Extrapair Fertilization in Monogamous Bull-Headed Shrikes Revealed by DNA Fingerprinting

We analyzed parentage in a wild population of the Bull-headed Shrike (Lanius bucephalus) by DNA fingerprinting. Male Bull-headed Shrikes pair with a single female and defend a large all-purpose territory. Generally, both parents provide substantial parental care. Among 99 nestlings from 24 clutches, 10 (10%) nestlings from 4 (17%) clutches had extrapair paternity, but were offspring of the females. These results show that copulation frequency is not necessarily a good measure of reproductive success even in monogamous birds where no extrapair copulations are observed. The average band-sharing proportion between full siblings was the same as that between parent and offspring, but the variance was greater. This is likely the case because the band-sharing proportion reflects the gene-sharing proportion. Received 6 May 1991, accepted 9 February 1992. OvER 90% of birds are thought to be monogamous (Lack 1968), and males of these species employ mate guarding to avoid being cuckolded (M0ller 1985). Nevertheless, recent evidence from field observations and genetic analyses has clearly demonstrated that extrapair copulations (EPCs) and extrapair fertilizations (EPFs) occur in monogamous species. Among colonially breeding species of monogamous birds like Bank Swallows (Riparia riparia; Beecher and Beecher 1979) and Cattle Egrets (Bubulcus ibis; Fujioka and Yamagishi 1981), EPCs occur frequently. In Purple Martins (Progne subis; Morton et al. 1990) and Zebra Finches (Taeniopygia guttata; Birkhead et al. 1990), EPCs often result in fertilization, and intraspecific brood parasitism (ISBP) has been documented. In House Sparrows (Passer domesticus), which have a loosely colonial breeding system, up to 8% of the offspring were not fathered by their putative male parent (Wetton et al. 1987). EPCs have been recorded even in species with large all-purpose territories, where individuals are relatively difficult to observe. In Indigo Buntings (Passerina cyanea), EPCs often resulted in EPFs (Westneat 1987a, b), while in monogamous Willow Warblers (Phylloscopus trochilus), EPCs resulted in few EPFs (Gyllensten et al. 1990). These results show that copulation frequency is not necessarily a good measure of reproductive success. However, there are only a few genetic studies of strongly monogamous and territorial species; all have found low frequencies of EPFs. The Bull-headed Shrike (Lanius bucephalus) has a strongly monogamous breeding system and has a large all-purpose territory (radius ca. 100 m; Yamagishi 1982b). No EPCs have been observed (Yamagishi and Saito 1985). In monogamous birds for which no EPCs have been observed, there are few studies involving the determination of parentage; an exception is an analysis of Dunnocks (Prunella modularis) under a monogamous system (Burke et al. 1989). Their study showed that in monogamous pairs no EPFs occurred, so that the reproductive success of a pair was related to the number of fledged young. To test if this was also the case for Bull-headed Shrikes, we looked for evidence of EPFs and ISBP using DNA fingerprinting, which can be a powerful tool for determining parentage (Jeffreys et al. 1985b). STUDY AREA AND METHODS Study area and capture techniques.-The study was conducted from March to June 1989 in Oizumi city park, Osaka Prefecture, central Japan (34?34'N, 135?32'E). The park covers an area of about 70 ha, is about 20 m in elevation, and is planted mainly with oaks (Quercus). The incubating female and/or the female and the male that fed their nestlings were captured with a mist net, and were regarded as the putative father and mother of the brood. All males that produced extrapair offspring were captured during incubation or nestling period (i.e. when their female mate was not fertilizable, therefore, a capture of the male did not affect our results). Age determination was based on the coloration of the wing coverts (Yamagishi 1982a). DNA fingerprinting.-DNA was prepared from 100 ,ul blood samples collected by jugular venipuncture