Ian Rowley

Promiscuity: an inbreeding avoidance mechanism in a socially monogamous species?

SummaryPaternity likelihood was tested in a population of splendid fairy-wrens Malurus splendens by allozyme electrophoresis. A total of 91 offspring of 24 dams and 37 putative sires were typed at 10 polymorphic loci. All young were compatible with their dams but at least 65% were not fathered by any of the males in their group. A long-term study of this wren population has shown that the males are sedentary, show little evidence of dispersal and help care for the nestlings and fledglings in their group. Had the senior male sired all the offspring in his group, there would have been a high incidence of close inbreeding. The promiscuous mating system demonstrated here would reduce the level of inbreeding in the population but still allow individuals the security of group-living in a stable year-round territory.

Inbreeding: Benefits may outweigh costs

found that budgerigars are capable of resolving amplitude fluctuations taking place as fast as every 2 ms. The species can detect changes in fluctuation rate of about 10% over a range of 40-320 amplitude fluctuations/s. In the kurriet call of the crested tern, as a rule, segment duration lasts longer than 20 ms (70 out of 72 cases), whereas differences in duration of successive segments exceed 10% (88 out of 96 cases). Amplitude fluctuations occur at frequencies of 7-12l/s, whereas differences in fluctuation rate between different parts of the call were in all cases (N=96) far greater than 10%. If the temporal resolving power of budgerigars is taken as representative for birds in general, crested terns can thus be expected to be capable of perceiving the information encoded in the temporal structure of the kurriet call. The analogy between the patterning of kurriet calls and Universal Product Codes suggests a new approach to problems in animal communication. Playback experiments with artificially modified calls may help to find the minimum necessary differences between codes to ensure unequivocal encoding of individual identity. This, in turn, could lead to a better knowledge of the feature-detecting capabilities of the terns. Given the ease of quantifying bar code similarity, the system would also seem to be a very promising one to use in examining inheritance of codes and kin recognition. I thank G. P. Baerends, J. G. van Rhijn, G. Thomas and an anonymous referee for comments on the manuscript. R. D. Wooller and J. N. Dunlop provided the opportunity to work in the crested tern colony on Rottnest Island.

Helper contributions to reproductive success in the splendid fairy-wren (Malurus splendens)

SummaryIn Malurus splendens, helpers were present in 65% of 226 group-years with at least one helper female in 37% of group-years. Most females helped for only one year, while many males did so for at least two years. Most were offspring of one or both present breeders, and in 53% of helper-years, helped both parents. For 159 helpers of known age and parentage, the mean coefficient of relatedness to the offspring was 0.47. Novice females with or without helpers produced fewer fledglings per season than females with one year breeding experience and the same level of help. Helpers did not affect production of fledglings per year by females with one year of experience. Females with two or more years experience and at least two helpers produced more fledglings than equivalent birds with one or no helpers. Experience and helpers have little effect on production of fledglings per nest but they lead to more females renesting after a first brood has been raised. Fewer than 20% of novices renest after fledging one brood, while for females with at least two years experience, the percent renesting after success is 40% with no help, 56% with one helper and 69% with 2 or more helpers. Experienced females begin their first clutch earlier than novices, and helpers reduce the time to renest after success from 66 days for an experienced female with no helpers to 50 days for females with at least two years experience and two or more helpers. Breeding females with helpers survive better (76%) than those with no helpers (55%), and helpers thus gain future indirect fitness. Despite their close relatedness to breeders and offspring, in only 19% of group-years did helpers increase their indirect fitness from an increase in productivity.

