Tail Streamers Research Articles

FUNCTION OF TAIL STREAMERS OF RED-TAILED TROPICBIRDS (PHAETHON RUBRICAUDA) AS INFERRED FROM PATTERNS OF VARIATION

Abstract We investigated the function of Red-tailed Tropicbird (Phaethon rubricauda) tail streamers by quantifying patterns of variation in streamers and other measurable traits on the basis of measurements of 659 individuals, including 422 of known age and 459 of known sex, measured in the field from 2000 to 2002. Our data were consistent with the idea that Red-tailed Tropicbirds' tail streamer ornaments function for mate attraction, but it suggested that variability in streamer expression is arbitrary and unlikely to provide a meaningful signal of individual quality during mate choice. The two elongated red tail streamers are composed of bilaterally symmetrical filamentous central rectrices with a black rachis and narrow red vane averaging 399 ± 1.8 mm (SE) in length when fully grown. Prior to breeding, individuals of opposite sex conspicuously display elongated red tail streamers during complex aerial courtship. Streamers exhibited measurement asymmetries (the difference between new and old fully grown...

Five main tail streamers of Comet Halley

It is of much interest that Comet Halley displayed long and straight main tails (tail streamers) even at large heliocentric distances. Streamers are usually thought to be plasma in nature. However, a dust streamer may be seen superimposed on the main tail when the Earth is close to the comet's orbital plane. Because of the broad sensitive wave range of the photographic plate and of projection effect, both plasma and dust streamers may be superimposed in the image. In this paper, photometric and geometric measurements of five main tail streamers of Comet 1P/Halley at different heliocentric distances are made and compared. The brightness profiles along streamer axes, the full widths at half maximum (FWHMs) of several transverse cuts, the apparent and actual aberration (or lag) angles of the axes as well as the tail lengths are obtained. Among the streamers recorded on blue plates, two before the perihelion passage are definitely plasma streamers. The similarity of the photometry and geometric features of the five streamers and other lines of evidence suggest that the other three streamers are also mainly plasma streamers, while any contamination by dust would be secondary.

The function and evolution of the tail streamer in hirundines

The morphology of a bird’s tail may result from compromises between aerodynamic efficiency, phylogenetic constraints and selection for non-aerodynamic characteristics, such as mate attraction. A good example of a trait shaped by trade-offs between aerodynamic efficiency and reproductive benefits mediated through female preference is the tail streamer of the barn swallow. Here we use a standardized task to measure the impact of manipulated tail streamer lengths on maneuvering flight in the barn swallow and in the sand martin, a closely related species that lacks a streamer. Our results show that the tail streamer of the barn swallow has a role in maneuvering flight. However, the outer tail feather is approximately 12 mm (9–20%) longer than the aerodynamic optimum for maneuvering flight. Furthermore, we show that the addition of artificial tail streamers to the sand martin, enhances maneuverability even at small streamer lengths, thereby implying that tail streamers may have evolved via natural selection for increased flight performance. Our results therefore suggest that initial tail streamer elongation in the barn swallow has a functional explanation in terms of increased aerodynamic performance. However, female choice has become associated with this trait, promoting the development of a costly handicap.

Sexy streamers? The role of natural and sexual selection in the evolution of hirundine tail streamers.

Matyjasiak et al. (2000) reported recently on experiments designed to mimic the early stages of tail streamer evolution in barn swallows (Hirundo rustic) by adding small streamers onto a closely related species that lacks them, the sand martin (Riparia riparia). This work was interpreted as providing evidence that there is a cost associated with the initial evolutionary stage of a tail streamer and that higher-quality individuals are better able to withstand this cost than lowerquality individuals. The authors suggest that these results give support to the hypothesis that tail streamers initially evolved as a handicap (Zahavi 1975), rather than as a result of Fisherian selection (Fisher 1958), to advertise male quality. We would like to raise a number of issues, which may offer an alternative interpretation.

