Kerry N. Rabenold

Cooperation in breeding by nonreproductive wrens: kinship, reciprocity, and demography

SummaryStripe-backed wrens (Campylorhynchus nuchalis) often live as adults in large groups on permanent, communally defended territories. Nonbreeding adults cooperate in rearing the young of a single breeding pair; this aid substantially increases the reproductive success of the breeders. In a 6-year study in Venezuela of a completely colorbanded population of 25–30 groups, most adults participated in breeding only as helpers and priority to breeding status was strictly age-determined. Detailed behavioral observations at breeding nests with nestlings showed that, in a sample of 100, helpers nearly always contributed as much to the care of young as breeders. Further, aid-giving does not vary systematically with relatedness of ycung to helpers or with probability of future reciprocation by young. Young being raised are most often at least half siblings of helpers, but seldom return aid to adults that helped raise them. Even adopted helpers collaborate fully. Patterns of demography and dispersal show slow turnover of breeders, delayed reproduction, and a viscous population structure.Application of Hamiltons condition for selection for aid-giving reveals that most individuals in this population can maximize inclusive fitness in the first 2 years by helping instead of breeding. Variation in helping effort and in age of first breeding is related to variation in natal group size and competition resulting from variable demographic neighborhoods in different years or in different parts of the population. Because reciprocation in the form of specific alliance formation among nonreproductives is uncommon, nonspecific reciprocity between cohorts and kin selection account well for the observed pattern of age-dependence in first breeding. Nondiscriminating helping in this population is associated with stable monogamous pair boncs, stable territory boundaries and group membership, strict seniortiy for breeding position, high viscosity and consistent effectiveness of aid. Under these circumstances, very simple behavioral rules amounting to nearly automatic helping seem sufficient to confer critical inclusive fitness gain on helpers.

Beta diversity along environmental gradients: implications of habitat specialization in tropical montane landscapes

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.

Assessment, age and proximity in dispersal contests among cooperative wrens: field experiments

Abstract Experimental removals of breeding females were conducted in a completely marked population of 35–85 territories in the cooperatively breeding stripe-backed wren, Campylorhynchus nuchalis, over 3 years in central Venezuela. The ability of non-breeders to discriminate among 19 vacancies with varying reproductive potential was tested and competitive asymmetries among contestants were examined. Significantly more females responded to openings (both experimental and natural) in large groups (four or more adults) than to openings in smaller ones. Data from 12 years show that breeders in large groups have much greater reproductive success than those in smaller groups. As many as 10 contestants fought with intense aggression for several days over vacancies in large groups. In contrast, vacancies in small groups attracted few contestants with little aggression and frequent abandonment of the opportunity. Nearly all older females (greater than 1 year of age) in the population competed for at least one vacancy, while only half of the younger females did so. When competitors responded from both age classes, an older female normally gained the position. Females from adjacent territories won valuable vacancies in large groups more often than did competitors from farther away. The escalation of these contests to intense aggression can be attributed mainly to the scarcity and necessity of the resource (multiple helpers) and the potential for monopolizing it. Advantages of a e and proximity probably promote delayed short-distance dispersal that is characteristic of this and other cooperative species.

Reproductive enhancement by helpers and an experimental inquiry into its mechanism in the bicolored wren

SummaryAs part of continuing studies of sociality in the wren genus Campylorhynchus we have been studying the bicolored wren — a facultatively cooperative breeder — for the past 6 years in the central Venezuelan savanna. Reproductive groups have ranged in size from 2 to 5. In one of our study populations, only about 15% of the groups contained helpers, and nearly all these contained only a single male helper (Fig. 2). In an adjacent population, the majority of groups contained helpers, and more than half of these contained several helpers of either sex. Territory size is, on average, much smaller in the latter population. In these populations the presence of a single helper is associated with a three-fold increase in reproductive success (Table 1). Additional helpers are not associated with further reproductive enhancement. Enhancement is chiefly due to an increased proportion of nest starts that eventually produce independent juveniles. This reproductive enhancement is not merely an epiphenomenon resulting from the presence of helpers on territories which are superior for other reasons, such as greater resource availability or the quality of particular parents. It is also not a function of the mean or variance in nestling feeding rate. Predator exclusion experiments, in which certan nests were artificially protected from terrestrial predators, suggested that the mechanism of reproductive enhancement was heightened effectiveness of nest defense. Helpers are usually nondispersers from the parental territory, and have always been found to be close relative of the nestlings that they assist in rearing.

