Habitat Saturation Research Articles

Importance of habitat saturation and territory quality for evolution of cooperative breeding in the Seychelles warbler

COOPERATIVE breeding, which often involves young remaining on their natal territory and helping their parents to raise subsequent broods1–3 is mostly explained by habitat saturation: young are constrained from becoming independent breeders by a shortage of breeding territories2,4. Here I present two lines of evidence against this hypothesis for the cooperatively breeding Seychelles warbler Acrocephalus sechellensis. first, territory quality has a significant effect on dispersal: vacancies arising on territories are mostly filled by prebreeding birds from territories of the same or lower quality. Second, individuals that delay reproduction in high-quality territories, but which eventually breed there, have greater lifetime fitness than those that disperse at one year of age and breed immediately in lower-quality territories. These results support the 'benefits of philopatry,5,6 hypothesis, which emphasizes the lifetime inclusive fitness benefits from staying at home. The transfers of warblers to unoccupied islands was the strictest experimental test of this hypothesis. At first there was no cooperative breeding, but as all high-quality areas became occupied, young birds born on high-quality territories began to stay as helpers, rather than occupying breeding vacancies on low-quality territories. Therefore habitat saturation and territory quality are both involved in the evolution of cooperative breeding.

The evolution of delayed dispersal in cooperative breeders.

Why do the young of cooperative breeders--species in which more than two individuals help raise offspring at a single nest--delay dispersal and live in groups? Answering this deceptively simple question involves examining the costs and benefits of three alternative strategies: (1) dispersal and attempting to breed, (2) dispersal and floating, and (3) delayed dispersal and helping. If, all other things being equal, the fitness of individuals that delay dispersal is greater than the fitness of individuals that disperse and breed on their own, intrinsic benefits are paramount to the current maintenance of delayed dispersal. Intrinsic benefits are directly due to living with others and may include enhanced foraging efficiency and reduced susceptibility to predation. However, if individuals that disperse and attempt to breed in high-quality habitat achieve the highest fitness, extrinsic constraints on the ability of offspring to obtain such high-quality breeding opportunities force offspring to either delay dispersal or float. The relevant constraint to independent reproduction has frequently been termed habitat saturation. This concept, of itself, fails to explain the evolution of delayed dispersal. Instead, we propose the delayed-dispersal threshold model as a guide for organizing and evaluating the ecological factors potentially responsible for this phenomenon. We identify five parameters critical to the probability of delayed dispersal: relative population density, the fitness differential between early dispersal/breeding and delayed dispersal, the observed or hypothetical fitness of floaters, the distribution of territory quality, and spatiotemporal environmental variability. A key conclusion from the model is that no one factor by itself causes delayed dispersal and cooperative breeding. However, a difference in the dispersal patterns between two closely related species or populations (or between individuals in the same population in different years) may be attributable to one or a small set of factors. Much remains to be done to pinpoint the relative importance of different ecological factors in promoting delayed dispersal. This is underscored by our current inability to explain satisfactorily several patterns including the relative significance of floating, geographic biases in the incidence of cooperative breeding, sexual asymmetries in delayed dispersal, the relationship between delayed dispersal leading to helping behavior and cooperative polygamy, and the rarity of the co-occurrence of helpers and floaters within the same population. Advances in this field remain to be made along several fronts.(ABSTRACT TRUNCATED AT 400 WORDS)

