Florida Scrub-Jays (Aphelocoma coerulescens) are Sentinels More When Well-Fed (Even with no Kin Nearby)

Sentinels occupy high, exposed positions while other group members forage nearby. If sentinel behavior involves a foraging–predation risk trade‐off, animals should be sentinels more when fed supplemental food. When individual Florida scrub‐jays (Aphelocoma coerulescens) were fed fragments of peanuts, during the following 30 min they shifted 30% of their time from foraging to sentinel behavior. In a follow‐up experiment, we fed either one or two members in each group. As before, the jays reduced their foraging and spent much more time as sentinels when given supplemental food. In each treatment, pairs were sentinels simultaneously considerably less often than expected by chance. The dramatic shift from foraging to sentinel behavior suggests that for Florida scrub‐jays sentinel behavior brings substantial benefits for no greater cost than that of lost opportunities to forage. Because the results held for simple mated pairs of scrub‐jays, we argue that kin selection and social prestige are not necessary to explain sentinel behavior.     

Do Conversational Gutturals Help Florida Scrub‐Jays Coordinate Their Sentinel Behavior?

Florida scrub‐jays, Aphelocoma coerulescens, perform sentinel behavior in which individuals alternate bouts of watchfulness with little overlap. We examined how calls might facilitate sentinel coordination. Small soft calls labeled conversational gutturals were heard more often from sentinels than from foraging birds. Calls occurred infrequently throughout sentinel bouts but were more common later in bouts. The pattern of calling does not match predictions for a ‘watchman’s song’ at regular intervals nor for a signal at the end of a sentinel bout. Thus, our quantitative assessment of calling by sentinels did not find support for either standard hypothesis. Although Florida scrub‐jays clearly have information on each other’s sentinel behavior, our results suggest that calls provide perhaps a fraction of this information.     

Mutualism among safe, selfish sentinels: a dynamic game

Sentinels are group members that watch from prominent positions. Sentinel interchanges often appear orderly, and groups with sentinels rarely have zero or many sentinels. A dynamic game was constructed to examine if these observations about sentinels could be based on selfish actions by individual group members. In this game, each group member chose to forage or be a sentinel based on its own energetic state and the actions of others. Sentinels received a selfish antipredator benefit if their ability to detect approaching predators more than compensated for their increased exposure to undetected predators. Provided sentinels were relatively safe and that detection information spread to other group members when sentinels detected predators, sentinels appeared highly coordinated for all combinations of parameters. This apparent coordination was based on mutualism because each individual gained by being a sentinel when other group members were not (and foraging when other group members were being sentinels). The model was very robust, but exact level of sentinel behavior varied somewhat with changes in foraging and predation parameters. This model could best be tested by testing its assumptions about sentinel safety, foraging-predation trade-offs, and information transfer in groups.     

The Effects of Social Context on the Food-Caching Behavior of Florida Scrub-Jays (Aphelocoma coerulescens)

Florida scrub‐jays are cooperative breeders that live in family groups consisting of a breeding pair, often with several non‐breeding helpers. Florida scrub‐jays cache food by scatter‐hoarding items for later consumption. Within family groups, members have the opportunity to observe and pilfer the caches of other members. We observed jays harvesting experimentally provisioned peanuts alone and in the presence of other family members, to determine whether jays modify their food‐handling behavior relative to social context. Non‐breeding helpers were less likely to cache in the presence of the dominant male breeder than when alone and all jays tended to cache out of sight when observed by another jay. These changes in caching behavior are consistent with cache protection strategies employed by other species. However, the adaptive value of such cache protection within a sedentary cooperatively breeding family group on a year‐round territory is unclear.     

