Barn Swallow Research

This page provide an introduction, with references, to some of our current research topics in barn swallows.
Enlargements of all the photos can be obtained by clicking on them.


Barn swallows are small passerine birds belonging to the family of swallows and martins Hirundinidae. They weigh ca. 20 g, and most morphological characters are similar in males and females, with the exception of the outermost tail feathers which are longer in males than in females (5% difference in North Africa increasing to almost 25% in Northern Europe).

Barn swallows are socially monogamous, with males arriving to the breeding grounds before females. Arrival occurs in Southern Europe in March-April and in Northern Europe later until June. After establishing a small breeding territory, males attempt to attract a mate by flight displays and showing off their long tail feathers. Once established, the male and the female build a nest of mud pellets, where the female will lay 3-7 eggs, usually 4-5 eggs. The female incubates the eggs ca. 14 days (in North America males also incubate), and the nestlings are fed ca. 21 days before fledging. They are fed by the adults yet another week before becoming independent. The number of broods ranges from one in Northern Europe to three in Southern Europe. Fledglings roost in communal roosts in reed beds or similar sites, where more than 100,000 birds can sleep together. Adults join roosts later in autumn. Migration takes place in Northern Europe in September-October and in Southern Europe in August-September. In Southern Spain and Northern Africa breeding birds may return already in January-February and they leave again already in July-August.


A barn swallow nest-building in south-western Spain.

Measuring a barn swallow's famoulsy elongated tail near Chernobyl in Ukraine (Anders Pape Møller and Tim Mousseau)

Males have longer outermost tail feathers than females, and males enjoy an advantage in terms of mating success because of this effect. Long-tailed males are more often mated (Møller 1994), mate more quickly (Møller 1988, 1990, 1991, 1994), have more broods (Møller 1988, 1990, 1994), have females that provide more parental care (Møller 1991; de Lope and Møller 1993), and have more extra-pair copulations than other males (Møller 1988).

Male barn swallows also have other characters involved in sexual selection, including white tail spots on the tail feathers (Kose and Møller 1998; Kose et al. 1999), symmetric tail feathers (Møller 1992, 1994, 1995), a red facial plumage (Ninni et al. unpublished), and a high song rate (Møller et al. 1998).

These different traits are positively correlated, implying that they are generally indicating the same quality properties. Several of the traits have also been shown to be condition-dependent. Male tail length and tail asymmetry also have significant resemblance among relatives (Møller 1991, 1994).

Females tend to work harder when mated to attractive males. Such differential parental investment is a maternal effect that provide attractive males with a selective advantage (de Lope and Møller 1993; Møller 1991, 1995).

Long tails are condition-dependent secondary sexual characters, with long-tailed males surviving better (Møller 1991, 1994; Møller and Nielsen 1996). However, when tail length is manipulated, an experimental increase in tail length causes a reduction in survival, while a shortening in tail length causes an increase in survival (Møller and de Lope 1994).

Migration is much faster in spring than in autumn, with males and to some extent females competing for early arrival and hence access to preferred partners (Møller 1994). Males with long tails arrive earlier than others, and condition-dependent arrival date results in such males have a large advantage in terms of start of breeding and hence in annual reproductive success. Arrival date is heritable and there is a genetic correlation between arrival date and male tail length (Møller 2001).

Barn swallows from Europe and western parts of Asia mainly winter in Africa south of the Sahara. Birds from different parts of the breeding range winter in different parts of Africa. Survival rate is affected by rainfall in the African winter quarters (Møller 1989).

Weather conditions in the African winter quarters directly affect the annual survival rate of adult barn swallows from the Danish population (A. P. Møller and T. Szép unpublished data). Mortality during the rapid spring migration can be considerable in Northern Africa, mainly so in years when rainfall in the winter quarters have been low (A. P. Møller and T. Szép unpublished data). Thus, there is an added negative effect of poor weather conditions in Southern Africa on mortality during migration.

Rainfall patterns in Africa have an important effect on barn swallow survival rate.

Clutch size, breeding date, egg size, number of clutches, and survival are life history characters that together in a given combination may provide an optimal solution to partitioning of reproductive effort under given environmental conditions.

We have investigated clutch size decisions by manipulating clutch through an increase by one egg or a decrease by one egg. These small manipulations change the effort of attending parents, but also affects the quality of the resulting offspring (Saino et al. 1997, 1999). Importantly, when parental effort is increased, this reduces the probability of survival from one year to the next (Saino et al. 1999).

Parasites play an important role in mediating the costs of reproduction in barn swallows. Parasites may affect the optimal clutch size because parents may be able to rear a different number of offspring when parasitised than when free of parasites (Møller 1991), and also their quality and quantity (de Lope and Møller 1993). In an experiment in which both clutch size and mite load of nests were manipulated experimentally, parasites increased the within-season costs of reproduction by reducing the frequency and delaying the start of second clutches (Møller 1993).

