Social organization and communal breeding in the chestnut-bellied starling (Spreo pulcher)

Chestnut-bellied starlings (Spreo pulcher) live in social groups of 10–30 individuals and during their breeding seasons maintain group ranges which show little overlap with neighbouring groups. A social group may contain two to six breeding pairs, non-breeding adults of both sexes, and immatures. Each breeding female has her own nest and she alone incubates. The parents and up to 12 other starlings feed the nestlings. Individual helpers may successively or simultaneously attend the nests of different breeders. The percentage of nests attended within a group differs for helpers of different sex, age and breeding status: immatures of 1–2 years and non-breeding adult males help most and adult females least. Nests with more helpers have higher fledging success than those with fewer helpers. These results are discussed with reference to the tentative benefits of helping behaviour and kinship relationships within the social group.     

The Cooperative Breeding System of the Red-cockaded Woodpecker

The breeding system of the red-cockaded woodpecker is described based on data collected over six years from a population of 500 marked individuals in the Sandhills of North Carolina. Male-female pairs were the most common social unit (59%), but 30% of social units contained one or more adult helpers, and 11% consisted of solitary males. Helpers were almost exclusively male: 27% of males remained in their natal group as helpers for at least one year, whereas only four (1%) females did. Most breeding females remained as breeders in the same group from one year to the next (56%), but a surprising number (12%) moved to another group. Many movements were related to incest avoidance or mate death, but 39% involved deserting a mate, usually following successful reproduction. We suggest that females sometimes are forced from groups by immigrants or other group members. The median distance of movements by adult females was only 1.3 km. In contrast to females, no breeding males switched groups. Survival of both breeding (76%) and helper (80%) males was higher than that of breeding females (69%). Males exhibited two distinct life-history strategies. Some remained as helpers on their natal territory for one or more years, and became breeders by inheriting breeding status in the natal group (17% per year) or by replacing a deceased breeder in a nearby group (13% per year, median distance moved 1.0 km). Other males dispersed from their natal group permanently during their first year. Some of these males were floaters at age one year, others were solitary, and a few became helpers in a non-natal group, but many were breeders. In contrast to males that first functioned as helpers, those that dispersed after fledging moved long distances (median dispersal distance 4.5 km), longer even than dispersing female fledglings moved (median distance 3.2 km). The habitat saturation model of the evolution of cooperative breeding is based on selection between the two life-history strategies exhibited by male red-cockaded woodpeckers. The model therefore may be tested directly with this species. Another indication that this model is appropriate for this species is the existence of a resource (cavity trees) that might provide an ecological basis for habitat saturation.     

Male-biased sex ratios in broods of the cooperatively breeding bell miner Manorina melanophrys

We examined the sex ratios of adults and nestlings in the cooperatively breeding bell miner Manorina melanophrys . Males were over-represented among helpers (mean of 6.8 male helpers per nest compared to 0.3 female helpers). 58% of nestlings sampled were identified as male using a molecular genetic marker. This was a significant departure from parity, yet the magnitude of the bias varied between years. The beneficial and male-biased nature of helping behaviour in this species and the similar size of male and female nestlings suggest the net cost of raising males is lower than the cost of raising females. Consequently, the male-biased sex ratio of nestlings we observed is consistent with the predictions of the repayment hypothesis that females may bias the production of their young towards the more helpful sex. Difficulties of generating quantitative predictions from repayment models that can be tested in the field are discussed.     

Sex-bias and timing of natal dispersal in cooperatively breeding Puff-throated Bulbuls Alophoixus pallidus

While natal dispersal can have a significant impact on population dynamics, it is typically difficult to quantify. We investigated timing of natal dispersal of the cooperatively breeding Puff-throated Bulbul Alophoixus pallidus in a tropical evergreen forest by modelling the probability of staying in or dispersing from their natal territory whilst taking into account the effects of sex, group size, and the presence of helper(s). Birds did not disperse until the beginning of and during the breeding season following the hatching year. Dispersal was strongly female-biased both in frequency and distance: most females (95%) dispersed away from their natal territories, and of those relocated, traversed 2–7 territories. In contrast, 50% of males remained in the natal territory as helpers in their second year, while relocated dispersing males crossed 1–2 territories. Natal dispersal was not influenced by either group size or the presence of helpers. Males that fledged earlier in the breeding season exhibited higher rates of philopatry than the males that fledged later, but no correlation between fledging date and philopatry was observed in females. The probability of staying in the natal territory during the second year was 0.58 ± 0.14 SE and 0.05 ± 0.04 for males and females, respectively. These findings may add to our understanding of how natal dispersal can reflect social patterns and kin structure in cooperative breeding species from a little-studied tropical forest region.     

