Outermost Tail Feathers Research Articles

SURVIVAL RATE OF ADULT BARN SWALLOWS HIRUNDO RUSTICA IN RELATION TO SEXUAL SELECTION AND REPRODUCTION

Sexual selection often affects individuals of the two sexes differently by imposing severe survival costs on individuals of the chosen sex, usually males. These costs may be energetic, physiological, mechanical, or ecological, and they can give rise to sex differences in survival. We studied survival rates of male and female adult Barn Swallows Hirundo rustica in a Danish population during 1984–1998, for the first time using capture–mark–recapture techniques to estimate the consequences of sexual selection for sex differences in survival. Males had slightly, but not significantly, higher survival than females. Females had annual survival estimates that were significantly more variable than those for males. Male annual survival rates were not positively correlated with those of females. Hence, different factors were hypothesized to affect male and female survival rate. Consistent with this prediction we found that annual male survival decreased with increasing annual mean length of the outermost tail feathers, which is a secondary sexual character subject to intense directional sexual selection. Heavier females were more likely to survive. Furthermore, females with long tails survived better than short-tailed females. Since tail length in females is an indicator of female condition, both body mass and tail length relationships suggest that females survive better in years when they are in superior condition. Thus, sexual selection as reflected by differences in tail length of males and females contributes in different ways to survival of male and female Barn Swallows.

Great and blue tit feathers as biomonitors for heavy metal pollution

In this study we investigated the concentration of two non-essential (Cd and Pb) and two essential (Cu and Zn) heavy metals in the outermost tail feathers of great ( Parus major ) and blue tits ( Parus caeruleus ). Cd, Cu, Pb and Zn concentrations in the feathers of great tits were significantly higher at a polluted site (near a metallurgic factory) than at a reference site (situated 4 km east of the polluted site). Blue tits at the polluted site had significantly higher Cd, Cu and Pb concentrations in the outermost tail feathers than blue tits from the reference site. Apart from Pb concentrations that were significantly higher in blue tits than in great tits, no significant interspecific differences in metal levels occurred. Although feathers have been used intensively as a biomonitoring tool, few studies have addressed the effect of both age and gender on the metal accumulation. We did not observe any significant differences in the metal levels in great tit feathers between the two studied age classes (individuals of 4–9 months old and individuals older than 16 months). Cd, Cu and Pb levels did not differ significantly between male and female great tits. The Zn concentration however, was significantly higher in males than in females. We examined correlations between concentrations in the outermost tail feather and the liver, kidney, muscle, innermost tail feather and the second and seventh primary in great tits. For the essential elements Cu and Zn we found no significant correlations between levels in the outermost tail feather and levels in any of the internal tissues examined. Cd and Pb concentrations in the outermost tail feather, on the other hand, were significantly positively correlated with levels in the liver, kidney and muscle. We suggest that feathers of great and blue tits may be useful biomonitors for local contamination with heavy metals.

Heavy metals and selenium in feathers of great tits (Parus major) along a pollution gradient

We evaluated the use of great tit (Parus major) feathers as biomonitors for heavy-metal pollution and tested whether there were differences in metal levels in feathers as a function of location, age (first year or older), or gender. In 1998 and 1999, we collected the outermost tail feathers of 185 great tits from four sites along a presumed pollution gradient and one reference site in Antwerp (Belgium) and analyzed them for calcium, selenium, and 13 heavy metals (silver, aluminum, arsenic, cadmium, cobalt, chromium, copper, iron, mercury, manganese, nickel, lead, and zinc). Silver, arsenic, cadmium, cobalt, copper, mercury, nickel, lead, selenium, and zinc concentrations were significantly higher closest to the pollution source (UM site) compared with the other four sites. Heavy-metal and selenium levels were on average 2 to 40 times higher at the UM site compared with the reference site. Aluminum, calcium, and iron levels were not significantly different among sites. We found no general age- or gender-related differences in metal levels except for arsenic and iron, where a significant interaction between site and gender was observed. Our results suggest that feathers of great tits might be useful biomonitoring tools because they reflect the environmental contamination by heavy metals well.

