Subordinate males are more likely to help if unrelated to the breeding female in cooperatively breeding white-browed scrubwrens

Subordinates in communally breeding groups of birds usually help to provision nestlings, but in some species helping-at-the-nest is facultative. In species in which groups usually contain relatives, subordinates either always feed young or are more likely to do so when breeding dominants are close relatives, suggesting that benefits of helping collateral kin are important. In other species, adult group members are unrelated to each other and males may only feed young if they have gained paternity, showing that cooperation is related to the mating system. The white-browed scrubwren, Sericornis frontalis, is a communally breeding species in which most groups consist of a simple pair or a dominant pair with a subordinate male. Subordinate males either fed nestlings in a given nest at a rate comparable to the dominants, or did not feed them at all. Breeding groups usually formed through natal philopatry of males, so that about 80% of subordinates were closely related to one or both members of the dominant pair. However, because of death and dispersal, 54% of subordinates were unrelated to the resident female. Although subordinates with their mother fed nestlings in 48% of cases, they fed offspring in 75% of cases if their mother had been replaced by an unrelated female, suggesting that their decision to help is influenced by the opportunity to mate with the female. Supporting this conclusion, relatedness to the dominant male did not affect subordinate behaviour, and genetic studies showed that subordinates often gained paternity if unrelated to the female. Thus, paradoxically, provisioning nestlings is related to the opportunity for mating in a society in which there is natal philopatry and subordinates are usually related to one or both members of the dominant pair. Received: 25 January 1997 / Accepted after revision: 1 May 1997     

Influence of kinship on helping behavior in Galápagos mockingbirds

Summary The social organization of the Galápagos mockingbird (Nesomimus parvulus) in unusual in that groups frequently include more than one breeding pair (plural breeding), and helping behavior is flexible: some birds neither breed nor help, while others do both. To investigate the influence of kinship on helping behavior, I categorized each bird as a helper or non-helper with respect to each nest within its group where it had an opportunity to help. The incidence of helping varied with relatedness: more birds helped when nestlings available to be fed were close relatives than when not. This result was independent of a higher incidence of helping among males than among females and of variation with age among males. Proportionally more nonbreeding than breeding males helped, but breeding and nonbreeding females helped equally infrequently; breeders helped most often after their own nests failed. The incidence of helping was highest among birds with opportunities to feed offspring of breeders that had fed the potential helper as a nestling, suggesting a mechanism for kin discrimination based on associative learning. Juveniles with opportunities to choose among alternative recipients preferentially fed closely related nestlings, but insufficient information was available to determine if adults also did so. Kinship did not influence the rate at which nestlings were fed by helpers. Juveniles fed nestlings at lower rates than did adult helpers, but helping effort was otherwise unaffected by age, sex, or relatedness. Limitation of help to former feeders functions as a mechanism for directing aid to relatives in a plural breeding system where degrees of kinship vary among potential recipients within the same group.     

Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.     

A critical analysis of ‘false-feeding’ behavior in a cooperatively breeding bird: disturbance effects,satiated nestlings or deception?

‘False feeding,’ where helpers arrive at nests with food but fail to provision the young, has been reported in several cooperative species. This and other potentially ‘deceptive’ behavior has been interpreted as indicating that helping may operate as a signal within such social groups. We critically examine these phenomena in the provisioning behavior of the bell miner Manorina melanophrys. Excessively close observation distances can artificially elevate the rate of false feeding in this (and other) species, but once this had been accounted for, there was little evidence for any ‘deceptive’ behavior by helpers or breeders. Natural and experimentally induced variation in the presence of a potential conspecific audience at the nest did not have any consistent influence upon the rate of false feeds, which was low at 7.94% of 6,880 nest visits. Instead, encountering unexpectedly low levels of brood demand provided a more parsimonious explanation for those visits where helpers failed to feed nestlings or ate the food themselves. Failure to completely transfer a load to nestlings was more likely when the load contained a high proportion of sticky lerp, indicating a simple prey-transfer problem. Finally, individuals that arrived at nests without prey were often members of neighboring breeding pairs, suggesting that these few non-feeding visits may instead involve an information-gathering function. We, therefore, suggest that future studies explicitly exclude the possibility of observer disturbance and all aspects of normal provisioning behavior before applying the terms ‘false feeding’ or ‘deceptive’ and inferring anything more than straightforward helping at the nest.     

