nature. Efforts to understand mouth color in the context of offspring-parent communication and visual signal evolution may benefit from considering the component parts of color, as they may function, be constrained, and evolve independently.
Bioindicators of wildlife health are useful tools for studying the viability of various organisms and populations, and can include a range of phenotypic variables, such as behavior, body size, and physiological parameters, such as circulating hormones and nutrients. Few studies have investigated the utility of total plasma protein as a predictor of environmental or nutritional variation among birds, as well as variation across different habitats and life-history stages. Here, we examined relationships between plasma protein and season, urbanization, sex, body condition, molt status, and disease state in House Finches (Haemorhous mexicanus). We sampled blood from House Finches across three seasons (winter, summer, and fall 2021) and measured plasma protein levels using a Bradford assay. We also collected data including condition, sex, and poxvirus infection state at capture, as well as fecal samples to assess gut parasitism (coccidiosis). During the fall season, we also estimated molt status, as number of actively growing feathers. We found a significant relationship between circulating protein levels and capture site, as well as novel links to molt state and pox presence, with urban birds, those infected with pox, and those in more intense molt having higher protein levels. Our results support the hypotheses that plasma protein concentration can be indicative of a bird’s body molt and degree of habitat urbanization, although future work is needed to determine why protein levels were higher in virus-infected birds.
MHC genes play a fundamental role in immune recognition of pathogens and parasites. Therefore, females may increase offspring heterozygosity and genetic diversity by selecting males with genetically compatible or heterozygous MHC. In birds, several studies suggest that MHC genes play a role in mate choice, and recent evidence suggests that olfaction may play a role in the MHC-II discrimination. However, whether olfaction is involved in MHC-I discrimination in birds remains unknown. Previous studies indicate that house sparrow females with low allelic diversity prefer males with higher diversity in MHC-I alleles. Here, we directly explored whether female and male house sparrows (Passer domesticus) could estimate by scent MHC-I diversity and/or dissimilarity of potential partners. Our results show that neither females nor males exhibit a preference related to MHC-I diversity or dissimilarity of potential partners, suggesting that MHC-I is not detected through olfaction. Further studies are needed to understand the mechanisms responsible for mate discrimination based on MHC-I in birds.
The red-legged partridge is increasingly recognized as a good study model in ecophysiology and evolutionary biology. Most research in these fields has focused on the expression of coloured social signals. The fact that this species expresses the two main types of coloured ornaments among vertebrates (i.e. carotenoid- and melanin-based) has attracted the interest of scientists investigating the mechanisms behind the function and evolution of these traits. Intensive research on the colour of the beak, eye rings and legs of this species has revealed that the redness of these characters results from the accumulation of two main carotenoids (astaxanthin and papilioerythrinone) that the animal synthesizes at the integument from dietary precursors. Diet composition, intestinal parasites, body condition, oxidative stress and androgen levels significantly affect the expression of the characteristic redness of these traits, making them reliable indicators of individual quality. Melanin-based plumage ornaments of the red-legged partridge (black bib and barred flank feathers) have been relatively less studied. However, experimental studies have shown that their expression is connected to the oxidative status of the individual during feather growth. In addition, the pattern of the black bib —measured through its fractal dimension— is sensitive to the body condition of the individual during moult. Given that most of these studies have been performed in captivity, the main challenge in the short term is to validate and calibrate the conclusions obtained in captivity with studies performed in the wild. This would also help evaluate the interconnection among different signalling traits —including behavioural displays— and their relative importance under different social and environmental contexts.
We investigated potential information content in red carotenoid-based undertail coverts of 28 pairs of breeding Great Spotted Woodpeckers by comparing plumage reflectance to measures of body condition and reproductive success. Plumage coloration was not significantly associated with sex or age, and did not correlate with body condition or the number of offspring fledged. Further, we found no evidence of assortative mating related to plumage colour. Our research provides an example of an exception from the commonly held hypothesis that carotenoid-based plumage coloration signals individual condition. Future surveys incorporating other plumage traits and physiological measures of condition in Great Spotted Woodpeckers are needed to verify our findings.