Cooperative sentinel calling? Foragers gain increased biomass intake

Many foraging animals face a fundamental tradeoff between predation and starvation. In a range of social species, this tradeoff has probably driven the evolution of sentinel behavior, where individuals adopt prominent positions to watch for predators while groupmates forage. Although there has been much debate about whether acting as a sentinel is a selfish or cooperative behavior, far less attention has focused on why sentinels often produce quiet vocalizations (hereafter known as "sentinel calls") to announce their presence. We use observational and experimental data to provide the first evidence that group members gain an increase in foraging success by responding to these vocal cues given by sentinels. Foraging pied babblers (Turdoides bicolor) spread out more, use more exposed patches, look up less often, and spend less time vigilant in response to sentinel calling. Crucially, we demonstrate that these behavioral alterations lead to an increase in biomass intake by foragers, which is likely to enhance survival. We argue that this benefit may be the reason for sentinel calling, making it a truly cooperative behavior.     

A sentinel system in the Florida scrub jay

One postulated advantage of living in groups is increased protection from predators. Living in small, permanent groups of stable composition allows for the formation of a sentinel system, in which individuals coordinate their vigilance. Florida scrub jays, Aphelocoma coerulescens coerulescens, live in family groups, and coordinate their vigilance into a sentinel system. Observed vigilance behaviour was compared with models generating similar behaviour patterns at random. The occurrence of one jay terminating vigilance in the same minute that another initiated vigilance was greater than expected by chance, and the occurrence of more than one vigilant jay at one time was less frequent than expected by chance. The seasonal pattern of scrub jay sentinel behaviour suggests that it functions primarily for predator detection, especially of raptors. Most aerial predator alarm calls were given by sentinels, and a sentinel was most likely to give the alarm call first.     

Response of foraging group members to sentinel calls in suricates,Suricata suricatta

In the suricate (Suricata suricatta), a cooperatively breeding mongoose, one individual typically watches out for predators while the rest of the group is foraging. Most of the time these sentinels announce their guarding duty with special vocalizations. The response of foraging group members to these calls was investigated by analysing observational data, and by performing playback experiments. The use of special calls by sentinels, and the responses of the foraging group members to them, suggest that the coordination of vigilance is strongly influenced by vocal communication. Sentinel calls decreased the time spent alert by the foraging group members. Other group members were less likely to go on guard when a sentinel was vocalizing. Both the proportion of time during which guards overlapped, and the proportion of time the group was unprotected without a guard, decreased when sentinels announced their duty, due to better coordination of the rotation of sentinels. Suricates, however, do not appear to use sentinel calls to mediate a strict rotation of guarding duty.     

Correlates and Consequences of the Pair Bond in Steller's Jays

Individuals that maintain pair bonds over multiple breeding attempts are often able to improve reproductive success compared to conspecifics that switch partners. However, the behavioral mechanisms driving this ‘mate familiarity effect’ are still largely unknown. We investigated whether long‐standing pairs in the long‐lived, socially monogamous Steller's jay improved their coordination of movements and behaviors, invested more time in pair bond maintenance, or became more compatible in their tendency to take risks over time compared to newly established pairs. We then compared these pair bond characteristics for successful and unsuccessful partnerships in terms of producing offspring. Jay partners regularly perched together, gave soft contact calls and travelled as a pair even in the non‐breeding season. However, the proportion of observations jay partners spent in each other's company (pair tenacity) was unrelated to risk‐taking behavior of pair members, pair bond duration, or the performance of subtle pair bond maintenance behaviors (i.e., a principle component of behaviors, including soft contact calls, proximity to mates, and frequency of arrival and departure flights with mate). However, evidence suggests that reproductive performance still improved in continuing compared to new pair bonds in Steller's jays. Variation in pair tenacity and frequency of pair bond behaviors may be inconsequential because of jays’ overall high level of contact with partners. Additionally, if jays are able to maximize familiarity early in the pair bond through high overall pair tenacity, the additional benefit of increasing coordination and familiarity with increasing pair bond age may be limited.     

Calling by Concluding Sentinels: Coordinating Cooperation or Revealing Risk?

