Abstract.  Population size of the Socorro Mockingbird Mimodes graysoni, which represents a monotypic genus endemic to Socorro Island, México, declined dramatically within the last 40 years. Postulated causal factors include competitive exclusion by Northern Mockingbirds Mimus polyglottos, predation by feral cats, and overgrazing by feral sheep. Habitat degradation looms as the primary candidate because surviving Socorro Mockingbirds live mainly in areas of the island with little apparent habitat damage, and because Northern Mockingbirds and cats both arrived on Socorro after much of the decline had occurred. Isolating key factors is difficult, however, because the present distributions of cats and Northern Mockingbirds coincide broadly with patterns of habitat degradation. To investigate habitat requirements of the endemic mockingbird in detail, we measured vegetation characteristics at localities with and without Socorro Mockingbirds, and observed foraging behavior of the native birds. Socorro Mockingbirds occupied sites in montane regions covered with Ilex socorroensis, Guettarda insularis, Triumfetta socorrensis, and Eupatorium pacificum, and they were also abundant in pristine lowland forests. Socorro Mockingbirds were scarce in disturbed forests where Dodonaea viscosa has replaced the original understorey, and they were absent from low-elevation Croton masonii scrub, even in areas with no signs of degradation. Socorro Mockingbirds foraged predominantly on fruits, and also on invertebrates. The density of trees bearing fleshy fruits associated positively with the number and persistance of Socorro Mockingbirds. These results support the hypothesis that the distribution of Socorro Mockingbirds is limited by habitat degradation, through depletion of resources provided by certain plants, habitat features providing protection from predators, and nesting substrates. Restoration of degraded habitat could help the population of Socorro Mockingbirds grow and reoccupy more of its former range.

Abstract. Champion and Gardner are the only two satellite islands of Floreana in the Gal·pagos archipelago that support populations of the Floreana Mockingbird, Nesomimus trifasciatus. The population on the much larger island of Floreana became extinct approximately 125 years ago. We studied the Champion population in every year from 1980 to 1991. Since the island is small (9.4 ha) we determined the demographic stability of the population by repeated censusing of the whole population. The number of breeding adults varied from 16 to 24 witrh a mean of ~20. Using a standard population genetics formula we calculate that the Champion population lost half of the original level of selectively neutral heterozygosity since presumed immigration from Floreana ceased more than a century ago. The short-term future of the Champion population is likely to be secure providing that goats and rats are not introduced, as these have caused much devastation on Floreana and probably resulted in the extinction of the mockingbird population there.

Abstract. Because predation is a primary cause of nest failure and habitat characteristics may influence predation risk, we may expect birds to be selective in choosing nest-sites. We studied nest-site selection and success in Florida Scrub-Jays in periodically burned habitat and nearby fragmented, overgrown habitat, using comparison between nest-sites and background habitat as well as landscape-scale analysis using GIS. Jays in periodically burned habitat nested disproportionately often in oak-dominated patches, in particular oaks, and in concealed sites often associated with vines. Nests in patches of moderate height, in certain shrub species, and with concealment and vines were most often successful; habitat type did not correlate directly with success. Patterns were similar overall in overgrown habitat: Despite taller vegetation and correspondingly greater nest height, jays had otherwise similar nest-sites and levels of success as those in frequently burned habitat. Habitat changes following fragmentation and fire suppression therefore usually do not have major impacts on Florida Scrub-Jay nest success through alteration of nest-site characteristics.

Abstract. The Socorro Mockingbird Mimodes graysoni is endemic to Isla Socorro, Mexico, and has declined dramatically in this century. Study of its population size, distribution, and structure is currently underway to help assess its conservation status. In 1993 and 1994 215 Socrro Mockingbirds were ringed; a modified Lincoln index yielded a population estimate of 353 ± 66 individuals in 1994. Most of the birds observed occupied a small region at high elevation that covered less than 10% of the island's area. Average territory size was 1.48 ± 0.71 ha, n = 11. Of the 86 birds banded in 1993 67% were adults and 33% subadults. Of the 170 birds detected in 1994, including new captures and ringed individuals from the previous year, 44% were adults and 56% subadults. In 1993 62% were male and 38% female; in 1994 56% were male and 44% were female. The sex ratio of the population was similar to that of other undisturbed Mimid populations, which argues against the possibility that cats might prey disproportionately on incubating females. Moreover, the presence of young birds in both years indicates that successful reproduction was occurring and that the population was not suffering catastrophic nest failure, as might be caused by cat predation on nestlings or fledglings. Nothern Mockingbirds Mimus polyglottos, which first arrived on Socorro in the 1970s, occupied mainly open areas produced by sheep grazing, which suggests that the Socorro Mockingbird is not being competitively displaced. Our observations indicate that habitat degradation by sheep is the most probable cause of the Socorro Mockingbird's decline.