The Shared-Pathway Hypothesis offers a cellular explanation for the connection between ketocarotenoid pigmentation and individual quality. Under this hypothesis, ketocarotenoid metabolism shares cellular pathways with mitochondrial oxidative phosphorylation such that red carotenoid-based coloration is inextricably linked mitochondrial function. To test this hypothesis, we exposed Tigriopus californicus copepods to a mitochondrially-targeted protonophore, 2-4-dinitrophenol (DNP), to induce proton leak in the inner mitochondrial membranes. We then measured whole-animal metabolic rate and ketocarotenoid accumulation. As observed in prior studies of vertebrates, we observed that DNP treatment of copepods significantly increased respiration and that DNP-treated copepods accumulated more ketocarotenoid than control animals. Moreover, we observed a relationship between ketocarotenoid concentration and metabolic rate, and this association was strongest in DNP-treated copepods. These data support the hypothesis that ketocarotenoid and mitochondrial metabolism are biochemically intertwined. Moreover, these results corroborate observations in vertebrates, perhaps suggesting a fundamental connection between ketocarotenoid pigmentation and mitochondrial function that should be explored further.
Ontogeny is expected to be a determinant factor affecting production of colour patches in lizards, while immune challenges or sudden weight loss may impair the maintenance of pigment-based coloration within a breeding season. We translocated males of the lizard Psammodromus algirus between two sampling plots that differed in distance to a road, vegetation structure, and predator abundance. We analysed variation in spectral reflectance of their colour patches the same and the following year. The change in the reflectance of the lizard colour patches within the first breeding season was explained by the interaction between plot and treatment, but not body condition. The maintenance of the breeding coloration was impaired only in those males translocated close to the road, probably reflecting that it is a poor-quality habitat for P. algirus . The following year, lizards that produced a more elaborate coloration were those that increased their body condition and controlled some parasitic infections, although suffered an increase of others. This study shows that colour patch production is plastic in P. algirus . Lizards increasing parasites or losing weight reduced pigmentation, although habitat quality can cushion these negative effects on pigmentation. However, not all parasites constrain the investment in coloration. In fact, some increased in those lizards that allocated more pigments to colour patches. In conclusion, longitudinal studies following experimental manipulation can contribute to understand pigment allocation rules in lizards.
Significance statement
Pigments involved in colour patches of animals are limiting resources that can be reallocated off the skin to other functions. However, longitudinal evidence of this phenomenon is scarce in reptiles. We designed a manipulative mark-recapture experiment to investigate effects of habitat and parasitic infections on colour patch maintenance (within-year variation) and production (between-year variation) in male free-ranging lizards that were reciprocally translocated between two patches of habitat that differed in quality. During the first year, lizards translocated to the habitat with more predators and worse vegetation impoverished their coloration, while lizards translocated to the more favourable habitat maintained it despite all translocated lizards loose body condition. The next year we detected different effects on the coloration of three different parasites investigated, suggesting that coloration can reflect the virulence of the infections.
Poxviruses (family: Poxviridae) infect many avian species, causing several disease outcomes, the most common of which are proliferative lesions on the legs, feet and/or head. Few avian studies of poxvirus to date have combined molecular and ecological analyses to obtain a more comprehensive understanding of the identity and distribution of the disease in a population. Here we describe patterns of poxvirus infection in an urban population of house finches (Haemorhous mexicanus) in Arizona (USA) and use high‐throughput sequencing determine the genome sequence of the virus. We found that poxvirus prevalence, based on visual identification of pox lesions, was 7.2% (17 infected birds out of a total of 235 sampled) in our population during summer 2021. Disease severity was low; 14 of the 17 infected birds had a single small lesion on the skin overlaying the eye, leg and ear canal. All but two lesions were found on the feet; one bird had a lesion on the eye and the other in the ear opening. We also investigated possible temporal (i.e., date of capture) and biological correlates (e.g., age, sex, body condition, degree of infection with coccidian endoparasites) of poxvirus infection in urban‐caught house finches during this time but found that none of these significantly correlated with poxvirus presence/absence. Two complete poxvirus genomes were determined from two infected birds. These genomes are ∼354,000 bp and share 99.7% similarity with each other, and 82% with a canarypox virus genome, the most closely related avipoxvirus. This novel finchpox virus is the first to be reported in house finches and has a similar genome organization to other avipoxviruses. This article is protected by copyright. All rights reserved
The last decades have provided valuable information on how animals age in the wild. However, examples of male reproductive senescence are scarce. In particular, few studies have described an age-related decline in the expression of conspicuous traits influencing mating success. Red ornaments could be good candidates to detect this decline because their expression may depend on the availability of pigments (carotenoids) related to oxidative stress, the latter frequently linked to senescence. Furthermore, it has been argued that efficient mitochondrial metabolism is key to express red carotenoid-based ornaments, and mitochondrial dysfunction is usually associated with senescence. We studied the age-linked expression of a red carotenoid-based trait: the yellow-to-red plumage coloration of male common crossbills (Loxia curvirostra). This coloration has recently been experimentally related to mitochondrial function. Here, we analyzed longitudinal plumage coloration data obtained throughout 28 years in free-living birds. We detected an initial increase in redness during the first two years of life and a subsequent decline. The relationship between color and age was unrelated to within-individual body mass variability. As far as we know, this is the first demonstration of an age-related ketocarotenoid-based color decrease detected by simultaneously testing within- and between-individual variability in wild animals.
