Staffan Bensch, Dennis Hasselquist 
Molecular Population Biology, Department of Ecology, Lund University, Sweden. 

The great reed warbler Acrocephalus arundinaceus breeds in highly productive lakes and reed marshes over a large part of the Palaearctic temperate region. It is a long distance migrant that spends the winter in tropical Africa. The population of great reed warblers at Lake Kvismaren has been studied since 1983 whereafter almost all males, females and nestlings have been ringed. The frequency of polygyny in great reed warblers is one of the highest among European passerines. On average, 40% of the males have at least two females simultaneously breeding on their territories. 

Males arrive to the breeding area about two weeks before the females. Unmated males and males trying to attract additional females sing at high intensity and use long song phrases and this song type has been called long song (picture and sound, 1,1 Mb, picture and sound 500kb).  Newly paired males sing short song as they follow and guard their female when she moves around in his territory searching for a nest site and nest material (Hasselquist & Bensch 1991). Females seem to make an active choice of pair mate and those settling with already mated males have in most cases also visited the territories of unmated males (Bensch & Hasselquist 1992). In an experiment with females tagged with radiotransmitters, we found that they visit the territories of three to twelve males before choosing a breeding partner. Females that mate with already mated males receive little assistance from the male in feeding young (Bensch & Hasselquist 1994) and this seems to result in starvation of nestlings (Bensch 1996). However, random nest predation reduces the cost to females settling with already mated males (Bensch & Hasselquist 1991a) because when the nest of a primary female fails the male will re-allocate his assistance to the nest of the secondary female (Bensch 1996). In addition, secondary females seem to commit infanticide at nests of primary females in order to take over the role as the assisted female (Bensch & Hasselquist 1994). By  carrying out an experiment with artificial nests containing soft clay eggs we have found support for infanticide being a frequently adopted strategy among female great reed warblers (Hansson et al. 1997). Females that choose to breed with already  mated males enjoy similar fledging and recruitment success as females that choose to breed with unmated males, and this supports the view that the polygyny threshold model can explain this polygynous mating system. 
 
 

Male mating success is mainly governed by territory quality (Bensch & Hasselquist 1991b). We have used DNA-fingerprinting to show that most females (95%) are faithful to their males and only 3% of the chicks are the result of extra-pair copulations (Hasselquist et al. 1995). Interestingly, females that engage in extra-pair copulations do so with males that have a more variable song than their social male (Hasselquist et al. 1996). The fitness gain seems to be increased survival of young to adulthood as  male song repertoire size correlates with the relative post-fledging survival of offspring. Females engaging in extra-pair copulations obtain no direct benefits from the cuckolderer. This suggests that good singers are attractive because they carry genes that promote fledgling survival. 
The great reed warbler initially started to breed at Kvismaren in 1978. The following years the breeding population increased to reach a level of 20-30 males and 25-35 females. Since 1987 we have collected blood samples from almost all individuals in the population at Kvismaren. By carrying out DNA fingerprinting we found that parents sharing a high proportion of DNA fingerprinting bands experienced elevated levels of hatching failure (Bensch et al. 1994). Since we know the pedigrees for a substantial fraction of the birds we could rule out the possibility that this resulted from close inbreeding (i.e. half-sib or closer). Rather, we believe that recent population bottlenecks have increased the relative frequency of deleterious alleles and that inbreeding, through non-incestuous effects, in fact is much stronger than the pedigree data suggest. The mean band sharing value (DNA fingerprinting profiles) in the population decreased from 0.43 in 1987 to 0.27 in 1993 (unpublished) indicating that the population is currently recovering from the bottleneck. 

Current research aims at investigating the importance of the major histocompatibility complex (MHC) on mate choice and inbreeding (Helena Westerdahl), the relation between dispersal and inbreeding and effects on lifetime fitness (Bengt Hansson, Staffan Bensch),  the relationship between phenotypic quality and immunocompetence (Dennis Hasselquist), and male song (Dennis Hasselquist, Staffan Bensch). 
 

• Bensch, S. 1996. Female mating status and reproductive success in the great reed warbler: is there a potential cost of polygyny that requires compensation? Journal of Animal Ecology 65:283-296.
• Bensch, S. & Hasselquist, D. 1991a. Nest predation lowers the polygyny threshold: a new compensation model. American Naturalist 138: 1297-1306.
• Bensch, S. & Hasselquist, D. 1991b. Territory infidelity in the polygynous great reed warbler Acrocephalus arundinaceus: the effect of variation in territory attractiveness. Journal of Animal Ecology 60: 857-871.
• Bensch, S. & Hasselquist, D. 1992. Evidence for active female choice in a polygynous warbler. Animal Behaviour 44: 301-311.
• Bensch, S. & Hasselquist, D. 1994. Higher rate of nest lost among primary than secondary females: infanticide in the great reed warbler? Behavioral  Ecology and Sociobiology 35:309-317.
• Bensch, S., Hasselquist, D. & von Schantz, T. 1994. Genetic similarity between parents predicts hatching failure: non-incestuous inbreeding in the great reed warbler. Evolution 48:317-326.
• Hansson, B., Bensch, S. & Hasselquist, D. 1997. Infanticide in great reed warblers: secondary females destroy eggs of primary females. Animal Behavior 54, 297-304
• Hasselquist, D. and Bensch, S. 1991. Trade-off between mate guarding and mate attraction in the polygynous great reed warbler. Behaviour Ecology and Sociobiology 28: 187-193.
• Hasselquist, D., Bensch, S. & von Schantz, T.1995. Low frequency of extra-pair paternity in the polygynous great warbler. Behavioral Ecology 6:27-38.
• Hasselquist, D. Bensch, S. & von Schantz, T. 1996. Correlation between male song repertoire, extra-pair paternity and offspring survival in the great reed warbler. Nature 381: 229-232.

This page is maintained by Mats Grahn Mats.Grahn@zooekol.lu.se 
Last edited on 4 December 1997