Efficient cooperation requires effective coordination of individual contributions to the cooperative behaviour. Most social birds and mammals involved in cooperation produce a range of vocalisations, which may be important in regulating both individual contributions and the combined group effort. Here we investigate the role of a specific call in regulating cooperative sentinel behaviour in pied babblers (Turdoides bicolor). 'Fast-rate chuck' calls are often given by sentinels as they finish guard bouts and may potentially coordinate the rotation of individuals as sentinels, minimising time without a sentinel, or may signal the presence or absence of predators, regulating the onset of the subsequent sentinel bout. We ask (i) when fast-rate chuck calls are given and (ii) what effect they have on the interval between sentinel bouts. Contrary to expectation, we find little evidence that these calls are involved in regulating the pied babbler sentinel system: observations revealed that their utterance is influenced only marginally by wind conditions and not at all by habitat, while observations and experimental playback showed that the giving of these calls has no effect on inter-bout interval. We conclude that pied babblers do not seem to call at the end of a sentinel bout to maximise the efficiency of this cooperative act, but may use vocalisations at this stage to influence more individually driven behaviours.     

Dynamics of House Sparrow Biparental Care: What Contexts Trigger Partial Compensation?

Nest attendance during incubation is characterized by an inequitable division of labor in house sparrows, Passer domesticus, with females spending more time at the nest than males. Previous research has shown that if male contributions are reduced experimentally via testosterone (T) implants, females compensate partially for those reductions, consistent with predictions from most models of negotiated biparental care. In this study, we attempted to identify the cues and contexts generating partial compensation, using data from both unmanipulated parents and pairs with T‐males. Both males and females of this species sometimes leave the nest before their mate returns to relieve them, and we found that these unrelieved departures by unmanipulated individuals occur when partners are on lengthy recesses. Females compensated partially for long male recesses by marginally extending their bouts; most females also slightly reduced their next recess. By contrast, when males left before their mate returned, they left earlier than when they waited for the female. Neither males nor females adjusted their recess lengths after returning to the nest and discovering that their partner was absent. More pronounced changes in nest attendance of unmanipulated parents occurred in the context of ‘visits’, when individuals returned to the nest but then left without relieving their mate. Such visits effectively prolonged the bout of the on‐duty partner and extended the visitor’s recess. Analyses of behavior of T‐males and their mates revealed that T‐males had significantly longer recesses than control males, and that their mates, in turn, had elevated rates of unrelieved departures. T‐males also visited their on‐duty mates more often than control males, whereas female visits to T‐males were rare. Collectively, the predicted changes in female nest attendance associated with lengthy male recesses and male and female visits account reasonably well for the compensatory response of females paired to T‐males. The majority of female compensation was attributable to changes in visit behavior, however, suggesting that much of the negotiation over nest attendance in this species occurs during direct interactions between mates.     

No evidence that genetic compatibility drives extra‐pair behavior in female blue‐footed boobies

The function of female birds' extra‐pair (EP) behavior has remained an unresolved question in ornithology and behavioral ecology for > 30 yr. The genetic compatibility hypothesis (GCH) proposes that females benefit by acquiring biological sires that yield more heterozygous, and therefore fitter, offspring than their social mates. We used ten polymorphic microsatellite loci to test GCH predictions and its assumption that fitness increases with heterozygosity in blue‐footed boobies Sula nebouxii, a long‐lived tropical seabird. Our predictions were not supported. Heterozygosity was uncorrelated with quality indicators (fledging probability, fledgling or adult body size or mass, adult ornamentation, mean breeding success). Females were no more likely to have EP behavior or chicks when their social mates were less heterozygous or compatible, nor were EP males more heterozygous or compatible than the males they cuckolded. Finally, EP chicks were no more heterozygous than within‐pair chicks overall or in half‐sib comparisons, nor were within‐pair chicks from all‐within‐pair nests more heterozygous that those with EP nest‐mates. There are both methodological and biological explanations for these consistently negative results. Inadequate sample size is possible but unlikely, since our samples were comparable or larger than those of similar studies with significant findings. Lack of identity disequilibrium (within‐individual heterozygosity correlation) among our marker loci could be responsible, and suggests either insufficient marker coverage or lack of inbreeding, bottleneck, and/or admixture. An independent social pedigree revealed infrequent inbreeding, suggesting that pressure on females to select sires that maximize offspring heterozygosity may be genuinely lax. Alternatively, it is possible that the GCH is only upheld when selection on young is strongest; this would not be detected in our sample, which was taken during an extremely productive year. Whatever their cause, our results expand the taxonomic breadth of avian EP behavior analyses and should be considered in future evaluations of the GCH.     

