Prior residency as a key determinant of social dominance in the willow tit (Parus montanus)

Summary We studied how age, body size and prior residency affected social dominance in the willow tit (Parus montanus) groups. The contribution of each variable was experimentally tested in unisexual two-bird trials, in which the birds were matched for all variables except the one studied. Large birds were dominant over smaller ones (Fig. 1). The effect of body size was more prominent in males than in females. Age had no influence on dominance. Residents became dominant more often than newcomers (Fig. 2). Adulthood or larger body size did not override the advantage of prior residency (Fig. 2). Therefore, the proximate reason for the age-dependent dominance seen in natural willow tit flocks is most likely the prior residency advantage of the adults. Factors connected with fighting ability (body size and age) seem to be less important than the time of establishment of rank, which may reflect the importance of resource value differences between residents and newcomers in this context. The advantage of residency might make it advantageous to be a member of a flock even as a subordinate, rather than being solitary.     

Hierarchical dominance structure in reintroduced California condors: correlates, consequences, and dynamics

Populations of reintroduced California condors (Gymnogyps californianus) develop complex social structures and dynamics to maintain stable group cohesion, and birds that do not successfully integrate into group hierarchies have highly impaired survivability. Consequently, improved understanding of condor socioecology is needed to inform conservation management strategies. We report on the dominance structure of free-ranging condors and identify the causes and consequences of rank in condor populations by matching social status with the behavioral and physical correlates of individual birds. We characterized the hierarchical social structure of wild condor populations as mildly linear, despotic, and dynamic. Condor social groups were not egalitarian and dominance hierarchies regulated competitive access to food resources. Absence of kin-based social groups also indicated that condor social structure is individualistic. Agonistic interactions among condors were strongly unidirectional, but the overall linearity and steepness of their hierarchies was low. Although one aggressive male maintained the highest dominance rank across the 3-year observation period, there was considerable fluidity in social status among condors within middle and lower rank orders. Older condors were more dominant than younger birds and younger males supplanted older females over time to achieve higher status. Dominance rank did not predict the amount of time that a bird spent feeding at a carcass or the frequency that a bird was interrupted while feeding. Thus, younger, less dominant birds are able to obtain sufficient nutrition in wild social populations.     

The influence of kinship on foraging competition in Siberian jays

Foraging competition in Siberian jay groups was examined in relation to dominance and kinship to determine whether juvenile offspring, by associating with adults, gained in food acquisition relative to juvenile immigrants. Members of the adult pair were dominant over juvenile cohort members and males were dominant to females, although an inter-sexual hierarchy, with male juveniles occasionally overlapping adult females, was suggested. Few competitive asymmetries were found between adults and retained offspring or adults and immigrant juveniles when they were competing for food together, but in kin and non-kin foraging groups, respectively. Male offspring visited the bait site more frequently than adult males, and female immigrants spent less time at the bait site than adult females. Under these circumstances, hoarding activities may limit the ability of alpha members to control resources. In mixed groups containing both juvenile offspring and juvenile immigrants, no difference was found in the number of visits made to the bait site, although load sizes and foraging rates were lower for immigrant birds. Retained juveniles obtained greater load sizes and foraging rates when associating with adults. The social dominance of parents suggests that they control juvenile foraging. Although offspring benefit in the presence of adults, adults may incur a cost to their restraint by spending more time at the bait site when competing with immigrants. These results extend conclusions from previous work describing the role of selective tolerance by adults which relaxes competition with retained offspring in Siberian jay winter groups. The present findings suggest that offspring benefit in both immediate and future energy gains, which may have a direct influence on survival. Received: 18 September 1996 / Accepted after revision: 26 January 1997     

Effect of blue tit song syntax on great tit territorial responsiveness – an experimental test of the character shift hypothesis