Abstract. In birds and mammals with 'helpers-at-the-nest', some individuals not only feed unrelated offspring, but also compete to do so. Non-adaptive explanations for alloparental care do not predict competition for access to offspring that, in its most extreme form, can include kidnapping young from adjacent territories. A common adaptive explanation holds that allofeeding promotes a 'social bond', with non-relatives. This proximate hypothesis does not explain why the recipient later cooperates with or helps at the nest of its former benefactor. An extension of this hypothesis posits that, by helping to care for unrelated young, individuals may take advantage of a kin-recognition mechanism based on associations learned by nestlings while being fed. The deceived young may later offer assistance according to its perceived relatedness to the former helper. This mechanism, termed kinship deceit, may be a form of bet-hedging in cooperative breeding systems where mortality is high, where breeders can benefit from contributions by helpers, and where helpers normally assist relatives.

Abstract. We studied cooperative breeding of Nesomimus parvulus over 13 years (1978-90) across a broad range of climatic conditions. Variation in annual rainfall strongly influenced demography, but had few direct effects on social behavior. Indirect effects were pronounced. The frequencies of plygyny and associated joint nesting decreased with adult sex ratio. Along with density, mate availability determined the proportion of males that bred and, secondarily, the proportion that helped. Sex ratio did not affect the proportion of females that bred, but the proportion that helped decreased with the availability of males because mate-sharing females most often acted as helpers. Polygyny benefits males but is usually rare because monogamous females have higher reproductive succcess than females sharing mates. The frequency of plural breeding varied with population age structure. By favoring flexible reproductive behavior, climatic and demographic variation ultimately accounts for the occurrence of polygyny, joint nesting, and plural breeding.

Abstract

1. The Galapagos mockingbird is a group territorial and cooperatively breeding species. We studied its demography on Isla Genovesa for 11 years (1978-88), a period that included 7 years with moderate rainfall, 2 extremely wet years when El Niño-Southern Oscillation (ENSO) events occurred, and 2 drought years.
2. Annual survival of adults (x = 61%) was high during average conditions but lower in both drought years and during an epizootic. Males survived slightly better than females. Survival did not vary with age among adults, but survival of juveniles was lower (x = 35%) and twice as variable as adult survival.
3. Mockingbirds bred in response to rainfall, producing up to 5.2 fledglings per breeding female each season (x = 2.3). Clutch size, number of clutches and total production of fledglings increased in wet years. Helpers attended 34% of all nests and increased production of fledglings by 19%.
4. Differential mortality produced variation in the adult sex ratio. Breeding by males was constrained by the relative scarcity of females, and the proportion of males acting as helpers varied correspondingly because non-breeding males most often helped. The proportion of females breeding did not vary with adult sex ratio, because polygyny occurred in 2 years when females predominated, and most of the few females that helped also bred.
5. Variation in natality and survival caused substantial variation in population density, but the density of breeders remained relatively constant (C.V. = 21%) compared to density of non-breeders (C.V. = 93%). Group territories, which were defended year-round, filled all available habitat throughout virtually the entire study.
6. Group size averaged 4.2 adults, with up to four breeding females in each group. Groups were larger and plural groups more frequent in years when density was high and when yearlings predominated in the population; 67% of yearlings remained in their natal territories, where 47% of females and 62% of males also first bred.
7. Climatic variation influences mockingbird social organization mainly through its effects on natality and survivorship, which indirectly influence the population's density, age structure, and sex composition. Plural breeding is maintained in this species because climatically-induced demographic variation favours the tendency of young birds to breed without establishing independent territories.