Test of the Ecological Basis of Cooperative Breeding in Red-Cockaded Woodpeckers

-In many cooperatively breeding birds, the primary reason that helpers remain on the natal territory rather than disperse to breed independently may be to gain an advantage in competing for high-quality habitat. We hypothesized that cavities excavated in living pines, because they require much time to construct, are the critical determinant of habitat quality that has led to cooperative breeding in Red-cockaded Woodpeckers (Picoides borealis). These woodpeckers rarely colonize sites that lack existing cavities. To test our hypothesis we drilled cavities in 20 unoccupied sites. Eighteen were occupied subsequently, but none of 20 control sites were used. The manipulation added 12 new social units (breeding pairs or unpaired territorial males) to the population. New groups mostly comprised previous helpers and dispersing first-year birds. These results support our contention that variation in habitat quality dependent on the presence or absence of cavities is the ecological basis of group formation in Red-cockaded Woodpeckers. Cavity construction may be used to increase the number of groups in a population, and to prevent territory abandonment when bird-constructed cavities are lost. Received 3 October 1990, accepted 24 June 1991. COOPERATIVE breeding refers to a social system found in birds and mammals in which some reproductively mature individuals (helpers) assist others in raising young (Brown 1978, 1987, Emlen and Vehrencamp 1983, Emlen 1992). Understanding the evolution of cooperative breeding involves two interrelated but distinct issues (Emlen 1982a, b, 1992, Brown 1985, 1987, Ligon and Stacey 1989): (1) determining what leads individuals to remain with their natal groups or form groups rather than disperse to breed independently; and (2) determining why such individuals engage in helping behavior such as feeding young. Altruism evolved through kin selection is an accepted explanation of helping behavior (issue 2; Emlen and Wrege 1988,1989). In contrast, remaining in the natal group (issue 1) and thereby delaying reproduction and altering dispersal behavior generally is viewed as producing direct benefits to the individual under certain conditions (Emlen 1982a, Woolfenden and Fitzpatrick 1984, Brown 1985, 1987, Stacey and Ligon 1987). Emlen (1982a) outlined two conditions under which remaining with the natal group may result in greater lifetime reproductive success than early dispersal and breeding. One condition involves a harsh, unpredictable environment. In I Present address: RD1, Box 27, Spring Mills, Pennsylvania 16875, USA. poor years, inexperienced birds reproduce poorly, so that living with the natal group is favored over independent reproduction. This hypothesis may apply to species that inhabit the dry regions of Africa and Australia (Reyer 1980, Emlen 1981, 1982a, Clarke 1984). The second, perhaps more common condition has traditionally been termed habitat saturation, because it is thought to result from a shortage of vacancies in breeding habitat (Selander 1964, Brown 1969, Stacey 1979, Emlen 1982a). An apparent lack of unoccupied territories has been noted in many cooperative breeders (e.g. Selander 1964, Ridpath 1972, Woolfenden 1975, Ligon and Ligon 1978, Walters and Walters 1980, Zack and Ligon 1985, Koford et al. 1986), but not all (Rabenold 1985). In species to which the habitat-saturation hypothesis has been applied, helpers remain on their natal territory and compete for breeding vacancies on and in the immediate vicinity of the natal territory (stay-andforay, abbreviated SAF), rather than dispersing after fledging to wander in search of a breeding vacancy (depart-and-search, DAS; Brown 1987). Those practicing SAF appear to have an advantage in competing for positions in their vicinity over those practicing DAS (Zack and Rabenold 1989), perhaps because they can monitor the availability of those positions much more effectively. The demographic conditions under which SAF may be selected over DAS have been mod-

The Role of Territoriality in the Social Organization of Harris' Hawks

ABsTAcr.-We studied territoriality and sociality in a population of Harris' Hawks (Parabuteo unicinctus) in Arizona during breeding and nonbreeding periods from 1984 to 1986. Our study area contained 22 to 26 breeding groups and density of nests averaged 1/2.0 km2. The number of hawks in breeding groups ranged from 2 to 7 and averaged 3.8. Hawks in breeding groups rarely ranged beyond 0.8 km from active nests, except to visit sources of water, and we did not observe overlap of hunting ranges in any groups. Hawks from different groups formed social aggregations in zones between nesting areas. Aggregations formed only during nonbreeding periods (autumn and winter) and the frequency of aggregations peaked approximately 3 weeks before egg laying. Aggregations averaged 5.9 hawks (range = 4-11), and were composed of members of 2 or 3 adjoining groups and transient hawks. Aggregations may allow potential immigrants to assess and be assessed by group members, and may provide benefits to participants via cooperative hunting. All aggressive behaviors observed during the study, except supplanting (i.e. one hawk replacing another at a perch), were more common between individuals in aggregations than in groups. The most intense aggressive behaviors (e.g. chasing and foot grabbing) were never observed between group members. Resident hawks chased trepassing conspecifics out of the nest area in all incursions observed during breeding and nonbreeding periods. Residents also showed aggression toward a trained conspecific released at 13 of 14 active nests. Our evidence that Harris' Hawks are territorial contradicts part of the information that has been used to reject the habitat saturation model for the development of cooperative breeding in this species. We propose that water, an important resource during the summer, may represent an ecological constraint that favors group living in Harris' Hawks in the Sonoran Desert. Received 29 May 1990, accepted 15 January 1991.