A previous study of geographic variation in blue tit (Parus caeruleus) song structure showed that changes in blue tit song syntax (i.e. presence/absence of a trill) are correlated with the breeding density of a close competitor, the great tit (P. major), and are not correlated with other environmental factors such as vegetation structure or blue tit breeding density. We tested the hypothesis that blue tit trilled song represents a character shift that evolved because it reduced territorial interactions with more dominant great tits. We conducted five sets of playback trials in three study populations (mainland southern France, Corsica and Denmark) presenting male great tits with blue tit trilled and untrilled songs and great tit songs. We found that great tits respond equally strongly to both blue tit untrilled songs and to great tit songs, but show a significantly weaker response to blue tit trilled songs. These findings are the first experimental evidence that interspecific competition may play an important role in macrogeographic variation of bird song. Received: 10 October 1999 / Received in revised form: 2 April 2000 / Accepted: 15 April 2000     

Mass-dependent take-off ability in wintering great tits (Parus major): comparison of top-ranked adult males and subordinate juvenile females

When birds are attacked by predators, initial take-off is crucial for survival. Theoretical studies have predicted that predation risk in terms of impaired flight ability increases with body weight. However, studies in which attacks were simulated, and within-individual daily changes in body weight were used to test mass-dependent take-off outside migration period, have so far failed to show an effect of mass on velocity. In this field study I compared the mass/velocity relationships of alarmed adult male and juvenile female great tits, Parus major. Fattening strategies differ among members of the dominance-structured basic flocks of wintering great tits, and dominant individuals often carry significantly less amount of fat reserves than subordinates. Since the range of body weight gain/loss is the least among dominant males, it was expected that impaired flight ability is more likely in lower-ranked female great tits. The results show that the birds differed significantly in their daily increase of relative body weight. Average daily weight increase of adult males was 6.2%, while it was 12.2% in juvenile females. Males were faster than females at take-off both at dawn and at dusk. Flight velocity of males did not differ significantly between dawn and dusk, whereas females took off at a significantly lower speed at dusk than at dawn. The results suggest that the larger fat reserves of subordinate females needed to increase their chances of overwinter survival probably place them at increased risk of predation. Electronic Publication     

Female brown-headed cowbirds' (<Emphasis Type="Italic">Molothrus ater</Emphasis>) social assortment changes in response to male song: a potential source of public information

In many species, females' behavior appears to be influenced by that of other females, particularly regarding mate choice. Females theoretically can reduce the costs associated with independent male assessment by observing conspecifics. Studies of brown-headed cowbirds suggest that females pay attention to other females' behavior. Group-housed females modify their song preferences, whereas females housed in pairs do not. What information is available to females in a group environment? To address this question, we studied two groups of juvenile (i.e. hatch-year birds) and adult female cowbirds in a naturalistic group setting. We used a longitudinal ABA design, consecutively introducing and removing males that differed in age, amount of song production and stage of song development, to isolate the male characteristics that related to changes in female behavior. Juvenile and adult females assorted by age class when singing adult males were in the aviary, but not when singing juveniles or silent males of any age class were in the aviary. Results from playback tests confirmed that adult male song alone influenced female age class assortment. Videotape analysis from playback tests revealed that females also wing stroked in response to male song. Other females sometimes approached females who wing stroked and observed them. We hypothesize that group-level changes in social organization and individual females' responses can serve as visual signals for other individuals.     

Energy budgets,risk and foraging preferences in dark-eyed juncos (Junco hyemalis)

Summary Juncos' preferences for constant versus variable food rewards were tested in three series of aviary experiments. Deprivation and feeding rates were varied across the three treatments, but the mean of the variable reward equalled the constant reward in every experiment. When the birds gained energy faster than required to meet all 24-h costs, they preferred the constant reward. When the birds' energy intake was less than the minimally required rate, they preferred the variable reward. When energy intake just balanced average daily costs, the birds preferred the constant reward or were indifferent, and their response depended on the particular mean-variance combination presented.     

