Aerodynamic costs of long tails in male barn swallows Hirundo rustica and the evolution of sexual size dimorphism

Exaggerated tail feathers of birds constitute a standard exampleof evolution of extravagant characters due to sexual selection.Such secondary sexual traits are assumed to be costly to produceand maintain, and they usually are accompanied by morphologicaladaptations that tend to reduce their costs. The aerodynamiccosts for male barn swallows Hirundo rustica of having longtails were quantified using aerodynamics theory applied to morphologicaldata from seven European populations. Latitudinal differencesin tail length were positively correlated with differences inflight costs predicted by aerodynamics theory. A positive relationshipbetween aerodynamic costs of long tails and the degree of sexualsize dimorphism was found among populations. Latitudinal differencesin foraging costs may result in tail length being relativelysimilar in males and females in southern populations, whereasthe low foraging costs for males in northern populations mayallow them to cope with higher aerodynamic costs, giving riseto large sexual size dimorphism. Enlargement of wingspan inmales can alleviate but not eliminate the costs of tail exaggeration,and therefore differences in aerodynamic costs of male ornamentswere maintained among populations. Sexual size dimorphism in thebarn swallow arises as a consequence of latitudinal differencesin the advantages of sexual selection for males and the costsof long tails for males and females.     

Tail length in birds in relation to tail shape,general flight ecology and sexual selection

Relative tail length (longtailedness) of Palearctic birds was assessed by the standardized residuals of log–log regressions of tail length on wing length and tarsus length. The mean degree of tail shortening was greater than mean degree of tail lengthening, but there was a greater frequency of extreme long-tailed than short-tailed species. Longtailedness was greater in ornamental pin, lyre, deep forked and graduated shaped tails. These shapes (except graduated, for which data were lacking) were also relatively long-tailed according to shortest-rectrix lengths, this extra length potentially contributing compensatory lift. In forked tails, tail ratio increased linearly with longtailedness to above the aerodynamic optimum, and thus the most elongated forked tails were also more deeply forked. Tail shortening was marked for rounded tails, a surprising result in view of their slightly ornamental shape. Phylogenetically independent contrasts showed significantly greater longtailedness in graduated than square-tailed species, confirming the species-wide analysis. In phylogenetically independent contrasts of longtailedness and ecological factors, short-tailed species had significantly greater flight distances than medium-tailed species, but long- and medium-tailed species did not differ in migratory distance, foraging distance, overall flight distance or importance of aerial foraging. The data suggest that ecological factors, i.e. natural selection, are more important in the evolution of short-tailedness than longtailedness in birds, and that an additional influence of sexual selection on tail length and shape is also widespread.     

Sexual selection in the barn swallow Hirundo rustica. VI. Aerodynamic adaptations

Secondary sexual characters are assumed to be costly to produce and maintain, and this will select for morphological modifications that reduce the magnitude of such costs. Here we test whether a feather ornament, the sexually exaggerated outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject to a directional female mate preference, and other aspects of the morphology used for flight have been modified to increase aerodynamic performance. This was done by making comparisons among sexes within populations, among individuals varying in tail length within populations, and among populations from different parts of Europe. Male barn swallows experienced reduced drag from their elongated tail feathers by morphological modifications of the ornamental feathers as compared to females. Morphological features of the outermost tail feathers were unrelated to tail length in both males and females within populations. Wing and tail morphology (length of central tail feathers and wings, wing span, wing area, wing loading, and aspect ratio) was modified in males compared to females. Barn swallows with long tails had morphological tail and wing modifications that reduced the cost of a large ornament, and similar modifications were seen among populations. The costs of the exaggerated secondary sexual character were therefore reduced by the presence of cost-reducing morphological modifications. The assumptions of reliable signalling theory, that signals should be costly, but more so to low than to high quality individuals, were not violated because long-tailed male barn swallows had the largest cost-reducing morphological characters.     