Reverse sex-biased philopatry in a cooperative bird: genetic consequences and a social cause

The genetic structure of a group or population of organisms can profoundly influence the potential for inbreeding and, through this, can affect both dispersal strategies and mating systems. We used estimates of genetic relatedness as well as likelihood-based methods to reconstruct social group composition and examine sex biases in dispersal in a Costa Rican population of white-throated magpie-jays ( Calocitta formosa , Swainson 1827), one of the few birds suggested to have female-biased natal philopatry. We found that females within groups were more closely related than males, which is consistent with observational data indicating that males disperse upon maturity, whereas females tend to remain in their natal territories and act as helpers. In addition, males were generally unrelated to one another within groups, suggesting that males do not disperse with or towards relatives. Finally, within social groups, female helpers were less related to male than female breeders, suggesting greater male turnover within groups. This last result indicates that within the natal group, female offspring have more opportunities than males to mate with nonrelatives, which might help to explain the unusual pattern of female-biased philopatry and male-biased dispersal in this system. We suggest that the novel approach adopted here is likely to be particularly useful for short-term studies or those conducted on rare or difficult-to-observe species, as it allows one to establish general patterns of philopatry and genetic structure without the need for long-term monitoring of identifiable individuals.     

Mating system, philopatry and patterns of kinship in the cooperatively breeding subdesert mesite Monias benschi

In the first molecular study of a member of the threatened avian family, Mesitornithidae, we used nine polymorphic microsatellite loci to elucidate parentage, patterns of within-group kinship and occurrence of extra-group paternity in the subdesert mesite Monias benschi, of southwest Madagascar. We found this cooperatively breeding species to have a very fluid mating system. There was evidence of genetic monogamy and polygynandry: of the nine groups with multiple offspring, six contained one breeding pair with unrelated helpers and three contained multiple male and female breeders with related helpers. Although patterns of within-group kinship varied, there was a strong positive relationship between group size and relatedness, suggesting that groups form by natal philopatry. There was also a strong positive correlation between within-sex and between-sex relatedness, indicating that unlike most cooperatively breeding birds, philopatry involved both sexes. In contrast to predictions of kin selection and reproductive skew models, all monogamous groups contained unrelated individuals, while two of the three polygynandrous groups were families. Moreover, although between-group variation in seasonal reproductive success was related to within-group female relatedness, relatedness among males and between the sexes had no bearing on a group's reproductive output. While kin selection may underlie helping behaviour in females, factors such as direct long-term fitness benefits of group living probably determine helping in males. Of the 14 offspring produced by fully sampled groups, at least two were sired by males from neighbouring groups: one by a breeding male and one by a nonbreeding male, suggesting that males may augment their reproductive success through extra-group paternity.     

THE PRESENT AND PAST DISTRIBUTION OF THE BALD IBIS IN THE PROVINCE OF THE CAPE OF GOOD HOPE

Tarboton, W. R. 1981. Cooperative breeding and group territoriality in the Black Tit. Ostrich 52:216-225.

In a small, colour-ringed population of Black Tits Parus niger in central Transvaal, 11 of 19 observed breeding units comprised pairs with one to three helper-males. These pairs and groups defended permanent territories, the size of which correlated with the size of the group. There were significantly more territorial disputes during winter when less food was available than in summer. Breeding occurred in summer and the female alone built the nest, incubated the eggs and brooded the young while they were small. During this time she was fed by the alpha male and helper males, although before egg-laying the alpha male prevented helpers from courtship-feeding her. On average, unassisted pairs reared 0,88 young/season whereas pairs with helpers reared 1,55 young/season. However the feeding rate of nestlings of pairs with helpers was not higher than that of unassisted pairs and the number of young reared per group did not correlate with the number of helpers within the group.