Extra-pair paternity uncommon in the cooperatively breeding bicolored wren

Abstract We investigated parentage using multilocus DNA fingerprinting for 222 juveniles produced during 99 group-years in the bicolored wren Campylorhynchus griseus, a cooperatively breeding bird of the Venezuelan savanna. Young adult bicolored wrens (auxiliaries) remain in their natal territories and substantially enhance the production of young there. We have previously used behavioral indicators of dominance by a single male/female pair (principals) to infer breeding status, resulting in the commonly applied model of helping in which current fitness accrues to auxiliaries only indirectly, in proportion to their relatedness to the principals and the effect of their assistance on breeding success. Our parentage analysis has demonstrated that 8.6% of the juveniles found on territories were not produced by the principal pair. Parentage of 4.1% of the juveniles was completely outside the social group; these appear to result from early dispersal of juveniles rather than from brood parasitism, most likely resulting from breakup of nearby groups. Principal females mated outside of their group (2.3%), or with an auxiliary male (2.3%), in the remaining cases of parentage outside the principal pair. No matings were detected between close relatives (e.g. mother-son); matings detected between the principal female and an auxiliary male followed a typical replacement of the principal female by an unrelated immigrant female. Our finger-printing results indicate that: (1) current fitness benefits accruing to most auxiliaries do not exceed their likely reproductive success had they dispersed successfully to a breeding position; (2) nearly all wren mating is monogamous and (3) behavioral dominants (especially females) can monopolize breeding.

Using landscape metrics to model source habitat for Neotropical migrants in the midwestern U.S

Size of a forest patch is a useful predictor of density and reproductive success of Neotropical migratory birds in much of eastern North America. Within these forested landscapes, large forest tracts appear to be sources – fragments in which surpluses of offspring are produced and can potentially colonize new fragments including woodlot sinks where reproduction fails to balance adult mortality. Within agricultural landscapes of the midwestern U.S., where forests are severely fragmented, high levels of brood parasitism by brown-headed cowbirds (Molothrus ater) and intense predation on nests generally result in low reproductive success for Neotropical migrants regardless of forest size. In some midwestern U.S. landscapes, however, the variation in reproductive success among forest fragments suggests that `source habitat could still exist for Neotropical migrants. We used vegetation, fragment and landscape metrics to develop multivariate models that attempt to explain the variation in abundance and reproductive success of Neotropical migrants nesting in an agricultural landscape in northern Indiana, USA. We produced models that reasonably described the pattern of species richness of Neotropical migrants and the abundance of wood thrushes (Hylocichla mustelina) and several other Neotropical migrant species within 14 forest fragments. In contrast, we were unable to produce useful models of the reproductive success of wood thrushes breeding in the same forest fragments. Our results suggest that (1) abundance patterns of Neotropical migrants are probably influenced by both landscape- and fragment-scale factors; (2) multivariate analyses of Neotropical migrant abundance are not useful in modeling the corresponding patterns of reproductive success; and (3) the location of any remaining `source habitat for Neotropical migrants breeding within agricultural landscapes in North America will be difficult to predict with indirect measures such as vegetation composition or landscape context. As a result, the potential for developing conservation strategies for Neotropical migrants will be limited without labor-intensive, direct measurements of demographic parameters.

Effects of aggregation on feeding and survival in a communal wren

SummaryForaging by a social wren, Campylorhynchus nuchalis (Troglodytidae), in a tropical savanna habitat is not enhanced by aggregation. Data for marked individuals show that solitary foraging results in a higher capture rate than foraging near others. We find no evidence of imitative foraging, as individuals actively avoid successful foragers following a capture and successful foragers do not restrict their search to recently productive stations or techniques. Captures are seldom temporally clumped, and clumping is probably not pronounced enough to favor imitation. Juveniles show no greater tendency to respond to captures of others, or to succeed in foraging in a group, than do adults. Aggregation is probably disadvantageous for foraging because of dispersed, scarce, cryptic, and noneruptive prey and because of the searching technique of these foliage-gleaning insectivores. If predator avoidance is enhanced by aggregation, it does not result in either increased survival or increased foraging efficiency in large groups, even by juveniles.