Patterns of morphometric variation in birds' tails: length, shape and variability

In Palaearctic birds, tail length was more variable than wing and tarsus, but not bill lengths. Tails of the ornamental shapes pin, lyre and graduated varied more in length than did non-ornamental shapes. Log coefficient of variation (C V) tail length showed an overall U-shaped relationship with longtailedness, but although the CV for most tail shapes increased in short-tailed species, only in ornamental shapes was C V also high in long-tailed species. C V of fork depth was lowest at a fork depth of 2, and considerably higher in shallow forked tails. CV streamer lengths were similar to CV deep fork depths. The more deeply forked tails thus seem ornamental. Phylogenetically independent contrasts confirmed in males, but not females, that long-tailed species had greater CV than medium-tailed species, and the greater CV of graduated than square tails, but the CV of short- and medium-tailed species did not differ. These comparisons, however, did not control for tail shape. The greater elongation and CV of tails with ornamental shapes are consistent with an influence of sexual/signal selection on these tails, and the increase in CV with longtailedness suggests that Weber's law applies to the perceptual threshold for tail length. Sexual selection may have a widespread, but moderate, influence on tail traits in birds.     

Haematocrit correlates with tail ornament size in three populations of the Barn Swallow (Hirundo rustica)

1. Handicap models of sexual selection propose that male ornaments are indicators of male quality and that honesty is enforced by the costs imposed by the exaggerated ornamental traits. In long-distance migratory birds that feed on the wing, the aerodynamic cost of exaggerated ornamental characters should be particularly high because the size of the ornaments deviates from the natural selection optimum. During migration, birds are expected to raise their oxygen consumption in relation to the energetic demands imposed by their morphology. An increase of haematocrit is an adaptive response to enhance oxygen uptake and efficiency of transfer to the muscular tissues during spells of intense muscular activity.
2. The change of haematocrit of Barn Swallows ( Hirundo rustica ) after their arrival to the breeding sites, and the relationships between haematocrit values recorded after migration and the size of ordinary and sexually selected morphological characters in three Barn Swallow populations were analysed.
3. Males had higher haematocrit values than females. Individual haematocrit values declined after arrival to the breeding sites. Haematocrit values of males were significantly and positively correlated with the size of their ornamental tail but not correlated with other characters, thus suggesting that well-ornamented males, in order to arrive early, have to raise their haematocrit above the level of short-tailed males.
4. Males and females of similar tail length did not differ in their haematocrit, thus suggesting that sexual dimorphism in haematocrit might be functionally related to dimorphism in tail length.
5. Our results are consistent with the handicap principle because long-tailed males experience lower mortality and larger seasonal reproductive success compared with short-tailed males.     

Evolution of the structure of tail feathers: implications for the theory of sexual selection

Bird tails are extraordinarily variable in length and functionality. In some species, males have evolved exaggeratedly long tails as a result of sexual selection. Changes in tail length should be associated with changes in feather structure. The study of the evolution of feather structure in bird tails could give insight to understand the causes and means of evolution in relation to processes of sexual selection. In theory, three possible means of tail length evolution in relation to structural components might be expected: (1) a positive relationship between the increase in length and size of structural components maintaining the mechanical properties of the feather; (2) no relationship; that is, enlarging feather length without changes in the structural components; and (3) a negative relationship; that is, enlarging feather length by reducing structural components. These hypotheses were tested using phylogenetic analyses to examine changes in both degree of exaggeration in tail length and structural characteristics of tail feathers (rachis width and density of barbs) in 36 species, including those dimorphic and nondimorphic in tail length. The degree of sexual dimorphism in tail length was negatively correlated with both rachis width and density of barbs in males but not in females. Reinforcing this result, we found that dimorphism in tail length was negatively associated with dimorphism in tail feather structure (rachis width and density of barbs). These results support the third hypothesis, in which the evolution of long feathers occurs at the expense of making them simpler and therefore less costly to produce. However, we do not know the effects of enfeeblement on the costs of bearing. If the total costs increased, the enfeeblement of feathers could be explained as a reinforcement of the honesty of the signal. Alternatively, if total costs were reduced, the strategy could be explained by cheating processes. The study of female preferences for fragile tail feathers is essential to test these two hypotheses. Preferences for fragile tails would support the evolution of reinforcement of honesty, whereas female indifference would indicate the existence of cheating in certain stages of the evolutionary process.     