Importance of monogamous male birds in determining reproductive success

Summary Several recent studies have questioned whether the presence and activities of monogamous male birds are of value to their young during the nesting attempt. We addressed this issue in house wrens (Troglodytes aedon) by removing males early in the nestling stage. In one of four periods survivorship among experimental nestlings was 63% lower than among control nestlings. In the other periods survivorship was about equal in experimental and control nests indicating that in this species the male's activities may only benefit his nestlings during unfavorable periods. We summarize the results of 15 other male removal studies, concluding that when males appear to be helping, their absence usually results in decreased survival of young whereas when the males render little apparent care their absence does not decrease survival of the young. A possible exception to this generalization, illustrated by our study and others, is that during the nestling stage of many altricial species, the female appears able to raise the young about as well without the male as with him, except in poor periods. A male caring for his nestlings during good periods may benefit by maintaining his mate's health for the next nesting attempt or by insuring her willingness to mate with him again. Alternatively, it may cost males so little to help during good periods, and benefit them so much during poor periods, that there has been little selective pressure for them to assess conditions and vary their level of care accordingly. A series of more specific predictions about which males are likely to be of greatest, and least, value to their nestlings is presented. Male care after young leave the nest has not been well studied, but is probably substantial in many species, and warrants attention in future studies.     

Parent blue-footed boobies suppress siblicidal behavior of offspring

Behaviorally dominant nestlings routinely kill sibling nestmates in blue-footed booby (Sula nebouxii) broods during periods of food shortage. Previous work demonstrated that these dominant, first-hatching “A-chicks” regulate the lethality of their behavior towards subordinate, second-hatching “B-chicks,” showing tolerance towards B-chicks except during chronic food shortages. Siblicide by A-chicks usually occurs after the hatchling stage. Results of an interspecific cross-fostering experiment indicated that A-chicks also attempt siblicide shortly after hatching, but parents apparently exert control over these attempts, and thwart them, when chicks are young. Theory predicts selection for such regulation in siblicidal birds that are likely to experience genetic parent-offspring conflict over the value of subordinant nestlings; our evidence of post-hatching parental regulation is consistent with that prediction. Received: 6 June 1998 / Accepted after revision: 12 July 1998     

Central place food caching: a field experiment with red-backed shrikes (Lanius collurio L.)

Summary Red-backed shrikes (Lanius collurio L.) stored experimentally presented mice (Mus musculus L) by reimpaling them on thorns of sloe bushes (Prunus spinosa L.) in the vicinity of the nest. Large mice (20 g) were stored further away from the nest than small mice (4 g), while the smallest mice (1 g) were transported directly to the nest and cut up there. Large prey required more round trips to deliver than smaller prey (4 g). Time to immobilize, load and deliver the prey and the proportion of undigestable tissue increased with prey size. Birds used stored prey as an alternative to hunting in other patches as expected from patch use models. The birds maintained a high rate of food delivery to the young by using stored mice during periods when their foraging success of natural prey was low. Several aspects of the shrike's food storing behaviour are in qualitative accord with suggestions derived from models assuming maximization of energy delivery rate.     

Do helpers increase reproductive success?

