Avian Cooperative Breeders Research Articles

Intraspecific variation in group structure arises due to environmentally-mediated directional dispersal in a cooperative breeder.

Many cooperatively breeding species live in groups with complex structure-large group sizes, low and variable kin structure, and multiple breeding pairs. Since these mixed-kin groups typically form because of immigration of unrelated individuals of both sexes in addition to limited offspring dispersal, differences in patterns of dispersal can generate variation in group structure, even within the same species or population. Here, we examine how environmentally mediated dispersal patterns influence variation in group structure in the plural breeding superb starling (Lamprotornis superbus), an avian cooperative breeder that inhabits a spatiotemporally variable savanna environment and forms mixed-kin groups with variable group sizes and more than one breeding pair per group. Using 4068 genome-wide polymorphic loci and fine-scale, remotely sensed ecological data from 22 groups sampled across a nearly 200 km2 environmental gradient in central Kenya, we find evidence of not only frequent and long-distance dispersal in both sexes (low isolation-by-distance and weak genetic structure), but also directional dispersal from small groups in lower quality habitat with low normalised difference vegetation index (NDVI) to large groups in higher quality habitat with high NDVI. Additionally, we find stronger genetic structure among groups in lower quality habitat, and higher genetic diversity and lower relatedness of groups in higher quality habitat. Previous work using long-term data from groups in the same population has shown that groups with lower relatedness are larger and have more breeding pairs. Long-distance, directional dispersal to maximise individual fitness can thus lead to smaller and simpler kin-based social groups in lower quality habitat, but larger and more complex mixed-kin groups in higher quality habitat. Such intraspecific, within-population variation in group structure, including variation in kin structure of social groups, could have profound implications for the relative importance of the evolutionary mechanisms (i.e. direct vs. indirect fitness benefits) underlying the formation of cooperative societies.

Plasticity in social behaviour varies with reproductive status in an avian cooperative breeder.

Cooperatively breeding vertebrates are common in unpredictable environments where the costs and benefits of providing offspring care fluctuate temporally. To balance these fitness outcomes, individuals of cooperatively breeding species often exhibit behavioural plasticity according to environmental conditions. Although individual variation in cooperative behaviours is well-studied, less is known about variation in plasticity of social behaviour. Here, we examine the fitness benefits, plasticity and repeatability of nest guarding behaviour in cooperatively breeding superb starlings (Lamprotornis superbus). After demonstrating that the cumulative nest guarding performed at a nest by all breeders and helpers combined is a significant predictor of reproductive success, we model breeder and helper behavioural reaction norms to test the hypothesis that individuals invest more in guarding in favourable seasons with high rainfall. Variation in nest guarding behaviour across seasons differed for individuals of different reproductive status: breeders showed plastic nest guarding behaviour in response to rainfall, whereas helpers did not. Similarly, we found that individual breeders show repeatability and consistency in their nest guarding behaviour while individual helpers did not. Thus, individuals with the potential to gain direct fitness benefits exhibit greater plasticity and individual-level repeatability in cooperative behaviour.

Social organization, demography and genetic mating system of a Tibetan cooperative breeder

Knowledge of cooperative breeding in birds from longitudinal studies is available only for a small proportion of species. This paper reports data from a 12‐year study on the Tibetan Ground Tit Pseudopodoces humilis. On average, 27.2% (range: 13.0–36.1%) of monogamous pairs in each year contained one (85.4%) or more (14.6%) male helpers, 83.7% of which were yearlings staying on natal territories. Most helpers (89.6%) helped once and then bred independently. Adults had male‐biased sex ratios, low annual survival rates (averaging 0.50) and shorter longevities (averaging 1.8 years) compared with low‐altitude avian cooperative breeders, suggesting that mate shortage promotes helping behaviour in this species. Incest occurred rarely (2.1% of pairs), probably because kin recognition occurs through year‐around living in family groups. There was a low level (3.1% of broods) of extra‐pair parentage, which could facilitate the maintenance of cooperative breeding.