Mating systems, sperm competition, and the evolution of sexual dimorphism in birds

Comparative analyses suggest that a variety of factors influence the evolution of sexual dimorphism in birds. We analyzed the relative importance of social mating system and sperm competition to sexual differences in plumage and body size (mass and tail and wing length) of more than 1,000 species of birds from throughout the world. In these analyses we controlled for phylogeny and a variety of ecological and life-history variables. We used testis size (corrected for total body mass) as an index of sperm competition in each species, because testis size is correlated with levels of extrapair paternity and is available for a large number of species. In contrast to recent studies, we found strong and consistent effects of social mating system on most forms of dimorphism. Social mating system strongly influenced dimorphism in plumage, body mass, and wing length and had some effect on dimorphism in tail length. Sexual dimorphism was relatively greater in species with polygynous or lekking than monogamous mating systems. This was true when we used both species and phylogenetically independent contrasts for analysis. Relative testis size was also related positively to dimorphism in tail and wing length, but in most analyses it was a poorer predictor of plumage dimorphism than social mating system. There was no association between relative testis size and mass dimorphism. Geographic region and life history were also associated with the four types of dimorphism, although their influence varied between the different types of dimorphism. Although there is much interest in the effects of sperm competition on sexual dimorphism, we suggest that traditional explanations based on social mating systems are better predictors of dimorphism in birds.     

BODY SIZE AND HAREM SIZE IN MALE RED-WINGED BLACKBIRDS: MANIPULATING SELECTION WITH SEX-SPECIFIC FEEDERS

We experimentally manipulated the strength of selection in the field on red-winged blackbirds (Agelaius phoeniceus) to test hypotheses about contrasting selective forces that favor either large or small males in sexually size dimorphic birds. Selander (1972) argued that sexual selection favors larger males, while survival selection eventually stabilizes male size because larger males do not survive as well as smaller males during harsh winters. Searcy (1979a) proposed instead that sexual selection may be self limiting: male size might be stabilized not by overwinter mortality, but by breeding-season sexual selection that favors smaller males. Under conditions of energetic stress, smaller males should be able to display more and thus achieve higher reproductive success. Using feeders that provisioned males or females but not both, we produced conditions that mimicked the extremes of natural conditions. We found experimental support for the hypothesis that when food is abundant, sexual selection favors larger males. But even under conditions of severe energetic stress, smaller males did not gain larger harems, as the self-limiting hypothesis predicted. Larger males were more energetically stressed than smaller males, but in ways that affected their future reproductive output rather than their current reproductive performance. Stressed males that returned had smaller wings and tails than those that did not return; among returning stressed males, relative harem sizes were inversely related to wing and tail length. Thus, male body size may be stabilized not by survival costs during the non-breeding season, nor by energetic costs during the breeding season, but by costs of future reproduction that larger males pay for their increased breeding-season effort.     