Female‐Biased Helping in a Cooperatively Breeding Bird: Female Benefits or Male Costs?

There is often a sex bias in helping effort in cooperatively breeding species with both male and female helpers, and yet this phenomenon is still poorly understood. Although sex‐biased helping is often assumed to be correlated with sex‐specific benefits, sex‐specific costs could also be responsible for sex‐biased helping. Cooperatively breeding brown jays (Cyanocorax morio) in Monteverde, Costa Rica have helpers of both sexes and dispersal is male‐biased, a rare reversal of the female‐biased dispersal pattern often seen in birds. We quantified helper contributions to nestling care and analyzed whether there was sex‐biased helping and if so, whether it was correlated with known benefits derived via helping. Brown jay helpers provided over 70% of all nestling feedings, but they did not appear to decrease the workload of breeders across the range of observed group sizes. Female helpers fed nestlings and engaged in vigilance at significantly higher levels than male helpers. Nonetheless, female helpers did not appear to gain direct benefits, either through current reproduction or group augmentation, or indirect fitness benefits from helping during the nestling stage. While it is possible that females could be accruing subtle future direct benefits such as breeding experience or alliance formation from helping, future studies should focus on whether the observed sex bias in helping is because males decrease their care relative to females in order to pursue extra‐territorial forays. Explanations for sex‐biased helping in cooperative breeders are proving to be as varied as those proposed for helping behavior in general, suggesting that it will often be necessary to quantify a wide range of benefits and costs when seeking explanations for sex‐biased helping.     

Mated pairs of owl monkeys (Aotus nancymaae) exhibit sex differences in response to unfamiliar male and female conspecifics

In socially monogamous species, mate‐guarding could be a reproductive strategy that benefits both males and females, especially when males contribute to parental care. By actively guarding mates, males may reduce their chances of being cuckolded, whereas females that mate‐guard may reduce the likelihood that their mates will desert them or acquire additional mates, and hence limit or reduce paternal care of offspring. Owl monkeys (Aotus spp.) are socially monogamous with biparental care of young and, hence, potential beneficiaries of mate‐guarding. We presented mated pairs of captive owl monkeys (A. nancymaae) with unfamiliar male and female conspecifics, to determine if either member of the pair exhibits intraspecific aggression toward an intruder or stays close to its mate, behaviors indicative of mate‐guarding. Male mates were more responsible for the maintenance of close proximity between mates than females. Male mates also exhibited elevated levels of behavior that signify arousal when presented with a male conspecific. These responses by mated male owl monkeys are consistent with patterns that may help prevent cuckoldry. Am. J. Primatol. 72:942–950, 2010. © 2010 Wiley‐Liss, Inc.     

Sexual selection in pinyon jays II: male choice and female-female competition

Experiments were performed on a group of captive pinyon jays, Gymnorhinus cyanocephalus, to determine which female traits are favoured by female-female competition and which are favoured by male choice of mates. Brightness of malar feathers and body weight were weakly associated with female dominance. Female rank, weight and bill depth were related to success in pairing. Competition and choice favoured specific traits less strongly in females than in males.     