Assessing the aerodynamic effects of tail elongations in the house martin ( Delichon urbica ): implications for the initial selection pressures in hirundines

Of the three species of hirundine that breed sympatrically across the U.K., one, the barn swallow, has outer tail feathers elongated into streamers, whereas the other two species, the house martin and the sand martin, do not. The tail streamer of the barn swallow is regarded as a classic example of a sexually selected trait. Recent evidence, however, has suggested that streamers may have evolved largely through natural selection for enhanced flight performance and increased maneuverability. We tested the hypotheses that small streamers (1) increase performance in turning flight, but (2) decrease performance in flight variables related to velocity. We manipulated the lengths of house martin outer tail feathers and measured changes in their free-flight performance, using stereo-video to reconstruct the birds" three-dimensional flight paths. Five flight variables were found to best describe individual variation in flight performance. Of these five, the three variables determining maneuverability predicted that flight performance would be optimized by a 6- to 10-mm increase in the length of the outer tail feathers. In contrast, for mean velocity and mean acceleration, extension of the outer tail feathers appears to have a detrimental effect on flight performance. We suggest that the initial selection pressure for streamers in ancestral short-tailed "barn swallows" was via natural selection for increased maneuverability. In addition, we propose that the benefits of increased maneuverability have differed between hirundines in the past, such that the cost of increasing the length of the outer tail feather has, to date, outweighed the benefits of doing so in streamerless hirundines.

The effect of tail streamer length on aerodynamic performance in the barn swallow

The elongated tail of the male barn swallow (Hirundo rustica) is regarded as one of the classic examples of a male trait exaggerated by female choice. However, recently a hypothesis has been proposed suggesting that the streamers, or elongated outer tail feathers, may aid aerodynamic performance through the Norberg mechanism, providing lift at slow speeds and high angles of attack when the tail is fully spread. The possibility exists that the tail streamer has evolved under natural selection, sexual selection, or a combination of both selection pressures. We tested these three hypotheses by reducing the streamer length of free-flying swallows and measuring their aerodynamic performance, using stereo-video. Measurements of flight performance were made from the digitized three-dimensional flight paths. Five flight variables best described the individual variation in flight performance. Four of these five parameters—mean velocity, mean curvature, maximum agility, and mean rate change of curvature in the XY plane—had significant second-order polynomial relationships with tail streamer manipulation. The first and second principal components (from principal components analysis of the flight variables) also showed similar relationships with streamer manipulation. The combination of a curvilinear relationship between flight performance and streamer length and an aerodynamic optimum between 0 and 20 mm reduction is only predicted if both natural and sexual selection have been acting on streamer morphology. Our data therefore suggest that sexual selection has extended streamer length by around 10 mm beyond its aerodynamic optimum. We suggest that both natural and sexual selection have been important in shaping tail morphology in the barn swallow, and the relative importance of both selection pressures is discussed. Key words: barn swallows, flight performance, Hirundo rustica, maneuverability, natural selection, Norberg mechanism, sexual selection. [Behav Ecol 11: 228–238 (2000)]

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.

Foraging Cost of a Long Tail Ornament: An Experiment with Sand Martin Females

The handicap hypothesis assumes that sexual ornaments impose a viability cost upon the bearers. There have been few empirical tests of this assumption. Previous studies show evidence for the cost of a tail ornament in male birds: a negative relationship between an experimentally increased tail ornament (long tail streamers) and efficiency at foraging for nestlings. However, it must be admitted, that the apparent impairing effect of an elongated tail could be a result of a decrease in male parental effort in response to an increase of female parental effort, which might have occurred in response to increased male attractiveness (differential allocation of female parental effort). In this study, the effect of differential parental expenditure was eliminated by lengthening the tail in female, rather than male, sand martins (Riparia riparia). Tail‐elongated females decreased the rate at which they fed nestlings, and captured more but smaller insects. There was no simultaneous increase of feeding rate in the males that could explain the decrease of feeding rate in the females. These results confirm the existence of a cost of a tail ornament in birds feeding in flight, as is expressed in terms of impaired flight and foraging capacity.