The Benefits-of-Philopatry Hypothesis for the Evolution of Cooperative Breeding: Variation in Territory Quality and Group Size Effects

Cooperative breeding systems are characterized by the presence of individuals that remain as nonbreeders in their natal groups past the age of reproductive maturity. Currently, the most widely accepted explanation of this nondispersal is that immature members of cooperative species are faced with ecological constraints on independent breeding, such as habitat saturation. However, current evidence indicates that reproductive opportunities in many noncooperative breeders (where immature individuals always disperse) are just as limited as those in cooperative species. Thus ecological constraints alone cannot explain the differences in the two dispersal patterns. As an alternative, we describe a benefits of philopatry hypothesis, which proposes that nonbreeding helpers remain at home only when there is a net fitness benefit to doing so. This is most likely to occur either when territories in a local area vary greatly in quality or when group cooperation leads to high variance in fitness of individuals in di...

The effect of conspecifics on habitat selection in territorial species

Despite widespread interest in habitat selection, many of the assumptions about how territorial animals choose habitats have not been tested. This study of juvenile Anolis aeneus lizards focuses on the relationship between the number of previous settlers in a habitat and the subsequent behavior of new arrivals at that habitat. Clearings containing the types of microhabitat preferred by juveniles were established in the field, several juvenile residents were allowed to establish territories in enclosures in the center of each clearing, and then naturally occurring immigrants were allowed access to the empty microhabitat surrounding the enclosures. Arrival rates and the probability of settlement were monitored on a daily basis from the day the first juveniles arrived until several days after the last juvenile had settled (=saturation). In each of seven trials, arrival rates were comparable early and late in the settlement process, and were unrelated to the degree of habitat saturation. Arrival rates did vary on a temporal basis, probably as a result of environmental factors affecting egg laying and hatching schedules, and habitats with high arrival rates saturated more quickly than those with lower arrival rates. All of the individuals arriving at the clearings did not settle, but settlers and non-settlers did not differ with respect to competitive ability, as measured by body size. The probability of settlement increased as settlement proceeded in each of seven trials, up to the day of saturation. These results refute the commonly held assumptions that prospective territory owners avoid entering relatively full habitats, and that they prefer to settle in relatively unsaturated habitats. The discussion considers why assumptions about the behavioral processes of habitat selection are so widely accepted, given the dearth of empirical information on the subject.

Territory Size as a Predictor of the Upper Limit to Population Density of Juvenile Salmonids in Streams

We examined the old, but untested hypothesis that territory size limits the maximum population density of salmonids in streams. We used published data to derive an interspecific regression of territory size (m2) on fork length (cm) (log10 territory size = 2.61 log10 length—2.83, r2 = 0.87, n = 23). Growth and mortality trajectories of salmonid cohorts from eight experimental studies were compared to the maximum-density regression, the inverse of the territory–size regression. In shallow habitats, such as riffles and raceways, the cohort trajectories followed the maximum density regression quite closely and were consistent with the territory–size hypothesis. In addition, natural densities in eight other studies did not exceed the predicted maximum density and tended to fail within the 95% C.L. of the maximum-density regression. Data from shallow habitats, therefore, provide strong support for the territory–size hypothesis. A linear logistic response model showed that the probability of observing density-dependent growth, mortality, or emigration increased significantly with increasing values of an index of habitat saturation, developed from the territory–size regression. Our results suggest that the territory–size regression has practical value for predicting the maximum densities of stream-dwelling salmonids in shallow habitats and the occurrence of density-dependent population responses.

