Assortative mating by multiple ornaments in northern cardinals (Cardinalis cardinalis)

In positive assortative mating, individuals of similar phenotypemate together more frequently than expected by chance. Assortativemating by a variety of qualities, including ornamentation, iswell documented in birds. Studies of assortative mating by ornamentshave focused on single, highly conspicuous ornaments, but manyspecies of birds possess multiple ornaments in both sexes. Wecompared ornament expressions between mates of northern cardinals(Cardinalis cardinalis) to determine if assortative mating occurredby one or more of the four ornaments displayed by both sexes.All cardinals possess tall head crests and red-orange bills.In addition, males have black face masks and entirely red bodyplumage, whereas females have blackish face masks and red underwingcoverts. We predicted that cardinals mate assortatively by plumagecolor because red plumage expression has been shown to indicatequality in both sexes. We found that cardinals mate assortativelyby plumage and bill color, the two ornaments colored by carotenoidpigments, but not by mask expression or crest length. Whetherthis mating pattern arises by mutual mate choice or intrasexualselection is not known.     

Patterns of variation in ornaments of Crested Auklets Aethia cristatella

We investigated patterns of variation of feather and bill ornaments of Crested Auklets Aethia cristatella , a monogamous seabird, based on 963 individuals measured in the years 1990 to 1998. Three prominent ornaments were displayed: a forehead crest, composed of 11–31 curved feathers averaging about 40 mm in length, bilaterally symmetrical white auricular plumes on the sides of the head behind the eyes, averaging about 30 mm in length, and brightly coloured semi-circular rictal plates at the corners of the bill. As in other putative sexually selected traits, auklet ornaments were more variable across individuals than non-ornamental traits. Crest length and auricular plume length were positively correlated within individuals but not across years. Among the traits measured there was evidence for slight sexual dimorphism for the auricular plume and rictal plate ornaments and for culmen length and tarsus (males were slightly larger than females) but not for the crest ornament. Breeding adult females and males had greater crest and plume ornament expression than non-breeding adults. Paradoxically, females' crests and rictal plates were more variable than males' crests and rictal plates. Based on independent samples, the expression of feather ornaments and rictal plate varied among years between 1990 and 1998. Crested Auklet ornaments did not vary in concert with the ornaments of Whiskered Aethia pygmaea and Least Auklets Aethia pusilla during this period. Crested Auklet subadults had smaller ornaments than adults. Based on adults remeasured after an interval of one to seven years, the size of individuals' feather ornaments increased with age. We found no relationship between auricular plume length and asymmetry. Male auricular plumes and female crests were weakly correlated with body condition.     

Divorce and asynchronous arrival in common terns, Sterna hirundo

We investigated which of three hypotheses (better option, incompatibility or asynchronous arrival) best explains divorce in the common tern. One partner did not return the next year in 18.5% of 150 pairs. Among the 106 pairs in which both mates returned, the divorce rate was 18.9%. We found no significant differences in: breeding performance or condition in relation to the probability of divorce; quality of previous mates and new mates, mean age in relation to pair bond status; breeding success before and after divorce nor did this differ from breeding success of reunited pairs. Hence the better option and incompatibility hypotheses were not supported. However, divorce was more likely in pairs in which mates arrived asynchronously on the breeding grounds, supporting the asynchronous arrival hypothesis. Median arrival asynchrony for divorced pairs was 7.5 days and for reunited pairs 2 days; mates arriving more than 16 days apart always split up. About 20% of divorced birds lost breeding status in the year of divorce, probably as a consequence of their late arrival. Our results suggest that terns search for a new mate as soon as they arrive on the breeding grounds and that mates remain faithful to each other to avoid the costs of searching for a new partner. Thus, synchrony in arrival facilitates pair bond maintenance rather than asynchrony promoting divorce, since divorce appears to be a side-effect of asynchrony and not an active decision. Copyright 1999 The Association for the Study of Animal Behaviour.     

Is Divorce in Young Common Terns,Sterna hirundo,after Recruitment just a Question of Timing?