Intraspecific transmission of Mycobacterium bovis among penned feral pigs in New Zealand

We investigated intraspecific transmission of Mycobacterium bovis (etiologic agent of bovine tuberculosis [Tb]) among penned pigs (Sus scrofa) in New Zealand. Between 2002 and 2006, we conducted two trials, each involving two 1-ha pens in natural habitat in which uninfected sentinel pigs were kept with wild-caught feral pigs, many of which were infected with M. bovis. The rate of M. bovis transmission to sentinels was used to assess whether intraspecies transmission between live pigs could explain the high levels of infection in the wild population. In trial 1, no new infection was detected in 18 penned sentinels after total exposure of 3.9 yr (2.6 mo/sentinel); three of 11 sentinels in the wild became infected. In trial 2, a more heavily infected source pig population (94% prevalence compared with 42% in the first trial) was used, and one (4%) of 25 penned sentinels became infected. However, 75% of the eight sentinels released to the wild became infected. Combining trials, the difference in apparent annual incidence was significant (mean ± 95% confidence interval = 0.03 ± 0.12 for penned sentinels vs. 1.06 ± 6.74 for released sentinels; t=-3.51, P=0.04). In the pens, infected pigs were kept in contact with susceptible sentinels for 7 yr in total, but only one transmission event was detected. Taking pig longevity into account, the R(0) value (the basic reproductive rate of disease) for intraspecies infection between live pigs seems unlikely to exceed 0.25, even under highly conducive conditions. We suggest that live pigs are unlikely to transmit M. bovis to wild conspecifics often enough to sustain Tb in pigs by this route of transmission alone. The high prevalences seen in some New Zealand feral pig populations are likely to result from transmission from another host or route.     

Duration and patterns of transmission of Theiler's mouse encephalomyelitis virus infection

The duration and patterns of Theiler's murine encephalomyelitis virus (TMEV) transmission were studied in eight index mice inoculated orally. Transmission was monitored by testing for seroconversion to TMEV in sentinel mice in direct contact with index mice and in other sentinel mice in contact with bedding soiled by index mice. For the first 14 weeks after inoculation, two contact sentinels were housed with each index mouse for 1 week, then replaced with two new sentinels. For the remaining 16 weeks, contact sentinels were changed monthly. All index mice transmitted TMEV continuously (weekly) for 4 to 9 weeks. Thereafter, six index mice transmitted virus intermittently. All index mice ceased transmitting TMEV 7 to 22 weeks post-inoculation. Results obtained from sentinel mice in contact with bedding soiled by index mice were 86% concordant with those using contact sentinel mice. Seven index mice were treated with cyclophosphamide or hydrocortisone 30 weeks post-inoculation. One cyclophosphamide treated mouse reinitiated virus shedding.     

Causes and consequences of pre‐laying weight gain in a food‐caching bird that breeds in late winter

Animals that reside at high latitudes and altitudes year‐round often use cached food to survive over the winter months, but a few species also rely on stored food to sustain them during the breeding season when the nutritional requirements of females are higher than normal. Gray jays Perisoreus canadensis rely on perishable cached food during the winter and females begin breeding in late winter when fresh food is rarely available. To examine pre‐laying patterns of weight gain, as well as the causes and consequences of weight gain among individuals, we weighed females regularly throughout the pre‐laying period. Females began increasing their weight approximately nine days prior to their first egg date, and on average increased their body weight by 25%, which is on par with other bird species that rely on non‐cached food. Final pre‐laying weight was positively influenced by the percent of conifers on territories, providing some support for previous results showing that coniferous trees are better able to preserve cached food. We also found that both final pre‐laying weight and the rate of weight gain were positively related to female age, supporting the hypothesis that female caching ability improves with age. With increasing final weight, females tended to lay larger clutches and hatched more nestlings, despite the fact that final weight was not influenced by weight at the beginning of the weighing period. Our results confirm that gray jays are able to reach breeding condition while relying primarily on food stored before winter, and suggest a novel mechanism by which habitat‐mediated carry‐over effects and female age may influence reproductive performance in a food‐caching animal.     