Coupling Delayed Breeding with Short‐distance Dispersal in Cooperatively Breeding Birds

AbstractShort‐distance dispersal in cooperatively breeding birds is demonstrated and a new model describes how it could be causally related to delayed breeding. Nonbreeders are hypothesized to wait for specific, adjacent territories of high quality. Strong, predictable differences in territory quality select for extended natal philopatry in comparison with species whose young disperse upon maturity. A nonbreeder's ability to discriminate territory quality, and competitive advantages of age and proximity in obtaining nearby breeding positions, are necessary conditions of this model. Direct benefits to both males (potential for inheritance of a breeding position on the natal territory) and females (priority to nearby territories of high quality) result from philopatry. This model differs from most others by emphasizing potential direct benefits of delayed dispersal, rather than assuming constraints, such as habitat saturation. Short‐distance dispersal is widespread in other bird and mammal social systems characterized by natal philopatry and delayed breeding, suggesting a general application of this model.

Habitat Saturation and Ecological Constraints: Origin and History of the Ideas

Lawton suggests that my book, Helping and Communal breeding in Birds, must be fascinating to sociologists of science because the book received favorable reviews in Science, Nature, and American Scientist (to which could be added, Animal Behavior, Trends in Ecology andEvolution, Journalfur Ornithologie, and others) but embarrasses some of her anonymous acquaintances who work in this area. Using Hull's concept of conceptual lineages and literature on delayed breeding, she argues that I have not credited certain unspecified workers adequately. Lawton's extreme accusations concern matters that are centrally important in the history of the ornithological study of helping behavior and that apparently need further clarification. Therefore, I accept her invitation to comment.

Geographic variation in social organization of Galápagos mockingbirds: ecological correlates of group territoriality and cooperative breeding

To investigate ecological influences on cooperative social organization, I studied the four allopatric species of mockingbirds (Nesomimus spp.) endemic to the Galapagos archipelago on four islands. On three small, low and arid islands (Genovesa, Champion and Espanola), mockingbird 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 Cristobal is 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 establish new territories only on San Cristobal. Helping behavior has not yet been observed on San Cristobal, 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 Cristobal, 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 Espanola. Differences in food supplies could also affect interand intra-island variation in population density. Variation in social organization among arid coastal sites on the four islands, and similarity between climatically different sites on San Cristobal, 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 overriding influence on cooperative social organization in Galapagos mockingbirds.

Territory acquisition and loss in male song sparrows

Male song sparrows, Melospiza melodia, contested for territories year-round in a non-migratory population. Over 5 years, most settlers were yearlings, but many were older floaters that had previously owned a territory. Most turnovers occurred when floaters evicted territory owners from all or part of their territories. This finding contrasts with the assumptions of most models of dispersal and habitat settlement, i.e. that owners are always dominant to floaters, and that floaters gain territories only after an owner's death. Territory owners often evicted neighbours in the breeding period, when they were also likely to gain access to additional females. Floaters usually gained territories just prior to breeding and as breeding began. These patterns are consistent with the idea that male territory defence primarily serves to provide access to females. Middle-aged males more often expanded their territories and regained them after being evicted than 1- or 4-year-old males. Birds that floated as yearlings were more likely to float after owning a territory as an adult than were birds that gained a territory before April of their first breeding season. Individual differences in competitive ability persist throughout life, but vary with age. Factors that affect patterns of aggressive versus passive territory acquisition between populations include: migratory tendency, habitat saturation, length of the breeding period, the potential to breed outside the natal population, the relative survival rates of territory owners versus floaters, and the mortality rate of territory owners in the absence of challenges by floaters. An understanding of the territorial system is crucial to predicting patterns of dispersal and habitat distribution.