Fork tails evolved differently in swallows and swifts

Whether sexual or viability selection drives the evolution of ornamental traits is often unclear because current function does not clarify evolutionary history, particularly when the ornamentation is a modified version of the functional traits. Here, using a phylogenetic comparative approach, we studied how deeply forked tails—a classic example of sexually selected traits that might also be a mechanical device for enhancing aerodynamic ability—evolved in two groups of aerial foragers, swallows (family: Hirundinidae) and swifts (family: Apodidae). Although apparent fork depth, the target of sexual selection, increases with increasing outermost tail feather length, fork depth can also increase with decreasing central tail feather length, which impairs the lift generated by the tail. Thus, we predicted that sexual selection, but not viability selection, should favour the evolution of short central tail feathers in species with deeply forked tails, particularly in swifts, which are less reliant on the lift generated by their tail than in swallows. We found support for these predictions because central tail feather length decreased with increasing tail fork depth, particularly in swifts. Instead, the increase in outermost tail feather length per unit tail fork depth was higher in swallows than in swifts, indicating that a similar sexual ornamentation (i.e. forked tails) differently evolved in these two aerial insectivores perhaps due to the differential cost of ornamentation. We also found support for an optical illusion that changes the relative importance of central and outermost tail feather length in sexual selection.     

Comparative evidence for costs of secondary sexual characters: adaptive vane emargination of ornamented feathers in birds

We used a comparative approach, by comparing bird species with tail ornamentation with sister taxa without ornamentation, to deduce the aerodynamic function of extravagant feather ornaments and the costs of such ornaments in birds. First, the aerodynamic function of tail feather ornaments in birds can be deduced from asymmetry in the width of tail feather vanes, since flightless birds have symmetrical vanes while flying birds without feather exaggeration by sexual selection have asymmetrical vanes. Distal inner vanes at the tip of tail feathers were more narrow in ornamented as compared to nonornamented birds, and vane asymmetry at the tip of the feather was therefore reduced in ornamented species, suggesting marginal aerodynamic function of the distal part of extravagant feather ornaments. Second, the cost of feather ornaments due to parasite drag is proportional to the area of feathers extending beyond the maximum continuous width of the tail, and aerodynamic costs of long tails could therefore be diminished by a reduction in feather width. Consistent with this prediction, the outermost tip of feather ornaments was narrower than the homologous character in nonornamented sister taxa, while the base of the feather had similar width in the two groups of birds. These results suggest that the costs of extravagant ornamentation have been diminished by a reduction in feather width, leading to a reduction in drag. Costs of feather ornaments, as demonstrated by their fine morphology, thus appear to have been extensive during the evolution of these characters.     

A comparative study on the evolution of reversed size dimorphism in monogamous waders

Sexual dimorphism in size is common in birds. Males are usually larger than females, although in some taxa reversed size dimorphism (RSD) predominates. Whilst direct dimorphism is attributed to sexual selection in males giving greater reproductive access to females, the evolutionary causes of RSD are still unclear. Four different hypotheses could explain the evolution of RSD in monogamous birds: (1) The ‘energy storing’ hypothesis suggests that larger females could accumulate more reserves at wintering or refuelling areas to enable an earlier start to egg laying. (2) According to the ‘incubation ability’ hypothesis, RSD has evolved because large females can incubate more efficiently than small ones. (3) The ‘parental role division’ hypothesis suggests that RSD in monogamous waders has evolved in species with parental role division and uniparental male care of the chicks. It is based on the assumption that small male size facilitates food acquisition in terrestrial habitats where chick rearing takes place and that larger females can accumulate more reserves for egg laying in coastal sites. (3) The ‘display agility’ hypothesis suggests that small males perform better in acrobatic displays presumably involved in mate choice and so RSD may have evolved due to female preference for agile males. I tested these hypotheses in monogamous waders using several comparative methods. Given the current knowledge of the phylogeny of this group, the evolutionary history of waders seems only compatible with the hypothesis that RSD has evolved as an adaptation for increasing display performance in males. In addition, the analysis of wing shape showed that males of species with acrobatic flight displays had wings with higher aspect ratio (wing span/²wing area) than non-acrobatic species, which probably increases flight manoeuvrability during acrobatic displays. In species with acrobatic displays males also had a higher aspect ratio than females although no sexual difference was found in non-acrobatic species. These results suggest that acrobatic flight displays could have produced changes in the morphology of some species and suggest the existence of selection favouring higher manoeuvrability in species with acrobatic flight displays. This supports the validity of the mechanisms proposed by the ‘display agility’ hypothesis to explain the evolution of RSD in waders.     