Summary Although several different hypotheses have been proposed to explain the evolution of helping behavior, most are based on the assumption that helping enhances the reproductive success of recipient breeders. I tested this assumption by removal experiments in the cooperatively breeding Florida scrub jay (Aphelocoma c. coerulescens). This species lives in permanently territorial social units containing a single breeding pair and none to six nonbreeders, which are usually offspring of the breeding pair and which usually act as helpers by feeding the nestlings and fledglings produced by the breeding pair. Although experimental removals of non-breeders in 1987–1988 had no significant effect on breeder survival, egg production, or hatching success, experimental groups suffered higher rates of predation on nestlings (1987) and lower rates of fledgling survival (both years) than did unmanipulated controls. As a result, experimental groups produced an average of only 0.56 independent juveniles, compared to 1.62 young for controls. Analysis of the factors contributing to nestling and fledgling mortality indicates that helping behavior per se (i.e., the aid that nonbreeders provide to dependent young), not the mere presence of nonbreeders, was responsible for the greater reproductive success observed in control groups. Because survival rates of allofeeders (i.e., those nonbreeders that provisioned dependent young) were virtually identical to those of non-allofeeders, the costs of helping behavior in this species appear to be small. Furthermore, nonbreeders are more likely to provision dependent young within their social unit when those young are closely related. I therefore conclude that nonbreeders increase their indirect fitness by serving as helpers and that helping behavior in the Florida scrub jay is a trait that has current selective utility. It remains debatable, however, whether helping in this species is an adaptation that has been shaped by the process of natural selection.     

Migratory fuelling in blackcaps (Sylvia atricapilla) under perceived risk of predation

It has been argued that the body mass levels achieved by birds are determined by the trade-off between risks of starvation and predation. Birds have also been found to reduce body mass in response to an increased predation risk. During migration, the need of extra fuel for flights is obvious and crucial. In this study, migratory blackcaps (Sylvia atricapilla) were subject to an experimental stopover situation where the predation risk was manipulated by exposure to a stuffed predator. Blackcaps that perceived an imminent risk of predation increased their food intake and fuel deposition rate during the first period of stopover compared with a control group. The pattern of night activity indicates that birds that were exposed to the predator also chose to leave earlier than birds in the control group. Since there was no cover present at the stopover site, birds might have perceived the risk of predation as higher regardless of whether they were foraging or not. Under such circumstances it has been predicted that birds should increase their foraging activity. The findings in this study clearly indicate that birds are able to adjust their stopover behaviour to perceived predation risk. Received: 8 January 1997 / Accepted after revision: 11 April 1997     

Age-dependent vigilance in winter aggregations of cooperatively breeding white-winged choughs (Corcorax melanorhamphos)

Summary Winter aggregation of cooperatively breeding white-winged choughs (Corcorax melanorhamphos) was examined to elucidate the advantages for different group members. Adults have higher vigilance burdens than juveniles when foraging in their regular group. Aggregation of groups in winter enables the older members to reduce vigilance and increase the time spent foraging. Vigilance was lowest and foraging time highest in aggregations of 20 individuals but the size of cooperatively breeding groups did not exceed fifteen. Despite increased foraging time, choughs did not consume more food items when aggregating.     

Helping at the nest,allofeeding and social status in immature arabian babblers

Summary Arabian babblers, Turdoides sqamiceps, are cooperatively breeding, group territorial birds, occurring in desert habitats. Non-breeders participate in several types of cooperative behaviour, including care of eggs, nestlings and fledglings, and bringing food to one another (allofeeding). This paper reports on observations of helping with eggs and young, and peer allofeeding, performed by immature babblers, fledged the previous season or earlier in the same season. The birds were from a babbler population studied since 1971 in the Arava valley of eastern Israel. They belonged to groups in which all individuals had been colourringed and were accustomed to human observers. Individuals and nests in the territories of these groups could be watched from distances of 1–2 m without causing alarm, so fine details of behaviour could be observed in the field. In most allofeeding interactions observed, a more dominant individual brought food to a subordinate. Sometimes the subordinate bird avoided accepting the proferred food, and was then hit or chased by the bird that had attempted to feed it. Occasionally a subordinate bird attempted to feed a more dominant individual. Dominant birds always refused to take food offered by a subordinate, and hit or chased any subordinate attempting to feed them. Frequencies of visits to nests were correlated with helper rank for those visits where the incoming bird displaced a previous visiter from the nest. Frequences of visits to unattended nests, however, were not correlated with helper rank. Similarly, frequencies of feeding visits to fledglings were correlated with helper rank until incubation of the next brood began. After this, the correlation disappeared. Numerous instances of aborted nest visits were observed in which a helper, often carrying food, arrived at the nest-tree but left again without visiting the nest. In many other cases, a helper arrived at the nest-tree but delayed visiting the nest for several minutes. Aborted and delayed visits usually occurred when a more dominant bird was in the vicinity of the nest. This suggests interference between helpers. Direct interference between helpers visiting fledglings was occasionally observed. In such cases, a more dominant helper snatched food from a subordinate approaching fledglings, and then fed this food to the fledglings itself. Interference between helpers, and conflict between proferers and recipients of food during allofeeding, are not easily explained by kin selection or reciprocity. On the other hand, such behaviour is readily explained by a hypothesis which suggests that individuals may increase their social status in the group by performing cooperative behaviour. Babblers that establish status by demonstrating ability to bear the short-term costs of cooperative behaviour, rather than through direct aggression towards rivals, are likely to forge collaborative relationships with other group members. Since babblers must collaborate to establish and defend a territory, such relationships are essential to reproductive success.     