Climate predicts which sex acts as helpers among cooperatively breeding bird species.

Among avian cooperative breeders, help in raising offspring is usually provided by males or by both sexes. Sex bias in helping should evolve in response to sex-specific ecological constraints on independent reproduction, with mate shortage for males and breeding vacancy shortage for each sex. Given that male-biased adult sex ratios are prevalent among birds, we predict that male-only helping mainly occurs in temperate species where fast population turnovers deriving from low adult annual survival allow all adult females to hold breeding vacancies, whereas some males overflow as helpers, and both-sex helping in tropical species where saturated habitats prevent not only males, but also females from breeding themselves. As expected, we found that across species, adult survival increased towards tropical zones and warmer climates, and higher adult survival tended to be associated with both-sex helping. Furthermore, sex bias in helping was predicted by latitude and ambient temperature. Our findings of demographic response of species to climate as a potential determinant of bias in helper sex uncover how ecological constraints operate to limit independent reproduction in sex-specific ways.

Provisioning patterns in the cooperatively breeding acorn woodpecker: does feeding behaviour serve as a signal?

Acorn woodpeckers, Melanerpes formicivorus, are cooperative breeders in which social groups consist of both nonbreeding helpers at the nest (offspring from prior reproductive attempts) and cobreeders of one or both sexes (usually siblings or a parent and his/her offspring). Regardless of composition, groups generally have one nest at a time at which all individuals participate in provisioning offspring. We tested the hypothesis that provisioning behaviour serves a signalling function used to gain social advantages within groups by enhancing dominance or social prestige, or by reducing the likelihood of being expelled from the group (‘pay-to-stay’). We found that birds adjusted their provisioning behaviour based on the activities of other group members by clumping their visits and by alternating their visits with other group members, thus synchronizing and coordinating provisioning within groups. Despite this evidence that acorn woodpeckers respond to the provisioning behaviour of other group members, analyses of feeding rates and patterns of overlap revealed no support for the hypothesis that provisioning functions as a signal to other group members in any of three ways: breeder males signalling to breeder females to increase their probability of mating; helpers signalling to other helpers to enhance their dominance or social prestige; or helpers signalling to breeders to reduce the probability that they will be considered ‘lazy’ and be evicted from the group. Our results add to previous studies that have thus far failed to support a signalling function for provisioning behaviour in avian cooperative breeders.

Population Viability and Vital Rate Sensitivity of an Endangered Avian Cooperative Breeder, the White-Breasted Thrasher (Ramphocinclus brachyurus).

Social behaviors can significantly affect population viability, and some behaviors might reduce extinction risk. We used population viability analysis to evaluate effects of past and proposed habitat loss on the White-breasted Thrasher (Ramphocinclus brachyurus), a cooperatively breeding songbird with a global population size of <2000 individuals. We used an individual-based approach to build the first demographic population projection model for this endangered species, parameterizing the model with data from eight years of field study before and after habitat loss within the stronghold of the species’ distribution. The recent habitat loss resulted in an approximately 18% predicted decline in population size; this estimate was mirrored by a separate assessment using occupancy data. When mortality rates remained close to the pre-habitat loss estimate, quasi-extinction probability was low under extant habitat area, but increased with habitat loss expected after current plans for resort construction are completed. Post-habitat loss mortality rate estimates were too high for projected populations to persist. Vital rate sensitivity analyses indicated that population growth rate and population persistence were most sensitive to juvenile mortality. However, observed values for adult mortality were closest to the threshold value above which populations would crash. Adult mortality, already relatively low, may have the least capacity to change compared to other vital rates, whereas juvenile mortality may have the most capacity for improvement. Results suggest that improving mortality estimates and determining the cause(s) of juvenile mortality should be research priorities. Despite predictions that aspects of cooperative systems may result in variation in reproduction or juvenile mortality being the most sensitive vital rates, adult mortality was the most sensitive in half of the demographic models of other avian cooperative breeders. Interestingly, vital rate sensitivity differed by model type. However, studies that explicitly modeled the species’ cooperative breeding system found reproduction to be the most sensitive rate.