Sexual selection in the northern pintail (Anas acuta): the importance of female choice versus male-male competition in the evolution of sexually-selected traits

We experimentally studied the relative importance of plumage, dominance status, and courtship behavior in determining male pairing success in the northern pintail Anas acuta and assessed whether these traits function in female choice, male-male competition or both. In an experiment (experiment IA) that eliminated the confounding effects of male-male competition and social courtship, females chose males with pure white breasts and colorful scapular feathers. When the same group of birds were free to interact (experiment 1B), male behavior was more important: females chose males that courted them intensely and were attentive to them, although preferred males again had whiter breasts and more colorful scapulars. In a second experiment (experiment 2), testing the effect of age on pairing success, females showed a significant preference for 2-year-old males over yearlings: 2-year-old males courted more and were more attentive to the female than yearlings; they were also more colorful than yearlings in a number of plumage measurements. Although males (in both experiments 1B and 2) were aggressive to one another while courting the female and dominant males were sometimes able to exclude subordinates from social courtship, contrary to expectation, we found no relationship between initial dominance rank and pairing success or dominance rank and age. In addition, dominance was not correlated with any of the morphological traits measured. Once chosen, however, subordinate males typically initiated fights with the higher-ranked male(s) and quickly achieved dominance. These results suggest that (1) females choose males based on a suite of morphological and behavioral characteristics, (2) male dominance relationships do not constrain active female choice, (3) a male's position in a dominance hierarchy is largely a result rather than a cause of female choice, and (4) female choice plays a more significant role than male-male competition in the evolution of several secondary sexual traits in male northern pintails.     

Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a correlation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptions and predictions of the phenotype-limited interference model in Andean condors (Vultur gryphus), a New World vulture with strong sexual size dimorphism (males are 30–40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scavenging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspecific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juvenile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juvenile females were more often observed searching in low-quality areas (plains), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the number of condors at arrival was high. Additionally, low-ranking condors spent less time feeding at carcasses when individuals of higher hierarchical levels were present. On the other hand, the number of condors present had a positive effect on feeding rates of dominant individuals, probably because of a reduction in individual vigilance. These results support most of the assumptions and predictions of the phenotype-limited distribution model, although a spatial truncated distribution between phenotypes was not observed. Asymmetric feeding pay-off, unequal parental roles and sexual selection constraints could favour sexual divergence in body size in Andean condors. Received: 6 April 1998 / Accepted after revision: 11 July 1998     

Fitness consequences of within-brood dominance in the cooperatively breeding acorn woodpecker

Social dominance confers potential advantages in terms of access to superior resources, habitats, and breeding opportunities. In the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), within-brood dominance among juveniles is correlated with relative body size as nestlings. Capitalizing on this relationship, we investigated the fitness consequences of dominance by means of paired comparisons of broodmates. We found that (1) larger fledglings retained at least some of their size advantage as adults; (2) overwinter survival of larger, dominant fledglings was significantly greater than subordinates, but was not relatively greater when resources were poor than when they were good; (3) among birds surviving their first winter, there were no differences vis-à-vis dominance in terms of the proportion of birds acting as helpers or inheriting their natal territory. However, larger, dominant males were present in the study area longer than subordinates, suggesting that they either survived better or were more successful at gaining reproductive opportunities; (4) if only one male broodmate became a helper instead of dispersing, he was significantly more likely to be the smaller subordinate, consistent with the view that helping is a best-of-a-bad-job strategy; and (5) there were no significant differences in reproductive success among pairs of male broodmates that cobred together as adults, consistent with prior work failing to detect a phenotypic correlation of reproductive skew. Our results indicate that within-brood dominance relationships established as juveniles have significant effects on first-year survivorship and at least some aspects of adult fitness.     