Do males and females differ in the feeding of large and small siblings? An experiment with the bluethroat

Males and females have been reported to differ in their feeding of large and small siblings in several species of birds. According to recent hypotheses, this phenomenon may be related to a sexual conflict over avian hatching patterns. We designed an experiment to test for the existence of such a sex difference by manipulating nestling size hierarchies of the bluethroat (Luscinia s. svecica) in two directions; half the broods were “asynchronized” to yield large size-differences within broods and the other half were “synchronized” to yield small size-differences. In all broods, nestlings were categorized as being either large or small according to body mass. We recorded male and female food distribution by video early (day 4 after hatching) and late (day 8) in the nestling period. Males and females did not differ in their distribution of food among different-sized nestlings. With large size-differences, both males and females fed large nestlings nearly twice as often as small ones. In contrast, when the size-differences were small, food was more evenly distributed among nestlings. Early in the nestling period, males fed more nestlings during each feeding visit than did females. Our finding that male and female bluethroats do not differ in the feeding of large and small siblings is in contrast to most previous studies. Variation in costs and benefits to males and females from feeding different-sized nestlings, and restrictions to parental choice due to nestling interactions, may explain interspecific variation. Received: 27 June 1997 / Accepted after revision: 26 January 1998     

Social and mating system of cooperatively breeding laughing kookaburras (Dacelo novaeguineae)

DNA fingerprinting was combined with field observations over four breeding seasons to investigate the social structure and mating system of the laughing kookaburra (Dacelo novaeguineae). Groups comprised a socially dominant pair and up to six helpers of either sex. Helpers were always recruited from young hatched in the group. Territorial inheritance, which is a feature of other cooperative breeders and an oft-cited benefit of philopatry, did not occur. Helpers only attained dominant status in an established group by dispersing into a vacant dominant position in that group. However, helpers could also form new groups by excising a new territory, often through a ”budding” process. The mating system was overwhelmingly monogamous. There were no cases of extra-group parentage in a sample of 140 nestlings; within groups of three or more birds, dominance predicted parentage almost perfectly (99.2% of 129 nestlings), irrespective of whether helpers in the group were related to one or both dominant birds. This is contrary to predictions from models of reproductive skew, possibly because they currently fail to incorporate the willingness of females to share reproduction among males. Received: 15 May 1999 / Received in revised form: 2 November 1999 / Accepted: 6 November 1999     

Nestling American robins compete with siblings by begging

Summary The evolution of intense begging by dependent nestling birds has recently been the subject of several theoretical papers. The interesting problem here is that nestlings should be able to communicate their nutritional status to parents in ways that are less costly energetically and less likely to attract predators. Thus, conspicuous begging behaviour is thought to have evolved as a result of either competition among nestmates or the manipulation of their parents to provide more food than would otherwise be favoured by selection. We studied sibling competition for parental feedings in the American robin (Turdus migratorius). We demonstrate that the probability that an individual nestling received food was related to several indices of begging. When we experimentally prevented parents from feeding part of their brood, both the intensity of begging and the number of feedings subsequently received by food-deprived nestlings increased. Furthermore, the begging intensity of those nestlings that were not food-deprived also increased in response to the begging of their hungrier siblings.Offprint requests to: R. Montgomerie     