Philopatry, Dispersal, and Habitat Saturation in the Banner‐Tailed Kanagaroo Rat, Dipodomys Spectabilis

We report natal dispersal distances for 331 banner—tailed kangaroo rats (Dipodomys spectabilis) at high and low population density. The data were collected in 29 censuses of marked individuals in two populations over 8 yr. One population underwent four—fold variation in density. The other was consistently at high density. Because the number of burrow systems (i.e., breeding sites) remained fairly constant, we interpret changes in density as changes in degree of habitat saturation. Dispersal distances were shorter and the proportion of philopatric young (moving less than a home range diameter) was greater at high density than at low density. This result does not conform to models of dispersal based on studies of cycling small mammals or to a simple competition model of dispersal. All data are consistent with a habitat saturation model, which holds that high—density saturated conditions favor philopatric tendencies in offspring.

Offspring quality and the evolution of cainism

Obligate siblicide, known as ‘cainism’ in large raptors, is a taxonomically widespread avian phenomenon that remains inexplicable as a simple consequence of food stress: two young can be raised to independence in experimentally manipulated nests, and food supplements do not decrease sibling aggression.A review of the Falconiformes identified 23 species in which obligate and facultative cainism is regular. All species have small clutches and deferred acquisition of adult plumage. Obligate cainists in particular are large, long‐lived species characterized by extreme subadult mortality and intense competition for breeding sites. Hence, it can be suggested that early sibling conflict, in the absence of food stress, is the end result of selection for quality (survival) and competitive ability. Cain's domination or killing of Abel insures (1) an increase in Cain's chances of survival through the high‐risk, pre‐breeding period via improved nestling weight gain, and/or (2) domination of surviving sibs, enhancing Cain/s competitive abilities and thereby increasing the probability of achieving breeding status. Only among long‐lived species can the benefits of enhanced survival and competitive ability outweigh the major costs of sibling loss.Facultative cainists. which in more than 10% of cases raise more than one young (despite aggression and sibling hierarchies), not only lay larger, more variable clutches, but on average attain adult plumage earlier than obligate cainists. Their shorter lives and higher population turnovers are consistent with their less extreme siblicidal tendencies. Similar life‐history traits and cainistic habits in other avian orders parallel those in the Falconiformes, indicating several independent evolutionary pathways to cainism.Retention of the second egg by obligate cainists, usually explained as insurance against failure, may instead allow parents adaptively to track population stability. Thus when breeding places are numerous (habitat saturation and competition low), parents laying two eggs and rearing two young may achieve greater fitness than single‐young parents. When populations become saturated (competition high), selection should favour high‐quality, competitive young and levels of siblicide should increase. Aproximate mechanism is proposed linking population saturation with the incidence of cainism, based on demonstrable population characteristics found in several long‐lived species.

Offspring quality and the evolution of cainism

Obligate siblicide, known as ‘cainism’ in large raptors, is a taxonomically widespread avian phenomenon that remains inexplicable as a simple consequence of food stress: two young can be raised to independence in experimentally manipulated nests, and food supplements do not decrease sibling aggression.A review of the Falconiformes identified 23 species in which obligate and facultative cainism is regular. All species have small clutches and deferred acquisition of adult plumage. Obligate cainists in particular are large, long‐lived species characterized by extreme subadult mortality and intense competition for breeding sites. Hence, it can be suggested that early sibling conflict, in the absence of food stress, is the end result of selection for quality (survival) and competitive ability. Cain's domination or killing of Abel insures (1) an increase in Cain's chances of survival through the high‐risk, pre‐breeding period via improved nestling weight gain, and/or (2) domination of surviving sibs, enhancing Cain's competitive abilities and thereby increasing the probability of achieving breeding status. Only among long‐lived species can the benefits of enhanced survival and competitive ability outweigh the major costs of sibling loss.Facultative cainists, which in more than 10% of cases raise more than one young (despite aggression and sibling hierarchies), not only lay larger, more variable clutches, but on average attain adult plumage earlier than obligate cainists. Their shorter lives and higher population turnovers are consistent with their less extreme siblicidal tendencies. Similar life‐history traits and cainistic habits in other avian orders parallel those in the Falconiformes, indicating several independent evolutionary pathways to cainism.Retention of the second egg by obligate cainists, usually explained as insurance against failure, may instead allow parents adaptively to track population stability. Thus when breeding places are numerous (habitat saturation and competition low), parents laying two eggs and rearing two young may achieve greater fitness than single‐young parents. When populations become saturated (competition high), selection should favour high‐quality, competitive young and levels of siblicide should increase. A proximate mechanism is proposed linking population saturation with the incidence of cainism, based on demonstrable population characteristics found in several long‐lived species.