Delayed juvenile dispersal and monogamy, but no cooperative breeding in white-breasted mesites (Mesitornis variegata)

Although cooperative breeding is known from only about 9 % of bird species, it has received substantial attention because individuals foregoing their own reproduction to help others represent a long-standing evolutionary puzzle. We studied group formation, breeding system, spatial distribution and several life-history traits of white-breasted mesites (Mesitornis variegata). Based on field observations across 3 years, we found that white-breasted mesites live in year-round stable pairs, and that groups are formed by juvenile philopatry. As other family-living birds, M. variegata exhibit a slow pace-of-life, characterized by high annual adult survival, low productivity, long chick dependence and extended parental care. However, although reproduction is monogamous and juveniles showed interest in their parents’ nests, we found no evidence of cooperative breeding. We suggest that slow life-histories, extended parental care and year-round territoriality predispose juvenile mesites to delay dispersal. However, adult intolerance toward older juveniles may prevent them from adopting a cooperative lifestyle. Comparisons with other species of mesite indicate that monogamy and delayed juvenile dispersal are necessary, but not sufficient for the evolution of cooperative breeding in this family of birds, and that particular ecological and social conditions have facilitated the transition from pair-living to a type of group that may represent a stepping stone in the evolution of cooperative breeding in mesites and other birds.     

Urbanized landscapes favored by fig-eating birds increase invasive but not native juvenile strangler fig abundance

Propagule pressure can determine the success or failure of invasive plant range expansion. Range expansion takes place at large spatial scales, often encompassing many types of land cover, yet the effect of landscape context on propagule pressure remains largely unknown. Many studies have reported a positive correlation between invasive plant abundance and human land use; increased propagule pressure in these landscapes may be responsible for this correlation. We tested the hypothesis that increased rates of seed dispersal by fig-eating birds, which are more common in urban habitats, result in an increase in invasive strangler fig abundance in landscapes dominated by human land use. We quantified abundance of an invasive species (Ficus microcarpa) and a native species (F. aurea) of strangler fig in plots spanning the entire range of human land use in South Florida, USA, from urban parking lots to native forest. We then compared models that predicted juvenile fig abundance based on distance to adult fig seed sources and fig-eating bird habitat quality with models that lacked one or both of these terms. The best model for juvenile invasive fig abundance included both distance to adult and fig-eating bird habitat terms, suggesting that landscape effects on invasive fig abundance are mediated by seed-dispersing birds. In contrast, the best model for juvenile native fig abundance included only presence/absence of adults, suggesting that distance from individual adult trees may have less effect on seed limitation for a native species compared to an invasive species undergoing range expansion. However, models for both species included significant effects of adult seed sources, implying that juvenile abundance is limited by seed arrival. This result was corroborated by a seed addition experiment that indicated that both native and invasive strangler figs were strongly seed limited. Understanding how landscape context affects the mechanisms of plant invasion may lead to better management techniques. Our results suggest that prioritizing removal of adult trees in sites with high fig-eating bird habitat may be the most effective method to control F. microcarpa abundance.     

Supply and demand: causes and consequences of assortative mating in common terns <Emphasis Type="Italic">Sterna hirundo</Emphasis>

Although assortative mating is widespread among long-lived monogamous birds, the underlying mechanisms still remain unclear in many species. In this study, we analysed assortative mating by age and body mass in the common tern Sterna hirundo with special regard to mate choice decisions of newly paired birds. To assess whether assortative mating by age is an active decision involving either homotypic or directional preferences or rather a passive process due to restricted availability of potential mates, we analysed the influence of age-dependent arrival date and cohort size. Furthermore, we looked for direct benefits in terms of reproductive success. Common terns mated assortatively by age and arrival date but not by body mass at arrival. Assortative mating by age was age dependent and was detected mainly in younger birds, whereas birds older than 8 years rather re-paired with younger ones. The availability of same-aged mates was restricted by cohort size and arrival date. We found no general evidence for better reproductive performance of same-aged pairs. Instead, relative reproductive success was related to own age and the relative age of mate: It should be advantageous for any bird to acquire an old mate; hence, assortative mating by age seems to be beneficial only for old terns but not for young ones. Age-assortative mating in common terns occurred by both passive and active processes, which are not mutually exclusive. Our results do not indicate a homotypic but a directional preference and support the theory that high-quality (older/experienced) individuals tend to mate assortatively if same-aged mates are available, which leaves low-quality (young/inexperienced) individuals to mate among themselves.     