The helper system in Black Tits was associated with a skewed sex-ratio (1,7:1 males: females) in the adult population and the data are consistent with the “hopeful reproductive” hypothesis for cooperative breeding.     

Male Seychelles warblers use territory budding to maximize lifetime fitness in a saturated environment

In cooperatively breeding species, helping at the nest and buddingoff part of the natal territory have been advanced as strategiesto increase fitness in an environment that is saturated withterritories. The importance of helping or territory buddingas a determinant of lifetime reproductive success (LRS) hasbeen debated because the potential benefits of both strategiescould not be separated. Here we test the causes and the immediateand future fitness consequences of single dispersal decisionstaken by male Seychelles warblers (Acrocephalus sechellensis).Males breeding in high-quality territories (high food abundance)have significantly higher LRS than similar-aged males buddingoff part of the parental territory. Initially, budders havea low reproductive success (because of limited food resourcesor absence of a breeding partner). However, they have a longlife span and inherit high-quality territories through sitedominance, by which they gain higher LRS than breeders on low-qualityterritories, helpers, or floaters. Experimental creation ofmale breeding territory vacancies showed that most young malesbecame budders because of intense competition for high-quality territories. The translocation of warblers to the previouslyunoccupied Aride Island shows that males behave according tothe expected fitness benefits of each dispersal strategy. Inthe absence of competition for territories on Aride, all youngmales bred in high-quality territories. However, after saturationof high-quality habitat with territories, most males becamebudders rather than breeders on low-quality habitat, helpers,or floaters.     

Cooperative enhancement of reproductive success in white-winged choughs

Summary White-winged choughs are a cooperatively breeding species which provide parental care to their young over an entire year. I traced the reproductive success of groups of white-winged choughs from the start of one breeding season to the next over 3 years. I examined the effect of helper number on timing of breeding, the success of each effort, the number of efforts made in a season, and the final reproductive success at the end of each year. Timing of commencement of breeding varied between years but was not related to group size. Early broods were not more successful than late broods. Nest building (July–September) commenced earlier in years which had high rainfall in July; choughs rely on rainfall for supplies of mud for nest construction. Most nest failures occurred gradually and were attributed to starvation of nestlings, although some sudden failures were attributed to predation. Large groups have more young by the beginning of the following season; this is due to higher fledging success and a greater likelihood of having second broods. Disappearance of young after fledging and during the transition to independence was not dependent on group size. Only groups of seven and above produce more than one young on average over the entire year; choughs provide one of the most marked cases for helpers enhancing the reproductive success of breeders. Large groups are virtually guaranteed of reproductive success over the whole year and grow more quickly than small groups. These results highlight the need to consider the effect of helpers over the entire period of reproduction and care of young, rather than just at fledging.     

Group structure,within-group conflict and reproductive tactics in cooperatively breeding Galápagos mockingbirds,Nesomimus parvulus

Reproductive conflict within groups can be an important feature of cooperative breeding systems, especially when more than one individual of a sex breeds within a social group. Relationships between group structure, dominance, within-group conflict and reproductive tactics of cooperatively breeding Galápagos mockingbirds were examined on Isla Genovesa. Territorial groups of 2–24 adults included up to three breeding females, with 42% of the groups containing more than one (plural groups); females in most plural groups nested separately. Territory size increased with group size, but the area available per pair in plural groups was smaller than in singular groups (groups with only one breeding pair). Most pairings were monogamous, and males usually outnumbered females; high-ranking males obtained mates more frequently than subordinate males. In 3 relatively dry years, but not in a wet El Niño year, subordinate pairs in plural groups fledged fewer young than dominant pairs or pairs breeding in singular groups. Interference by dominant breeders, often leading to abandonment of nests by subordinate pairs, appears to account for these differences: through nest disruption in drier years, dominant individuals may reduce the cost of sharing their territories and increase the chances of recruiting helpers. Dominant males in plural groups may also father young through extra-pair copulations with subordinate females. Despite costs imposed by within-group conflict, subordinate breeders have higher long-term reproductive success than birds that defer breeding. Plural group structure is maintained because unpredictable climatic variation favours opportunistic breeding by subordinates.     

