INTENSE NATURAL SELECTION ON BODY SIZE AND WING AND TAIL ASYMMETRY IN CLIFF SWALLOWS DURING SEVERE WEATHER

Extreme climatic disturbances provide excellent opportunities to study natural selection in wild populations because they may cause measurable directional shifts in character traits. Insectivorous cliff swallows (Petrochelidon pyrrhonota) in the northern Great Plains must often endure periods of cold weather in late spring that reduce food availability, and if cold spells last four or more days, mortality due to starvation may result. We analyzed morphological shifts associated with viability selection, and how patterns of bilateral symmetry were affected by survival selection, during a four-day period of cold weather in 1992 and a six-day period in 1996 in southwestern Nebraska. Birds that died during the cold were compared to those still alive when the severe weather ended. The event in 1992 killed relatively few birds, but the cold spell in 1996 killed thousands of cliff swallows and reduced their population by about 53%. Climatological records suggest that mortality events comparable to that of 1996 have occurred in only one other year since 1875. Larger birds were favored in the 1996 event. Selection was more intense in 1996 than in 1992 because of more stressful conditions in 1996. Directional selection gradient analysis showed that measures of skeletal body size (tarsus length, culmen width and length) and wing length were targets of selection in 1996. Survivors had lower wing and outer tail asymmetry, and wing and tail asymmetry were targets of selection in both events. Mortality patterns did not differ by sex, but older birds suffered heavier mortality; morphological traits generally did not vary with age. Nonsurvivors were not in poorer apparent condition prior to the weather event than survivors, suggesting that selection acted directly on morphology independent of condition. Selection on body size in cliff swallows was more intense than in studies of body size evolution in other bird species. Larger swallows were probably favored in cold weather due to the thermal advantages of large size and the ability to store more fat. Swallows with low asymmetry were favored probably because low asymmetry in wing and tail made foraging more efficient and less costly, conferring survival advantages during cold weather. This population of cliff swallows may have undergone relatively recent body size evolution.     

SEXUAL SELECTION IN THE BARN SWALLOW (HIRUNDO RUSTICA). IV. PATTERNS OF FLUCTUATING ASYMMETRY AND SELECTION AGAINST ASYMMETRY

The patterns of variation in fluctuating asymmetry were studied in four morphological characters of the barn swallow Hirundo rustica. The level of absolute and relative asymmetry was larger in the secondary sexual character “outer tail length” than in three nonsexual morphological traits (wing, central tail, and tarsus length). The extent of individual asymmetry in outer tail length was negatively correlated with tail-ornament size, whereas the relationship between asymmetry of all other morphological characters and their size was flat or U-shaped. Asymmetry in outer tail length was unrelated to asymmetry in other morphological characters, whereas asymmetries in the length of wing, central tail, and tarsus were positively correlated. Male bam swallows exhibited larger asymmetry in outer tail length than females. Asymmetry of most morphological traits exhibited intermediate repeatabilities between years, with the exception of male and female outer tail length, which were highly repeatable. Tail asymmetry of offspring weakly, though significantly, resembled that of their parents. Asymmetry in wing and outer tail length was also significantly related to several fitness components. Male barn swallows that acquired a mate were less asymmetric in wing and outer tail length than unmated males. Females with more asymmetrical tails laid eggs significantly later. Annual reproductive success was unrelated to fluctuating asymmetry. Male barn swallows that survived were less asymmetric in wing and outer tail length than nonsurvivors, whereas female survivors were less asymmetric in outer tail length than nonsurvivors. These results suggest that levels of fluctuating asymmetry in barn swallows are associated with differences in fitness.     