Parent Barn Swallow fluctuating asymmetry and offspring quality

Fluctuating asymmetry may impair locomotion but may also reflect intrinsic phenotypic quality. I tested whether fluctuating asymmetry of adult Barn Swallows Hirundo rustica negatively influenced offspring quality, by estimating the relationship between parental asymmetry and offspring size, condition and immunocompetence during three breeding seasons. Controlling for timing of breeding, brood size and the size of a secondary sexual character (tail length), wing and outermost tail feather asymmetry of male and female parents was not significantly correlated with offspring size, condition and immunocompetence. This was the case in spite of clear differences in nestling quality among years. In addition, parents with extreme asymmetries due to tail feather damage (not representing fluctuating asymmetry) did not have nestlings of lower quality than parents with undamaged tail feathers. These results indicate that there is only a weak relation between parental asymmetry and offspring quality.

Survival and reproductive rate of mites in relation to resistance of their barn swallow hosts.

Parasite resistance may act via a number of different mechanisms that regulate or control the survival and the reproductive rate of parasites. Observations and experiments were used to test for effects of host resistance on parasite survival and rate of reproduction. Natural levels of infestation of barn swallow Hirundo rustica nests by the tropical fowl mite Ornithonyssus bursa were positively related to brood size, inversely related to the length of the outermost tail feathers of male nest owners (a secondary sexual character) and affected by time of reproduction by the host. A mite inoculation experiment, in which 50 adult mites were introduced into nests during the laying period of the host, was used to test for differential survival and reproduction of mites as a function of host resistance. The relationship between survival and reproduction of parasites, male tail length and host resistance was investigated. There was a negative relationship between mite numbers per nest after fledging of nestlings and male tail length. This relationship was mainly caused by a reduction in the number of mites in the first and second nymph stage with increasing tail length of male hosts, implying a reduction in rate of reproduction of mites. The proportion of mites that had recently fed was inversely related to tail length of male hosts. The proportion of nymph stages was positively related to the proportion of mites that had recently had a blood meal. Parasite resistance of barn swallows to the tropical fowl mite thus appeared to act through increased mortality rate of adult and nymph stages of mites, and through reduced reproductive rates of mites on resistant hosts. This is the first study demonstating a direct relationship between fitness components of a parasite and the expression of a secondary sexual character of a host.

European Swallows Hirundo rustica in Botswana during three non-breeding seasons: the effects of rainfall on moult

van den Brink, B., Bijlsma, R.G. & van der Have, T.M. 2000. European swallows Hirundo rustica in Botswana during three non-breeding seasons: the effects of rainfall on moult. Ostrich: 71 (1): 198–204. The rate of moult of European Swallows spending the non-breeding season in Botswana was studied during December-January of 1992/93,1993/94 and 1994/95 to investigate the effects of variability in rainfall and roosting habitat availability. In January 1994, 2–3 million European Swallows were counted at a traditional roost along the Boteti River. The rate of moult was relatively slow, about one feather (primary, secondary or tail feather) was replaced every two weeks in both adults and juveniles. The speed of moult in juveniles was generally lower than in adults, in particular of secondaries and tail feathers. Moulting rate of both primaries and tail feathers was lowest in 1994/95 during a period of drought and coincided with the almost complete destruction of roosting habitat. In 1992/93, moulting rate was highest when rainfall was moderate and roosting habitat abundant. Moulting rate was intermediate in 1993/94 when rainfall was frequent but roosting habitat reduced because of the low water level in the Boteti River. The combined effect of reduced food availability during droughts and higher densities and longer foraging flights when roosting habitat is scarce might explain the annual variation in moulting rate. From the second week of January onwards many adults started moulting the outermost tail feather before the penultimate feathers. This phenomenon could indicate the importance of long tail streamers in aerial manoeuvring when foraging during the return migration to the breeding grounds.

Carotenoids, sexual signals and immune function in barn swallows from Chernobyl.