Sexual selection and tail streamers in the barn swallow: appropriate tests of the function of size-dimorphic long tails

A.B. was supported by a Marie Curie post-doctoral grant from the European Union and A.P.M. was supported by grants from the Swedish and Danish Natural Science Research Councils

Length of tail streamers in barn swallows

Length of tail streamers in barn swallows

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.

Selection on swallow tail streamers

Sexual selection results when one sex, usually females, discriminates among members of the opposite sex during mate choice. It is interesting because it is a mechanism by which those traits used in mate choice can become exaggerated in a way that is apparently detrimental, except in terms of mating, to the bearer of the ornaments1. One of the classic examples of an exaggerated trait produced by sexual selection is the tail streamers of swallows (Hirundo rustica)2. Here, however, by using tail-manipulation experiments, I show that the tail streamers of swallows are mainly the product of natural selection, with sexual selection only causing the streamer to extend beyond the aerodynamic optimum by a small amount.

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.

Testing the functional significance of tail streamers

Studies of the evolution of elaborate ornaments have concentrated on their role in increasing attractiveness to mates. The classic examples of such sexually selected structures are the elongated tails of some bird species. Elongated tails can be divided into three categories: graduated tails, pin tails and streamers. There seems to be little debate about whether graduated and pin tails are ornaments; i.e. costly signals used in mate choice. However, in the case of streamers there is considerable discussion about their function. It has been suggested that tail streamers could be (i) entirely naturally selected, (ii) entirely sexually selected, (iii) partly naturally and partly sexually selected. The prime example of a species with tail streamers is the swallow (Hirundo rustica) in which both sexes have tail streamers. In this paper we discuss the aerodynamic consequences of different types of manipulation of the streamer and/or outer tail feather. We make qualitative predictions about the aerodynamic performance of swallows with manipulated tail streamers; these predictions differ depending on whether streamers have a naturally or sexually selected function. We demonstrate that these hypotheses can only be separated if tail streamers are shortened and changes in aerodynamic performance measured during turning flight.

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 dust tail of Comet 1P/Halley after its perihelion in 1986 and the rotation of the nucleus

The dust tail of Comet 1P/Halley in 1986 and the evolution of its streamers after perihelion are thoroughly investigated by means of direct modelling, determination of streamer densities, and synchrone and syndyne grids. For this purpose, 16 selected wide-field images of the comet in the red dust continuum, taken from 1986 February 22 to April 16, are used to cover the whole time interval of the observable dust tail evolution during its most active phase. In total, 10 significantly distinct streamers have been detected, which were emitted from 22 d before to 12 d after perihelion. The analysis of these streamers yields a synodic ‘basic period’ for the activity of the cometary nucleus of T0 = 2.10 ± 0.11 d. Moreover, ‘missing’ streamers indicate a complex interplay between the rotation of the nucleus and possibly its precession/nutation, and two or more distinct active regions on the nucleus. Periodicities of about 3.3 and 5.0 d or longer also seem to be present in the activity pattern. Thus the morphological analysis of the dust tail streamers independently confirms the complicated rotational pattern of Comet Halley's nucleus found from previous studies. The ‘decay’ of the dust tail and its streamers, i.e. the disappearance of the smallest particles from the tail, seems to be governed by a time-scale of ####t1/e ≈ 9 d. Typical ‘survival times’ of dust particles in the tail range from about 25 d for β = 1.0 (a ≈ 1 μm) to more than 65 d for β < 0.01 (a > 100 μm).

Infrared Observations of an Outburst of Small Dust Grains from the Nucleus of Comet P/Halley 1986 III at Perihelion

A close examination of the 0.7- to 23-μm infrared data base acquired by Gehrz and Ney (1992, Icarus 100, 162-186) suggests that the nucleus of Comet P/Halley 1986 III emitted a burst of small dust grains during a 3-day period commencing within hours of perihelion passage on 1986 February 9.46 UT. The outburst was characterized by significant increases in the coma's grain color temperature T obs, temperature excess (superheat: S = T obs/ T BB), infrared luminosity, albedo, and 10-μm silicate emission feature strength. These changes are all consistent with the sudden ejection from the nucleus of a cloud of grains with radii of approximately 0.5 μm. This outburst may have produced the dust that was responsible for some of the tail streamers photographed on 1986 February 22 UT. The peak of the dust outburst occurred about 3 days before a pronounced increase in the water production rate measured by the Pioneer Venus Orbiter Ultraviolet Spectrometer. We suggest that jets that release large quantities of small particles may be largely responsible for some of the variable infrared behavior that has been reported for P/Halley and other comets during the past two decades. Such jets may also account for some of the differences between IR Type I and IR Type II comets.

