Monogamous Swallow Research Articles

Repeatability of female choice in a monogamous swallow

Abstract. Models of the evolution of female mate preferences often assume that there is a genetic basis for female choosiness, although there are few empirical tests of this assumption. One way in which to estimate consistency in female mate preferences is to calculate the repeatability of characters of males chosen by females on different mate choice occasions. Repeatability of female mate choice for morphological characters of males chosen in different years was estimated in the monogamous barn swallow, Hirundo rustica . There was no significant repeatability of mate preferences for the absolute size of male traits. However, females have different numbers of males and phenotype ranges available at different mate choice events. The rank of chosen male phenotypes demonstrated significant repeatability for the length of the outermost tail feather, but not for wing length, the length of the short central tail feather, or tarsus length. Consistency in female mate preferences could be due to genetic or environmental factors.

Preferred males acquire mates of higher phenotypic quality

Several field studies have shown that ornamented males of both polygynous and monogamous animals gain an advantage in terms of mate acquisition. Male reproductive success is strongly positively related to the number of mates acquired, but differential quality of female mates may also play an important role. Darwin (The descent of man, and selection in relation to sex (1871)) and Fisher (The genetical theory of natural selection (1958)) suggested that preferred males of monogamous species should experience a reproductive advantage because of differences in female phenotypic quality. Good condition is supposed to allow females to breed early and to grant them high fecundity. Preferred males should acquire early-breeding females in good condition and for that reason have greater reproductive success. Several genetic models confirm the possible significance of this sexual selection mechanism. Alternatively, preferred males may acquire mates that invest more in reproduction because they have acquired males of higher quality. Field data on birds suggest that preferred males do acquire early-breeding mates, but it has never been shown that such mates are of a higher phenotypic quality in terms of condition. Long-tailed males of the monogamous swallow, Hirundo rustica , are preferred by females. In this study, observations using males with naturally varying and experimentally manipulated tails show that females mated to long-tailed males were heavier, bred earlier, produced more offspring per season and were much more likely to survive to the next breeding season that were females mated to short-tailed male swallows. This is the first demonstration that preferred males actually obtain mates of high phenotypic quality. Preferred males had a direct reproductive benefit of mating with long-lived females, because pairs consisting of mate-faithful individuals reproduced earlier and produced more offspring than did pairs without previous common experience.

Density‐dependent Extra‐pair Copulations in the Swallow Hirundo rustica

AbstractDensity‐dependence of extra‐pair copulation rates and mate guarding intensity in the monogamous swallow Hirundo rustica was studied from 1983 to 1988 in N. Jutland, Denmark. Changes in local population size paralleled changes in national population size from 1976 to 1988. Median and mean colony size increased and minimum, mean and median nearest neighbour distance decreased with increasing population density. The percentage of female swallows involved in extra‐pair copulations increased significantly with increasing population density between years and with increasing colony size within years in spite of the fact that male mate guarding intensity changed similarly. Mate guarding did reduce the probability of the female being involved in extra‐pair copulations, but not to the extent that it compensated for the density‐dependent increase in extra‐pair copulation attempt rates. Natural and sexual selection arising from extra‐pair copulations is frequency‐dependent and varies spatially and temporally in relation to population density.

Male tail length and female mate choice in the monogamous swallow Hirundo rustica

Male and female swallows benefit from an early start to breeding because it allows the raising of many young and a high proportion of two broods within a single season. Long-tailed males arrived early in spring while female arrival date was independent of their tail length. Male tail length was significantly positively related to success in mate acquisition and negatively to the duration of the pre-mating period and the start of breeding. Arrival date also influenced success in mate acquisition and mating date. Females chose mates using tail length as a cue. Female mate choice was apparently not affected by territory quality. Female swallows could not predict the intensity of mate guarding or the extent of paternal care (feeding intensity of offspring) from male arrival date or male tail length.

Viability costs of male tail ornaments in a swallow

MALE sexual ornaments that increase mating success may evolve even when they decrease other components of fitness such as survival1–9. But natural covariation between the apparent level of investment in such ornaments and fitness components such as mating success, fecundity and survival, does not provide incontrovertible evidence that the ornaments are costly, because uncontrolled variables such as overall health5 may affect several components of fitness at the same time10–12. By experimental manipulation of male tail length in the monogamous swallow, Hirundo rustica, however, the effects of tail endorment can be tested directly. I show here that in such experiments, females prefer males with elongated tails over those with shortened tails13, but that males with experimentally elongated tail ornaments captured smaller, less profitable prey than those with shortened tails. Impaired foraging efficiency of tail-elongated males increased the frequency of fault bars in their tail feathers, probably as a result of food deficiency during moult. Males with experimentally elongated tail ornaments also decreased their natural tail size during moult, thereby causing a fitness loss in terms of delayed breeding and a reduced annual production of offspring resulting from reduced sexual attractiveness during the following year.

Paternity and paternal care in the swallow, Hirundo rustica

Paternal care in the monogamous swallow, Hirundo rustica, measured in terms of frequency of feeding of young, was positively related to the certainty of paternity. Male parental care increased with the absolute number of pair copulations and decreased with the absolute and relative number of extra-pair copulations by his mate. Nearly identical results were obtained using either relative or absolute number of male feedings as a measure of the intensity of parental care. Male swallows apparently estimated their certainty of paternity from the interest that neighbouring males were taking in their mates during the fertile period. When the number of extra-pair copulations involving their mates was experimentally increased, males significantly decreased their feeding rates compared with controls. Paternal care was not related to several measures of male quality (body condition, arrival time, mite infection, song rate, or mate guarding rate).

Female choice selects for male sexual tail ornaments in the monogamous swallow

Male secondary sexual ornaments are widespread among polygynous animals1 where they apparently arise through female choice and differential mating success of males2–5. Darwin1 and Fisher2 suggested that female choice should also select for extravagant male ornaments in monogamous birds because female preference for ornamented males should result in earlier breeding which would increase male reproductive success by enhancing the quality and/or the quantity of offspring raised each year. In such cases female reproductive success should be positively related to body condition6 and a heritable component of the time of breeding should be affected by stabilizing selection7. Female preferences for orna-mented males would be favoured by sexual selection because such choosy females would have a greater probability of bearing orna-mented sons with an increased chance of pairing up early. I report here that in the monogamous swallow, Hirundo rustica, males with experimentally elpngated tail ornaments obtain mates more quickly than males with shorter tails, and enjoy increased reproductive output in one breeding season. Such males are also preferred by females seeking extra-pair-bond copulations. Thus male sexual ornaments may also be maintained by female mate choice in monogamous species.