Summary 1.Understanding how species in a diverse regional pool are spatially distributed with respect to habitat types is a longstanding problem in ecology. Tropical species are expected to be specialists along environmental gradients, and this should result in rapid compositional change (high beta diversity) across landscapes, particularly when alpha diversity is a small fraction of regional diversity. Corollary challenges are then to identify controlling environmental variables and to ask whether species cluster into discrete community types along a gradient. 2. We investigated patterns of avian species' distributions in the Tilarán mountains of Costa Rica between 1000 m and 1700 m elevation where a strong moisture gradient exists. High beta diversity was found with both auditory counts adjusted for detectability and extensive capture data, revealing nearly complete change in community composition over a few kilometres on the Pacific slope. As predicted, this beta diversity was roughly twice as high as on temperate mountainsides. 3. Partial Mantel analyses and canonical correspondence analysis indicate that change in species composition is highly correlated with change in moisture (and correlated epiphyte cover) at different distances from the continental divide on the Pacific slope. Altitude was not a good predictor of change in species composition, as species composition varies substantially among sites at the same elevation. 4. Detrended correspondence analysis and cluster analysis revealed a zone of rapid transition separating a distinct cloud forest community from rainshadow forest. On the Caribbean slope, where a shallower moisture gradient was predicted to result in lower beta diversity, we found lower rates of compositional change and more continuous species turnover. 5. Results suggest that habitat specialization of birds is likely a strong ecological force generating high beta diversity in montane landscapes. Despite overall rapid rates of species turnover, zones of relatively coherent composition could be identified. 6. Landscapes with such high beta diversity are common in the tropics, although little studied. They offer high benefit/cost opportunities for conservation, particularly as climate change threatens to alter the species composition of communities of habitat specialists.

Summary Dispersal in many species is sex‐biased. Greenwood (1980, Animal Behaviour, 28, 1140–1162) hypothesized that the direction of sex bias was dependent on the mating system, and suggested that female‐biased dispersal in birds is associated with monogamy and resource defence by males, while mammalian male‐biased dispersal is associated with polygyny and male defence of mates. Cooperatively breeding brown jays Cyanocorax morio live in large territorial groups and are one of the few bird species known to exhibit male‐biased dispersal. Survivorship of adults is high, and both males and females delay breeding for many years. In this study, dispersers almost always moved to neighbouring social groups, rarely in response to a known breeding vacancy, and both sexes dispersed into groups with fewer members of the same sex than their origin group. Dispersal and extra‐group forays were strongly male biased. Forays were more common during nest building and egg laying, suggesting that males were monitoring mating opportunities, and many males later dispersed into the groups they visited. Genetic parentage analyses indicated that males mainly obtain paternity outside their natal groups through extra‐group forays and dispersal. The major routes to breeding status for females are inheritance of breeding positions on their natal territories and/or breeding there concurrently with an established breeder. Females usually dispersed on their own into groups where reproductive success had previously been poor. Kin facilitation appears to affect male dispersal options: related males formed coalitions and tended to disperse to groups where male relatives were already established. Male dispersal, female philopatry, and a matrilineal society correspond to three of Greenwood's seven predictions for a ‘mate‐defence’ system, but overt mate defence itself is absent, territorial defence is communal, and genetic polyandry is common. Social behaviour appears to affect a reversal in sex‐specific costs and benefits of dispersal, so that females benefit from philopatry and matrilineal inheritance, while males benefit from expanded dispersal options in part through kin facilitation, but without effective mate defence or clear cut sex‐specific resource defence.