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Animals inhabiting urban areas often experience elevated disease threats, putatively due to factors such as increased population density and horizontal transmission or decreased immunity (e.g. due to nutrition, pollution, stress). However, for animals that take advantage of human food subsidies, like feeder-visiting birds, an additional mechanism may include exposure to contaminated feeders as fomites. There are some published associations between bird feeder presence/density and avian disease, but to date no experimental study has tested the hypothesis that feeder contamination can directly impact disease status of visiting birds, especially in relation to the population of origin (i.e. urban v. rural, where feeder use/densities naturally vary dramatically). Here we used a field, feeder-cleaning experimental design to show that rural, but not urban, house finches ( Haemorhous mexicanus ) showed increased infection from a common coccidian endoparasite ( Isospora spp.) when feeders were left uncleaned and that daily cleaning (with diluted bleach solution) over a 5-week period successfully decreased parasite burden. Moreover, this pattern in rural finches was true for males but not females. These experimental results reveal habitat- and sex-specific harmful effects of bird feeder use (i.e. when uncleaned in rural areas). Our study is the first to directly indicate to humans who maintain feeders for granivorous birds that routine cleaning can be critical for ensuring the health and viability of visiting avian species.
Carotenoids color many of the red, orange and yellow ornaments of birds and also shape avian vision. The carotenoid-pigmented oil droplets in cone photoreceptors filter incoming light and are predicted to aid in color discrimination. Carotenoid use in both avian coloration and color vision raises an intriguing question: is the evolution of visual signals and signal perception linked through these pigments? Here, we explore the genetic, physiological and functional connections between these traits. Carotenoid color and droplet pigmentation share common mechanisms of metabolic conversion and are both affected by diet and immune system challenges. Yet, the time scale and magnitude of these effects differ greatly between plumage and the visual system. Recent observations suggest a link between retinal carotenoid levels and color discrimination performance, but the mechanisms underlying these associations remain unclear. Therefore, we performed a modeling exercise to ask whether and how changes in droplet carotenoid content could alter the perception of carotenoid-based plumage. This exercise revealed that changing oil droplet carotenoid concentration does not substantially affect the discrimination of carotenoid-based colors, but might change how reliably a receiver can predict the carotenoid content of an ornament. These findings suggest that, if present, a carotenoid link between signal and perception is subtle. Deconstructing this relationship will require a deeper understanding of avian visual perception and the mechanisms of color production. We highlight several areas where we see opportunities to gain new insights, including comparative genomic studies of shared mechanisms of carotenoid processing and alternative approaches to investigating color vision.
Coloration can evolve in response to selection targeting colorful traits that affect survival or reproductive success. Trait covariation can complicate evolutionary dynamics when selection on covarying traits acts antagonistically. In prairie lizards (Sceloporus consobrinus), males exhibit blue ventral patches that are reduced in females. The development of ventral coloration corresponds with increases of circulating testosterone at sexual maturity. Previous work has illustrated that testosterone and coloration covary, such that individuals with higher testosterone have bluer patches. However, testosterone is known to increase mortality in many organisms, including Sceloporus, so selection against high testosterone can lead to indirect selection against bluer males. While previous work has suggested that patch coloration does not affect fitness, this has not yet been explicitly tested using estimates of survival and reproductive success. Here, we quantify patch coloration, apparent survival, encounter probability, and reproductive success in a wild population of male prairie lizards to assess the role of patch color variation on fitness. Specifically, we tested how patch coloration affects survival and reproductive success across 2 years. We found that none of the patch phenotypes we measured covaried with fitness, suggesting that neither natural nor sexual selection act, or act very weakly, on patch morphology. We found evidence of sexual selection on body size, such that larger males sired more offspring, likely through increased activity. However, this result was inconsistent, as it was only found in one year. Interestingly, we found that, in both years, bluer males were more likely to sire at least one offspring, suggesting some role of patch coloration in reproduction, although we cannot detect what that role may be. Overall, our study suggests that neither apparent survival nor reproductive success is strong drivers of the evolution of ventral coloration in S. consobrinus.