Reproductive biology of captive male cottontop tamarin monkeys as a function of social environment

The cottontop tamarin, Saguinus oedipus oedipus, is a cooperatively breeding monkey in which mature male and female offspring serve as helpers to assist in rearing younger siblings. Generally, only one female per social group reproduces; breeding restriction is mediated in postpubertal female offspring through low and acyclic levels of reproductive hormones. We investigated (1) reproductive activity of postpubertal male offspring, and (2) whether aggression towards male offspring and a cortisol-mediated stress response might restrict breeding of male offspring in the natal group. We examined sexual behaviour, olfactory communication and urinary hormone levels (testosterone, dihydrotestosterone, luteinizing hormone, cortisol) of the subject males while we manipulated their social environment from housing in natal groups to pairing with a novel female, and after the production of their own offspring. Mounting and erection rates of the male subjects were as high in the natal group as when paired with a novel female. However, most mounts in the natal group were directed towards other males, and complete copulation sequences did not occur with natal-group females. Social environment had no significant effect on olfactory investigation of breeding females. Although hormone levels increased significantly after the subjects were removed from the natal group, the elevation was transient; the hormone levels of subjects in their natal groups did not differ from the levels shown by the same males when successfully producing their own offspring. Male offspring received more contact aggression in the natal group than when paired with the novel female. However, most of the aggression was received from siblings rather than the breeding pair, and levels of cortisol did not correspond with levels of aggression. Thus, at both a behavioural and endocrine level, mature male offspring in captive natal groups were potentially fertile, but sexual activity with natal-group females appeared to be behaviourally restricted and directed instead towards group males. In wild cottontop tamarin groups, this reproductive potential may allow male helpers flexibility to respond to breeding opportunities. Copyright 2001 The Association for the Study of Animal Behaviour.     

Complex sex allocation in the laughing kookaburra

In groups of the cooperatively breeding laughing kookaburra(Dacelo novaeguineae), offspring sex varied with the type ofsocial group and with hatch rank. Groups with female helpers,especially if all helpers were female, had male-biased clutchand fledging sex ratios. Groups without female helpers (unassistedpairs or male-only helpers) had female-biased clutch and fledgingsex ratios. Breeding females responded facultatively to increasesin the number of female helpers in their group by producingmore male eggs. These biases may occur if breeding femalestry to limit the number of daughters recruited into their groupbecause unlike male helpers, female helpers depress the breedingsuccess of their parents. Across all nests, two-thirds of first-hatchedyoung were male, two-thirds of second-hatched young were female, and the sex ratio of third-hatched young was even. Hatch ranksex ratios also varied dramatically between different typesof social groups, from 16.7% for second-hatched nestlings ofunassisted pairs to 100% for first-hatched nestlings of groupswith only female helpers. A corollary of the relationship betweenhatch rank and sex was that hatching sex sequences were distributed nonrandomly: all groups avoided hatching a daughter first followedby a son (FM). Sibling competition is aggressive and sometimesfatal. Since females grow to be 15% larger than males the hatchingsequence of sexes could affect nestling growth and mortality.However, an exhaustive analysis found little evidence thatgrowth or survival of males was compromised if hatched aftera sister. The small number of FM sequences may only have occurredin nests that were able to ameliorate any negative consequences.Alternatively, when clutch size is small and fledging successunpredictable because of brood reduction, the preferred broodsex ratio may be contingent on the number of fledged young,making it advantageous to order the sexes in the brood.     

Helper contributions in the cooperatively breeding laughing kookaburra: feeding young is no laughing matter

I studied the contributions of individuals to incubation and nestling feeding in a population of cooperatively breeding laughing kookaburras, Dacelo novaeguineae. In most cooperatively breeding birds where nest success is limited by nestling starvation, related helpers increase the overall level of provisioning to the nest, thus boosting the production of nondescendent kin. However, although partial brood loss is the largest cause of lost productivity in kookaburra nests, additional helpers failed to increase overall provisioning. Instead, all group members, but especially helpers, reduced their feeding contributions as group size increased. Breeders and helpers reduced the size of prey delivered, and helpers also reduced the number of feeding visits. An important benefit of helping in kookaburras may be to allow all group members to reduce their effort. Within groups, contributions to care depended on status, sex, group size and the brood size. Breeding males delivered the most food. Breeding females provisioned less than their partner, but their effort was comparable to that of male helpers. Female helpers contributed the least food. Incubation effort followed similar patterns. The relatedness of helpers to the brood had no impact on their provisioning. Across all group sizes, helpers generally brought larger items to the nest than breeders. Copyright 2000 The Association for the Study of Animal Behaviour.     