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Dwarf mongooses (Helogale parvula) are small, communally breeding carnivores found in woodland and tree-savanna throughout Africa. Within a pack, socially subordinate mongooses do not normally breed, yet they invariably participate in all aspects of parental care. The primary alternative to tolerating reproductive suppression is dispersal, which shortens the wait for dominance and breeding. Here, we calculate the annual inclusive fitness payoffs to the dispersing and nondispersing strategies for males and females of ages one to seven, using data from a 14-year study in Serengeti National Park, Tanzania. Factors with effects on inclusive fitness included relatedness to pack mates before and after dispersal, the effect of help on recipients' reproductive success, the probability of dispersing successfully, the probability of attaining dominance, and reproductive success after attaining dominance. All of these factors differed between male and female dwarf mongooses. We compared the contributions of direct and indirect fitness to the total fitness of mongooses pursuing each of the strategies, across a range of ages. In our population, dispersal and nondispersal both yielded direct and indirect payoffs at most ages. For dispersers of both sexes, direct fitness was the primary component of total fitness but indirect fitness was substantial for young (< 2 years old) dispersers. For nondispersers of both sexes, indirect fitness was the major component of total fitness among young mongooses (up to 2 or 3 years), but direct fitness was the major component among older mongooses. By comparing the inclusive fitness payoffs for the two strategies, we determined the range of ages at which dispersal should be favored for each sex. These comparisons correctly predicted that males should be more dispersive than females at all ages, and that males should disperse over a broader range of ages.

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Stripe-backed Wrens, Campylorhynchus nucha/is, live in cooperative groups of up to 14 adults in which all members share in defense of the common territory, in construction and defense of roosting and breeding nests, and in the care of young produced by a single breeding pair. A 5-yr study of a completely marked population of 25-30 groups in the Venezuelan savanna shows that breeders (principals) assisted by two or more nonreproductive auxiliaries have much greater reproductive success than those with less help. Per capita production of independent juveniles is higher, as well. Only groups with at least two auxiliaries successfully rear two clutches in a season. The proportion of nesting attempts successful in producing fledglings is much higher for groups of four adults or more than for smaller groups, probably because oflesser predation on the nests oflarge groups. The success of large groups is not likely due to differences in territory quality or breeder experience.Rates of food delivery to nestlings and number of fledglings per successful nest are as high for pairs and trios as for larger groups. However, feeding of nestlings and fledglings by auxiliaries frees the principals from these tasks and probably makes multiple successful clutches possible. Auxiliaries directly defend the breeding nest from predators and competitors, although the principal male is most active in defense. Feeding efforts of auxiliaries are probably linked to increased nesting success primarily through enabling the principal male to defend the nest better. Aid-giving by auxiliaries is explained (I) by its effectiveness in improving the reproductive success of close kin through reducing risk of predation for nestlings, (2) by the probability of reciprocation of such effective help, and (3) by the low productivity of breeding outside a large established group. Data from this population of wrens constitute some of the strongest evidence to date that group defense against predators can provide the impetus for cooperative breeding.

Summary 1.Dispersal from the natal area, or between breeding sites, marks a critical transition for individual animals, and a fundamental demographic and genetic process. Limitations on dispersal in natural populations are important in predicting population dynamics and structure. Tropical habitats that can be naturally patchy are also being fragmented rapidly, and these habitats harbour many social species. 2. We studied dispersal in cooperatively breeding stripe-backed wrens ( Campylorhynchus nuchalis ) in the Venezuelan savanna, using 20 years of data from marked populations over 40 km 2 . We test the hypotheses that dispersal is limited by competition for social position and that such limitation is sex-specific. 3. Dispersal is strongly female-biased, and results mainly in attainment of breeding status in neighbouring groups. Reproductive success is determined by the structure of the breeding group, and is concentrated in a small minority of individuals. 4. Natal dispersal is delayed, and represents a dramatic shift from indirect contribution to kin production (helping) to the direct approach of breeding. Monogamy limits breeding opportunities, compared to other social species, and both natal and breeding dispersal are constrained to short distances, particularly when compared to noncooperative breeders. 5. Males are more philopatric compared to females, for which more frequent and distant dispersal leads to greater reproductive success. Competition for breeding status in an established social order impedes dispersal, in part because experience and previous familiarity are important, particularly in males that rarely disperse to productive large groups. 6. Diverse patterns of sex bias in dispersal among cooperatively breeding birds can be explained by the comparative diversity of reproductive alternatives available to males and females. A 'breeding diversity' hypothesis is illustrated by comparison with malebiased dispersal in brown jays. 7. Social resistance to movement establishes a brake on gene flow and demographic interaction among populations that has important implications for population viability.