Synopsis
For decades, scientists have noted connections between individual condition and carotenoid-based coloration in terrestrial and aquatic animals. Organisms that produce more vibrant carotenoid-based coloration tend to have better physiological performance and behavioral displays compared with less colorful members of the same species. Traditional explanations for this association between ornamental coloration and performance invoked the need for color displays to be costly, but evidence for such hypothesized costs is equivocal. An alternative explanation for the condition-dependence of carotenoid-based coloration, the Shared-Pathway Hypothesis (SPH), was developed in response. This hypothesis proposes that red ketocarotenoid-based coloration is tied to core cellular processes involving a shared pathway with mitochondrial energy metabolism, making the concentration of carotenoids an index of mitochondrial function. Since the presentation of this hypothesis, empirical tests of the mechanisms proposed therein have been conducted in several species. In this manuscript, we review the SPH and the growing number of studies that have investigated a connection between carotenoid-based coloration and mitochondrial function. We also discuss future strategies for assessing the SPH to more effectively disentangle evidence that may simultaneously support evidence of carotenoid-resource tradeoffs.
Producing colored signals often requires consuming dietary carotenoid pigments. Evidence that food deprivation can reduce coloration, however, raises the question of whether other dietary nutrients contribute to signal coloration, and furthermore, whether individuals can voluntarily select food combinations to achieve optimal coloration. We created a 2-way factorial design to manipulate macronutrient and carotenoid access in common mynas ( Acridotheres tristis ) and measured eye patch coloration as a function of the food combinations individuals selected. Mynas had access to either water or carotenoid-supplemented water and could eat either a standard captive diet or choose freely between three nutritionally defined pellets (protein, lipid, carbohydrate). Mynas supplemented with both carotenoids and macronutrient pellets had higher color scores than control birds. Male coloration tended to respond more to nutritional manipulation than females, with color scores improving in macronutrient- and carotenoid-supplemented individuals compared to controls. All mynas consuming carotenoids had higher levels of plasma carotenoids, but only males showed a significant increase by the end of the experiment. Dietary carotenoids and macronutrient intake consumed in combination tended to increase plasma carotenoid concentrations the most. These results demonstrate for the first time that consuming specific combinations of macronutrients along with carotenoids contribute to optimizing a colorful signal and point to sex-specific nutritional strategies. Our findings improve our knowledge of how diet choices affect signal expression and, by extension, how nutritionally impoverished diets, such as those consumed by birds in cities, might affect sexual selection processes and ultimately population dynamics.
The virulence‐transmission trade‐off hypothesis has provided a dominant theoretical basis for predicting pathogen virulence evolution, but empirical tests are rare, particularly at pathogen emergence. The central prediction of this hypothesis is that pathogen fitness is maximized at intermediate virulence due to a trade‐off between infection duration and transmission rate. However, obtaining sufficient numbers of pathogen isolates of contrasting virulence to test the shape of relationships between key pathogen traits, and doing so without the confounds of evolved host protective immunity (as expected at emergence), is challenging. Here, we inoculated 55 isolates of the bacterial pathogen, Mycoplasma gallisepticum, into non‐resistant house finches (Haemorhous mexicanus) from populations that have never been exposed to the disease. Isolates were collected over a 20‐year period from outbreak in disease‐exposed populations of house finches and vary markedly in virulence. We found a positive linear relationship between pathogen virulence and transmission rate to an uninfected sentinel, supporting the core assumption of the trade‐off hypothesis. Further, in support of the key prediction, there was no evidence for directional selection on a quantitative proxy of pathogen virulence and, instead, isolates of intermediate virulence were fittest. Surprisingly, however, the positive relationship between virulence and transmission rate was not underpinned by variation in pathogen load or replication rate as is commonly assumed. Our results indicate that selection favors pathogens of intermediate virulence at disease emergence in a novel host species, even when virulence and transmission are not linked to pathogen load.
Historically, studies of condition-dependent signals in animals have been male-centric, but recent work suggests that female ornaments can also communicate individual quality (e.g., disease state, fecundity). There also has been a surge of interest in how urbanization alters signaling traits, but we know little about if and how cities affect signal expression in female animals. We measured carotenoid-based plumage coloration and coccidian (Isospora spp.) parasite burden in desert and city populations of house finches Haemorhous mexicanus to examine links between urbanization, health state, and feather pigmentation in males and females. In earlier work, we showed that male house finches are less colorful and more parasitized in the city, and we again detected such patterns in this study for males; however, urban females were less colorful, but not more parasitized, than rural females. Moreover, contrary to rural populations, we found that urban birds (regardless of sex) with larger patches of carotenoid coloration were also more heavily infected with coccidia. These results show that urban environments can disrupt condition-dependent color expression and highlight the need for more studies on how cities affect disease and signaling traits in both male and female animals.