Shared parentage and incest avoidance in the cooperatively breeding acorn woodpecker

Social groups of acorn woodpeckers (Melanerpes formicivorus) range in size from unaided pairs to 15 adults. Behavioural indicators of mate guarding, assumed incest avoidance and observations of egg-laying indicate that social organization ranges from monogamous pairs to groups with up to seven male and three female putative cobreeders plus up to 10 nonbreeding helpers. In addition, groups occasionally lack a putative breeder throughout the breeding season. Here we report results from multilocus DNA fingerprinting of 372 nestlings from 123 nests in groups with putative cobreeders of one or both sexes. No extra-group fertilizations were found. Putative cobreeding males within social groups shared paternity. However, the most reproductively successful male was, on average, almost three times as successful as the next most successful and additional males only occasionally sired offspring. In contrast, cobreeding females shared parentage equally. Helpers never bred incestuously when their opposite-sex parent (or another relative, such as their uncle) held breeding status in the group. However, during breeding male vacancies, 14 nestlings were produced when helper males bred incestuously with their mother. Both male and female helpers usually became successful cobreeders with their same-sex parent following replacement of the opposite-sex breeder(s) by unrelated individuals.     

Testis size depends on social status and the presence of male helpers in the cooperatively breeding cichlid Julidochromis ornatus

Cooperatively breeding animals, in which helpers may participatein reproduction with dominant breeders, are ideal species forexamining intraspecific variation in testis size because theyoften exhibit both monogamous breeding (low risk of sperm competition)and polyandrous breeding (high risk) within a population. However,little is known about testis investment as a result of spermcompetition in these animals. The substrate-brooding cichlidfish Julidochromis ornatus has a cooperatively breeding system,in which some males mate monogamously and other males reproduceas dominant breeders or helpers within cooperatively breedinggroups, in which male helpers frequently sire young. We examinedthe relationship between testis investment and male social statusin relation to the risk of sperm competition. As predicted fromsperm competition models, in groups with male helpers, boththe male breeders and the male helpers invested more in testesmass, compared to breeding males without male helpers. We alsofound a positive relationship between the testes mass of malebreeders and their male helpers, suggesting that males increasetheir investment in reproductive capability under the risk ofsperm competition. Sperm competition models also predict thatlarger testes are associated with increased siring success.Our paternity analysis supported this prediction; we found apositive relationship between testis investment by male helpersand the number of offspring they sired.     

Resource availability, breeding site selection, and reproductive success of red-winged blackbirds

Red-winged blackbirds are polygynous and show strong breeding site preferences, but it is unclear which environmental factors regulate their reproductive success and are ultimately responsible for shaping their patterns of habitat selection and their mating system. We evaluated the effect of variation in insect emergence rates on the reproductive success of male and female redwings nesting on replicate ponds. The number of male and female redwings that settled on a pond varied two- to three-fold among ponds, but was not related to insect emergence rates. Insect emergence rates had a positive effect on the number of nestlings successfully fledged by females, the number of nestlings fledged from male territories, and on the mass of nestlings at fledging. Typha stem density also varied widely among ponds, and was positively related to male and female settling density and mass of nestlings at fledging, but not to the number of nestlings fledged by females or males. We conclude that alternative breeding sites differ in their ability to support redwing reproduction, and that the availability of emerging odonates is an important environmental factor influencing the reproductive success of both male and female red-winged blackbirds. Received: 31 March 1997 / Accepted: 3 July 1997     

Cooperative Breeding and Group Structure in the Lake Tanganyika Cichlid Neolamprologus savoryi