Experimental analyses of sexual and natural selection on short tails in a polygynous warbler

We believe that no experimental study has yet tested Darwin's idea that, as well as generating trait elaboration, intersexual selection might sometimes drive sex-biased trait reduction. Here we present the results of two experiments exploring the negative relationship between tail length and reproductive success in male golden-headed cisticolas (Cisticola exilis). In the first experiment, artificially shortening a male's tail produced a dramatic increase in his reproductive success, measured as either the number of females nesting or number of chicks Hedged on his territory. A second experiment, in which manipulated birds were flown through a maze, revealed that short tails also impose costs by reducing aerodynamic performance during slow-speed foraging flight. Because tail shortening yields reproductive benefits and viability costs, we conclude it has evolved via sexual selection. Disentangling exactly how short tails enhance male reproductive success is more difficult. Male-male competition appears partly responsible: aerodynamic theory predicts that tail reduction enhances high-speed flight and, in line with this, shortened-tail males spent more time engaged in high-speed aerial chases of rivals and defended higher-quality territories. However, shortened-tail males had higher reproductive success independent of territory quality and spent more time in aerial displays which may be directed at females. This suggests that tail shortening is also favoured via female choice based on male phenotype.     

Subtle,pervasive genetic correlation between the sexes in the evolution of dimorphic hummingbird tail ornaments*

Male hummingbirds have repeatedly evolved sexually dimorphic tails that they use as ornaments during courtship. We examine how male ornament evolution is reflected in female morphology. Lande's two-step model of the evolution of dimorphism predicts that γ (the genetic correlation between the sexes) causes trait elaboration to first evolve quickly in both sexes, then dimorphism evolves more slowly. On the hummingbird phylogeny, tail length does not fit this two-step model; although hummingbirds repeatedly evolved ornamental, elongated tails, dimorphism evolves on the same phylogenetic branch as elongation, implying that γ quickly evolves to be low over phylogenetic timescales. Male “bee” hummingbirds have evolved diverse rectrix shapes that they use to produce sound. Female morphologies exhibit subtle, pervasive correlations with male morphology. No female-adaptive hypotheses explain these correlations, since females do not also make sounds with their tail. Subtle shape similarity has arisen through the genetic correlation with males, and is subject to intralocus sexual conflict. Intralocus sexual conflict may produce increased phenotypic variation of female ornaments. Other evolutionary constraints on tail morphology include a developmental correlation between neighboring tail-feathers, biasing tail elaboration to occur most often at the ends of the feather tract (rectrix 5 or 1) and not the middle.     

Sexual dimorphism in wing and tail length as shown by the Swallow Hirundo rustica

In both sexes of Hirundo rustica two metric traits are significantly different. One of these traits, tail length, shows a positive correlation between mates and a negative correlation with the date of first egg laying. On the other hand, wing lengths are not significantly related in any of the corresponding tests. Similar relations were recorded both for all analysed birds and for at least two-year-old individuals. It is suggested that sexual selection, as proposed by Darwin (1871) for monogamous birds, is responsible for sexual dimorphism in the tail length of the Swallow. It seems that intersexual difference in wing length could be a result of such mechanisms as stabilizing the body sizes of males and females at different energetically optimum levels by natural selection.     

