Nest building is a sexually selected behaviour in the barn swallow

Abstract

Females may use male nest building to assess male parental quality, and nest size would then be a sexually selected trait. In the barn swallow, Hirundo rustica, females select their partner by his tail length, a character believed to signal good genes. Both sexes participate in nest building, although male participation is negatively related to his attractiveness as reflected by tail length. We tested the hypothesis that nest building is a sexually selected trait: females paired with males of high parental quality (as shown by the male during nest building) may obtain a mate providing large amounts of parental investment, while, as has been shown previously, females mated to attractive (long-tailed) males will acquire mates with good genetic quality. Therefore, since nest building in barn swallows occurs after mating, we predicted a postmating sexual selection process by which the female invests differentially in reproduction depending on the male’s nest-building effort (reflecting his willingness to invest in reproduction). The volume of material in a nest was related to the male’s contribution to nest building and, in agreement with our hypothesis, in a multiple regression analysis, male tail length and nest material volume were negatively related to laying date and positively to female investment in reproduction (total number of eggs laid during the breeding season). Moreover, females paired with long-tailed males (which contribute very little to nest building), but using the same amount of nest material as females paired with short-tailed males, reduced the thickness of the nest and hence increased its capacity. Therefore, in the barn swallow two different traits appear to be sexually selected: tail length of males owing to the good genes process and nest-building ability owing to the good parent process.  1998 The Association for the Study of Animal Behaviour Correspondence: J. J. Soler, Departamento de Biología Animal y Ecología, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain (email: jsolerc@goliat.ugr.es). J. J. Cuervo is at the Estación Biologica de Doñana, C.S.I.C., Apartado 1056, E-41080 Sevilla, Spain. A. P. Møller is at the Laboratoire d’Ecologie, CNRS URA 258, Université Pierre et Marie Curie, Bâtiment A, 7éme étage, 7 quai St Bernard, Case 237, F-75252 Paris cedex 05, France. F. de Lope is at the Departamento de Zoología, Facultad de Ciencias, Universidad de Extremadura, E-06071 Badajoz, Spain. Nest-building behaviour is often associated with courtship and pair formation in birds. The degree to which this behaviour is used in courtship varies from mere manipulation of a piece of nest material or display of a potential nest site to the building of an entire nest by the male (Collias & Collias 1984). Nest-building behaviour is also used in sexual display by both polygynous and monogamous bird species (see examples in Collias & Collias 1984) in a postmating sexual selection process (Møller et al. 1995). 0003–3472/98/121435+08 $30.00/0 143 Nest-building behaviour may signal the reproductive condition of individuals and physiologically stimulate a partner (Collias 1964), but there is very little information on the importance of the nest itself and its role in mate choice (Hoi et al. 1994). The nest may indicate parental quality, experience or genetic quality, and females could therefore benefit from mating with a superior nest builder. Nest-building behaviour could provide information to pair members about the quality of the potential partner, and such assessment of mate quality may allow individuals to choose a mate in nonmonogamous species, while in monogamous species assessment may also allow partners to invest differentially in reproduction relative to the quality of the mate (Burley 1986; Møller 1994). In a comparative study of nest size in relation to parental effort in birds, Soler et al. (1998) showed that bird species in which both sexes build the nest have larger nests than those in which only the female builds. Nest size (relative to body size) was positively correlated with  1998 The Association for the Study of Animal Behaviour 5 1436 ANIMAL BEHAVIOUR, 56, 6 the amount of parental investment (estimated as the relative duration of the nestling period) among passerines. Thus, Soler et al. (1998) concluded that nest size could indicate the willingness of males to invest in reproduction and it could therefore be a postmating sexually selected trait. Several studies have shown that the nest-building ability of males is related to female mate choice and the reproductive success of the pair. For example, Hoi et al. (1994, 1996) showed that female penduline tits, Remiz pendulinus, choose their partner based on nest quality. Evans & Burn (1996) showed that female wrens, Troglodytes troglodytes, select their mates depending on the number of nests a male builds. Hoi et al. (1996) also showed a positive relationship between song rate of male penduline tits, a generally accepted sexually selected trait in passerines, and nest quality. Perhaps the species in which it has been most clearly shown that nest size is a sexually selected trait is the black wheatear, Oenanthe leucura: individual males of better parental quality build larger nests (Moreno et al. 1994), and morphological adaptations for stone carrying (stones are used in the nest structure) result in an extreme display used in postmating sexual selection (Møller et al. 1995). An experimental increase in the number of stones carried resulted in increased reproductive success owing to differential female investment in reproduction (Soler et al. 1996). We decided to investigate the relationship between nest building and sexual selection in the barn swallow, Hirundo rustica, because sexual selection has been particularly well studied in this species (Møller 1994). The barn swallow is a migratory, insectivorous, monogamous and sexually dimorphic passerine rearing one to three broods per year. Males have considerably longer tails than females (Møller 1994) and tail length is a sexually selected trait (Møller 1988, 1990) that indicates relative quality of the male (Møller & de Lope 1994). However, tail length is negatively related to parental quality: long-tailed males feed the nestlings less often and with poorer quality food than short-tailed males (de Lope & Møller 1993; Møller 1994). Tail