There is widespread contemporary interest in causes and consequences of blood glucose status in humans (e.g., links to diabetes and cardiovascular disease), but we know comparatively less about what underlies variation in glucose levels of wild animals. Several environmental factors, including diet, disease status, and habitat quality, may regulate glucose circulation, and we are in need of work that assesses many organismal traits simultaneously to understand the plasticity and predictability of glucose levels in ecological and evolutionary contexts. Here, we measured circulating glucose levels in a species of passerine bird (the house finch, Haemorhous mexicanus) that has served as a valuable model for research on sexual selection, disease, and urban behavioral ecology, as these animals display sexually dichromatic ornamental coloration, harbor many infectious diseases (e.g., poxvirus, coccidiosis, mycoplasmal conjunctivitis), and reside in both natural habitats and cities. We tested the effects of sex, habitat type, body condition, coccidiosis and poxvirus infections, and expression of carotenoid plumage coloration on blood glucose concentrations and found that the body condition and poxvirus infection significantly predicted circulating glucose levels. Specifically, birds with higher blood glucose levels had higher body condition scores and were infected with poxvirus. This result is consistent with biomedical, domesticated-animal, and wildlife-rehabilitation findings, and the premise that glucose elevation is a physiological response to or indicator of infection and relative body weight. The fact that we failed to find links between glucose and our other measurements suggests that blood glucose levels can reveal some but not all aspects of organismal or environmental quality.
We analyse data from a cross-fostering experiment in which house finches were fostered by canary parents. Some individuals received canary song input, while others received no input, after a period of masking noise. We compare audio recordings of songs by these individuals to each other and to species-typical house finch and canary songs. Canary-tutored house finches learn to trill as part of their song. Since trills are not present in typical house finch song, naturally occurring song patterns underestimate what a species is capable of learning and producing. These results highlight a potential avenue for the origin of novel syntax in songbirds.
Colorful ornaments are hypothesized to have evolved in response to sexual selection for honest signals of individual quality that provide information about potential mates. Red carotenoid coloration is common in diverse groups, and in some vertebrate taxa, red coloration is a sexually selected trait whereby mates with the reddest ornaments are preferred. Despite being widespread among invertebrates, whether red carotenoid coloration is assessed during mate choice in these taxa is unclear. The marine copepod Tigriopus californicus displays red coloration from the accumulation of the carotenoid astaxanthin. Previous research on copepods has shown that astaxanthin provides protection from UV radiation and xenobiotic exposure and that carotenoid production is sensitive to external stressors. Because of the condition dependency of the red coloration, we hypothesized that Tigriopus would use it as a criterion during mate choice. To test this hypothesis, we conducted trials in which males chose between females that were wild‐type red (carotenoid‐rich algae diet) or white (carotenoid‐deficient yeast diet). To control for dietary differences and to isolate the effect of carotenoid coloration, we also presented males with restored‐red females fed a carotenoid‐supplemented yeast diet. We found that wild‐type red females were weakly preferred over white females. After controlling for diet, however, we found that restored‐red females were avoided. Our observations do not support the hypothesis that male copepods prefer the carotenoid coloration of females during mate choice. We hypothesize that algal‐derived compounds other than carotenoids play a role in mate choice. Red coloration in copepods appears to be a condition‐dependent trait that is not assessed during mating.
Carotenoid coloration is widely recognized as a signal of individual condition in various animals, but despite decades of study, the mechanisms that link carotenoid coloration to condition remain unresolved. Most birds with red feathers convert yellow dietary carotenoids to red carotenoids in an oxidation process requiring the gene encoding the putative cytochrome P450 enzyme CYP2J19. Here, we tested the hypothesis that the process of carotenoid oxidation and feather pigmentation is functionally linked to mitochondrial performance. Consistent with this hypothesis, we observed high levels of red ketolated carotenoids associated with the hepatic mitochondria of moulting wild house finches (Haemorhous mexicanus), and upon fractionation, we found the highest concentration of ketolated carotenoids in the inner mitochondrial membrane. We further found that the redness of growing feathers was positively related to the performance of liver mitochondria. Structural modelling of CYP2J19 supports a direct role of this protein in carotenoid ketolation that may be functionally linked to cellular respiration. These observations suggest that feather coloration serves as a signal of core functionality through inexorable links to cellular respiration in the mitochondria.
