Variable postfledging care in a cooperative bird: causes and consequences

Prolonged postfledging care is a commonly observed behaviorin many cooperatively breeding species and has been shown toprovide young with both survival and developmental benefits.However, the causes of intraspecific variation in postfledgingcare and the consequences of this variation on the developmentof young remain unclear. Here we investigate factors affectingthe duration of postfledging care in the cooperatively breedingpied babbler (Turdoides bicolor). We show that the durationof care is variable (40–97 days) and is determined primarilyby the cost of care. Adults in groups with a low adult:fledglingratio were unable to maintain body mass during the period ofchick provisioning and subsequently ceased care of young earlier.This had a strong influence on offspring development: fledglingsthat received longer periods of care attained higher foragingefficiency and body mass than their counterparts at 6 monthsof age. The duration of postfledging care also had long-termeffects, with individuals that received longer periods of postfledgingcare more likely to successfully disperse from their natal group.This had important fitness implications as successful dispersersbecame reproductively active at an earlier age than their "failed-disperser"counterparts. These findings highlight the importance of consideringlong-term influences when assessing the benefits of prolongedpostfledging care on offspring fitness and development in cooperativesocieties.     

Can we measure the benefits of help in cooperatively breeding birds: the case of superb fairy-wrens Malurus cyaneus?

1. Correlational studies of reproductive success are plagued by difficulty over the direction of causation. For example, improved reproductive success with age can result from increased experience or reproductive effort, or selection against low-quality phenotypes that survive poorly. An association between supernumeraries and reproductive success in cooperative breeders can arise either because supernumeraries boost productivity, or productive territories accumulate supernumeraries. 2. Paired comparisons of parents sampled with and without supernumeraries have recently been widely applied to quantify help. However, Dickinson & Hatchwell (2004) have argued that this approach is flawed. They conjectured that those groups that gain supernumeraries are a biased superior sample of those that initially lack supernumeraries, while groups that lose supernumeraries will be a sample of inferior cooperative groups. They predict that these biased comparisons will underestimate the effect of help. 3. This conjecture has neither been explored theoretically, nor empirically tested. We use data from a 19-year study of the superb fairy-wren Malurus cyaneus to examine the conjecture and derive predictors of annual reproductive success in this species. 4. We introduce statistical models of reproductive success based on a zero-inflated Poisson link function to identify three strong correlates of reproductive success: high spring rainfall, progress from the first to later years of life, and acquisition of supernumeraries. 5. First year females that died after breeding and those that survived to breed again had similar productivity. As female productivity improves with age, increased reproductive skill or effort is implicated rather than selection against inferior phenotypes. 6. We argue that the Dickinson-Hatchwell conjecture does not constrain paired comparisons in M. cyaneus. The dominant male and breeding female gain no immediate fecundity advantage from supernumeraries. 7. Effects on the future survival of dominants are even more difficult, as while helpers could enhance survival of dominants, a territory that facilitates survival should also accumulate philopatric supernumeraries. Males, the philopatric sex, did not survive better on territories with supernumeraries. However, females, the dispersive sex, had higher survival as the number of supernumeraries increased, because helpers allowed them to reduce the costs of reproduction. These data exacerbate the paradox posed by previously reported costs that supernumeraries impose on dominant males.     

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.     

Reproductive success and helper effects in the cooperatively breeding grey-crowned babbler

Cooperative breeding, where some individuals help to raise offspring that are not their own, is a relatively rare social system in birds. We studied the breeding biology of a declining cooperative breeder, the grey-crowned babbler Pomatostomus temporalis , with the aim of isolating the social factors that affect its reproductive success. Most breeding pairs were assisted by philopatric offspring, although pairs could breed successfully without helpers. Females laid up to four clutches (usually three eggs per clutch) per season. Male (but not female) helpers increased the number of young fledged from individual nests and the likelihood of re-nesting, resulting in higher seasonal fledgling production. Helper effects on brood size and fledgling production were greater in the second year of the study, which was also characterized by higher nest failure. This suggests that helpers enhance reproduction more in poor conditions. Our study demonstrates the interacting effects of social and ecological factors on reproductive success, and that retention of offspring is not always beneficial for the breeders in cooperative species.     

Influence of age on reproductive performance in the Seychelles warbler

I studied age-related breeding performance of the cooperativelybreeding Seychelles warbler (Acrocephalus sechellensis) on CousinIsland, Seychelles, during 14 years. The annual number of youngthat fledged is significantly related to territory quality andnumber of helpers in the breeding group. Accounting for thesefactors and for the partner's breeding experience, annual productionof fledglings by breeding birds increases from 2 to 5 yearsand decreases beyond 5 years of age. Age-related changes inreproductive success within breeding individuals, paired withthe same experienced partner and occupying the same breedingterritory with similar amount of food from 2 to 8 years of age,show that Seychelles warblers have higher hatching success andproduce more hatchlings and fledglings as they become older.This is probably not a response to decreasing residual reproductivevalue caused by decreasing life expectancy as they become older.Primiparous warblers produce the same number of fledglings asmultiparous warblers of the same age, have a similar life expectancyas multiparous warblers, and occupy territories with similaramounts of resources available for reproduction. As all warblershave similar access to food before reproduction and similarforaging efficiency, the low reproductive success in youngerwarblers cannot be ascribed to differences in environment butto the bird's ability to breed successfully (e.g., a resultof previous helping and/or breeding experience). Effects ofsenescence on reproduction begin to occur from age 6 for bothsexes. From that age, eggs have lower hatching success, butfledging success remains the same. Birds that start breedingat a young age on a given quality territory produce more fledglingsin their lifetime than birds that delay breeding. Many old birdsnever fledge young of their own. They have not been able tobreed because of a shortage of breeding territory vacancies.[BehavEcol 7: 417–425 (1996)]     

Bi‐parental vs. cooperative breeding in a passerine: fitness‐maximizing strategies of males in response to risk of extra‐pair paternity?