A Comparative Study of the Breeding Ecology of Harris' and Swainson's Hawks in Southeastern New Mexico

I compared productivity, habitat, and diet of breeding Harris' (Parabuteo unicinctus) and Swainson's hawks (Buteo swainsoni) to examine predictions that cooperatively breeding birds should produce fewer young and be more specialized in their use of habitat or food than closely related noncooperatively breeding species. Reproductive success of Harris' Hawk (1.73 young/year) and Swainson's Hawk (1.67) breeding units was similar. Harris' Hawks tended to exploit habitats with greater mean densities of large mesquites, less mean grass cover, and more exposed ground than Swainson's Hawks; but both species used a broad range of the habitats available. Swainson's Hawks were opportunistic generalists taking the most available prey, while Harris' Hawks were relative specialists on larger mammals and some birds.My data do not support habitat saturation or resource localization as important to the evolution of cooperative breeding in the Harris' Hawk.

Food Habits and Social Organization of Acorn Woodpeckers in Colombia

Colombian populations of Acorn Woodpeckers (Melanerpes formicivorusflavigula) inhabit the Andes and are geographically isolated from Central American populations. Their diverse food habits include acorns from Quercus humboldtii, the only oak in the Colombian Andes. Unlike birds from North America, however, Colombian Acorn Woodpeckers do not store acorns or rely heavily upon them. Instead, they feed on insects, sap, and fruit year-round. Although oak woodlands seem to be their preferred habitat, Colombian Acorn Woodpeckers are found residing permanently and breeding cooperatively in other kinds of habitat. The limited availability of storage facilities (habitat saturation hypothesis) does not seem to be adequate to explain cooperative breeding in these populations.

The Cooperative Breeding System of the Red‐cockaded Woodpecker

AbstractThe breeding system of the red‐cockaded woodpecker is described based on data collected over six years from a population of 500 marked individuals in the Sandhills of North Carolina. Male‐female pairs were the most common social unit (59%), but 30% of social units contained one or more adult helpers, and 11% consisted of solitary males. Helpers were almost exclusively male: 27% of males remained in their natal group as helpers for at least one year, whereas only four (1%) females did.Most breeding females remained as breeders in the same group from one year to the next (56%), but a surprising number (12%) moved to another group. Many movements were related to incest avoidance or mate death, but 39% involved deserting a mate, usually following successful reproduction. We suggest that females sometimes are forced from groups by immigrants or other group members. The median distance of movements by adult females was only 1.3 km. In contrast to females, no breeding males switched groups. Survival of both breeding (76%) and helper (80%) males was higher than that of breeding females (69%).Males exhibited two distinct life‐history strategies. Some remained as helpers on their natal territory for one or more years, and became breeders by inheriting breeding status in the natal group (17% per year) or by replacing a deceased breeder in a nearby group (13% per year, median distance moved 1.0 km). Other males dispersed from their natal group permanently during their first year. Some of these males were floaters at age one year, others were solitary, and a few became helpers in a non‐natal group, but many were breeders. In contrast to males that first functioned as helpers, those that dispersed after fledging moved long distances (median dispersal distance 4.5 km), longer even than dispersing female fledglings moved (median distance 3.2 km).The habitat saturation model of the evolution of cooperative breeding is based on selection between the two life‐history strategies exhibited by male red‐cockaded woodpeckers. The model therefore may be tested directly with this species. Another indication that this model is appropriate for this species is the existence of a resource (cavity trees) that might provide an ecological basis for habitat saturation.