The probability of divorce in birds has been linked with age, breeding experience, reproductive output and synchrony in return. Here, we investigate the consequences of first breeding attempts in common terns for mating in the subsequent season. Nearly 20% of all first‐time breeders disappeared or skipped at least one season after recruitment. In 84 pairs, which consisted of at least one recruit and of which both partners returned to the colony, the divorce rate was 45%. We compared reproductive success, arrival dates, and asynchrony in arrival dates of pairs of the first breeding season against the second season, for both reunited and divorced pairs and males and females separately. First, in pairs of which both members came back to the colony, we found an increase of reproductive success most pronounced in males. In the second season reproductive success of divorced compared with reunited pairs was higher, as only divorced pairs significantly improved the number of fledglings, and again this relation was stronger in males. Secondly, females of reunited pairs arrived significantly earlier from the first to the second season and by far more days than their males. However, in divorced pairs former mates did not differ in the number of days they advance their arrivals. Finally, divorced males arrived on average 4 d earlier than their former mates, whereas divorced females arrived 5 d later compared with their former mates of the recruitment season. Contradictory to nearly all other divorce studies in birds so far, we found a clear fitness gain in divorced males. We suggest that the improvement in reproductive success of young males stems from a side‐effect of the birds’ quality and ability to reach the breeding site in appropriate time and earlier as potential competitors. In long‐lived bird species the heterogeneity among young individuals in the timing of arrival at the colony seems to explain why former recruit‐pairs reunite or split. For young males we suggest as best explanation of divorce that they profit from ‘pushing for an empty chair’, while females seem to profit from their choosiness and may actively decide between former and other mates.     

Divorce and its consequences in the Common Blackbird Turdus merula

An urban population of Common Blackbirds Turdus merula was studied between 1995 and 1998 in the Botanic Garden in Bonn, Germany. The number of breeding pairs varied from 32 to 39 per breeding season. Mean divorce rate (pair separation with remating of at least one of the partners) between two breeding seasons was 19.3% and 5.1% within breeding seasons. The number of fledglings produced per breeding season, of pairs which subsequently divorced, was significantly lower than that of pairs that stayed together. Females, but not males, produced more fledglings in the season after divorce than with their former mates the year before, and as many as females of the same age in stable pair bonds. The increase in reproductive success was thus unlikely to be due to age effects. Nestling mass correlated positively with offspring survival to the following year. Females had heavier nestlings after divorce than in their former pair bond. There was no difference in nestling mass for males before and after divorce. The data were used to test the predictions of several hypotheses to explain divorce. Because divorce was more likely after low reproductive success in the previous year, and only females benefited, the findings best support the 'better option' hypothesis.     

Sex-specific costs of reproduction in Eastern Bluebirds Sialia sialis

In species with bi-parental care, individuals must partition energy between parental effort and mating effort. Typically, female songbirds invest more than males in reproductive activities such as egg-laying and incubation, but males invest more in secondary sexual traits used in attracting mates. Animals that breed more than once within a season must also allocate time and energy between first and subsequent breeding attempts and between current and future breeding seasons. To investigate strategies of reproductive investment by males and females and the consequences of such strategies, we manipulated the size of broods of Eastern Bluebirds Sialia sialis . Pairs with enlarged first broods were less likely to produce a second clutch or took longer to initiate one than pairs with reduced broods. After rearing enlarged broods, females were less likely than males to survive to the following year. Although plumage coloration is a sexually selected trait in Eastern Bluebirds that is influenced by nutritional stress, we did not detect an effect of brood-size manipulation on female coloration. Past research, however, demonstrates that, in males, plumage colour is negatively affected by increasing brood size. We suggest that there are sex-specific strategies of reproductive investment in Eastern Bluebirds, and that researchers should incorporate measures of residual reproductive value in studies of life-history evolution.     