Evolution of Cooperative Breeding in Birds: A Long-Term Fieldwork with the Ground Tits Endemic to the Tibet Plateau as a Model System

Cooperative breeding in birds has been interesting to behavioral and evolutionary biologists as many basic questions associated with the mysterious phenomenon remain unsolved. The wasteland and meadow on the Tibetan plateau represent extremely harsh environments in the world. Since 2004, we have studied cooperative breeding behaviors of the ground tits Parus humilis endemic to the plateau. After knowing the basic structure of this species society, we are accumulating data to analyze lifetime fitness of helpers vs. independent breeders to test the kin-selection theory, cooperative breeding in relation to climate conditions to explore the mechanisms underlying ecological constraints, the relationship between cooperative breeding and extra-pair paternity across populations to elucidate genetic monogamy as the driver of helping behavior, and environmental conditions as extrinsic factors together with social interaction as intrinsic factors to give insight into population dynamics of avian cooperative breeders. Life-long efforts of researchers are required to uncover the mysteries of the ground tit society.

Do avian cooperative breeders live longer?

Cooperative breeding is not common in birds but intriguingly over-represented in several families, suggesting that predisposing factors, similar ecological constraints or a combination of the two facilitate the evolution of this breeding strategy. The life-history hypothesis proposes that cooperative breeding is facilitated by high annual survival, which increases the local population and leads to a shortage of breeding opportunities. Clutch size in cooperative breeders is also expected to be smaller. An earlier comparative analysis in a small sample of birds supported the hypothesis but this conclusion has been controversial. Here, I extend the analysis to a larger, worldwide sample and take into account potential confounding factors that may affect estimates of a slow pace of life and clutch size. In a sample of 81 species pairs consisting of closely related cooperative and non-cooperative breeders, I did not find an association between maximum longevity and cooperative breeding, controlling for diet, body mass and sampling effort. However, in a smaller sample of 37 pairs, adult annual survival was indeed higher in the cooperative breeders, controlling for body mass. There was no association between clutch size and cooperative breeding in a sample of 93 pairs. The results support the facilitating effect of high annual survival on the evolution of cooperative breeding in birds but the effect on clutch size remains elusive.

Validation of an automated data collection method for quantifying social networks in collective behaviours

The social network of preferences among group members can affect the distribution and consequences of collective behaviours. However, the behavioural contexts and taxa in which social network structure has been described are still limited because such studies require extensive data. Here, we highlight the use of an automated passive integrated transponder (PIT)-tag monitoring system for social network analyses and do so in a novel context—nestling provisioning in an avian cooperative breeder, for which direct observation of social behaviours is difficult. First, we used observers and cameras to arrive at a suitable metric of nest visit synchrony in the PIT-tag data. Second, we validated the use of this metric for social network analyses using internal nest video cameras. Third, we used hierarchical regression models with ‘sociality’ parameter to investigate structure of networks collected from multiple groups. Use of PIT tags led to nest visitation duration and frequency being obtained with a high degree of accuracy for all group members, except for the breeding female for whom accurate estimations required the use of a video camera due to her high variability in visitation time. The PIT-tag dataset uncovered significant variability in social network structure. Our results highlight the importance of combining complementary observation methods when conducting social network analyses of wild animals. Our methods can also be generalised to multiple contexts in social systems wherever repeated encounters with other individuals in closed space have ecological implications.