Bird song is the primary animal model system for cultural evolution. Longitudinal studies of bird song across many generations can provide insights into patterns and mechanisms of change in socially transmitted traits. In this study, we conducted a comparative analysis of songs of the House Finch (Haemorhous mexicanus) across an interval of 37 years (in 1975 and in 2012). Recordings from both years were collected in western Long Island, New York, which is thought to be the initial site of introduction of the House Finch around 1940 from the West Coast. Song types experienced a complete turnover during this period, although half of the syllable types were represented in both samples. Song length, frequency bandwidth and several other spectrographic features were the same in both years, and no structural features predicted recurrence of individual syllables. Consistent with the fact that our study population expanded substantially following a cultural founder effect, song and syllable sharing and similarity between individuals were lower in 2012 than in 1975, reflecting an increase in song diversity at the population level. However, in the more recent sample individual songs had fewer syllables, and were sung with less sequence stereotypy across renditions than in the earlier sample. Syllable prevalence in 2012 was associated both with complexity (as gauged by frequency excursion) and increased minimum frequency. Thus, over nearly 4 decades, Eastern House Finch songs remained structurally similar at the whole-song level, diversified between individuals, but became simpler and less consistent within an individual.
Many birds use conspicuous plumage coloration to signal quality for sexual or social selection. However, it is still often unclear how intraspecific coloration and associated signaling vary spatially. Plumage coloration that is dependent on carotenoids may be ideal for studying spatial color variation because birds cannot naturally synthesize this pigment; therefore, bird coloration from carotenoids is at least partially contingent upon diet. As food availability often varies spatially, so might color and its signaling strength. While male coloration has received more research focus, less is known about female coloration and its relationships with social rank or sexual quality. To further improve our understanding of spatial variation in plumage coloration and correlations with individual quality, we compared Prothonotary Warblers breeding at 2 ecologically disparate sites separated by 1,300 km: in bottomland forests of Arkansas, USA, and the forests near the tidal freshwater James River in Virginia, USA. We assessed crown and breast plumage coloration for both sexes and compared several color metrics between sites. We then assessed surrogates of female quality (number of young fledged, number of eggs laid, provisioning rate, apparent annual survival, and nest depredation) and compared coloration-quality relationships between sites. We found that coloration of birds breeding in Arkansas was generally more elaborate than those breeding in Virginia. However, this was somewhat dependent on sex: females showed greater differences than males between sites. Additionally, color metrics of females breeding in Virginia showed stronger relationships with quality (all 5 quality metrics) than for birds breeding in Arkansas (only provisioning rate and nest depredation). Proximately, spatial variation in plumage coloration and the associated signaling may be explained by differences in diet between sites. Ultimately, spatial variation in intra- and intersexual selection pressures may explain how spatial variation in plumage signal strength evolved.
Urban environments create a unique suite of conditions, leading to changes in animal behavior, morphology, phenology, and physiology. Condition-dependent traits such as the carotenoid-based coloration offer a unique opportunity to assess the impacts of urbanization on organisms because they reflect the nutrition, health, or other resource-based attributes of their bearers and they play an essential role in intra and intersex interactions. To determine if and how the carotenoid-based coloration of male house finches (Haemorhous mexicanus) varies along a gradient of urbanization, we quantified the plumage coloration of more than 1000 individuals in urban, suburban , rural, and desert habitats over the course of 17 months. We also examined for the first time the preference of females for male plumage coloration across the urban-rural gradient, to test if and how female preferences varied relative to the plumage coloration displayed by males in their local population. We found that carotenoid-based coloration decreased along the gradient of urbanization, suggesting that the enzyme-driven metabolic conversion of dietary carotenoids into red carotenoids used to color plumage is sensitive to urban stressors. The stronger negative effect of urbanization on carotenoid-based plumage coloration during breeding than during molt and winter suggests that urbanization affects color fading rate, maybe through modifications of feather-degrading bacterial load. Finally, we have shown that urbanization influences female mate-choice behavior, suggesting that female color preferences may track the variation in male coloration across the gradient of urbanization.
Although temperature‐correlated shifts in the timing of egg laying have been documented in numerous bird species, the vast majority of species examined to date have been those that breed in Europe and have an animal‐based diet during breeding. However, given that the timing of breeding can be driven, either in the proximate or ultimate sense, by seasonal fluctuations in food availability, the relationship between temperature and laying may differ with diet. Here, we report on patterns of reproductive timing in House Finches Haemorhous mexicanus, a North American species that breeds on a primarily seed‐based diet. Analyzing nest records from House Finches in California spanning more than a century, we found that egg laying occurred significantly earlier in warmer springs. We also found that while the timing of egg laying does not show long‐term changes in most of California, in the hottest region of the state (the southeast desert basin) it has advanced significantly.