Alula size signals male condition and predicts reproductive performance in an Arctic‐breeding passerine

While studies of achromatic plumage signaling are scarce relative to chromatic ornaments, achromatic ornaments have the potential to act as an efficient form of visual communication due to the highly conspicuous contrast between black and white body regions. Recently, achromatic plumage reflectance has been shown to indicate condition, yet the condition‐dependence of achromatic patch size remains unstudied. Here we show the first evidence that alula size, an achromatic plumage patch, has the potential to signal a male’s condition and predict reproductive performance. In Arctic‐breeding snow buntings Plectrophenax nivalis, the size of the alula simultaneously predicted pre‐breeding physiological health and the number of offspring produced, through an intermediate variable (lay date). Snow buntings appear to pair assortatively; males and females arriving earlier pair together, and changes in body condition over the breeding season are positively related within pairs. We suggest that simple achromatic plumage patches, like alula size, have the potential to act as condition‐dependent signals. Consequently, females may benefit from assessing these signals to reliably evaluate a male’s condition and reproductive potential as a means of maximizing their reproductive success.     

Carotenoid-based ornaments of female and male American goldfinches (Spinus tristis) show sex-specific correlations with immune function and metabolic rate

Conspicuous ornamentation has been linked to immunological and physiological condition in males of many species. In species where both sexes are ornamented, it is unclear whether the signal content of ornaments differs between males and females. We examined the immunological and physiological correlates of carotenoid-based bill and plumage ornamentation in American goldfinches Spinus tristis, a species in which bright orange bills are sexually monomorphic but yellow plumage is sexually dimorphic during the breeding season. Because bill color is dynamic over short periods while plumage color is static over longer time frames, we tested whether these signals have the potential to provide temporal information about immunity and condition. In both sexes, bill color (but not plumage color) was negatively related to leukocyte differential, a measure of recent stress, while plumage color (but not bill color) was positively related to resting metabolic rate. In females, bill color also positively correlated with immunoglobulin Y, a component of acquired immunity, while plumage color positively predicted natural antibody levels, a component of innate immunity. In males, neither bill color nor plumage color predicted immune function, suggesting that the mechanisms underlying these signals vary with sex. Our results demonstrate that dynamic signals such as bill coloration do not merely reflect the same information provided by static signals but that these two classes of signal provide information about different temporal aspects of phenotypic quality. Furthermore, our results indicate that a signal expressed in both sexes has the potential to provide different information depending on the sex of the bearer.     

Is the citrus-like plumage odorant of crested auklets (Aethia cristatella) a defense against lice?

Plumage odors may function as chemical defenses against ectoparasites in birds. We tested this hypothesis for crested auklets (Aethia cristatella), a species of colonial seabird that emits a very strong citrus-like odor from its plumage. This odorant contains known chemical repellents. We evaluated evidence for chemical defense in this species using two approaches. First, we exposed pigeon lice to the volatiles emitted by freshly plucked plumage and by whole specimens. Louse survivorship was compared between these treatments and two controls. Second, we compared louse abundance on crested auklets versus a closely related congener that nests in close association. Louse survivorship did not differ between crested auklet treatments and controls. Comparison of ectoparasite loads showed that crested auklets had significantly higher louse abundance than least auklets (A. pusilla), even after controlling for body size. Our results failed to support two expectations for chemical defense. Presence of the crested auklet plumage odorant in our experiments did not reduce louse life span. Presence of the aldehyde odorant in nature did not reduce louse abundance on crested auklets. Hence we conclude that the aldehyde odorant is not immediately lethal to lice at natural concentrations in plumage.     

A test of the Darwin–Fisher theory for the evolution of male secondary sexual traits in monogamous birds

The Darwin–Fisher theory proposes that the presence of male secondary sexual traits in monogamous birds is selected for by early season breeding of females that are in good condition. These early breeding females have high fecundity because of their good condition, and they select mates based on secondary sex traits. We tested whether this hypothesis may be responsible for the presence of male sexual ornaments in the great frigatebird, a socially and genetically monogamous seabird. Consistent with the Darwin–Fisher theory, we found that reproductive success declined over the season. However, males with more exaggerated ornaments were not chosen as mates earlier in the season than males with less exaggerated ornaments, and selection gradients on these ornaments were not significantly different from zero.     