Social context and the lack of sexual dimorphism in song in an avian cooperative breeder

Elaborate male traits are typically used to attract mates and to compete with other males for mating opportunities. However, similarly ornate secondary sexual characteristics are also found in females in many species and may be used in competition in both sexual and nonsexual contexts. Trait elaboration in females may be particularly important in cooperatively breeding species where reproduction is monopolized by a few individuals in a group and where both sexes must compete for these limited mating opportunities. Previous work in African starlings has shown that females in cooperatively breeding species are larger and more ornamented than those in noncooperative species, resulting in reduced plumage and size dimorphism. To further examine patterns of sexual dimorphism in signalling traits and to better understand their role in mediating social competition, we investigated the form and function of song in cooperatively breeding superb starlings, Lamprotornis superbus . In addition to comparing song between sexes, we contrasted song in dominant breeders and subordinate helpers of both sexes and examined its use in various social contexts. We found that song was indistinguishable between males and females, with both sexes showing similar complexity, pitch, versatility and structure. However, the song of breeders was more versatile than that of helpers. Moreover, song structure differed when birds sang in chorus from when they sang alone. These findings suggest that, like male song, female song may be used in social competition, and that song may be an important signal in both sexes in species where intrasexual competition is high in males and females. ► Both sexes are predicted to have elaborate traits in cooperatively breeding species. ► Song form and function was examined in cooperatively breeding superb starlings. ► Both sexes sang equally complex songs. ► The song of dominant breeders differed from that of subordinate helpers. ► Song structure also differed in different social contexts.

Reproductive skew in an avian cooperative breeder: an empirical test for theoretical models

Reproductive skew models predict how animal groups containing multiple males maintain stable through partitioning reproduction. However, these models have rarely been tested in vertebrates, especially birds where tests have been made for less than 10 of more than 800 species that regularly exhibit cooperative breeding behavior. In the ground tit Parus humilis, a cooperative group consists of primary pair members and secondary individuals that are mainly failed male breeders. A previous study showed that primary males of low quality and thus facing high cuckoldry risks preferred to accept relatives as secondary members and yielded paternity to them. After showing no probability for secondary males to leave the group and live solitarily, the current study revealed that paternity share by a secondary male was positively correlated with his relatedness to the primary male and group productivity but independent of his body mass ratio to the primary male as a measure of relative competitive ability. These results are consistent with restraint transactional models, which predict that when a subordinate can claim unsanctioned reproduction for itself even though the dominant controls membership, both relatedness and group productivity favor the dominant to tolerate increased reproduction share by the subordinate.

Reduced Egg Investment Can Conceal Helper Effects in Cooperatively Breeding Birds

Cooperative breeding systems are characterized by nonbreeding helpers that assist breeders in offspring care. However, the benefits to offspring of being fed by parents and helpers in cooperatively breeding birds can be difficult to detect. We offer experimental evidence that helper effects can be obscured by an undocumented maternal tactic. In superb fairy-wrens (Malurus cyaneus), mothers breeding in the presence of helpers lay smaller eggs of lower nutritional content that produce lighter chicks, as compared with those laying eggs in the absence of helpers. Helpers compensate fully for such reductions in investment and allow mothers to benefit through increased survival to the next breeding season. We suggest that failure to consider maternal egg-investment strategies can lead to underestimation of the force of selection acting on helping in avian cooperative breeders.

Female extrapair mate choice in a cooperative breeder: trading sex for help and increasing offspring heterozygosity

Sexual conflict between males and females over mating is common. Females that copulate with extrapair mates outside the pair-bond may gain (i) direct benefits such as resources or increased paternal care, (ii) indirect genetic benefits for their offspring, or (iii) insurance against infertility in their own social mate. Few studies have been able to demonstrate the different contexts in which females receive varying types of benefits from extrapair mates. Here, I examined sexual conflict, female extrapair mate choice, and patterns of extrapair paternity in the cooperatively breeding superb starling Lamprotornis superbus using microsatellite markers. Although extrapair paternity was lower than many other avian cooperative breeders (14% of offspring and 25% of nests), females exhibited two distinct mating patterns: half of the extrapair fertilizations were with males from inside the group, whereas half were with males from outside the group. Females with few potential helpers copulated with extrapair mates from within their group and thereby gained direct benefits in the form of additional helpers at the nest, whereas females paired to mates that were relatively less heterozygous than themselves copulated with extrapair mates from outside the group and thereby gained indirect genetic benefits in the form of increased offspring heterozygosity. Females did not appear to gain fertility insurance from copulating with extrapair mates. This is the first study to show that individuals from the same population mate with extrapair males and gain both direct and indirect benefits, but that they do so in different contexts.