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Studies of animal contests have focused on the probability of winning an encounter, because it directly affects the benefits of competition. However, the costs (e.g., physiological stress) and benefits of competition should also depend on the number of aggressive encounters per unit time (combat rate, hereafter) in which the focal individual is involved. Using colourful and drab male house finches (Haemorhous mexicanus) from urban and rural sites, we showed that combat rate was repeatable across the same and different group sizes for birds who won competitions. In addition, colourful urban males exhibited the lowest propensity for frequent aggression (and hence low combat rate). However, male bill size (another trait we previously found to correlate with male competitiveness in this species) was not related to aggressive propensity. Combat rate can be predicted by male identity and some, but not all, predictors of male competitiveness.
Temperature-correlated shifts in reproductive timing are now well documented in numerous bird species. However, whether temperature directly influences reproductive timing or if its effects are mediated by an intermediate environmental cue, such as plant phenology, remains poorly understood. In this study, we investigated the direct effects of temperature on reproductive timing in house finches (Haemorhous mexicanus), which have a range and breeding diet not well represented in previous studies of temperature and reproductive timing. We conducted experiments with captive male house finches in which temperature was elevated within realistic ranges and the effects on the timing of preparations for reproduction, as well as on the termination of reproduction and the onset of prebasic feather molt were examined. We found no adjustments in the timing of reproductive preparations of males in direct response to temperature. However, elevated temperature did advance the breeding-molt transition. Our results suggest elevated temperatures in the range tested here do not directly impact physiological preparations for reproduction in male house finches, but may constrain the timing of the breeding-molt transition in this species.
Colourful plumage is typical of males in species with conventional sex roles, in which females care for offspring and males compete for females, as well as in many monogamous species in which both sexes care for offspring. Reversed sexual dichromatism—more colourful females than males—is predominant in species with sex role reversal. In the latter species, males care for offspring and females compete for mates, the mating system is mainly polyandrous and there is reversed size dimorphism—females are larger than males. Here, we document a case of reversed dichromatism, in the greater flamingo Phoenicopterus roseus, in which there is no sex role reversal and no reversed size dimorphism. Although theoretical models postulate that cases of reversed dichromatism should be rare among monogamous ornamented birds, our findings show that the use of cosmetics might be a mechanism for the occurrence of more ornamented females than males. Indeed, the concentrations of carotenoids in the uropygial secretions used as make-up were higher in females than in males. Apparently, there was a trade-off between coloration and antioxidant defence, as the concentrations of carotenoids in the uropygial secretions were lower during chick provisioning than in other periods, contrary to those in plasma. In this system, the application of make-up would act as a dynamic signal, which would allow a rapid reallocation of resources used for signalling among functions depending on needs. Cases like this may have evolved to signal the ability to provide parental care when females are more physiologically stressed than males.
Significance statement
For species in which there is no sex role reversal, but females are ornamented and the resources allocated to ornaments are important for offspring viability, it has even been suggested that females should be less ornamented than males. This may be because for females, it would be better to invest directly in fecundity rather than in costly ornaments. We show a case of reversed sexual dichromatism in a monogamous bird with no sex role reversal, the greater flamingo, where females apply make-up over feathers. In the case of this species, there could be directional male mate preferences for female plumage coloration because the costs of signalling would not affect breeding investment in females, since cosmetic coloration is not used after it is no longer required, thus allowing the resources used in make-up (carotenoids) to be used in other functions.
Global urban development continues to accelerate and have diverse effects on wildlife. Although most studies of anthropogenic impacts on animals have focused on indirect effects (e.g. environmental modifications like habitat change or pollution), there may also be direct effects of physical human presence and actions on wildlife stress, behavior, and persistence in cities. Most studies on how humans physically interact with wildlife have focused on the active, daytime phase of diurnal animals, rarely considering effects of our night-time activities. We hypothesized that, if night-time human presence is a stressor for wildlife that are not commonly exposed to humans, night-disturbed rural animals would show stronger physiological signs of elevated stress than would urban individuals. Specifically, we experimentally investigated the effects of human presence at night on disease, body mass, and mass-specific metabolic rates in urban- and rural-caught house finches (Haemorhous mexicanus) in captivity. Our human-presence-at-night treatment consisted of a human entering the housing room of the birds and briefly jostling the home cages of each finch as the person walked around the room for a 3-minute period on 5 randomly selected nights per week. Compared to a control (night-undisturbed) group, we found that human presence at night greatly increased the odds finches were awake for ca. 33 minutes post-disturbance, but that chronic treatment this did not alter body mass, parasitic infection by coccidian endoparasites, or mass-specific basal metabolic rates. Additionally, finches caught from urban and rural sites did not differ in their response to the treatment. Overall, our results are consistent with those showing that brief but regular human disturbances can have acute negative effects on wildlife, but carry few if any long-term metabolic or disease-related costs in fast-lived birds. However, these findings contrast with the broad, chronic physiological effects of other anthropogenic changes, such as artificial light at night, and highlight the differential impacts that various human activities (which differ in sensory stimulus type, perceived threat, duration and intensity, etc.) can have on wildlife health and behavior.