Mate Choice and Colored Beak Spots of King Penguins

The theory of sexual selection explains sexual dimorphisms in ornaments used in mate choice. Mutual mate choice is a form of sexual selection that might explain sexually monomorphic ornamental traits. Under mutual mate choice, both sexes select partners based on the same ornament. We tested the mutual mate choice hypothesis in a mutually ornamented seabird, the king penguin (Aptenodytes patagonicus), through observations of the pair formation process in the field. Penguins that were ready to mate formed displaying pairs at the edge of the colony. Some of these pairs moved into the breeding colony and produced an egg (definitive pairs), while other pairs separated and often switched to another potential partner (temporary pairs). Colored ornaments were quantified using color vision modeling. We predicted that birds would mate assortatively by their elaborate ornamental traits (specifically, colors of beak spots, auricular patches of feathers, and breast patch of feathers). We also predicted that definitive pairs would exhibit more elaborate ornaments than temporary pairs. The mutual mate choice hypothesis was supported by assortative pairing for color of the beak spots, but not for color or size of the auricular patches or for the color of the breast patch. An alternative hypothesis was also consistent with our results, that female choice for a male ornamental trait and superior female condition associated with the same trait produced assortative pairing patterns. More UV‐ and yellow‐colored beak spots for females in definitive than temporary pairs supported the female choice hypothesis over the mutual mate choice hypothesis, but previous experimental results from altered beak spot colors supported the latter. Evidence to date thus supports both the mutual mate choice and female choice hypotheses.     

Coloration,Paternity, and Assortative Mating in Western Bluebirds

Coloration in birds can act as an important sexual signal in males, yet in many species, both sexes display bright colors. Social selection may account for this pattern, with more brightly colored individuals pairing together on the best territories. Mutual mate choice may also explain this, as males investing a great deal of parental care in the offspring should be choosy about their social mates. It is less clear whether this pattern of mate choice can apply to extra‐pair partners as well. We examined western bluebirds (Sialia mexicana) to determine whether more colorful individuals tended to pair with one another, both in social pairs and between females and their extra‐pair partners. Both male and female western bluebirds display both UV‐blue structural plumage and a melanin‐based chestnut breast patch, although females are duller than males. Social pairs mated assortatively with regard to UV‐blue brightness, but not chestnut coloration. There was no evidence that extra‐pair partners mated assortatively, but males with brighter UV‐blue coloration had fewer extra‐pair offspring in their nests. Older males were more successful at siring extra‐pair offspring, despite displaying no differences in coloration compared to younger males. Coloration did not play a role in determining extra‐pair male success. These results suggest that coloration plays a role in the formation of social pairs, but not mate choice for extra‐pair partners.     

Parental Effort in Relation to Structural Plumage Coloration in the Mountain Bluebird (Sialia currucoides)

Indicator models of sexual selection suggest that costly ornaments signal reliable information regarding an individual's quality to potential mates. In species that produce altricial offspring, the amount of parental care provided by both males and females can impact reproductive success. The Good Parent Hypothesis proposes that ornamentation in biparental species can act as an honest signal of parental ability to potential mates. We tested this hypothesis using the mountain bluebird (Sialia currucoides), a sexually dichromatic, socially monogamous species in which both sexes have structurally based ornamental plumage coloration. A male's plumage color predicted neither the rate at which it provisioned nestlings nor brood growth rate. The same was true for females. We also found no indication of assortative mating by color or body condition. Feeding rates within pairs were positively correlated, which we suggest may be due to pairs responding similarly to the perceived needs of nestlings or to local area prey availability. In sum, our results do not support the Good Parent Hypothesis as an explanation for the evolution of ornamental plumage color in mountain bluebirds. We suggest alternative hypotheses for the evolution of ornamental plumage color in this species.     