The higher the better: sentinel height influences foraging success in a social bird

In all social species, information relevant to survival and reproduction can be obtained in two main ways: through personal interaction with the environment (i.e. ‘personal’ information) and from the performance of others (i.e. ‘public’ information). While public information is less costly to obtain than personal information, it may be inappropriate or inaccurate. When deciding how much to rely on public information, individuals should therefore assess its potential quality, but this possibility requires empirical testing in animals. Here, we use the sentinel system of cooperatively breeding pied babblers (Turdoides bicolor) to investigate how behavioural decisions of foragers are influenced by potential variation in the quality of anti-predator information from a vigilant groupmate. When sentinels moved to a higher position, from where their probability of detecting predators is likely to be greater, foragers reduced their vigilance, spread out more widely and were more likely to venture into the open. Consequently, they spent more time foraging and increased their foraging efficiency, resulting in a profound increase in biomass intake rate. The opposite behavioural changes, and consequent foraging outcomes, were found when sentinels moved lower. A playback experiment demonstrated that foragers can use vocal cues alone to assess sentinel height. This is the first study to link explicitly a measure of the potential quality of public information with a fitness measure from those relying on the information, and our results emphasize that a full understanding of the evolution of communication in complex societies requires consideration of the reliability of information.     

To forage,mate, or thermoregulate: Influence of resource manipulation on male rattlesnake behavior

Male animals should preferentially allocate their time to performing activities that promote enhancing reproductive opportunity, but the need to acquire resources for growth and survival may compete with those behaviors in the short term. Thus, behaviors which require differing movement patterns such as ambushing prey and actively searching for mates can be mutually exclusive. Consequently, males that succeed at foraging could invest greater time and energy into mate searching. We radio‐tracked sixteen male massasauga rattlesnakes (Sistrurus catenatus) and supplemented the diets of half the snakes with mice across an active season. We tested the predictions that reduced foraging needs would allow fed snakes to move (i.e., mate search) more, but that they would consequently be stationary to thermoregulate less, than unfed controls. Contrary to our first prediction, we found no evidence that fed snakes altered their mate searching behavior compared to controls. However, we found controls maintained higher body temperatures than fed snakes during the breeding season, perhaps because fed snakes spent less time in exposed ambush sites. Fed snakes had higher body condition scores than controls when the breeding season ended. Our results suggest the potential costs incurred by devoting time to stationary foraging may be outweighed by the drive to increase mating opportunities. Such instances may be especially valuable for massasaugas and other temperate reptiles that can remain inactive for upwards of half their lives or longer in some cases, and for female rattlesnakes that generally exhibit biennial or more protracted reproductive cycles.     

Pair bond characteristics and maintenance in free‐flying jackdaws Corvus monedula: effects of social context and season

Most birds rely on cooperation between pair partners for breeding. In long‐term monogamous species, pair bonds are considered the basic units of social organization, albeit these birds often form foraging, roosting or breeding groups in which they repeatedly interact with numerous conspecifics. Focusing on jackdaws Corvus monedula, we here investigated 1) the interplay between pair bond and group dynamics in several social contexts and 2) how pair partners differ in individual effort of pair bond maintenance. Based on long‐term data on free‐flying birds, we quantified social interactions between group members within three positive contexts (spatial proximity, feeding and sociopositive interactions) for different periods of the year (non‐breeding, pre‐breeding, parental care). On the group level, we found that the number of interaction partners was highest in the spatial proximity context while in the feeding and sociopositive contexts the number of interaction partners was low and moderately low, respectively. Interactions were reciprocated within almost all contexts and periods. Investigating subgrouping within the flock, results showed that interactions were preferentially directed towards the respective pair partner compared to unmated adults. When determining pair partner effort, both sexes similarly invested most into mutual proximity during late winter, thereby refreshing their bond before the onset of breeding. Paired males fed their mates over the entire year at similar rates while paired females hardly fed their mates at all but engaged in sociopositive behaviors instead. We conclude that jackdaws actively seek out positive social ties to flock members (close proximity, sociopositive behavior), at certain times of the year. Thus, the group functions as a dynamic social unit, nested within are highly cooperative pair bonds. Both sexes invested into the bond with different social behaviors and different levels of effort, yet these are likely male and female proximate mechanisms aimed at maintaining and perpetuating the pair bond.