Food-dependent benefits of hatching asynchrony in American kestrels Falco sparverius

Food supply and hatching asynchrony were manipulated for 90 broods of American kestrels (Falco sparverius) during 1989–1991. We measured the growth and mortality of nestlings within four treatment groups (asynchronous, synchronous, food-supplemented, unsupplemented) to test the brood reduction hypothesis of Lack (1947, 1954). Fledging success did not differ between synchronous and asynchronous broods when food was poor but consistent with the brood reduction hypothesis, nestlings died at a younger age in asynchronous broods. When food was supplemented, mortality did not occur in the synchronous broods but youngest nestlings still died in asynchronous nests despite apparently adequate food for the brood. Oldest nestlings in asynchronous broods fledged with a greater mass than their younger siblings, also consistent with Lack's hypothesis. Average nestling quality in synchronous broods was very dependent on food levels. Synchronous young that were supplemented were, on average, the heaviest of any treatment group but young from unsupplemented synchronous broods were the lightest. Overall, patterns of mortality and growth for kestrels support the brood reduction hypothesis when food is limited, but not when it is abundant. This food-dependent benefit of asynchrony in the nestling period is a prerequisite for facultatively adjusted hatching spans during laying.     

Costs of switching social groups for dominant and subordinate dark-eyed juncos (Junco hyemalis)

I quantified the costs of switching from a familiar to an unfamiliar flock for captive dark-eyed juncos (Junco h. hyemalis) by measuring several physiological and behavioral variables before and after flock switching. Birds that were initially dominant dropped in status in unfamiliar flocks, and experienced increased metabolic rates, while subordinate birds appeared to undergo less physiological change when switching flocks. This difference occurred despite a lack of any rank-related differences in the effects of joining a new flock on rates of aggression, weight change, access to food, or plasma corticosterone levels. These results suggest that for dominant, but not subordinate, individuals there is a measurable metabolic cost to joining a new social group, even in the absence of adverse factors such as food limitation. Dominant individuals may be less likely than subordinates to leave familiar flocks because of their higher metabolic costs when joining a new social group.     

Adjustment of parental effort to manipulated foraging ability in a pelagic seabird,the thin-billed prion Pachyptila belcheri

An experiment was designed to examine in a long-lived seabird, the thin-billed prion (Pachyptila belcheri), how adults adjust their food provisioning strategy when their foraging abilities are reduced and when the chick's needs are increased. To reduce the foraging abilities of adults we impaired their flying ability by removing some flight feathers (handicapped), and to increase the food needs of the chick one parent was retained (single). Birds made either short foraging trips lasting 1–3 days, or long trips lasting 5–9 days. Control birds alternated long and short trips whereas single birds or handicapped birds made several successive short trips and thereafter a long trip. In each treatment, food loads tended to be heavier after long trips than after short trips, and single birds tended to bring heavier loads than control or handicapped birds. Birds in the three treatments lost similar amounts of mass after short trips and gained similar amounts of mass after long trips. However, the mass of handicapped birds declined through the experiment, while that of control and single birds remained stable. Although the proportion of chicks that died during the experiment was similar among the three treatments, the chicks fledged by a single bird were lighter than those in control nests. The results of the experiment suggest that thin-billed prions adjust their breeding effort differently to decreased flying ability or increased food demand by the chick. Single birds increase foraging effort without allowing their condition to deteriorate. Conversely, handicapped birds are unable to maintain their body condition while sustaining the chick at the same rate as control birds. It is suggested that in this long-lived seabird, adults probably adjust their breeding effort so that they do not incur the risk of an increased mortality, this risk being monitored by the body condition.     