Latitudinal Gradients in Avian Species Diversity and the Role of Long-Distance Migration

Understanding geographic patterns of variation in the species composition of natural communities, as well as increasing species diversity from temperate latitudes to the tropics in particular, remains one of the great challenges to ecology. The extreme diversity of plant and animal species in some tropical habitats, compared to those of the temperate zone, have inspired evolutionary biologists since Darwin and Wallace (Wallace, 1878; Dobzhansky, 1950; Mayr, 1969; MacArthur, 1969). Biologists have responded to the challenge of biogeographic patterns with explanatory schemes on a variety of spatial and temporal scales: evolutionary (including geographical patterns of adaptive radiation), biogeographical (including regional history), and ecological (factors operating locally within a generation). Explanations on these different levels are generally complimentary (reviews in Fischer, 1960; Pianka, 1966; MacArthur, 1972), but the theoretical foundation for these schemes, especially concerning the roles of species interactions and regional patterns of speciation, is still being developed (Prance, 1982; Cody, 1985; Cox, 1985; Diamond and Case, 1986; Kikkawa and Anderson, 1986). To some degree, altitudinal diversity gradients have mirrored latitudinal ones and have been explained in similar ways (Terborgh, 1971,1977,1985; Able and Noon, 1976).

Forest Succession in an Oak-Hickory Dominated Stand During a 40-Year Period at the Ross Biological Reserve, Indiana

Abstract Deciduous forests in the eastern United States have been dynamic over both geological and historical time scales, particularly since humans have modified the landscape. Historically, the central hardwood forest has been subject to considerable human disturbance, especially fire, and these anthropogenic disturbances have contributed to the dominance of shade-intolerant oak (Quercus) and hickory (Carya) species within the central hardwood region. Current research indicates that many of these forests are changing to dominance by shade-tolerant species, mainly sugar maple (Acer saccharum Marsh.). This change has been attributed to a lack of disturbance. The objective of this study was to document long-term changes in structure and composition of a mature oak-hickory stand at the Ross Biological Reserve in Indiana. There has been little disturbance at the Ross Biological Reserve in the past 55 years, and a similar trend of increased shade-tolerant species was expected. Results of decadal tree censuses suggest that a successional change toward the dominance of sugar maple has been occurring during the 40-year study period, and an abundance of sugar maple saplings suggests that the increasing importance of sugar maple will continue with a lack of disturbance. The increased abundance of sugar maple may be having a negative effect on regeneration of oak-hickory species as well as on understory species such as flowering dogwood (Cornus florida L.). Such changes in the plant community also suggest changes in resources for animals. Management of the Ross Biological Reserve, and similar areas, requires an appreciation of forest dynamics on a variety of time scales.

TERRITORIALITY AND NEIGHBOR ASSESSMENT IN BROWN JAYS (CYANOCORAX MORIO) IN COSTA RICA

Abstract Defense of group-held resources is a common and widely accepted function of territorial interactions between neighboring groups. In addition, territorial interactions could provide opportunities to assess members of neighboring groups and reproductive opportunities there, or to solidify status in the home group. We studied group-level characteristics and individual participation in territorial encounters in the cooperatively breeding Brown Jay (Cyanocorax morio). Intergroup encounters at stable territory boundaries include both aggressive and affiliative behavior, which suggests that a territorial encounter could function as both a resource defense mechanism and as an arena for social interactions. Territory characteristics that increase the probability of contact between groups (long boundaries, large combined group size, and home range overlap) explain much of the variation in frequency of territorial encounters. Male-biased dispersal was more common to neighboring groups with long boundaries, supporting the idea that frequent interactions between neighbors facilitate dispersal. Females usually inherit breeding positions on their natal territories, and participation in intergroup encounters by females does not vary with age or breeding status. In addition to defending group resources, females on their natal territories could be defending their positions in the breeding queue. Immigrant females are not likely to breed successfully, or to disperse again, and they participated less than expected. Participation by both natal and immigrant males varied by age; young males, at the ages when dispersal and intergroup forays are most likely, participated more than expected, whereas older males (≥4 years) participated less. That is consistent with the hypothesis that participation in intergroup encounters facilitates dispersal and improves integration into social groups. Because extragroup matings occur in this population, both breeding females and males could be assessing neighboring individuals for mating opportunities. Resource defense and social facilitation are not mutually exclusive hypotheses, and our observations suggest that both are important components of territorial encounters in Brown Jays.