Carotenoids have been hypothesized to facilitate immune function and act as free-radical scavengers, thereby minimizing the frequency of mutations. Populations of animals exposed to higher levels of free radicals are thus expected to demonstrate reduced sexual coloration if use of carotenoids for free-radical scavenging is traded against use for sexual signals. The intensity of carotenoid-based sexual coloration was compared among three populations of barn swallows Hirundo rustica differing in exposure to radioactive contamination. Lymphocyte and immunoglobulin concentrations were depressed, whereas the heterophil:lymphocyte ratio, an index of stress, was enhanced in Chernobyl swallows compared to controls. Spleen size was reduced in Chernobyl compared to that of two control populations. Sexual coloration varied significantly among populations, with the size of a secondary sexual character (the length of the outermost tail feathers) being positively related to coloration in the two control populations, but not in the Chernobyl population. Thus the positive covariation between coloration and sexual signalling disappeared in the population subject to intense radioactive contamination. These findings suggest that the reliable signalling function of secondary sexual characters breaks down under extreme environmental conditions, no longer providing reliable information about the health status of males.

Sexual selection, feather breakage and parasites: the importance of white spots in the tail of the barn swallow ( Hirundo rustica )

The long outermost tail feathers of barn swallows Hirundo rustica have white spots that are larger in males than in females and in adults than in juveniles. Spot size increases with age among adults and is positively correlated with tail length. We tested the functional significance of these white spots by randomly assigning males to either of three groups during spring: (1) subjected to a considerable reduction in the size of all spots using a black permanent pen; (2) subjected to a small reduction in the size of all spots; or (3) a control group with no reduction. Experimental treatment significantly affected the total number of offspring produced per season: breeding tended to be delayed and second broods were less frequent after a considerable reduction in the size of the males' tail feather spots. White parts of feathers lacking melanin were more often found to be the site of feather breakage than the melanised parts of similar width. Furthermore, white tail spots were the preferred feeding site of feather-eating Mallophaga and, since long-tailed males have fewer Mallophaga than short-tailed males, long-tailed males may pay a lower cost for their large white tail spots than short-tailed males. Hence, the cost of increased risk of feather breakage at white tail spots and the non-random distribution of feather lice among individuals will render tail spots a reliable signal of phenotypic quality.

Length of tail streamers in barn swallows

Both natural and sexual selection affect the expression of secondary sexual characters1, so any secondary sexual character will be affected by a mixture of selection pressures. Evans2 has compared the flight performance of male barn swallows (Hirundo rustica) whose tail lengths have been altered with that of controls, and concludes that the outermost tail feathers of these birds are mainly the outcome of natural selection.

Senescence in a short‐lived migratory bird: age‐dependent morphology, migration, reproduction and parasitism

1. Senescence reflects age‐dependent changes in residual reproductive value. Annual survival rates of the barn swallowHirundo rusticaL. increased from 1‐ to 2‐year‐old individuals, but decreased among 5 years old or older individuals. Estimates of age‐dependent reproductive value showed a similar pattern.2.Longitudinal data from two long‐term population studies were used to test whether a number of different measures of performance (condition‐dependent morphological traits, migratory performance, reproductive success, intensity of parasitism) changed among individuals when reaching old age.3.The length of the outermost tail feathers (a secondary sexual character) decreased among old individuals, while two measures of individual developmental instability increased with age. Migratory performance decreased in old barn swallows as reflected by a delay in spring arrival at the breeding grounds. Reproductive performance measured as seasonal reproductive success decreased with age. The intensity of infestations with an haematophagous mite and a mallophagous ectoparasite increased among old barn swallows.4.These results suggest that the condition‐dependent secondary sexual character, developmental stability, and measures of migratory and reproductive performance deteriorated, and the frequency of parasitism increased among old individuals. Ageing was thus associated with a general deterioration of performance.

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.

Immunocompetence, ornamentation, and viability of male barn swallows (Hirundo rustica).