Social behavior of free-ranging juvenile sooty mangabeys (Cercocebus torquatus atys)

Sooty mangabeys are terrestrial monkeys exhibiting female philopatry and male dispersal. Studies in captivity as well as in the wild have found that adult females form linear dominance hierarchies. However, while captive studies found no evidence for a matrilineal social system, a previous study in Taï National Park, Ivory Coast, suggested that relatedness could influence both dominance rank and affiliation pattern among adult females. Here I test whether the dominance rank, coalitionary behavior, and affiliative behavior of juveniles in a group of free-ranging mangabeys in the Taï National Park are in accordance with a matrilineal, individual, or age-related dominance system. I found that juvenile females' dominance ranks remained stable over time and were highly correlated with the dominance ranks of their mothers, whereas juvenile males' dominance ranks were initially correlated with the ranks of their mothers, but showed greater instability with increasing age. Moreover, coalitions occurred mainly between juveniles and animals that were close in rank, including their mothers and siblings. Finally, juvenile females associated and groomed preferentially with close-ranking juvenile and adult females. Juvenile males showed similar preferences in affiliation with adult females, but when associating with juvenile males, they preferred peers. The observed social behavior of free-ranging juvenile sooty mangabeys resembled the social behavior described for juveniles of many matrilineal primate species.     

The territorial flight of male houseflies (Fannia canicularis L.)

Summary The heart rate (HR) of three male and five female European blackbirds (Turdus merula) was monitored by radiotelemetry under three conditions: in a dark cage, in a lighted cage, and in an outdoor aviary. In all three, the response to recorded bird song was tested.The resting HR in the cage ranged from 4.8 to 6.3 beats per second. The HR of all the birds changed in response to playback of conspecific song as well as of the songs of other species. In 68% of trials a typical biphasic HR curve was obtained, with acceleration followed by deceleration (Fig. 2). The time from stimulus onset to the second curve inflection (t _iII) averaged 15–18 s; this was significantly longer for conspecific than for heterospecific song. The parameter t _iII was used as a basic measure of the response.The reaction of the males was longer-lasting than that of the females. The possibility that this reflects the territorial role of the songs is discussed. Response durations were longest in the dark cage, and shortest in the aviary. The reason is thought to be the gradation in total stimulus input available to the bird.The results suggest that the HR response consists of an unspecific component (acceleration) plus a component specific to bird sounds (deceleration). At this fundamental level of stimulus processing it is possible to study reactions to single strophes not only in males, as with more traditional methods, but also in females and young birds.     

Dominance, not kinship, determines individual position within the communal roosts of a cooperatively breeding bird

Kin selection has played an important role in the evolution and maintenance of cooperative breeding behaviour in many bird species. However, although relatedness has been shown to affect the investment decisions of helpers in such systems, less is known about the role that kin discrimination plays in other contexts, such as communal roosting. Individuals that roost communally benefit from reduced overnight heat loss, but the exact benefit derived depends on an individual's position in the roost which in turn is likely to be influenced by its position in its flock's dominance hierarchy. We studied the effects of kinship and other factors (sex, age, body size and flock sex ratio) on an individual's roosting position and dominance status in captive flocks of cooperatively breeding long-tailed tits Aegithalos caudatus. We found that overall, kinship had little influence on either variable tested; kinship had no effect on a bird's position in its flock's dominance hierarchy and the effect of kinship on roosting position was dependent on the bird's size. Males were generally dominant over females and birds were more likely to occupy preferred roosting positions if they were male, old and of high status. In this context, the effect of kinship on social interactions appears to be less important than the effects of other factors, possibly due to the complex kin structure of winter flocks compared to breeding groups.     