Genetic and environmental effects on morphology and fluctuating asymmetry in nestling barn swallows

A barn swallow Hirundo rustica partial cross‐fostering experiment with simultaneous brood size manipulation was conducted in two years with contrasting weather conditions, to estimate heritable variation in tarsus, tail and wing size and fluctuating asymmetry. Environmental stress had contrasting effects depending on trait type. Significant heritabilities for tarsus, tail and wing size were found only in enlarged broods irrespective of year effects, while tarsus asymmetry was significantly heritable in the year with benign weather conditions irrespective of brood size manipulation effects. Tail, wing and composite (multicharacter) asymmetry were never significantly heritable. The environment with the higher heritability generally had higher additive genetic variance and lower environmental variance, irrespective of trait type. Heritability was larger for trait size than for trait asymmetry. Patterns of genetic variation in nestlings do not necessarily translate to the juvenile or adult stage, as indicated by lack of correlation between nestling and fledgling traits.     

Ectoparasites cause increased bilateral asymmetry of naturally selected traits in a colonial bird

Abstract Parasitism has been shown to correlate with levels of bilateral symmetry in some organisms, with more asymmetric individuals often having more parasites. However, few studies have shown experimentally that parasitism directly causes increased asymmetry. By fumigating some cliff swallow (Petrochelidon pyrrhonota) colonies and leaving others untreated, we investigated experimentally whether ectoparasitism by the cimicid swallow bug led to higher levels of asymmetry in length of wings, outer tail feathers, and tarsus among juvenile and adult birds. Juveniles from fumigated colonies measured soon after fledging had significantly less asymmetry in wing and outer tail length than juveniles from nonfumigated colonies; asymmetry in tarsus length was unaffected by parasitism. Adults that had undergone one or more post‐juvenal molts on the wintering grounds showed no differences in asymmetry between those reared in fumigated vs. nonfumigated colonies. These results show that ectoparasitism directly leads to increased feather asymmetry in cliff swallows, probably through parasite‐induced nutritional stress. Because wing and tail asymmetry impair flight performance and reduce foraging efficiency, the increased asymmetry caused by parasites represents a fitness cost to cliff swallows. This is among the few experimental studies to show an effect of parasites on asymmetry of naturally selected characters.     

Tail injuries increase the risk of mortality in free-living lizards (Uta stansburiana)

Summary Caudal autotomy is an effective anti-predator mechanism used by many lizard species. Fitness benefits of surviving a predatory attack are obvious, although lizards that autotomize their tails may be at greater risk during subsequent encounters with predators than lizards with complete tails. In previous laboratory studies, tail-less lizards were more vulnerable to capture by predators, but little is known about the relative survival of tailed versus tail-less lizards in nature. This study reports on significant associations between naturally incurred tail injuries and the subsequent risk of mortality in 7 populations of the lizard Uta stansburiana. I used standard mark-recapture techniques to document survival and quantified tail injuries by estimating tail completeness. I then used sampled randomization tests to compare intitial tail completeness values of surviving versus non-surviving lizards. I evaluated overall patterns by comparising the means of tail completeness values of survivors versus non-survivors among mark-recapture sequences. Lizards with incomplete tails suffered higher mortality in the field, although this was not true for every comparison considered (i.e., for every mark-recapture sequence analyzed), and the overall trend was much stronger for adult males than for either adult females or juveniles. Higher mortality among lizards with incomplete tails is presumably a consequence of increased vulnerability to capture by predators. Vulnerability to predation of tail-injured lizards may be confounded by reduced social status in this species, because social subordination can result in the occupation of an inferior home range.     