Immunocompetence (i.e., the ability to produce an immune response to pathogens) can be predicted to influence the chances that organisms have to survive and reproduce. In this study we simulated a challenge to the immune systems of male barn swallows (Hirundo rustica) by injecting them intraperitoneally with a multigenic antigen, sheep red blood cells, and we analyzed long-term survival in relation to their immunocompetence. Males were assigned to four groups that differed for the treatment of the length of the outermost tail feathers, a sexually dimorphic ornamental character that is currently under directional sexual selection. Immunocompetence was measured as change of concentration of gamma globulins relative to plasma proteins. The intensity of the immune response was independent of age. Males that showed the highest short-term response to sheep red blood cells were more likely to survive until the breeding season following that in which they had been inoculated, a pattern consistently observed within each experimental group. Males with comparatively long tails were more likely to survive than those with short tails. To our knowledge, the results of this study are the first to demonstrate that immunocompetence can predict long-term survival in a free-ranging vertebrate. Moreover, they are compatible with current models of parasite-mediated sexual selection because long-tailed males are more immunocompetent than short-tailed ones, and females, by preferring to mate with the most ornamented males, may acquire the "good genes" for high immunocompetence and, hence, for high viability of their offspring.

Energetic cost of tail streamers in the barn swallow (Hirundo rustica)

Different hypotheses stress the importance of natural or sexual selection to explain the evolution and maintenance of long outermost tail feathers in the barn swallow (Hirundo rustica). Since energy costs are predicted to arise from tail length manipulation, we measured the daily energy expenditure in three experimental groups (tail-shortened, tail-elongated, and control birds) with the doubly labelled water technique. Though we did not directly measure flight cost, we assumed this to be positively related to daily energy expenditure. Mass independent daily energy expenditure (kJ/mass0.67 day) average daily metabolic rate (ml CO2/g h), and water flux (ml H2O/g day) did not show any significant difference among treatments in either sex. Males had higher values than females for the three parameters. Males with short original tail length experienced a higher water flux than originally long-tailed males. Females that laid more eggs during the breeding season or had heavier broods also showed higher levels of water flux which could imply a higher food intake. Our expectation of finding energetic costs of manipulated tail length in barn swallows with an integrated measure of metabolism was not fulfilled, and we did not find evidence for behavioural changes in the birds involved in the experiment.

The function of long tails in female barn swallows (Hirundo rustica): an experimental study

The outermost tail feathers in male barn swallows (Hirundo rustica) are the target of a strong directional female mate preference. The tail ornament is also expressed in females, since females have considerably longer tails than juveniles, either due to (1) a strong genetic correlation between the characters in the two sexes, or (2) direct sexual selection on females. To discriminate between these two hypotheses, we manipulated the length of the outermost tail feathers in female barn swallows shortly after arrival by either shortening or elongating the outermost tail feathers, or maintaining their length among control individuals. Start of laying of the first clutch, reproductive performance, or provisioning of offspring did not show any significant differences among treatments. Original female tail length before manipulation was unrelated to reproductive performance, while male tail length explained some variation in the number of clutches and, to some extent, the total number of eggs laid per year. Females with longer tails arrived earlier at the breeding grounds. Manipulated female tail length was positively correlated to the tail length of their mates. Our results support the correlated response hypothesis but do not support the sexual selection explanation for the existence of exaggerated tail feathers in female barn swallows.

THE DIFFERENTIAL-ALLOCATION HYPOTHESIS: DOES THE EVIDENCE SUPPORT IT?