While direct contact may sometimes be sufficient to allow a pathogen to jump into a new host species, in other cases fortuitously adaptive mutations that arise in the original donor host are also necessary. Viruses have been the focus of most host shift studies, so less is known about the importance of ecological versus evolutionary processes to successful bacterial host shifts. Here we tested whether direct contact with the novel host was sufficient to enable the mid-1990s jump of the bacterium Mycoplasma gallisepticum from domestic poultry into house finches ( Haemorhous mexicanus ). We experimentally inoculated house finches with two genetically distinct M. gallisepticum strains obtained either from poultry (Rlow) or from house finches at epizootic outbreak (HF1995). All 15 house finches inoculated with HF1995 became infected, whereas Rlow successfully infected 12 of 15 (80%) inoculated house finches. Comparisons among infected birds showed that, relative to HF1995, Rlow achieved substantially lower bacterial loads in the host respiratory mucosa and was cleared faster. Furthermore, Rlow-infected finches were less likely to develop clinical symptoms than HF1995-infected birds and, when they did, displayed milder conjunctivitis. The lower infection success of Rlow relative to HF1995 was not, however, due to a heightened host antibody response to Rlow. Taken together, our results indicate that contact between infected poultry and house finches was not, by itself, sufficient to explain the jump of M. gallisepticum into house finches. Instead, mutations arising in the original poultry host would have been necessary for successful pathogen emergence in the novel finch host.
This study was conducted to investigate the influence of atmospheric factors on the body and egg of great tits (Parus major), which were bred in artificial nest boxes from March to July of 2012 and 2013. The mean temperature and relative humidity were significantly higher in the spring season of 2012 than in 2013. Body masses of incubating parent birds differed between both years. Eggs which were laid in 2012 were larger and heavier than those of 2013. Hue and saturation values for background and spot colors were significantly different between 2012 and 2013. In 2012, there were warmer and wetter atmospheric conditions, which may influence vegetation and food availability for the birds. The large, heavy, and less vividly-colored eggs were laid in 2012 by parents with good body condition. Results showed that spring atmospheric factors influenced both body and egg of great tits. Further studies on the relationships between atmosphere, vegetation, and food availability of the birds are needed.
Significance
The yellow, orange, and red colors of birds are produced through the deposition of carotenoid pigments into feathers and skin, and often function as signals in aggressive interactions and mate choice. These colors are hypothesized to communicate information about individual quality because their expression is linked to vital cellular processes through the mechanisms of carotenoid metabolism. To elucidate these mechanisms, we carried out genomic and biochemical analyses of the white recessive canary breed, which carries a heritable defect in carotenoid uptake. We identified a mutation in the SCARB1 gene in this breed that disrupts carotenoid transport function. Our study implicates SCARB1 as a key mediator of carotenoid-based coloration and suggests a link between carotenoid coloration and lipid metabolism.
The evolution of male ornamentation has long been the focus of sexual selection studies. However, evidence is accumulating that
sexually selected traits can also be lost, although the process is ill-understood. In male fairy-wrens (Malurus spp.), early molt into
the seasonal breeding plumage is critical for obtaining extra-pair paternity (EPP), which reaches very high levels in these socially
monogamous songbirds. A notable exception is the purple-crowned fairy-wren, Malurus coronatus, which, like its congeners, breeds
cooperatively, but where EPP is very rare. Nevertheless, males develop a conspicuous seasonal breeding plumage at highly variable
times. Based on 6 years of molt data collected for 137 individuals, we investigated the adaptive significance of pre-breeding molt timing
as a sexual signal under (near) genetic monogamy. Molt timing varied between and within individuals with age and climate: molt was
completed earlier in older males and after wetter years. Despite its potential to act as a sexual signal of male quality, fitness benefits
and costs of early molt appear limited: molt timing did not correlate with 1) the likelihood of gaining a breeding position; 2) female mate
preference (EPP/cuckoldry, divorce); 3) female reproductive investment (breeding timing, clutch size, number of clutches); 4) breeding performance (hatching success, fledging success, fledgling survival, annual reproductive success); and 5) male survival. However, although molt timing did not predict which subordinates would become breeders, breeders molted earlier than subordinates. The lack of EPP in this species might imply relaxed sexual selection on early molt with potential to lead to trait disappearance.
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