Sexual Signals of the Male American Goldfinch

Male American goldfinches (Carduelis tristis) exhibit conspicuous yellow plumage, orange bills, and black caps during the breeding season. These secondary sexual characteristics may serve as criteria by which females evaluate males as potential mates because the traits vary among individuals and are likely to be costly. We quantified plumage and bill color and cap characteristics of wild, free‐ranging American goldfinches during the breeding season and tested for relationships between those features, body condition and individual genetic diversity in males. Overall, male and female goldfinches were highly sexually dichromatic, with plumage saturation and brightness and bill brightness contributing strongly to the dimorphism. Body condition decreased significantly with Julian date, even over the 2‐wk period immediately prior to the onset of nesting during which we collected our color and cap measurements. Principal components describing color of the back and the bill significantly predicted date‐corrected body condition based on quadratic regressions, suggesting that there is reliable information in back and bill color that females could use when choosing mates. In a subset of captive males, we found that bill hue faded from orange to yellow within 24 h of capture, suggesting that bill color may reflect short‐term changes in health status or carotenoid availability. Individual genetic diversity based on a panel of eight microsatellite loci was correlated with back brightness and perhaps with cap symmetry. Based on the results of this field study, ornamentation of male American goldfinches appears to signal both long‐ and short‐term aspects of phenotypic quality.     

Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis)

Many of the brilliant plumage coloration displays of birds function as signals to conspecifics. One species in which the function of plumage ornaments has been assessed is the Eastern bluebird (Sialia sialis). Studies of a population breeding in Alabama (USA) have established that plumage ornaments signal quality, parental investment, and competitive ability in both sexes. Here we tested the additional hypotheses that (1) Eastern bluebird plumage ornamentation signals nest defense behavior in heterospecific competitive interactions and (2) individual variation in plumage ornamentation reflects underlying differences in circulating hormone levels. We also tested the potential for plumage ornaments to signal individual quality and parental investment in a population breeding in Oklahoma (USA). We found that Eastern bluebirds with more ornamented plumage are in better condition, initiate breeding earlier in the season, produce larger clutches, have higher circulating levels of the stress hormone corticosterone, and more ornamented males have lower circulating androgen levels. Plumage coloration was not related to nest defense behavior. Thus, plumage ornamentation may be used by both sexes to assess the physiological condition and parental investment of prospective mates. Experimental manipulations of circulating hormone levels during molt are needed to define the role of hormones in plumage ornamentation.     

Offspring sex ratio allocation in the parasitic jaeger: selection for pale females and melanic males?

The maintenance of plumage color polymorphism in the parasiticjaeger (Stercorarius parasiticus) is still not well understood.Earlier studies indicated that selection may favor pale femalesand melanic males. If so, females would maximize their fitness,producing pale female and melanic male offspring. We thereforepredicted that females might bias their offspring sex ratiotoward daughters in pale pairs and toward sons in melanic pairs.Females might also choose to mate assortatively in relationto plumage color, thereby maximizing the probability of producingeither pale or melanic offspring. Because females are largerthan males, differential rearing costs may affect the offspringsex ratio independent of parental plumage color. We examinedoffspring sex ratio allocation, breeding variables indicativeof parental quality, and mating pattern in relation to plumagecolor in a colony of parasitic jaegers in northern Norway. Jaegerstended to mate assortatively in relation to plumage color. Thereproductive performance declined with season, and matched pairsappeared to be of lower quality than mixed pairs. The proportionof male offspring increased with hatching date in matched paleand mixed pairs, whereas the situation was reversed in matchedmelanic pairs. Matched pale pairs produced an overall surplusof favorable pale but costly daughters despite their lower quality,while melanic pairs produced a surplus of favorable melanicsons. However, differential offspring rearing costs and parentalrearing capacity may have additionally affected the realizedoffspring sex ratio. Mixed pairs producing an overall surplusof pale and melanic daughters allocated their resources accordingto differential rearing costs and parental quality only. Wesuggest that both strategies of sex ratio allocation togetherwith differences in reproductive success in matched versus mixedpairs may have a balancing effect on the mating pattern betweenplumage morphs and may contribute to the maintenance of thecolor polymorphism in this species.     