In a recent paper, de Lope and M0ller (1993) investigated the differential-allocation hypothesis in a Spanish population of barn swallows Hirundo rustica near Badajoz in 1991, and interpreted the results as support for this hypothesis. I believe that another interpretation could explain their results. The differential-allocation hypothesis predicts that mates of attractive individuals are to contribute more than the average (of their sex) to parental investment in order both to obtain and keep their attractive mates (Burley 1986). Consequently, the attractive partner can afford to reduce its parental effort and thus gain a survival advantage. Offspring of pairs with an attractive individual gain an advantage by increased body mass when fledging and survive better than other offspring. This hypothesis has only been tested and supported once in a laboratory study by manipulating the attractiveness of male and female zebra finches Taeniopygia guttata, by changing the color of their leg bands (Burley 1988). De Lope and M0ller changed the attractiveness of male barn swallows by shortening or lengthening the outermost tail feathers and addressed the question of whether tail manipulation of already mated males affects the reproductive effort of their female partners. They assigned males randomly to one of three groups: Shortened males had a 20-mmlong piece of outermost feathers cut; males had 20-mm-long pieces of outermost feather added to their own tail feathers; control males were not manipulated. Since previous studies have shown that female barn swallows prefer males with artificially elongated tails (M0ller 1990), the authors conclude that males with tails become unattractive and elongated-tailed males become attractive. De Lope and M0ller made clear predictions for the differentialallocation hypothesis: they expected females mated to elongated-tailed males to have increased reproductive success, but also to make a greater provisioning effort; by comparison, they expected the provisioning rate of males to decrease with increasing tail length. The data on reproductive effort in females supported the differential-allocation hypothesis. Female mates of elongatedtailed males indeed bred earlier and produced more eggs and nestlings per nest and season. But contrary to the prediction (if females adjusted their reproductive effort to experimental tail length, females mated to long-tailed males should . . . produce nestlings of higher body mass; de Lope and M0ller 1993, p. 1155) nestlings of pairs with elongated-tailed males did not have higher body mass than those of the shortened and control group. Because the survival prospects of nestlings are positively related to body mass (Perrins 1965), nestlings of pairs with an attractive partner do not survive better. The authors estimated the feeding rates of each partner in randomly sampled pairs for 1 h on two different days when nestlings were 1O-15-d old. Additionally, they provided nestlings with a ligature and collected food boluses brought by the attending males. They recorded the number of prey items per bolus and measured the length of the prey items to the nearest 0.1 mm. They found a significant decrease in the feeding rate of shortened males over control males to males with elongated tails and a significantly increasing trend in female feeding rate in the same ordering, and concluded that these results support the differential-allocation hypothesis. However, this effect could be explained more simply by a compensation effect. Because an elongated tail is a handicap and reduces foraging efficiency (M0ller 1989) female partners of elongated-tailed males have to feed more frequently in order to provide enough food for the young. Thus, because males in the group were unable to feed more due to an impairment imposed on them, their females had to increase their feeding rates. Accordingly, females were forced to invest more to maintain the integrity of their genetic investment. The differential-allocation hypothesis, however, predicts that the females are willing to do more than the average of their sex because they have acquired an attractive partner (Burley 1986). It is difficult to distinguish between the compensationeffect and the attractiveness-effect predicted by the differential-allocation hypothesis because the feeding rate of one partner depends on that of the other partner. To bypass this difficulty, one could, for example, provide additional food to the young fathered by the elongated males up to a level at least matching the deficit imposed by those males. Assuming that the compensation-hypothesis is true, females should no longer increase their feeding rate. However, if the differentialallocation hypothesis were true, the same females would feed their young more frequently than females of the other groups, that is, they would feed more than necessary. There is yet another snag: Do the elongated-tailed males indeed reduce their feeding effort? The differential-allocation hypothesis focuses on the provisioning effort of the individual mated to an attractive partner. Because de Lope and M011er determined the attractiveness of the males, they should

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

AbstractSecondary 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.

Foraging costs of a tail ornament: Experimental evidence from two populations of barn swallows Hirundo rustica with different degrees of sexual size dimorphism

Secondary sexual characters are assumed to be costly to produce or maintain. A test of this assumption was performed using the sexually exaggerated outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject of a directional female mate preference. A possible cost of sexual signalling in male barn swallows arises from increased flight cost during foraging in this aerially insectivorous species. A longer tail may impose a greater drag during flight and thereby affect foraging ability. This was tested by determining the relationship between experimentally modified male tail lengths and number and size of prey delivered to offspring in Spain, where sexual size dimorphism in tail length is small, compared to Denmark, where dimorphism is large. Food boluses contained significantly fewer small insects in Spain than in Denmark. Males with elongated tails captured more and smaller insects while males with shortened tails captured fewer and larger prey items at both sites. Males with naturally long tails were less affected by experimental treatment in terms of effects on the number and the size of prey delivered to their offspring, a finding consistent with a long tail being a condition-dependent viability indicator. The effect of a given degree of tail manipulation on prey size and number of prey per bolus was larger in Spain than in Denmark. These results demonstrate that (1) tail length in male barn swallows affects foraging, and (2) larger sexual size dimorphism occurs where the foraging cost of an increment in ornament size is smallest.

Inter- and intra-sexual patterns of fluctuating asymmetry in the red-billed streamertail: should symmetry always increase with ornament size?