Parentage and reproductive success in the white-throated magpie-jay, Calocitta formosa, a cooperative breeder with female helpers

I conducted a 3-year study investigating parentage in a Costa Rican population of white-throated magpie-jays, a cooperatively breeding corvid inhabiting the tropical dry forests of Central America. This species is unusual among avian cooperative breeders in that helpers are mostly females. Using 12 microsatellite loci, I estimated parentage for 110 offspring in 32 broods and eight social groups. Primary breeders were not the sole producers of offspring within a brood. At the 95% confidence level, female helpers within the group produced 16% of young, and floater males sired approximately 30% of young. Helpers that bred mated either with the primary male (three cases) or with floaters (five cases). When the analysis was limited to the most reliable data, the 20 nests and 63 offspring where all candidate mothers had been sampled, at the 95% confidence level only one helper produced young, while floater males sired 22% of offspring. The results of this study cast doubt on the simple classification of white-throated magpie-jays as cooperative breeders and suggest a more complex combination of cooperative and parasitic behaviours.

The Parental Investment Strategy of an Avian Cooperative Breeder Differs between a Fragmented and an Unfragmented Landscape

Previous articleNext article No AccessThe Parental Investment Strategy of an Avian Cooperative Breeder Differs between a Fragmented and an Unfragmented LandscapeGary W. Luck, and Associate Editor: Allen J. MooreGary W. LuckCenter for Ecosystem Management, School of Natural Sciences, Edith Cowan University, 100 Joondalup Drive, Joondalup, Western Australia 6027, Australia*Present address: Center for Conservation Biology, Department of Biological Sciences, Stanford University, 371 Serra Mall, Stanford, California 94305‐5020; e‐mail: [email protected]. Search for more articles by this author , and Associate Editor: Allen J. Moore Search for more articles by this author Center for Ecosystem Management, School of Natural Sciences, Edith Cowan University, 100 Joondalup Drive, Joondalup, Western Australia 6027, AustraliaPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 160, Number 6December 2002 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/343881 Views: 38Total views on this site Citations: 7Citations are reported from Crossref HistoryReceived March 20, 2002Accepted June 11, 2002 Keywordsparental investmentcooperative breedinghabitat fragmentationreproductive successbirdsnestling provisioning© 2002 by The University of Chicago.PDF download Crossref reports the following articles citing this article:K.J. MacLeod, L. Brouwer Social context-dependent provisioning rules in red-winged fairy-wrens do not vary with signals of increased chick need, Animal Behaviour 143 (Sep 2018): 105–111.https://doi.org/10.1016/j.anbehav.2018.07.010Stephanie A.J. Preston, James V. Briskie, Ben J. Hatchwell Adult helpers increase the recruitment of closely related offspring in the cooperatively breeding rifleman, Behavioral Ecology 13 (Jun 2016): arw087.https://doi.org/10.1093/beheco/arw087Xin Lu, Tonglei Yu, Dianhua Ke Helped ground tit parents in poor foraging environments reduce provisioning effort despite nestling starvation, Animal Behaviour 82, no.44 (Oct 2011): 861–867.https://doi.org/10.1016/j.anbehav.2011.07.024Juan Carranza, Vicente Polo, Juliana Valencia, Concha Mateos, Carlos de la Cruz How should breeders react when aided by helpers?, Animal Behaviour 75, no.44 (Apr 2008): 1535–1542.https://doi.org/10.1016/j.anbehav.2007.10.006Rebecca L. Boulton, Yvan Richard, Doug P. Armstrong Influence of food availability, predator density and forest fragmentation on nest survival of New Zealand robins, Biological Conservation 141, no.22 (Feb 2008): 580–589.https://doi.org/10.1016/j.biocon.2007.12.007A.F Russell, N.E Langmore, J.L Gardner, R.M Kilner Maternal investment tactics in superb fairy-wrens, Proceedings of the Royal Society B: Biological Sciences 275, no.16301630 (Oct 2007): 29–36.https://doi.org/10.1098/rspb.2007.0821Juliana Valencia, Carlos De La Cruz, Juan Carranza, Concha Mateos Parents increase their parental effort when aided by helpers in a cooperatively breeding bird, Animal Behaviour 71, no.55 (May 2006): 1021–1028.https://doi.org/10.1016/j.anbehav.2005.06.021