Oxidative damage to DNA related to survivorship and carotenoid-based sexual ornamentation in the common yellowthroat

Carotenoid-based sexual ornaments are hypothesized to be reliable signals of male quality, based on an allocation trade-off between the use of carotenoids as pigments and their use in antioxidant defence against reactive oxygen species. Carotenoids appear to be poor antioxidants in vivo, however, and it is not clear whether variation in ornament expression is correlated with measures of oxidative stress (OXS) under natural conditions. We used single-cell gel electrophoresis to assay oxidative damage to erythrocyte DNA in the common yellowthroat (Geothlypis trichas), a sexually dichromatic warbler in which sexual selection favours components of the males' yellow 'bib'. We found that the level of DNA damage sustained by males predicted their overwinter survivorship and was reflected in the quality of their plumage. Males with brighter yellow bibs showed lower levels of DNA damage, both during the year the plumage was sampled (such that yellow brightness signalled current OXS) and during the previous year (such that yellow brightness signalled past OXS). We suggest that carotenoid-based ornaments can convey information about OXS to prospective mates and that further work exploring the proximate mechanism(s) linking OXS to coloration is warranted.     

Effects of site fidelity and breeding performance on mate retention in a short‐lived passerine,the tree swallow Thachycineta bicolor

We studied the relationship among re‐mating, site fidelity and breeding performance in the tree swallow Tachycineta bicolor using 16 y of data on reproductive biology in a population breeding in nest boxes near Ithaca, New York. Of 217 pairs for which both members survived the non‐breeding season, 76% mated with a new partner and 24% reunited with their previous mate. Pairs did not increase their breeding success by breeding together for more than one breeding season. Males produced fewer fledglings after breeding with a new partner, but females neither increased nor decreased their success when breeding with a new mate. Females who bred with a new partner were younger than females that reunited with their previous mates, and they were more likely to move to a different nest box. Males that bred with a new mate were of similar age to males that reunited, and they did not move more often. The probability of breeding with a new partner was better predicted by female age than by previous breeding success, suggesting that re‐mating was not strongly affected by past breeding performance. Because younger females change breeding sites more frequently than do older females and females that mated with a new partner were younger than females that reunited with their previous mates, we suggest that the tendency of tree swallows to change partners between years is a by‐product of lower site fidelity of younger females rather than a strategy for increasing breeding success.     

A label of health: a previous immune challenge is reflected in the expression of a female plumage trait

Studies of ornaments as indicators of individual quality have hitherto focused on males, while studies of female ornaments have been almost absent. However, females within the same species may show large variation in both outer appearance and individual quality. We experimentally examined the effect of an immune challenge on the size of white fringes in the wing feathers produced one to two months after the challenge in female common eiders (Somateria mollissima), a long-lived sea duck. Immune-challenged females produced feathers with smaller white fringes, showing that a previous immune challenge may affect the expression of a plumage trait. We also report an unexpected difference in the expression of this white plumage trait within the immune-challenged group, related to the specific immune response against one of the injected antigens.     

Vocal and Optical Indicators of Individual Quality in a Social Seabird,the Crested Auklet (Aethia cristatella)

Indicators of individual quality in ornamentation or in vocalizations have been reported for different animal species. However, no studies have jointly investigated ornamentation and vocalizations in one species. The crested auklet (Aethia cristatella, Alcidae) is a small colonial seabird of the North Pacific and is unusual for a bird in using optical, vocal, and olfactory signals. We estimated the potential for coding individual quality in vocalizations and plumage ornamentation and the relationship between vocal and optical traits in crested auklets. During the summer seasons of 2008–2009, we recorded 359 trumpet calls from 28 individually marked males and measured indices of body size, condition, and plumage patterns of 58 male and 48 female crested auklets from a breeding colony on Talan Island, Sea of Okhotsk. We found strong interindividual differences in trumpet call characteristics. Furthermore, we found that the maximum fundamental frequency of the loudest notes of the trumpet call is negatively correlated with body condition, and head crest length is positively correlated with body size. However, we found no relationship between vocal and ornamental traits. The results suggest that advertising calls of crested auklets signal caller individuality and quality, but future experimental studies should test whether or not this is indeed case.