Recent work on fluctuating asymmetry has suggested that ornaments should have higher levels of fluctuating asymmetry than (1) non-ornaments and (2) homologous structures in the non-ornamented sex. In addition, as both ornament size and symmetry should increase with individual quality there should be a tendency for ornament symmetry to increase with ornament size. In non-ornaments, a U-shaped relationship between symmetry and size is expected, with the individuals at the extremes being more asymmetrical than individuals around the optimum. We tested these predictions in the red-billed streamertail (Trochilus polytmus), a sexually dimorphic endemic Jamaican hummingbird. The lengths of four bilaterally symmetrical traits (first and second outermost tail feathers, tarsi and wings) in 43 adult males and 42 females were measured. The second outermost tail feathers of adult males (which are elongated into streamers) were absolutely but not relatively more asymmetrical than non-ornaments (including the homologous feathers in females). When character size was controlled for, wings were shown to be relatively more symmetrical than other traits. Symmetry did not increase with increasing trait size in any of the morphological traits measured. There was a U-shaped relationship between asymmetry and trait size for four traits (adult male streamers, adult male wings and female outer tail feathers). These results do not support any of the predictions made by fluctuating asymmetry hypotheses and suggest that stabilising selection may act on ornaments as well as non-ornaments. These predictions have been supported in swallows and peafowl but not in sunbirds; this may be due to differences in female perception of tail ornaments. Perhaps male tails do not convey information about quality in some species, or there may be inter-specific differences in the relative costs of tail ornaments and the benefit of marginal increases in tail length and symmetry.

Phenotypic quality and molt in the barn swallow, Hirundo rustica

Phenotypic quality may determine the development and expression of secondary sexual characters. We studied the relationship between molt and several measures of phenotypic quality in the sexually size-dimorphic barn swallow (Hirundo rustica) in its winter quarters in Namibia. Males were in a more advanced stage of molt than females and juveniles, and the speed of molt as determined from the residual of the regression of the size of the gap in wings caused by missing and growing feathers on wing molt score (residual wing raggedness) was also higher in males than in females and juveniles. Male barn swallows with long and symmetric tail feathers had a more advanced stage of molt and molted at a higher speed than males with short and asymmetric tails. Long-tailed females had a delayed molt, and females with asymmetric tails had less advanced molt and lower rates of feather growth than females with symmetric tails. Molt of secondaries in juveniles also appeared to be less advanced if they had long tails. Adult barn swallows molted their tail feathers in an irregular sequence with the longest, outermost tail feather usually replaced before the second or the third outermost feathers. Good body condition was positively associated with a high molt score for some feather tracts and a rapid wing molt in adult females and tail molt in juveniles. Mallophaga were only weakly negatively associated with primary and secondary molt score in adult females and speed of wing molt in adult males. In conclusion, phenotypic quality of adult male barn swallows as reflected by the expression of their secondary sexual character during the previous molt reliably reflected stage and speed of current molt.

Secondary sexual characters, parasites and testosterone in the barn swallow, Hirundo rustica

Abstract. The immuno-competence hypothesis predicts that the expression of secondary sex traits should be positively related to testosterone levels, but that androgens should simultaneously have negative effects on the immune defence system. Relatively high levels of circulating androgens should therefore result in elevated levels of parasite infections. Parasite load and testosterone levels will be uncorrelated, or even negatively correlated, however, if males with high testosterone levels and large secondary sexual characters are reliable advertisers of genetic resistance to parasites. In male barn swallows, testosterone concentrations peaked during the pre-laying period. Males that had high levels of testosterone (adjusted for the stage of the breeding cycle) at one stage of the breeding cycle also had relatively high levels at other stages. The length of the outermost tail feathers, which are secondary sexual characters currently involved in sexual selection, was positively related to adjusted circulating levels of testosterone, even when potentially confounding variables were controlled statistically. The prevalence (proportion of hosts infected) and intensity (number of parasites per host individual) of ecto-parasites (two species of Mallophaga, one species of Acari) infecting barn swallows were unrelated to adjusted testosterone concentrations. Intensities of ecto-parasite infections were negatively related to tail length of male barn swallows, and unmated males had higher intensities of infections than mated males. These results are consistent with the immuno-competence version of the handicap principle.