The Parental Investment Strategy of an Avian Cooperative Breeder Differs between a Fragmented and an Unfragmented Landscape

The Parental Investment Strategy of an Avian Cooperative Breeder Differs between a Fragmented and an Unfragmented Landscape

Dispersal costs set the scene for helping in an atypical avian cooperative breeder.

The ecological constraints hypothesis is suggested to explain the evolution of cooperative breeding in birds. This hypothesis predicts that the scene for cooperative breeding is set when ecological factors constrain offspring from dispersal. This prediction was tested in the atypical cooperative breeding system of the long-tailed tit, Aegithalos caudatus, by comparing the degree of philopatry and cooperation in an isolated and a contiguous site whilst experimentally controlling for confounding aspects of reproduction. No difference was found between the two sites in the survival of offspring but a greater proportion were found to remain philopatric in the isolated site. This difference was caused by greater philopatry of normally dispersive females suggesting, as predicted, that dispersal costs were greater from this site. Furthermore, a greater proportion of males and females cooperated following breeding failure in the isolated site than in the contiguous site. Thus, as has been suggested for typical avian cooperative breeders, dispersal costs, relative to philopatric benefits, appear to set the scene for cooperative breeding in long-tailed tits.

Can incest within cooperative breeding groups be detected using DNA fingerprinting?

Although generally considered a rare phenomenon, in-cest has been observed in a number of populations ofbirds and mammals in which dispersal is limited (Green-wood et al. 1978; Bulger and Hamilton 1988; Gibbs andGrant 1989; Packer and Pusey 1993; Keller and Arcese1998). In cooperatively breeding animals where off-spring remain in their natal territory as adults, the oppor-tunity for incest may be substantial (Ligon and Ligon1990). Inbreeding avoidance mechanisms ensure thatcases of incest in the cooperatively breeding acornwoodpecker (Melanerpes formicivorus) are exceedinglyrare, and groups whose membership consists of onlyclosely related members of the opposite sex may foregobreeding for up to 3 years if membership remains unal-tered (Koenig et al. 1998). Incest is avoided in the su-perb fairy wrenMalurus cyaneusthrough a differentmechanism: although offspring remain on their natal ter-ritory, females seek extra-group matings, particularlywhen many sons remain as helpers in the group (Brookeret al. 1990; Mulder et al. 1994). Few other studies of co-operatively breeding populations have verified genetical-ly whether or not incest occurs.To date, most studies that have looked at genetic rela-tionships within groups of cooperatively breeding ani-mals have used multilocus DNA fingerprinting (Rabenoldet al. 1990; Jones et al. 1991; Packer et al. 1991; Bruce etal. 1996; Whittingham et al. 1997; Lundy et al. 1998). Inseveral cases, the authors appear not to have appreciatedthat this technique does not allow the detection of parent-offspring incest. While some have acknowledged the lim-itations of the data and were careful not to make inferenc-es beyond the scope of the technique (Haig et al. 1994;Dickinson et al. 1995; McRae 1996; Koenig et al. 1998),others have simply ignored the possibility of incest. Sincegenetic data of this nature are often compiled to drawgeneral conclusions about levels of reproductive skew(Reeve and Keller 1995), and inbreeding (Heinsohn et al.1990; Pusey and Wolf 1996), it is important that this pos-sibility is not overlooked. We outline here why conven-tional DNA fingerprinting is limited, and in some casesunsuitable, for studying parentage in social animals, anddiscuss the power of typing with microsatellite markers.The technique of multilocus DNA fingerprinting suf-fers from the constraint that individual bands cannot beassigned to known loci making it impossible to deter-mine whether a given band is of paternal or maternal ori-gin (Jones et al. 1991; McRae 1996; Danforth andFreeman-Gallant 1996). Careful analyses are required todetermine whether or not bands are segregating indepen-dently. For example, bands may be consistently scoredtogether (linkage), or they may appear mutually exclu-sive (allelism). Although segregation analyses can beused to calculate the probability of error due to non-inde-pendence, this is not always helpful for parentage analy-sis when parental candidates are the first-order relativesof other parental candidates.Paternity analysis with multilocus DNA fingerprintsrelies on the existence of ‘diagnostic’ bands to excludeindividuals from parentage. The problem with analyzingfingerprints of cooperative groups arises uniquely whenthere is an adult helper in the group that is the full off-spring of the dominant pair. This is a very commongroup structure among avian cooperative breeders re-ferred to as ‘simple’ family groups (Emlen 1995). Takethe example of a dominant pair whose adult son remainsin the group as a helper. When a chick is the offspring ofthe dominant male and female (i.e., the full sibling of thehelper), assuming the identity of the mother is reliable,there will often be (paternal) bands found in the finger-prints of the chick and the dominant male that are notfound in that of the helper son. Thus it is usually possi-ble to exclude the son from paternity. However, if achick is the product of mother-son incest, it is impossibleto exclude the dominant male (i.e., the actual grandfather

