Helper Contributions Research Articles

Contributions to cooperative rearing in meerkats

In vertebrate societies where young are reared communally, nonbreeding helpers are usually closely related to young but often vary widely in their contributions to feeding them. Evolutionary explanations of helping behaviour have focused on whether differences in the level of contributions between helpers are related to variation in kinship. We investigated the contribution of helpers in meerkats, Suricata suricatta. The helpers varied widely in the number of food items they gave to pups and individual differences were related to variation in foraging success as well as to sex and age. When we controlled the influence of these variables, the level of contributions that helpers made to rearing pups was not significantly correlated with variation in kinship to the litters they were rearing.

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.

Feeding Rates and Individual Contributions to Feeding at Nests in Cooperatively Breeding Western American Crows

Helpers increase the reproductive success of breeders in many species of cooperatively breeding birds. Several mechanisms by which they do so involve the feeding of nestlings. The presence of additional feeders may result in increased amounts of food brought to nests, which can increase growth rates of nestlings (Dickinson et al. 1996) and reduce starvation (Rowley 1978, Reyer 1980, Emlen and Wrege 1991, Heinsohn 1992). Alternatively, helpers may not increase total feeding rates at nests but instead reduce the workload of one or both breeders. Lightening the load of breeders may translate into increased success in several ways. Relief of feeding responsibilities may allow breeders to devote more time to vigilance and antipredator behavior (Rabenold 1984, Austad and Rabenold 1985). For some species, subsequent survival is higher for members of assisted pairs than for unassisted ones (Stallcup and Woofenden 1978, Reyer 1984, Russell and Rowley 1988). Additionally, in some multiple-brooded species, helper assistance results in shortened interbrood intervals, allowing for the production of more clutches per season (Rowley 1965, Brown and Brown 1981, Rabenold 1984, Russell and Rowley 1988). Here, I describe the feeding contributions made by individuals at assisted and unassisted nests of Western American Crows (Corvus brachyrhynchos hesperis) and examine the effects that the feeding contributions of helpers have on nestling feeding rates. I also examine other factors that could affect feeding rates, and the consequences of variation in feeding rates on breeding success. Methods.-I studied members of a resident population of crows on the Balboa and Encino golf courses in Encino, California, from March 1985 through August 1990. The habitat consisted of tracts of grass separated by rows of trees, with additional clumps of trees scattered throughout the site. The climate was southern Californian Mediterranean, with hot, dry summers and occasional winter rains between December and February. I captured 173 free-flying individuals (63 males, 54 females, 56 unsexed) using large walk-in traps and a cannon net. Trapped crows were weighed, measured, and marked with identical patagial tags bear-

Helping is costly to young birds in cooperatively breeding white-winged choughs

Cooperative breeding among birds is at its most extreme in white-winged choughs ( Corcorax melanorhamphos ). Choughs have never been observed to breed successfully without helpers, and reproductive success increases linearly across all group sizes (maximum = 16). Further, helpers contribute to every aspect of reproduction, including nest building and incubation. Here we show that the contribution of young helpers (one year old and less) to incubation depends on the group in which they live. In small groups (3-5 birds), young helpers contribute as much to incubation as older birds, but in large groups they contribute little. In large groups, help increases sharply with age. Old birds contribute equally, regardless of group size. Although choughs generally do not lose body mass over incubation, young helpers lose mass in proportion to the amount of incubation they perform, independent of any effect of group size. This provides evidence that helpers in cooperatively breeding birds suffer costs from providing help additional to the costs incurred from remaining philopatric. It also demonstrates that the needs of the group influence whether young birds provide help.

The contribution of helpers to feeding nestlings in grey-capped social weavers, Pseudonigrita arnaudi

Abstract. Data from feeding watches at nests of communally breeding grey-capped social weavers were used to examine the factors influencing feeding visit frequencies, food item size and the rate of food supply to the nestlings (feeding rate). Provisioning was not affected by environmental factors (time of day, temperature, rainfall, date and year) but nestling age, brood size and the number of birds feeding at the nest all had substantial effects. Having accounted for brood size and nestling age, feeding visit frequency and feeding rate increased when more birds fed, although the mean size of food items declined. The contribution of a single helper significantly increased the feeding rate (corrected for other effects) both within and across nesting attempts, with mean increases of 20% and 7%, respectively. Helpers fed less frequently than the breeding birds but brought items of a similar size. Breeders did not reduce their feeding effort when helped. Helpers were recorded more often in the later part of the nestling period, when nestling food demands were highest. Paired and unpaired helpers made similar contributions, but paired helpers assisted only when they had no young to feed in their own nests. Higher nestling survival and rates of weight increase in nests with helpers seem likely to be a consequence of the extra food supplied.