Correlates of male mating success in great bustard leks: the effects of age, weight, and display effort

We examined how mating success varied in relation to age, weight, body size, and display behavior among great bustard Otis tarda males. The estimated mating success was strongly skewed, with 45% of adult males being involved in copulation attempts and only 9.7% actually seen copulating successfully. Unlike most birds, body size continued increasing in great bustards several years after reaching sexual maturity. Age, weight, and display effort were all significant and independent predictors of male mating success. The higher display effort involved performing longer full-display bouts. Older males could detach from the male flock earlier in the season as well as on each day and spend longer seasonal and daily periods displaying as solitary birds, which contributed to increase their mating success. In contrast, males weighing more did not invest more in display, which suggests that they could be recognized as dominants by other males and selected by females through assessment of their plumage sexual traits. In contrast to most other bird species, the system described for great bustards resembles that found in some lek-mating ungulates, where social rank is a complex trait determined by both age and mass, and as in these mammals, it suggests that sexual selection continues to favor a high male weight in this extremely sexually dimorphic species.     

Singing and the establishment and maintenance of dominance hierarchies in captive brown-headed cowbirds

Summary The importance of singing in the establishment and maintenance of dominance rank was investigated in captive flocks of brown-headed cowbirds (Molothrus ater). Each flock consisted of 3 males and 1 female. In one experiment the two largest males in each of 4 flocks were temporarily devocalized by puncturing the interclavicular air sac just before group formation. The third male in each group was sham-operated. There was a significant tendency for the non-devocalized birds to become the dominant member of their group. Only sham-operated males sang immediately after the operation. In a second experiment, dominant males from flocks with established hierarchies were temporarily devocalized. None of these males lost their dominant positions, despite their inability to sing. These results suggest that song is important in the establishment of dominance rank; however, once established, social hierarchies may be maintained by non-vocal means, such as social inertia or individual recognition.     

Reproductive strategy and singing activity: blue tit and great tit compared

The costs and benefits of bird song are likely to vary among species, and different singing patterns may reflect differences in reproductive strategies. We compared temporal patterns of singing activity in two songbird species, the blue tit (Cyanistes caeruleus) and the great tit (Parus major). The two species live side by side year round, and they have similar breeding ecology and similar rates of extra-pair paternity. However, they differ in two aspects of reproductive strategy that may have an influence on song output: blue tits are facultatively polygynous and have a fairly short breeding season with almost no second broods, whereas great tits are socially monogamous but more commonly raise second broods. We found that great tit males continued singing at high levels during the egg-laying and incubation periods, while monogamously paired blue tit males strongly reduced singing activity after the first days of egg-laying by their female. Since males of both species sang much more intensely shortly before sunrise than after sunrise, at midday or in the evening, this difference was most conspicuous at dawn. No differences in singing activity were found within species when testing for male age. We suggest that in contrast to blue tits, great tit males continued singing after egg-laying to defend the territory and to encourage the female for a possible second brood.     

Social transmission of food handling in the context of triadic interactions between adults and young canaries (<Emphasis Type="Italic">Serinus canaria</Emphasis>)

We studied the factors that enhance food recognition and consumption in young canaries when confronted with adults. In contrast to previous studies on canaries, in which social transmission of food habits was studied in the context of dyadic interactions (one juvenile–one adult), we proposed a more realistic framework in which young canaries were studied in the context of triadic interactions, free or not, with adults of both sexes. We found that during free interactions, the young bird only eats with a familiar male and that this association enhances the social transmission of seed handling. When the juvenile was separated from the adults by a transparent partition, it only learned to husk seed if it was present at the feeder at the same time as a familiar adult acting as a demonstrator. The presence of adults that are familiar but do not act as demonstrators does not facilitate social transmission of handling. However, the presence of a familiar, demonstrating female had also no effect on this transmission. Coordination of the actions of the experienced bird and of the naive subject is required for social transmission to occur. Action coordination does not depend solely on the level of familiarity between partners but also on the role played by the demonstrator (here, the adult male) that looks after the juvenile during its transition towards independence.