Territory Quality and Dispersal Options in the Acorn Woodpecker, and a Challenge to the Habitat-Saturation Model of Cooperative Breeding

The central feature of cooperative breeding systems is the presence of one or more nonbreeding birds in a social group that devote time and energy to help raise the offspring of other group members. The most widely accepted model for the evolution of this behavior is that it arises when there is some ecological constraint to independent breeding by young birds that otherwise would disperse from their natal groups. Among territorial species, this constraint is usually hypothesized to be saturation of suitable habitat by sedentary established groups. We use data collected over a 10-yr period to test this model for a population of acorn woodpeckers (Melanerpes formicivorus) in central New Mexico. Acorn woodpeckers store large quantities of mast, and territories vary greatly in the amount of storage facilities present. Storage facilities significantly affect the survival and reproductive success of groups occupying those territories. Most helpers occur in high-quality territories; low-quality territories are frequently unoccupied and thus available for colonization. We estimate lifetime direct and indirect offspring production for yearlings that help in territories of differing quality and for varying numbers of years. Birds that forgo individual reproduction for one or more years in high-quality territories but eventually breed there can have greater lifetime fitness than those that disperse at 1 yr of age and breed immediately in low-quality territories. Had we combined data from all territories and all groups, these findings would have been obscured; rather, the resulting calculations would have erroneously indicated that, for yearlings, dispersing and breeding is always the preferred option. Our analyses indicate that habitat saturation or some other constraint on the opportunity to breed successfully is not required to explain the existence of cooperative breeding in this population of woodpeckers. Instead, we suggest that competition for space may be the outcome of factors that promote philopatry, such as access to a critical resource and/or mutualistic benefits of living in a group and eventually breeding in the natal territory. If this scenario is correct, habitat limitation is a result of other factors favoring cooperative breeding rather than the basis for this kind of social system.

Juvenile helping in the moorhen, Gallinula chloropus

In a 3-year study of the moorhen, Gallinula chloropus, some chicks from first broods stayed on their natal territory once they had reached independence, and helped to rear their younger, second brood, siblings. Juvenile dispersal was constrained by habitat saturation, and first brood young were forced to stay on their natal territory. Juveniles that hatched early in the year were forced to stay longer, and helped more than those that hatched late. The total feeding rates to broods with and without juvenile helpers were the same, but parents with helpers reduced their feeding rates relative to parents without helpers. Pairs with helpers (=pairs attempting second broods) reared more chicks per nesting, attempt than pairs rearing chicks at the same time of year without helpers (=pairs attempting first clutch renests). This was true both for all clutches, and for hatched clutches only, even when controlling for parental quality, territory size and scasonal effects.

A comparison of the social organization of three syntopic species of Australian thornbill, Acanthiza

Brown (Acanthiza pusilla), buff-rumped (A. reguloides) and striated thronbills (A. lineata) were studied over three years in eucalypt forest in northern New South Wales. All three are small, sexually monomorphic insectivores. A. pusilla is a shrub-feeder, reguloides a bark and ground forager and lineata gleans from eucalypt leaves. A. reguloides and lineata show a two-tiered social organization with territorial clans in the non-breeding season which split up into breeding pairs or groups of 3 or 4. Non-breeding birds feed nestlings and fledglings, and after reformation of the clan adults feed fledglings belonging to other groups. Females tend to disperse more than males before the next breeding season, males tend to stay and help. A. pusilla breeds in pairs, with no helpers, defends year-round territories and expels juveniles a few months after they fledge. We discuss possible reasons why A. pusilla is not cooperative whereas the other two species are. It seems not to be due to greater adult survival and habitat saturation in the latter two, nor to reduced seasonal fluctuations in food. We propose that a major factor is the low risk of dispersing relative to staying at home in pusilla compared with the other two species.