HOW TO COMPENSATE FOR COSTLY SEXUALLY SELECTED TAILS: THE ORIGIN OF SEXUALLY DIMORPHIC WINGS IN LONG-TAILED BIRDS

Recent work on birds suggests that certain morphological differences between the sexes may have evolved as an indirect consequence of sexual selection because they offset the cost of bearing extravagant ornaments used for fighting or mate attraction. For example, long-tailed male sunbirds and widowbirds also have longer wings than females, perhaps to compensate for the aerodynamic costs of tail elaboration. We used comparative data from 57 species to investigate whether this link between sexual dimorphism in wing and tail length is widespread among long-tailed birds. We found that within long-tailed families, variation in the extent of tail dimorphism was associated with corresponding variation in wing dimorphism. One nonfunctional explanation of this result is simply that the growth of wings and tails is controlled by a common developmental mechanism, such that long-tailed individuals inevitably grow long wings as well. However, this hypothesis cannot account for a second pattern in our data set: as predicted by aerodynamic theory, we found that, comparing across long-tailed families, sexual dimorphism in wing length varied with tail shape as well as with sex differences in tail length. Thus, wing dimorphism was generally greater in species with aerodynamically costly graduated tails than in birds with cheaper, streamer-shaped tails. This result was not caused by confounding phylogenetic effects, because it persisted when phylogeny was controlled for, using an independent comparisons method. Our findings therefore confirm that certain aspects of sexual dimorphism may sometimes have evolved through selection for traits that reduce the costs of elaborate sexually selected characters. We suggest that future work aimed at understanding sexual selection by investigating patterns of sexual dimorphism should attempt to differentiate between the direct and indirect consequences of sexual selection.     

Sexual size dimorphism and assortative mating for morphological traits in Passer domesticus

In this study, assortative mating for different morphological traits was studied in a captive population of house sparrows (Passer domesticus). Males were larger than females. Assortative mating was found for tail length, wing length and general body size. Males with larger badge size mated with females with longer tails. The strongest assortative mating occurred for tail length (r=0.77), and this assortative mating remained significant after controlling for wing length, mass and tarsus length, suggesting that it was not an artefact of assortative mating for body size. The possibility of sexual selection for tail length in the house sparrow is discussed.     

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.     

Size variation and reproductive success of female Aedes punctor (Diptera: Culicidae)

Abstract. 1. Variation in size (wing length) of females and males within a population of Aedes punctor (Kirby) at emergence was recorded in northern Britain during 1984 and 1985.
2. Wing length correlated well with body dry weight, confirming its usefulness as a measure of body size. In 1984, but not in 1985, mean size decreased significantly as emergence progressed.
3. The size of host-seeking females caught at human bait varied seasonally. This was partly accounted for by size variation during emergence.
4. Of females which fed to repletion, the largest spent least time in contact with the host.
5. In the laboratory, wing length correlated well with potential fecundity (number of ovarioles) and less well with actual fecundity (number of matured follicles) after feeding on human blood.
6. In the field, larger females were more successful at locating hosts, developed more egg clutches in a lifetime and generally lived longer than did smaller ones. Larger females appear to enjoy greater reproductive success than do smaller females.     

Phenotypic quality and molt in the barn swallow, Hirundo rustica

Phenotypic quality may determine the development and expressionof secondary sexual characters. We studied the relationshipbetween molt and several measures of phenotypic quality in thesexually size-dimorphic barn swallow (Hirundo rustica) in itswinter quarters in Namibia. Males were in a more advanced stageof molt than females and juveniles, and the speed of molt asdetermined from the residual of the regression of the size ofthe gap in wings caused by missing and growing feathers on wingmolt score (residual wing raggedness) was also higher in malesthan in females and juveniles. Male barn swallows with longand symmetric tail feathers had a more advanced stage of moltand molted at a higher speed than males with short and asymmetrictails. Long-tailed females had a delayed molt, and females withasymmetric tails had less advanced molt and lower rates of feathergrowth than females with symmetric tails. Molt of secondariesin juveniles also appeared to be less advanced if they had longtails. Adult barn swallows molted their tail feathers in anirregular sequence with the longest, outermost tail featherusually replaced before the second or the third outermost feathers.Good body condition was positively associated with a high moltscore for some feather tracts and a rapid wing molt in adultfemales and tail molt in juveniles. Mallophaga were only weaklynegatively associated with primary and secondary molt scorein adult females and speed of wing molt in adult males. In conclusion,phenotypic quality of adult male barn swallows as reflectedby the expression of their secondary sexual character duringthe previous molt reliably reflected stage and speed of currentmolt.     