Reproductive skew and the threat of eviction: a new perspective

Most recent models of the partitioning of reproduction attempt to explain patterns of skew on the assumption that dominant individuals have complete control over breeding opportunities within the group, but may nevertheless concede a share of direct reproduction to subordinates as an incentive to remain peacefully in the association. Although these models may be applicable to some animal societies, we argue that they fail to provide a comprehensive theory of skew. Instead, we suggest that subordinates may often be able to claim unsanctioned reproduction for themselves, but will be forced to exercise a degree of reproductive restraint lest they incite ejection by the dominant. Reproductive skew, in other words, may reflect the threat of ejection (inducing subordinate restraint) rather than the threat of subordinate departure (inducing reproductive concessions by dominants). We present a simple ESS model of reproductive skew under these circumstances, which demonstrates that a shift in emphasis from reproductive concessions by dominants to reproductive restraint on the part of subordinates, radically alters the predictions of skew models. High group productivity, high relatedness and (when group members are related) strong ecological constraints are all expected to lead to reduced skew (the opposite conclusions to those of previous, concession-based analyses). The reason is that these factors reduce the benefits (or increase the costs) of ejection to the dominant, who therefore does best to tolerate more subordinate reproduction.

Ecological Factors Affecting Group and Territory Size in White-Throated Magpie-Jays

White-throated Magpie-Jays (Calocitta formosa) breed cooperatively and defend permanent, all-purpose group territories. We measured territory area, resource levels, resource dispersion, group size, and group reproductive success for 14 groups over a threeyear period in Costa Rica. Larger groups possessed larger territories containing more bullhorn acacia trees (Acacia cornigera and A. collinsii), which provided a critical food resource during the dry season. On a per capita basis, however, the number of acacia trees and territory area were the same for group members regardless of group size, and survivorship did not vary significantly with group size. Variation in reproductive success among groups was influenced by two factors:(1) larger groups produced more successful nests per year, and (2) territories with a higher density of acacia trees fledged more offspring per successfunel nest. Magpie-jays bred in pasture and foraged primarily in woodland; both habitat types were patchily distributed. The dispersion of woodland and pasture patches in the landscape appeared to constrain where jays could form territories, whereas the amount of acacia and other food resources determined the size that groups could attain. We conclude that ecological factors are critical to understanding the White-throated Magpie-Jay social system, along with other social and demographic constraints typically found in avianc ooperative breeders.