length signals genetically based parasite resistance and offspring viability, and it is therefore maintained by sexual selection (Møller 1994). Barn swallows build cup-shaped nests out of mud, normally mixed with straw, and they line the nest with soft materials such as feathers, hair or straw. Both sexes participate in nest building but there is considerable variation in the male’s contribution, which is negatively related to male tail length. Short-tailed males invest more in nest building than long-tailed males (Møller 1994), and female nest-building activity is therefore greater when their partners have long tails. Both nest size and male tail length in the barn swallow are positively related to clutch size (Møller 1982, 1994). Since egg production is costly, females have to determine the number of eggs to lay relative to their own phenotypic quality and that of their mate (Møller 1992; de Lope & Møller 1993). Therefore, they may lay more eggs when mated to an attractive partner (owing to differential parental investment). Alternatively, a female may lay more eggs if large nests reflect high parental quality of the partner contributing to building the nest. If this is the case, in pairs that use the same nest for all clutches, total number of eggs laid during the breeding season should be positively related to nest size after controlling for the effect of male tail length. Nest size could also be a consequence of clutch size, because brood size may be constrained by nest size in passerines (Slagsvold 1989). Thus fieldfare, Turdus pilaris, with experimentally increased nest cup volumes produced more fledglings. In swallows, females paired with long-tailed males lay large clutches, but suffer most of the cost of reproduction and get little help in nest building. We predict that these females should increase the nest capacity (nest cup volume) while still using the same amount of nest material as other females paired with short-tailed males. In other words, for a given amount of nest material and a given clutch size, females put the same effort into building, whether their mates have long or short tails, resulting in thinner nest walls when it is only or mainly the female that builds. On the other hand, short-tailed males show their good parental quality by their nest-building behaviour, and their females will also lay large clutches. Hence, they need nests with a nest cup volume to fit this clutch size. However, their nests will contain more material than those of long-tailed males owing to the contribution to nest building of males of good parental quality. Therefore, nest cup volume, after controlling for the amount of nest material and clutch size, should be larger when males are long-tailed. Moreover, the amount of nest material should explain variation in total number of eggs laid during the breeding season, after controlling for male tail length, better than nest cup volume does. In this paper we tested the hypothesis that nest building is a postmating sexually selected trait signalling parental effort. We first analysed the relationship between the male’s contribution to nest building and final nest material volume. Second, we analysed the relationship between female quality (estimated by laying date) and investment in reproduction (estimated by the total number of eggs laid during the breeding season) and male sexual attractiveness as reflected by his tail length. We also determined the relationship between amount of nest material, which we hypothesized to be a sexually selected trait indicating future investment in reproduction by the male, and (1) nest cup volume, which is related to the number of eggs the female is going to lay (Slagsvold 1982; independent variable) and (2) female quality and reproductive investment (dependent variables). Finally, we distinguished between the effect of male tail length and amount of nest material on female quality and her investment in reproduction. We predicted that, in addition to the effect of male tail length, amount of nest material would explain variation in both laying date and total number of eggs laid in the barn swallow, the partial correlation coefficients being negative for laying date and positive for the total number of eggs. 1437 SOLER ET AL.: SEXUALLY SELECTED SWALLOW NESTS MATERIALS AND METHODS We carried out field work at Badajoz, southwest Spain, during the breeding season of 1995. The study area consisted of agricultural land with scattered groups of trees (de Lope 1983), and the barn swallows bred in rooms in farm houses. In 1997 we studied the relationship between the male’s contribution to nest building and final amount of nest material at Seville, southern Spain. Nest size variables (nest material amount and nest cup volume) did not differ significantly between these two areas. Early during the breeding season we captured adults by using mist nets placed across windows and doors of their breeding rooms. When a new bird was seen in the area, we caught it 1 or 2 days later. We measured tail length to the nearest mm and ringed each bird with a metal ring and a combination of coloured plastic rings. Barn swallow nests may persist for many years and high-quality nests from previous years may be refurbished and reused (Møller 1994). In this study we only considered pairs that built a new nest in 1995 or 1997 and that used the same nest during the whole breeding season. These pairs should be representative of the population because, for example, in the Badajoz population (1995) we have not found significant differences between the pairs included in this study and the rest of the pairs for laying date (nests in this study: X&SE=49.86&3.34, N=14; other nests: 43.95&1.52, N=97 (date 1 corresponding to 15 February), t test after log10 transformation of variables: t109=1.69, P=0.09) or male tail length (males in this study: X&SE=96.48&2.88, N=14; other males: 99.87&0.73, N=96, t test after log10 transformation of variables: t108=1.71, P=0.09). We measured inner diameter, external diameter and height and depth of the nests to the nearest 0.5 cm. We calculated nest volume and nest cup volume as a quarter of the ellipsoid determined by the measured radii by the equation

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@inproceedings{JOSEu1998NestBI, title={Nest building is a sexually selected behaviour in the barn swallow}, author={JUAN JOSEu and Florentino de Lope}, year={1998} }