Swallow tail streamer is a mechanical device for self-deflection of tail leading edge, enhancing aerodynamic efficiency and flight manoeuvrability

In some birds, sexual selection, owing to female preference for long tails in mate choice, has led to tails large enough to constitute serious handicaps in flight, incurring a cost in terms of natural selection. Elongated tails also occur among species that depend heavily on flight for foraging, but then usually only the outer tail feathers are long, often being drawn out into narrow streamers. Apart from their role in mate choice, no functional explanation has ever been given of these tail streamers. Based on high-speed motion-picture films of swallows, Hirundo rustica, in free foraging flight and wind-tunnel experiments, I here report on a mechanical and aerodynamic function of the tail streamers in the swallow. Owing to the aeroelastic properties of the outer tail feather, the streamer causes an automatic and instantaneous downward deflection of the tail's leading edge when the swallow spreads and lowers the tail. Thus flow separation on top of the tail may be delayed to higher angles of attack, enabling much higher aerodynamic lift to be elicited from the tail before stall sets in. This improves manoeuvrability by allowing tighter turns, and the development of tail streamers is therefore governed largely by natural selection.

The asymmetrical cost of tail elongation in red-billed streamertails

Ornaments used in signalling should vary in size with signaller quality. It has also been suggested that high-quality individuals should be less susceptible to asymmetrical development than low-quality individuals. It has been claimed that the degree of asymmetry in ornaments could be used by receivers to assess signaller quality. If these claims are correct, high-quality individuals should produce large symmetrical ornaments, and low-quality individuals small, asymmetrical ones. Negative relations between tail length and asymmetry have been demonstrated and taken as support for these predictions. However, these results could occur if individuals growing long tails were constrained to more symmetrical development. This mechanical constraints hypothesis predicts that the costs of a given amount of asymmetry should increase with increasing tail length and, when tails are asymmetrical, the costs of increasing tail length should rise faster than when symmetrical. To test these predictions, we manipulated tail streamer length and symmetry of male red-billed streamertails (Trochilus polytmus) which were released through a maze forcing the birds to manoeuvre to avoid collision. We show that if streamer length is experimentally reduced then manipulations of symmetry have no effect on the ability of the birds to negotiate this maze. However, when streamers are experimentally elongated, the time taken to negotiate the maze and the number of collisions increase with the degree of the asymmetry manipulation. This demonstrates that individuals producing long tails may be forced into greater symmetry to minimize costs.

The aerodynamic costs of asymmetry in the wings and tail of birds: asymmetric birds can’t fly round tight corners

Asymmetry in the wings and tails of birds is an unusual trait in that, because of their aerodynamic function, the optimum phenotype for the trait is known: perfect symmetry. There is considerable variation from this optimum both within and between species. Here I use simple aerodynamic theory to predict the aerodynamic costs of asymmetry in the wings and tail, and to examine the relation between the length of the wings or tail, and the cost of given amounts of absolute asymmetry (equal to a given fixed length) and relative asymmetry (asymmetry equal to a given proportion of the length of the wings or tail). The aerodynamic analysis shows that wing asymmetry is much more costly than tail asymmetry, and asymmetry in the aerodynamically functional parts of the tail is more costly than asymmetry in ornaments such as tail streamers. The cost of wing asymmetry and of asymmetry in aerodynamically functional parts of the tail is unaffected or decreases with trait length. In contrast, the aerodynamic cost of asymmetry in tail ornaments increases with trait size. Much of the pattern of variation in the level of fluctuating asymmetry in the wings and tail can thus be explained by natural selection for aerodynamic efficiency.