Abstract. To investigate ecological influences on cooperative social organization, I studied the four allopatric species of mockingbirds (Nesomimus spp.) endemic to the Galápagos archipelago on four islands. On three small, low and arid islands (Genovesa, Champion, and Española), mockingtbird territories filled all terrestrial habitat, mean group size varied from 4.5 to 14.2 adults, maximum group size ranged from seven to 24 birds, and 70-100% of groups contained more than two birds. San Cristóbal in larger and higher, and it supports a broader range of habitats. At one highland and two coastal sites on this island, mockingbirds did not hold territories in all available habitats, group size averaged 2.2 adults, only 25% of groups were larger than two, and none included more than three adults. Adults dispersed into vacant habitat to estrablish new territories only on San Cristóbal. helping behavior has not yet been observed on San Cristóbal, but it occurs on the other three islands. These results support the hypothesis that social groups and cooperative breeding are maintained where limited availability of preferred habitat constrains dispersal. The mechanism relaxing habitat saturation on San Cristóbal, however, remains undetermined. Predation by introduced rats and cats may reduce survival and indirectly reduce group size; these predators are absent from Genovesa, Champion, and Española. Differences in food supplies could also affect inter- and intra-island variation in npopulation density. Variation in social organization among arid coastal sites on the four islands, and similarity between climatically different sites on San Cristóbal, suggest that climatic conditions are less important as determinants of dispersal and breeding. Skews in adult sex ratios also fail to account for inter-island variation in sociality. Although they live in a climatically variable environment, territorial behavior and the physical limits of suitable habitat have an overrriding influence on cooperative social organization in Galápagos mockingbirds.

Abstract. The social organization of the Galápagos mockingbird (Nesomimus parvulus) is 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 offsprinbg 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 close relatives preferentially fed closely related nestlings, but insufficient information was available to determine if adults did so also. 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.

Abstract.Reproductive conflict within groups can be an important feature of cooperative breeding systems, especially when more than one individual of a sex breeds within a social group. Relationships between group structure, dominance, within-group conflict and reproductive tactics of cooperatively breeding Galápagos mockingbirds were examined on Isla Genovesa. Territorial groups of 2-24 adults included up to three breeding females, with 42% of the groups containing more than one (plural groups); females in most plural groups nested sparately. Territory size increased with group size, but the area available per pair in plural groups was smaller than in singular groups (groups with only one breeding pair). Most pairings were monogamous, and males usually outnumbered females; high-ranking males obtained mates more frequently than subordinate males. in 3 relatively dry years, but not in a wet El Ninño year, subordinate pairs in plural groups fledged fewer young than dominant pairs or pairs breeding in singular groups. Interference by dominant breeders, often leading to abandonment of nests by subordinate pairs, appears to account for these differences: through nest disruption in drier years, dominant individuals may reduce the cost of sharing their territories and increase the chances of recruiting helpers. Dominant males in plural groups may also father young through extra-pair copulations with subordinate females. Despite costs imposed by within-group conflict, subordinate breeders have higher long-term reproductive success than birds that defer breeding. Plural group structure is maintained because unpredictable climatic variation favours opportunistic breeding by subordinates.

Summary. Mockingbirds began breeding on Genovesa in December 1982 following rain that first fell in November. Breeding ocurred until July 1983, with a short break in late February and early March. Some females initiated as many as six clutches. Clutches were somewhat larger than average than in prvious years. However, hatching success was unuusually low. Fledging success was normal. The net result was a typical number of fledglings per nest, but repeated breeding yielded a higher overall breeding success per female than in previous years. Most nest failures were attributable to heavy rainfall which killed eggs and newly hatched chicks. Some losses were due to predation by short-eared owls and to parental abandonment.

Population density on genovesa was unusually low at the beginning of the breeding season. This may have been the result of heavy adult mortality vcaused by a pox-like disease that continued to appear throughout the breeding season. In areas where disease-related mortality was high, group structure broke down. The incidence of helping at the nest was low, as many subordinate birds bred in new territories instead of staying on the territories of the dominant birds and helping to raise their offspring.

Elsewhere in the archipelago the syptoms of a pox-like disease were observed on many islands, and population sizes did not increase greatly despite extensive breeding. For example, on the islet of Champion the breeding season apparently spanned eight months but the population size remained approximately stable at 45 birds.