Correlational selection leads to genetic integration of body size and an attractive plumage trait in dark-eyed juncos

When a trait's effect on fitness depends on its interaction with other traits, the resultant selection is correlational and may lead to the integration of functionally related traits. In relation to sexual selection, when an ornamental trait interacts with phenotypic quality to determine mating success, correlational sexual selection should generate genetic correlations between the ornament and quality, leading to the evolution of honest signals. Despite its potential importance in the evolution of signal honesty, correlational sexual selection has rarely been measured in natural populations. In the dark-eyed junco (Junco hyemalis), males with experimentally elevated values of a plumage trait (whiteness in the tail or "tail white") are more attractive to females and dominant in aggressive encounters over resources. We used restricted maximum-likelihood analysis of a long-term dataset to measure the heritability of tail white and two components of body size (wing length and tail length), as well as genetic correlations between pairs of these traits. We then used multiple regression to assess directional, quadratic, and correlational selection as they acted on tail white and body size via four components of lifetime fitness (juvenile and adult survival, mating success, and fecundity). We found a positive genetic correlation between tail white and body size (as measured by wing length), which indicates past correlational selection. Correlational selection, which was largely due to sexual selection on males, was also found to be currently acting on the same pair of traits. Larger males with whiter tails sired young with more females, most likely due to a combination of female choice, which favors males with whiter tails, and male-male competition, which favors both tail white and larger body size. To our knowledge, this is the first study to show both genetic correlations between sexually selected traits and currently acting correlational sexual selection, and we suggest that correlational sexual selection frequently may be an important mechanism for maintaining the honesty of sexual signals.     

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.     

Reproductive correlates of plumage coloration of female Mountain Bluebirds

Many studies have shown that the plumage coloration of male birds can act as an honest signal of quality, indicating benefits that a female could gain from pairing with a specific male. In some species, females also display ornamental plumage, but less is known about the function and potential adaptive significance of female coloration because most research has focused on male coloration. Male Mountain Bluebirds (Sialia currucoides) display full body, ultraviolet (UV)‐blue plumage, whereas female plumage is more subdued, with blue color focused on the rump, wing, and tail. During the 2011 and 2012 breeding seasons (May–July) near Kamloops, BC, Canada, we examined coloration of the rump and tail of female Mountain Bluebirds to determine if their plumage could act as an indicator of direct reproductive benefits (e.g., enhanced parental care or reproductive success) to potential mates. We found no relationship between female plumage coloration and either provisioning rate or fledging success. However, female coloration varied with age, with after‐second‐year (ASY) females having brighter, more UV‐blue tail feathers than second‐year (SY) females. In addition, ASY females with brighter, more UV‐blue tails had larger clutches. We also observed positive assortative mating by tarsus length. Because previous work with other species suggests that female body size may be a good predictor of breeding success, males could potentially benefit from pairing with larger females. However, reproductive success did not vary with female size in our study. Although our evidence that structural plumage coloration of female Mountain Bluebirds is a signal of direct reproductive benefits for males (e.g., higher reproductive success) is limited, our results (i.e., ASY females with brighter tails than SY females, and ASY females with brighter tails having larger clutches) do suggest the potential for sexual selection to act on female coloration.     

MINUTES OF THE ANNUAL GENERAL MEETING OF THE SOUTH AFRICAN ORNITHOLOGICAL SOCIETY,HELD AT CAPE TOWN, 26 JUNE 1959

Brown, C. J. 1989. Plumages and measurements of the Bearded Vulture in southern Africa. Ostrich 60: 165–171.