No evidence of sexual dimorphism in the tails of the swallowtail butterflies Papilio machaon gorganus and P. m. britannicus

The European swallowtail butterfly (Papilio machaon) is so named, because of the long and narrow prominences extending from the trailing edge of their hindwings and, although not a true tail, they are referred to as such. Despite being a defining feature, an unequivocal function for the tails is yet to be determined, with predator avoidance (diverting an attack from the rest of the body), and enhancement of aerodynamic performance suggested. The swallowtail, however, is sexually size dimorphic with females larger than males, but whether the tail is also sexually dimorphic is unknown. Here, museum specimens were used to determine whether sexual selection has played a role in the evolution of the swallowtail butterfly tails in a similar way to that seen in the tail streamers of the barn swallow (Hirundo rustica), where the males have longer streamers than those of the females. Previously identified sexual dimorphism in swallowtail butterfly size was replicated, but no evidence for dimorphism in tail length was found. If evolved to mimic antennae and a head to divert a predatory attack, and if an absolute tail size was the most effective for this, then the tail would probably be invariant with butterfly hindwing size. The slope of the relationship between tail length and size, however, although close to zero, was nonetheless statistically significantly above (tail length ∝ hindwing area ^{0.107 ± 0.011}). The slope also did not equate to that expected for geometric similarity (tail length ∝ hindwing area^1/2) suggesting that tail morphology is not solely driven by aerodynamics. It seems likely then, that tail morphology is primarily determined by, and perhaps a compromise of several, factors associated with predator avoidance (e.g. false head mimicry and a startling function). Of course, experimental data are required to confirm this.     

Sexual dimorphism in snake tail length: sexual selection,natural selection,or morphological constraint?

Male snakes typically have longer tails relative to body length than females, but the extent of this dimorphism varies among species. Three hypotheses have been suggested to explain tail dimorphism. The Morphological Constraint Hypothesis proposes that males have relatively longer tails to accommodate hemipenes and retractor muscles. The Female Reproductive Output Hypothesis proposes that females have relatively shorter tails as a secondary result of natural selection for increased reproductive capacity. The Male Mating Ability Hypothesis proposes that sexual selection favours relatively longer tails in males during courtship. These hypotheses make different predictions about the relationships among tail length, body size, male reproductive morphology, female reproductive output, mode of reproduction, and male mating behaviour among and within taxa. Predictions were tested using published data for 56 genera in the family Colubridae and original data for the water snake, Nerodia sipedon. Tail length dimorphism was more male-biased in tam having relatively short tails (r=–0.52, P < 0.001), hemipenes and retractor muscles occupied a greater proportion of the tail in taxa having relatively short tails (r=– 0.71, P < 0.00l and r=– 0.66, P = 0.001, respectively), and tail length dimorphism was more male-biased in taxa in which body size dimorphism was more female-biased (r=– 0.60, P < 0.001). These results support both the Morphological Constraint Hypotheses and the Female Reproductive Output Hypothesis. However, tests of other predictions, including those regarding patterns within N. sipedon , failed to support any of the three hypotheses. Comparisons among taxa suggest several species in which further tests of these hypotheses would be especially appropriate.     

Sexual selection and tail streamers in the barn swallow

The functional significance of elongated, narrow tips of the tail feathers of certain birds, so-called tail streamers, has recently been discussed from an aerodynamic point of view, and the effects of sexual selection on such traits have been questioned. We review our long-term field studies using observational and experimental approaches to investigate natural and sexual selection in the barn swallow, Hirundo rustica, which has sexually size-dimorphic outermost tail feathers. Experimental manipulation of the length of the outermost tail feathers has demonstrated sexual selection advantages of tail elongation and disadvantages of tail shortening, with opposite effects for natural selection in terms of foraging efficiency, haematocrit and survival. These findings are contrary to the prediction of a general deterioration from both shortening and elongation, if the tail trait was determined solely by its effects on aerodynamic efficiency and flight manoeuvrability. Patterns of sexual selection in manipulated birds conform with patterns in unmanipulated birds, and selection differentials for different components of sexual selection in manipulated birds are strongly positively correlated with differentials in unmanipulated birds. Age and sex differences in tail length, and geographical patterns of sexual size dimorphism, are also consistent with sexual selection theory, but inconsistent with a purely natural selection advantage of long outermost tail feathers in male barn swallows.     