In socially monogamous species, males that risk cuckoldry more than others might gain inclusive fitness by yielding paternity to relatives. We tested this prediction in the Tibetan ground tit Pseudopodoces humilis , an unusual facultative cooperative breeder wherein most helpers (87% males) join a mated pair shortly before clutch completion. Extra-pair paternity among bi-parental broods occurred less often (26% of broods, 9% of young) compared with cooperative broods (68%, 25%). In the former, most extra-pair sires (88%) were pair breeders unrelated to the cuckolded males, whereas in the latter, sires (87%) were mainly helpers related to the dominant males. Brood productivity did not differ between the bi-parental and cooperative breeders, but helpers' partitioning over group paternity reduced the realized reproductive success of helped males. After taking inclusive fitness into account, however, there was no difference in success of dominant males between the two social systems. One possible explanation for the differences in the rates of cuckoldry in the two systems was body size, because pair-bond males in bi-parental situations were significantly larger than those in cooperative ones. We propose two alternative strategies for males to maximize fitness: breed as a pair if large to avoid cuckoldry from helpers, or breed cooperatively if small but compromise some paternity to relatives. Our results provide an unusual route to the incidence of cooperative groups, based on constraints imposed by low competitive ability of breeding males rather than some external ecological or demographic factors.     

Influence of helping and breeding experience on reproductive performance in the Seychelles warbler: a translocation experiment

Reproductive success of the cooperative breeding Seychelleswarbler (Acrocephalus sechellensis) increases with age. Thisage effect is not due to differential survival or increasedreproductive effort, but to accumulated helping and breedingexperience. In their first year of breeding, reproductive performanceof inexperienced warblers with neither helping nor breedingexperience was significandy lower than that of warblers of thesame age with either previous helping or breeding experience.Reproductive performance was the same for primiparae with helpingexperience and for birds with breeding experience. Female primiparaewith helping experience or breeding experience built betternests and spent more time incubating than inexperienced females,which led to increased hatching success. Male primiparae withhelping experience or males with breeding experience guardedthe clutch better than inexperienced males, which led to reducedegg predation. Even-aged warblers with different previous experienceswere transferred to unoccupied islands, where birds startedbreeding immediately in high-quality territories. The experimentshowed that birds with helping experience produced their firstfledgling as fast as experienced breeders, and significandyfaster than inexperienced birds. Breeding performance did notimprove further with experience after the first successful breedingattempt. Only birds with previous breeding experience who pairedwith inexperienced birds, were likely to change mate. The otherpair combinations remained stable. Thus, primiparous birds withhelping experience have greater lifetime reproductive successthan inexperienced primiparae of the same age. This experimentshows that helping behavior has not only been selected for inthe context of promoting an individual's indirect fitness, butalso in the context of gaining helping experience which translatesinto improved reproductive success when a helper becomes a breeder.[Behav Ecol 7: 326-333 (1996)]     

Experimental evidence for helper effects in a cooperatively breeding cichlid

Neolamprologus pulcher is a cooperatively breeding cichlid fish,in which helpers stay in their natal territory and help withbrood care, territory defense, and maintenance. In this studywe investigated helper effects by an experimental group sizereduction in the field. After this manipulation, focal helpersin reduced groups tended to feed less, and small helpers visitedthe breeding shelter significantly more often than same-sizedhelpers in control groups. No evidence was found that remaininghelpers compensated for the removed helpers by increasing territorydefense and maintenance behavior. Breeders, however, did showa lower defense rate, possibly caused by an increase in broodcare effort. Survival of fry was significantly lower in removalthan control groups, which provides the first experimental proofin a natural population of fish that brood care helpers do effectivelyhelp. The data suggest that in small, generally younger, helpers,kin selection may be an important evolutionary cause of cooperation.Large helpers, however, who are generally older and less relatedto the breeders than small helpers are suggested to pay to beallowed to stay in the territory by helping. All group membersbenefit from group augmentation.     

Synchronous provisioning increases brood survival in cooperatively breeding pied babblers

1.  Behavioural synchrony typically involves trade-offs. In the context of foraging, for example, synchrony may be suboptimal when individuals have different energy requirements but yield net benefits in terms of increased foraging success or decreased predation risk.
2.  Behavioural synchrony may also be advantageous when individuals collaborate to achieve a common goal, such as raising young. For example, in several bird species, provisioners synchronize nest-feeding visits. However, despite the apparent prevalence of provisioning synchrony, it is not known whether it is adaptive or what its function might be.
3.  Here, we propose a novel explanation for provisioning synchrony: it increases brood survival by decreasing the number of temporally separate nest visits and accordingly the chance that the nest will be detected by predators. Using cooperatively breeding pied babblers, we showed experimentally that provisioners synchronized nest visits by waiting for another provisioner before returning to the nest. Brood survival increased with provisioning synchrony. Provisioners were more likely to synchronize feeding visits for older nestlings as they were louder and possibly more conspicuous to predators. Finally, provisioners in large groups were more likely to wait for other provisioners and synchronized a higher proportion of all visits than those in smaller groups. Thus, provisioning synchrony may be one mechanism by which large groups increase brood survival in this species.
4.  This study highlights a novel strategy that birds use to increase the survival of young and demonstrates the advantages of coordinated behaviour in social species.