Four different age classes of the southern African Bearded Vulture Gypaetus barbatus are recognized and their plumages described: juvenile (3–24 months old), immature (24–45 months), subadult (45–60 months) and adult (60+ months). There was no significant difference in size between adult male and female birds. Adults were larger than juvenile birds in bill width, beard length, wingspan and mass, and had a higher aspect ratio and wing loading, while juvenile birds were larger than adults in the length of their outer rectrices, tail area, wing breadth and wing area. These features are considered to be adaptive to young birds inexperienced in flying. Immature and subadult birds were intermediate in size between juveniles and adults. Bearded Vultures differ from other large raptors in two sets of physical characteristics, (a) those adapted to cold, mountainous habitat, e.g. feathered head and face, unusually long wings, a high aspect ratio and a particularly long tail, and (b) those adapted to their diet of mainly bones, e.g. wide gape, beard and relatively long talons for carrying food.     

The moult of the Little Stint Calidris minuta in the Kenyan rift valley

Moult data were collected during 1967–80 from some 6900 Little Stints in the southern Kenyan rift valley.
Adults typically moulted from summer to winter body and head plumage during September and early October, soon after arrival. The complete pre-winter wing and tail moult began in most adults between mid-September and early October. Some birds finished by December, but others continued until February and March. Individual duration was usually between 100 and 150 days. Adults which completed this moult early often remoulted outer primaries between January and early April.
Young birds acquired first-winter body plumage during October and early November. Some 90% had a complete pre-winter wing and tail moult. This usually began between December and early February, and finished during March or early April, taking about 70–100 days. In about 10% of young birds, flight feather moult was restricted to the outer primaries and inner secondaries. Birds adopting this strategy typically began moult late, during January or February. Short periods of suspension were common during pre-winter wing moult, particularly in adults. The difference in moult speed between adult arid first-winter birds was attributable in the primary, secondary and tail tracts to differences in numbers of growing feathers.
Practically all birds completed a pre-summer moult involving the entire body and head plumage, most of the tertials, some or all of the tail feathers and many wing coverts. Most birds began this moult between early February and late March, and finished between mid-April and early May. It was typically later and more rapid in first-year birds than adults. In late birds, the onset of pre-summer moult was linked to the final stages of pre-winter moult.
The wing moult of the Little Stint in different wintering areas is discussed. First-winter moult strategy is compared with that in other small Calidris species.     

Why do male snakes have longer tails than females?

In most snake species, males have longer tails than females of the same body length. The adaptive significance of this widespread dimorphism has attracted much speculation, but few tests. We took advantage of huge mating aggregations of red-sided gartersnakes (Thamnophis sirtalis parietalis) in southern Manitoba to test two (non-exclusive) hypotheses about the selective forces responsible for this dimorphism. Our data support both hypotheses. First, relative tail length affects the size of the male copulatory organs (hemipenes). Males with longer tails relative to body length have longer hemipenes, presumably because of the additional space available (the hemipenes are housed inside the tail base). Second, relative tail length affects male mating success. Males with partial tail loss (due to predation or misadventure) experienced a threefold reduction in mating success. Among males with intact tails, we detected strong stabilizing selection on relative tail length in one of the two years of our study. Thus, our data support the notion that sex divergence in tail length relative to body length in snakes reflects the action of sexual selection for male mating success.     

Morphological relationships of passerine birds from Australia and New Guinea in relation to their diets

Beak, wing, leg and intestinal lengths, and gizzard widths, were all significantly related to body mass in 51 honeyeater species from Australia, 48 honeyeater species from New Guinea and 31 purely insectivorous passerine bird species from Australia. The nectar-feeding honeyeaters had smaller gizzards and intestines than wholly insectivorous birds of comparable size, although their wing and leg lengths did not differ; New Guinean and Australian honeyeaters were similar in these respects. Overall, honeyeaters had longer beaks than pure insectivores. Among Australian honeyeaters, those genera consuming more nectar than insects had longer beaks than the less nectarivorous, more insectivorous genera. Indeed, the latter group had beaks comparable in length to wholly insectivorous birds. All morphological differences revealed were attributable to known differences in diet.     