As yet, cooperative breeding has been described only for some fish species. However, evidence is accumulating that it is widespread among Lake Tanganyika cichlids. We studied the cooperative breeding system of the substrate breeding cichlid Neolamprologus savoryi. Breeding groups typically consisted of a large breeding male with one to four breeding females and three to 33 helpers (mean group size: 14.3 members). Group size was significantly related to breeding male and female body sizes, and larger males had more breeding females and larger sized male helpers. The size of the largest female in the group was positively related to the number and sizes of secondary breeding females and female helpers. In case of multiple breeding females, these females usually divided the group's territory into sub‐territories, each with its own helpers (subgroups). Interspersed between groups, independent fish were detected defending an individual shelter (4.4% of all fish). In 9% of the groups no breeding female was present. All group members participated in territory defence and maintenance, and showed submissive behaviours to larger group members. As expected, the level of between‐subgroup conflicts was high compared with the level of within‐subgroup conflicts. We compare these results with data available from other cooperatively breeding fishes.     

Family status and contributions to breeding by Florida scrub jays

Nestling care by Florida scrub jay Aphelocoma c. coerulescens breeders and helpers is quantified. Breeding males and older male helpers deliver more food than first-year helpers and older female helpers. Breeding males adjust the quantity of food they deliver inversely to that delivered by their helpers. Older female helpers, who make many nest visits without delivering food, attempt to usurp the nest-sitting role of the breeding female. The advantages of group breeding to the breeders seem clear. Pairs with helpers live longer and produce more fledglings. The advantages of being a helper, though obscure, probably include non-altruistic factors such as increased survival and, for males, increased chances of obtaining space for breeding. Food contributions of helpers seem to parallel these opportunities.     

Reproductive senescence in a cooperatively breeding mammal

1. Senescence (or 'ageing') is a widespread and important process in wild animal populations, but variation in ageing patterns within and between species is poorly understood.
2. In cooperatively breeding species, the costs of reproduction are shared between breeders and one or more helpers. The effects of ageing in breeders may therefore be moderated by the presence of helpers, but there have been very few studies of senescence patterns in natural populations of cooperative breeders.
3. Here, we use 13 years of data from a long-term study population of wild meerkats ( Suricata suricatta ) to investigate age-related changes in several traits known to be key components of reproductive success in females of this species.
4. Four of the six traits studied exhibited significant declines with age, indicating senescence. Litter size, the number of litters produced per year and the number of pups that survived to emergence from the natal burrow per year all increased with female age up to a peak at c. 4 years, and declined steeply thereafter; the mean pup weight at emergence in a given litter declined steadily from age zero.
5. These results provide the first evidence of reproductive senescence in a wild population of a cooperatively breeding vertebrate. Breeding success declined with age despite the sharing of reproductive costs in this species, but further study is needed to investigate whether helping affects other aspects of senescence, including survival.     

Age-Related Changes in Nestling Diet of the Cooperatively Breeding Green Woodhoopoe

In many socially monogamous bird species, parents of altricial young respond to the increasing demands of growing nestlings by increasing their feeding rate and the size of prey items delivered and by altering the types of prey provided. In some cooperatively breeding species, similar changes in feeding rate and prey size have been documented. However, potential changes in the types of prey delivered, both as nestlings age and by different group members, remain largely unexplored. Moreover, studies rarely compare the diet fed to nestlings with that eaten by the provisioning adults themselves. Here, I show that green woodhoopoe ( Phoeniculus purpureus ) nestlings receive a smaller proportion of spiders and larger proportions of caterpillars and centipedes as they grow older. Both male and female adults delivered a higher proportion of spiders to young nestlings than they ate while self-feeding, probably in response to particular nutritional requirements of the chicks. However, only males altered the proportions of caterpillars and centipedes delivered, providing smaller proportions to young nestlings than eaten themselves. These prey items may be too large for young nestlings to handle, and males may make a greater adjustment in provisioning diet than females because they collect more caterpillars and centipedes than do females. Although there were sex differences in provisioning diet, there were no differences between same-sex breeders and helpers in terms of the overall proportions of prey delivered or the changes with nestling age. Hence, individuals of different reproductive status may be following the same provisioning rules, at least in terms of prey type.