A colour-ringed Spanish barn swallow.
When barn swallows become old, they tend to have reduced survival rates and deteriorate in quality. This is reflected by their tails and wings starting to become shorter, their parasite loads increasing, their arrival date becoming retarded, clutch size decreasing, and parasite loads increasing (Møller and de Lope 1999). Parental investment during the first year causes a reduction in reproductive performance in subsequent years, and the quality of offspring in terms of a component of immunity in one year is negatively related to the quality of offspring the subsequent year (Saino et al. 2001).

Barn swallows are colonial passerines that often reuse their nest sites from one year to the next. Hence, nests are often infested with large number of ectoparasites such as mites, fleas, louseflies and feather lice. Several populations also have high levels of helminth infections. The number of parasites build up over the breeding season (de Lope and Møller 1993). This is associated with a change in priority of nestling development from growth to immune function when comparing first and second broods (Merino et al. 2000).

Presence of blood sucking mites causes barn swallows to build new nests (Møller 1990). Mites reduces reproductive success and the quality of offspring produced, and second broods are reduced in frequency and delayed by mites (Møller 1990). Louseflies likewise reduce the quality of offspring produced (Saino et al. 1989). Barn swallows have relatively large reductions in reproductive success caused by parasites, as compared to other hirundines (Møller et al. 2001).

Mites suck blood from both adults and nestlings. Males with long tails have fewer mites than the average male (Møller 1990, 1991), and many mites reduce the length of the tail and increase the asymmetry of the tail after the subsequent moult (Møller 1990, 1992). Mites are less able to suck blood from the offspring of long-tailed birds, and their reproductive rates are reduced (Møller 2000). The abundance of mites has decreased dramatically during the last 15 years, as tail length in the adult barn swallows has increased (Møller 2001).

A barn swallow landing on its nest, showing the white spots on the tail feathers which are the preferred habitat of feather lice.

Feather lice and feather mites are common on nestling and adult barn swallows (Møller 1991). Males with long tails have fewer feather lice (Møller 1991), and so do males with large white spots on their tails (Kose and Møller 1999; Kose et al. 1999). Feather lice preferentially eat parts of the feather that are white, and habitat choice experiments have shown that they prefer this part of the feather (Kose and Møller 1999; Kose et al. 1999). Feather lice seem to time their reproduction in response to testosterone levels of their hosts, with experimentally increased testosterone causing an increase in abundance (Saino and Møller 1995).

Nestling barn swallows attempt to avoid ectoparasites by fledging early when a nest is heavily infested (Møller 1990). This is achieved by changing growth priorities from skeletal traits to feathers (Saino et al. 1999).

Barn swallows have relatively strong immune responses compared to solitarily nesting hirundines (Møller et al. 2001). We have measured immunoglobulins, T-cell mediated immunity and B-cell mediated immunity in adult and nestling barn swallows. T-cell mediated immunity in nestlings is reduced by an experimental increase in clutch size (Saino et al. 1997). When nestlings are provided extra nutrients (from a small amount of beef), they increase their T-cell response considerably (Saino et al. 1997). Parent birds trade their own survivorship against the T-cell mediated immunity of their offspring (Saino et al. 1999).

Tail length of male barn swallows reflects B-cell mediated immunity, and females may thus gain information on immunity of potential mates from their tail length (Saino and Møller 1996). This measure of immunity provides reliable information about male quality because males with strong responses were better able to survive the annual migration to Africa than were those with weak responses (Saino et al. 1997).

Immunoglobulin levels of adult barn swallows fluctuate in relation to the time of the reproductive cycle. Females show a peak during the period just prior to and during egg laying, while males do not show a similar pattern (Saino et al. 2000). Interestingly, females show a more dramatic increase when mated to a very attractive male, suggesting that females increase immunoglobulin levels as a way of increasing investment in maternal antibodies in their eggs (Saino et al. 2000).

Carotenoids have physiological functions related to immunity and free radical scavenging. If carotenoids are in short supply through the diet, we should expect that males with bright red facial coloration to be in prime health state. This prediction was supported in a study of immunoglobulins and leukocytes in Italian breeding barn swallows (Saino et al. 1999).

Male barn swallows produce an elaborate song that is used for attracting females. Males with experimentally increased mite loads reduced their song rate (Møller 1991). Subsequent studies revealed that features of the song reflect testosterone levels of males, but also information about their social environment (Galeotti et al. 199). Male song also provides information about health status since males with particular songs have low levels of circulating immunoglobulins and leukocytes (Saino et al. 1997).