Dominance Hierarchies and Helper Contributions in Harris' Hawks

We examined dominance hierarchies, mating relationships, and helping by individually marked Harris' Hawks (Parabuteo unicinctus) at 64 nests in Arizona (1984-1988). Dominance hierarchies were linear and always contained an adult female as the most dominant member. We placed hawks into three dominance categories (alpha, beta, and gamma) based on frequencies of supplanting. Pairs without helpers (16%, n = 64) and groups that contained a beta male helper (63%) were the most common social structures. Some groups (11%) contained a second alpha female (alpha-2) that was subordinate to the alpha female and dominant over the alpha male. Of 68 observations of copulatory behavior, 45 (66%) were copulations between alpha males and alpha females. Beta males attempted to copulate with alpha females (23 observations; 34%) but were unsuccessful because of a lack of female compliance. Observations of movements by marked hawks suggested that most gamma helpers were offspring of the alpha pair and that beta males and alpha-2 females were probably unrelated to the alpha pair. Alpha hawks restricted access to the nest by both beta and gamma helpers, but beta males were tolerated closer to the nest than were gamma helpers. Consequently, beta and gamma helpers rarely participated in the direct care of eggs and young but were active in procuring prey, transporting prey to the nest area, and defending the nest from predation by Great Horned Owls (Bubo virginianus).

Breeding Biology of White-Fronted Bee-Eaters at Nakuru: The Influence of Helpers on Breeder Fitness

SUMMARY (1) Among white-fronted bee-eaters (Merops bullockoides) near Nakuru, Kenya, helpers dramatically increased the fitness of breeders. Using 5 years of data, we examine the relative importance of food availability, breeder attributes and group size on reproductive success. (2) Both food availability and size of the group attending the nest had significant effects on productivity, while the characteristics of breeders (age, experience) did not. Helpers had their effect almost entirely through increased productivity per nesting attempt, each helper on average increasing the number of fledglings by 0 50 (+0.06 S.E.). The activities of helpers did not affect either breeder survival or the number of nesting attempts per year. (3) The presence of helpers did not affect clutch size, had no influence on the probability of successful hatching, but dramatically increased the rate of provisioning per nestling. As a consequence of their feeding contributions, helpers significantly decreased both the probability of nestling starvation and the degree of nestling developmental retardation due to food stress. More young fledged, and fledged in better condition, from helped nests. (4) The mechanisms by which helpers have their effects are largely ecologically determined. The ability of bee-eater helpers to increase fledging success through their feeding contribution is related to the environmentally unpredictable, and often harsh, conditions at Nakuru. Nestling starvation rates were high (48% of all hatchlings), whereas predation losses were low (4% of eggs laid). In other, cooperative breeding species where helper feeding contributions have increased nestling growth and survival, sparse or unreliable food resources have also been the rule. In contrast, predation has been implicated as the major source of mortality in many other cooperative breeding species, and it is in these species that the anti-predator activities of helpers have been reported to have an important impact on nesting success. (5) Helper effects of the magnitude recorded in the Nakuru population of beeeaters are unusual. The presence of one helper effectively doubled the fledging success of an unaided pair, and the effect of increasing numbers of helpers was linear across all commonly observed group sizes. This large helper contribution has important implications for the behavioural options of both breeders and helpers. Not only should the presence/absence of helpers influence the decision to breed, but breeders are expected to compete for potential helpers. From the perspective of a potential

Infant-care behavior of mothers and fathers in a communal-care primate, the cotton-top tamarin (Saguinus oedipus).

Infant care behavior was examined for 35 litters of 23 different groups of captive cotton-top tamarins. The behavior of mothers and fathers was compared to determine the effects of infant number and helper presence on each parent and to determine whether parents differed in responsiveness to infants. For young infants (1-4 weeks), the contribution of fathers to infant carrying was negatively correlated with helpers' contribution; as helpers carried more, fathers carried less. In contrast, the mothers' contribution to infant carrying was unaffected by helper contribution. Mothers carried older infants (5-8 weeks) less than did fathers, regardless of infant number or helper contribution. Fathers and mothers were equally likely to retrieve a non-harassed infant; however, fathers were more likely than mothers to retrieve an infant being harassed. These results suggest that fathers are more responsive to infants than are mothers. Mothers may limit their involvement in infant care, as has been proposed by field results, due to an energetically demanding reproductive strategy.

Co-Operative Breeding in Riflemen (Acanthissitta Chloris) Benefits To Parents, Offspring and Helpers

AbstractTwo types of helpers were observed feeding young of the New Zealand rifleman (Acanthisitta chloris): "regulars" and "casuals". The former contributed significantly to feeding young from only one nest while the latter contributed a trivial amount to young from a number of nests. Regulars were usually unpaired adult males some of which acquired a mate from the brood they helped. While some casuals were unpaired adult males, most were current season offspring helping with their parents' second clutch. The relative contributions of parents and helpers in raising broods were measured to establish the benefits of co-operative breeding to offspring and feeders. Whether or not helpers were present while feeding young, the breeding female's contribution was the same. By contrast, the work load of the breeding male was significantly reduced when helpers were present, but even then he usually contributed more than this mate. That male parents benefited most from helpers feeding young was probably related to their being responsible for most of the feeding in the absence of helpers. A helper's presence did not improve productivity or the male parent's chance of survival. However, female parents of nests with regular helpers survived better than those without. Offspring fed by helpers were not heavier upon fledging nor were their chances of survival improved. The interval between fledging first clutch broods and laying second clutches was the same whether helpers were present or not.

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.