The significance of territory size and quality in the mating strategy of the splendid fairy-wren

The role of territory in the splendid fairy-wren Malurus splendens was analysed using data from a long-term study of a wild population in which pairs were socially monogamous, even though more often than not mating was promiscuous. Territory sizes varied both between family groups and within groups from 1 year to the next. Variation between groups was related to habitat quality, while variation within groups was related to the number of males in the group. Large territories shared common boundaries with more neighbours than did small territories. The reproductive success of the group was positively related to both territory size and territory quality, although these variables had no effect on the survival of breeding females or adult males. It is suggested that the frequency of extra-pair copulation (promiscuity) is a function of the age and status of the male, where a male can enhance his status by (i) acquiring his own territory and social partner, (ii) producing helpers who assist him by supplementing his parental and territorial duties and by helping to expand his territory, thereby (iii) allowing him the opportunity to gain access to females outside his group. This results in a variable mating strategy where young males, typically with small territories and no helpers, practise sexual monogamy, despite a high likelihood of cuckoldry, while older males, with large territories and many helpers, are promiscuous. It is suggested that females maintain permanent social pairings because (i) this provides the safety of permanent territory with exclusive use of resources for building nests and raising offspring, (ii) it provides at least one permanent assistant in raising offspring, and (iii) it provides a focal point where she can be found by the most competent neighbouring males. This allows the breeding female the choice of better matings than might be provided by her social partner.

Splendid wren Malurus splendens response to cuckoos: an experimental test of social organization in a communal bird

A population of cooperatively breeding, group-living splendid wrens was tested with a mounted parasitic cuckoo. At all nests with incubated eggs or nestlings, wrens attacked the cuckoo. The timing and intensity of attacks was independent of the nest day and of the age and breeding experience of the wrens. The breeding female usually spotted and attacked the cuckoo first. Her mate and the nonbreeding helpers responded to her call and mobbed and attacked the cuckoo. Response was no quicker in groups with nonbreeding auxiliaries than in single pairs. Discovery time was independent of the number of birds in a group and depended on the movements of the breeding female. Most wrens fed the young and mobbed the cuckoo. When a wren did not attack, it usually was caring for the young of another breeding female or an earlier brood. Variance in helping behavior was not closely associated with variance in the genetic relationship between helper and the breeding female or the young beneficiaries of mobbing. Use of a common territory, attendance at a nest, feeding the young, and mobbing and hitting a cuckoo were all associated cooperative activities. The main limitation of cooperative behavior in defense against the cuckoo is the same as the observed constraint on care of the young during the prolonged period of parental feeding-a conflict of interest among breeding females for care of their own young.

“Philandering” — a mixed mating strategy in the splendid fairy-wren Malurus splendens

SummaryIn a population of the splendid fairy-wren Malurus splendens, we describe 202 intrusions by solitary adult males in full breeding plumage into the territories of conspecifics. Intrusions were not secretive and although silent they involved conspicuous flight above the vegetation and several characteristic display elements, including petal-carrying. Fifteen intrusions were observed in the 2 months before eggs were laid, but the rest occurred during the period of nest-building and egg-laying when the breeding female was presumed to be fertile, and were by males from adjacent territories. A high level of extra-pair fertilization has previously been determined in this species and we interpret many of these intrusions as attempts by males to obtain extra-pair copulations (EPC); we call this activity “philandering”. The costs and benefits of EPC for males and females are discussed and the promiscuous mating strategy of M. splendens is contrasted with the persistent social pairing in stable groups of this long-lived cooperatively breeding species.

The Purple-crowned Fairy-wren Malurus coronatus. II. Breeding Biology, Social Organisation, Demography and Management

Two populations of the Purple-crowned Fairywren Malurus coronatus were studied between 1982 and 1987. Both subspecies were represented: M. c. coronatus (Kimberleyan) at Drysdale River Crossing, Western Australia, and M. c. macgillivrayi (Carpentarian) at Riversleigh Station on the Gregory River, Queensland. Using playback of a territorial call, 206 birds were caught and individually colour-banded. At both locations, progeny remained with their parents after reaching maturity and helped to raise siblings; such cooperative breeding is a feature of the genus Malurus. Both males and females called loudly and frequently duetted. Territories were maintained throughout the year and were usually 200-300 m long in linear succession, embracing both banks of the river. Breeding can probably take place throughout the year but was commonest early or late in the dry season (March-September). Nests (25) were large, usually placed at the base of a Pandanus aquaticus leaf 0.25-2.5 m from the ground. Our few data suggest that the clutch size is 2-3, incubation 14 days and nestlings fledge when about ten days old. Competition with other species was slight for these specialist insectivores foraging at ground level or through dense cover, seldom more than 5 m from water. Annual productivity varied from 0.22 to 1.95 yearlings per group depending on season; the mean was 0.78 at Drysdale and 1.04 at Riversleigh. Survival of juveniles and adults was high; a third of yearlings reached four years old and 70% of breeders survived from one breeding season to the next. Our data suggest that the species will be slow to recover from devastation or to colonise vacant areas. They depend on a very specialised habitat that is easily damaged by stock or fire. Loss of this habitat could lead to local extinction of the species and management should focus on maintaining the riverside vegetation as a renewable resource.