Barn swallow females are constrained in their mate choice because only a single female may become mated with a very attractive male in a monogamous mating system. Hence, we should expect such females to copulate with already mated males. Males with long tails are more successful in obtaining extra-pair copulations than other males (Møller 1991), and experimental manipulation of tail length of males causes a parallel change in extra-pair mating success (Møller 1988). Extra-pair copulations result in extra-pair paternity, and males with experimentally elongated tails sire more offspring in their own nests and in other nests than control males (Saino et al. 1997). Male song rate reduces the risk of extra-pair paternity, but more so for short tailed than for long tailed males (Møller et al. 1998). Males with a high frequency of extra-pair paternity in their nests feed their nestlings relatively less than when their brood has less extra-pair paternity (Møller and Tegelström 1997).

Barn swallows about to copulate.

A brood of barn swallows begging in south-western Spain, showing their brightly colored gapes which appear to be a signal of health.

Barn swallow nestlings beg vigorously for food from their parents by producing loud calls and displaying their bright orange gapes. The gape display is a reliable signal of health status since a challenge of the immune system with sheep red blood cells reduces the level of coloration, while artificial provisioning with lutein (the carotenoid causing the coloration) increases the level of coloration (Saino et al. 2000). Parent barn swallows respond to experimental manipulation of gape colour by changing their allocation of food (Saino et al. 2000). Gape coloration reflects the viability of offspring (Saino et al. 2001). Nestling begging calls also reliably reflect health status (Sacchi et al. 2001). Both current hunger status and long-term condition of nestlings affect their begging rate and the response of parents to the displays (Saino et al. 2000).

Studies of micro-satellites in the barn swallow have revealed one of the most variable molecular markers discovered (Primmer et al. 1995, 1996). The very high mutation rates have allowed direct description of the modes of mutation (Primmer et al. 1996, 1998), and studies of changes in germline mutation rates caused by low levels of radiation in Chernobyl (Ellegren et al. 1997).
We have made many analyses of quantitative genetic variation in phenotypic traits, and we have found evidence of significant heritability of characters such as tail and wing length, abundance of ectoparasites and social environment (Møller 1991, 1994, 2001; N. Saino et al. unpublished data).

Barn swallows are common breeding birds in the area surrounding the radioactive contaminated sites near Chernobyl, Ukraine. Our studies have shown that barn swallows around Chernobyl have markedly elevated levels of asymmetry in tails and wings compared to museum samples from the same area before the contamination in 1986, and that birds from control areas (Møller 1992). Mutation rates in micro-satellite molecular markers are increased by a factor 5-10 over the rates in control areas (Ellegren et al. 1997). The red facial plumage of barn swallows consists of carotenoids, and birds from Chernobyl have much paler coloration that birds from other areas, in particular males (Camplani et al. 1999). The frequency of partial albinism of the red facial plumage of a genetic origin occurs at a rate of ca. 15% in Chernobyl, but at less than 1% elsewhere (Ellegren et al. 1997; Møller and Mousseau 2001). Birds from Chernobyl tend to be smaller than other birds, but since the size of barn swallows has not changed during the period 1991-2000, these must be selected against (Møller and Mousseau 2001).

Swallows from near Chernobyl: the bird on the left is partially albinistic.

Barn swallows have shown long-term changes in phenotype that can be related to changes in climate. A recent study showed that the relative importance of first and second clutches in a Danish population changed in response to changes in the North Atlantic Oscillation (NAO; Møller 2001). When the NAO index was high, implying rainy weather in Northern Europe, barn swallows produced low quality offspring in first broods, but high quality offspring in second broods. The reverse was the case when the NAO index was low and weather was sunny and warm early during the summer in Denmark (Møller 2001). Immigration to the study population increased following a year when annual productivity was caused mainly by fledglings from the first brood. This implies that increases in NAO increases the size of the barn swallow population through effects on the relative productivity of first broods.

Barn swallows have changed dramatically in phenotype in Denmark during the last 18 years, and this change appears to be associated with a micro-evolutionary change in phenotype, with males having increasingly long outermost tail feathers (A. P. Møller and T. Szép unpublished data).

Catching barn swallows breeding in a barn in Ukraine (Tim Mousseau and Alexander Peklo).
Barn swallows are closely associated with humans and dairy farming in most parts of the range. Hence, changes in farming practice have resulted in reductions in reproductive success and quality of offspring produced (Møller 2001). The number of cattle and the number of recent years with cattle are good predictors of population size on Italian and Danish farms (Ambrosini et al. 2001).

Population size has been decreasing in areas in Denmark, Germany and other countries. Based on demographic models and individual histories of recruitment and survival the breeding population in Northern Denmark has been predicted to go extinct with a high probability during the next few decades (Engen et al. 2001).

© Anders Pape Møller, 2002
Laboratoire d'Ecologie Evolutive Parasitaire, Université Pierre et Marie Curie, Paris, France
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