Hunger and food allocation among nestlings of facultatively siblicidal ospreys

Summary Previous studies of food allocation among nestlings of siblicidal birds have focussed on the proximate link between food and aggression. However, food sharing among sibs (and brood reduction) need not involve aggression. Here I examine the role of hunger (defined operationally in terms of the amount of food eaten recently) within the broader framework of passive and aggressive food-sharing among nestlings of facultatively siblicidal ospreys (Pandion haliaetus). Feeding among brood members was hierarchical; senior, dominant sibs ate more when hungry, leaving a smaller residual share of food for junior sibs. Senior sibs did not become more generous at large meals (i.e., the prospect of a large meal did not quell the selfish tendencies of senior sibs). That individuals ate more when hungry indicates a proximate link between food and the allocation of resources among siblings. Overall, aggression was infrequent and seemed to be used when junior sibs contested food allocation at meals. Aggression was diminished and food allocation was skewed toward the junior sib in broods with artificially exaggerated hatching intervals, suggesting that senior sibs were more generous when their dominant status was not threatened.Offprint requests to the present address     

Parent-absent begging in the Brown-headed Cowbird (<Emphasis Type="Italic">Molothrus ater</Emphasis>): the role of short-term need and nestmate size

Although it is well-established that nestlings of many altricial species beg when parents are away from the nest, we have a poor understanding of parent-absent begging in brood parasites, including the proximate factors that may influence begging frequency and intensity. In this study, I examined how parent-absent begging was influenced by competitive asymmetries between host and Brown-headed Cowbird (Molothrus ater) nestlings under disparate levels of short-term need. Food-deprived cowbird nestlings begged more frequently and for a greater proportion of parent-absent period than when food-supplemented, with similar patterns observed in hosts of different sizes. In contrast, three metrics of cowbird begging intensity varied relative to host size but not due to differences in short-term need. Cowbirds consistently begged more frequently and intensively than host nestlings for a given level of short-term need, providing evidence that cowbird begging displays are more frequent and intense than non-parasitic nestlings during both feeding visits and parent-absent periods. In sum, the frequency of begging by cowbirds was only influenced by short-term need, whereas begging intensity during parent-absent events was only influenced by the host against which cowbirds competed. This study demonstrates that host size and short-term need had differing influences on the frequency and intensity of parent-absent begging in cowbirds, although both factors are likely important in limiting the evolution of parent-absent begging in cowbirds. Because it appears to provide no immediate benefits yet may decrease fitness, parent-absent begging should be included in future theoretical models investigating the evolution of begging displays in nestling birds.     

Costs and benefits of surplus offspring in the lesser kestrel (Falco naumanni )

Lesser kestrels (Falco naumanni) lay clutches which appear excessive as only 3% of them yield as many young as eggs laid. Four hypotheses may explain the adaptive value of producing surplus eggs: (1) the bet-hedging hypothesis assumes that the environment varies unpredictably and surplus eggs serve to track uncertain resources; (2) the ice-box hypothesis suggests that surplus offspring serve as a reserve food during a period of shortage; (3) the progeny choice hypothesis says that parents produce surplus offspring in order to choose these with higher fitness; and (4) the insurance-egg hypothesis proposes that extra eggs are an insurance against the failure of any egg. To test the significance of this strategy in the lesser kestrel, an experiment manipu-lating brood size at hatching was carried out over 2 years, with good and bad feeding conditions. The experiment consisted of adding a chick to experimental broods where one egg failed to hatch or removing a randomly selected chick from experimental broods where all eggs had hatched. Independently of annual food availability, pairs with brood sizes reduced by one chick fledged more nestlings than pairs with brood size equalling their clutch sizes. Body condition of young was also better in the former group, but only in 1993 (a high-food year). Independently of year, mean local survival of parents with complete broods at hatching was lower than for parents raising reduced broods. These results supported only the insurance-egg hypothesis which says that surplus eggs may be an insurance against the failure of any egg, but parents may suffer reproductive costs when all eggs hatch. Received: 17 January 1997 / Accepted after revision: 27 April 1997