Convergent evolution of the tradeoff between egg size and tail fork depth in swallows and swifts

Convergent evolution provides strong evidence of the power of natural selection, particularly for distantly related taxa. Swallows and swifts are such distantly related taxa; both are specialised to feed on flying insects and have similar morphological features, such as long wings. These birds also exhibit deeply forked tails in some species, but their function remains unclear; some have argued that fork tails have evolved due to sexual selection to attract mates, while others claim that viability selection for efficient foraging favours fork tails. A recent phylogenetic analysis found the negative relationship between female tail fork depth and egg size in swallows perhaps due to foraging costs of fork tails during egg formation, but its generality remains unclear. Here, using swifts, which differ from swallows by foraging on weak‐flying insects, we found that egg size significantly decreased with increasing female fork depth. Because female fork depth was not significantly related to clutch size, clutch size would not compensate for the relationship between egg size and fork depth. The current finding using swifts, together with the previous finding in swallows, provide strong support for an evolutionary tradeoff between the female plumage ornament and reproductive investment, as predicted by sexual selection theory.     

Extra-pair paternity and sexual dimorphism in birds

Differences in the strength of sexual selection between males and females can lead to sexual dimorphism. Extra-pair paternity (EPP) can increase the variance in male reproductive success and hence the opportunity for sexual selection. Previous research on birds suggests that EPP drives the evolution of dimorphism in plumage colour and in body size. Because EPP increases the intensity of sexual selection in males, it should lead to increased dimorphism in species with larger or more colourful males, but decreased dimorphism in species with larger or more colourful females. We explored the covariation between EPP and sexual dimorphism in wing length and plumage colouration in 401 bird species, while controlling for other, potentially confounding variables. Wing length dimorphism was associated positively with the frequency of EPP, but also with social polygamy, sex bias in parental behaviour and body size and negatively with migration distance. The frequency of EPP was the only predictor of plumage colour dimorphism. In support of our prediction, high EPP levels were associated with sexual dichromatism, positively in species in which males are more colourful and negatively in those in which females are more colourful. Contrary to our prediction, high EPP rates were associated with increased wing length dimorphism in species with both male- and female-biased dimorphism. The results support a role for EPP in the evolution of both size and plumage colour dimorphism. The two forms of dimorphism were weakly correlated and predicted by different reproductive, social and life-history traits, suggesting an independent evolution.     

Sexual size dimorphism and sex identification using morphological traits of two Aegithalidae species

The black-throated tit, Aegithalos concinnus, and long-tailed tit, A. caudatus, are two widely-distributed species of Aegithalidae. They are thought to be monomorphic and thus difficult to differentiate between sexes in the field. We determined the sex of 296 black-throated tits and 129 long-tailed tits using DNA analysis, evaluated their sexual size dimorphism, and developed discriminant models to identify sex based on morphometries, including bill length, bill depth, bill-head length, maximum tarsus length, tarsus width, wing length, tail length, total body length, and body mass. Both species were sexually dimorphic in size, with males having larger measurements than females, except for bill length in black-throated tits, and both bill length and body mass in long-tailed tits. Moreover, both species showed similar sexual size dimorphism (SSD) among the morphological features, with tail length having the highest SSD value. The multivariate discriminant models for sex identification had an accuracy of 82% for both species, which was slightly higher than the best univariate discriminatory model for each species. Because of the complicated nature of the multivariate model, we recommend univariate models for sex identification using D = 0.491 × tail length - 24.498 (accuracy 80%) for black-throated tits and D = 0.807 × wing length - 45.934 (accuracy 78%) for long-tailed tits. Females in both species showed generally higher morphological variation than did males, resulting in lower accuracies in all discriminate functions regardless of univariate or multivariate approach. This could be the result of a sex-biased selective pressure in which males have higher selective pressures for these morphological features.