THE WINTERING AND MOULT OF RUFFS PHILOMACHUS PUGNAX IN THE KENYAN RIFT VALLEY

Some 5700 Ruffs were ringed in the southern Kenyan rift valley during 1967–79, mainly at Lakes Nakuru and Magadi. These have produced 15 recoveries outside East Africa, 14 in Siberia between 73° and 154°E and one in India. Adult males returned to Kenya mainly during August, and females during late August and early September. Females greatly outnumbered males at all times. Most wintering males departed late in March and early in April, but females not until about a month later. First-year birds appeared from the end of August, but remained in low numbers until late October or November. Most departed during April and May, but a few females oversummered. First-year birds typically accounted for about 25% of the wintering Nakuru females, but about 50% of those at Magadi. At both sites they accounted for a higher proportion of male birds than females. Most of the birds at Nakuru throughout late August to May appeared to be local winterers, and many individuals remained in the area for many months each year. Retrapping indicated that approximately 60% of each season's birds returned the following season. Adult males and most adult females commenced pre-winter wing moult before arrival, but completed most of it in Kenya. Males moulted 3–4 weeks ahead of females, and most had finished before December. Females typically finished during December and early January. Most second year birds timed their pre-winter moult similarly to older adults. Suspension was recorded in over 15% of all moulting birds examined. Adult pre-summer moult involved most or all of the tertials, some or all of the tail feathers, most of the inner wing coverts and the body and head plumage. It occurred mainly during January to March (males) or February to April (females), although tertial renewal commonly began a month earlier. Males showed no sign in Kenya of the supplementary prenuptial moult. First-year birds moulted from juvenile into first winter body plumage during late September to November. They underwent a pre-summer moult similar in extent and timing to that of adults, and again about a month earlier in males than females. Spring feathers acquired were often as brightly coloured as those of adults. About 15% of first-year birds renewed their outer 2–4 pairs of large primaries during January to April. Adult and first-year birds fattened before spring departure, commonly reaching weights 30–60% above winter mean. Weights of adult males peaked early in April, those of adult females early in May, and those of first-winter females later in May. Weights were relatively high also during August and September. This was due to the arrival of wintering birds carrying ‘spare’ reserves, and also apparently to the presence of a late moulting fattening passage contingent. The wing length of newly moulted adults was about 3 mm longer than that of newly arrived first-year birds, but there was no evidence of an increase in the wing kngth of adults with successive moults. Adult wing length decreased by 4–5 mm between the completion of one moult and the middle stages of the next. The migrations and annual timetable of Kenyan wintering Ruffs are discussed, and their moult strategy is compared with that of other Holarctic waders.     

Morphological shifts of the external flight apparatus across the range of a passerine (Northern Wheatear) with diverging migratory behaviour

We studied morphological differentiation in the flight apparatus of the four currently recognised sub-species of Northern Wheatears, Oenanthe oenanthe. Considering all measured birds without assigning them a priori to any sub-species we found a clinal morphological shift. Relative wing length, wing pointedness, and the degree of tail forking were positively correlated with migratory distance, whereas tail length (relative to wing length) was negatively correlated. The large-sized, long-distance migrant "Greenland" Wheatear, O. o. leucorhoa, is characterized by relatively longer, broader and more pointed wings and more forked tails, similar to the smaller-sized nominate Northern Wheatear, O. o. oenanthe, from North Europe, Siberia and Russia. In contrast, the short distance migrant "Seebohm's" Wheatear, O. o. seebohmi, from northwest Africa, possesses much rounder wings, and the tail is relatively longer and less forked. Sub-species with intermediate migratory habits (different populations of nominate Northern Wheatear, O. o. oenanthe, and "Mediterranean" Northern Wheatear, O. o. libanotica) show, as expected, intermediate features according to their intermediate migratory behaviour. Our results are congruent with other inter- and intraspecific studies finding similar adaptations for energy-effective flight in relation to migration distance (morphological migratory syndrome).     

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.