High levels of extra-group paternity in a population of Australian magpies Gymnorhina tibicen: evidence from microsatellite analysis

Breeding systems vary widely in birds, from monogamous pairs through to complex group systems where subordinates assist breeding individuals to rear young each season. The Australian magpie varies geographically both in plumage patterns and social organization. Some populations of both eastern and western plumage forms are plural breeders with group size varying from three to over 15 mature individuals. This study used variation at microsatellite loci to determine the level of extra‐group paternity in a population of the western form near Perth in Western Australia. Extra‐group paternity was the highest recorded for any bird species to date (82%) and indicates that few offspring within a territory are sired by the social partner of the female. In addition, the data indicated that nearly 10% of juveniles were not the genetic offspring of any female within their territory, suggesting some intraspecific brood parasitism. Taken together, these findings are remarkable considering the highly territorial nature of the species and the extent of territorial defence practised by all members of the group towards extra‐group conspecifics during daylight hours.

The Ecology and Breeding Biology of the Red-winged Fairy-wren Malurus elegans

In 33.5 ha of Kam forest in Smiths Brook Reserve, 841 individually colour-banded Malurus elegans, a Western Australian endemic, were studled from 1980-1986 in 24-30 territorial groups. This species is multi-brooded but less so than other Malurus due to their short breeding season; 90% of eggs were laid in October and November. Most nests (85%) were within 300 mm of the ground and more than half in accumulated litter on the forest floor, nest sites liable to be lost following fuel-reduction burning. Clutches of two and three were equally common (mean = 2.43 eggs); fertility was 94%; incubation lasted 14-15 days; nestlings hatched synchronously and fledged 11-12 days later, juveniles were independent of adult provisioning when one month old. Parasitism by cuckoos was rare and predation variable between years. Of eggs laid, 79% hatched and 52% produced fledglings. Repeat nesting was rapid and groups produced an average of 2.5 fledglings and 1.9 independent young per year. M. elegans are co-operative breeders; 82% of groups had helpers, the mean size being four adults. Both adult and juvende survival were high and the limited forest habitat was fully occupied.

Demography and social organisation of the red-winged fairy-wren, Malurus elegans

The red-winged fairy-wren, Malurus elegans, is endemic to the high-rainfall region of south-western Australia. We studied it in Eucalyptus diversicolor (karri) forest near Manjimup, Western Australia from 1980 to 1995. After a detailed study of breeding biology during 1980–86, we monitored dispersal and survival in known groups during 1987–95. M. elegans bred cooperatively, with 83% of groups (mean size 4.1) including one or more non-breeding males or females that helped to rear young and defend the territory. Survival of breeding adults (78%) and helper males (76%) was high. Territories and groups persisted from year to year, even though one or other of the breeding pair was replaced. Most known dispersals were to a group only 1–2 territories distant. Dispersal was female-biased, mostly in their third or fourth year. A behaviour not recorded in other Malurus spp. was that some birds, chiefly females, joined groups as helpers. The feeding rate of nestlings was not related to group size, but in larger groups the share of work done by the breeding female decreased. Helpers did not enhance the survival of breeding females, and had little overall effect on the production of fledglings. Females produced a mean of 2.4 fledglings, 1.8 independent young and 1.1 yearlings per year; survival of fledglings to the start of the following breeding season was44.2% (31–61%). We argue that the high levels of adult and juvenile survival influence many aspects of the social system in M. elegans, such as large groups, the presence of female helpers, occurrence of immigrant helpers and delayed dispersal. We suggest that an important benefit of delayed dispersal and group living is in promoting the survival of young birds, and increasing their chance of acquiring a territory.