Mixed-species Associations Research Articles

Social and personal information use by squirrel monkeys in assessing predation risk.

The threat of predation can significantly influence prey behaviors through altered perceptions of risk. Prey risk perception is constantly updated via collection of personal and social information about predators. Better understanding of the links between information availability, its use, and prey species' perception of risk will aid in explaining how animals adapt to predation. The goal of this study was to determine the environmental and social cues-available to prey via personal and social information, respectively-that influence wild squirrel monkey (Saimiri sciureus) reactivity to potential predators, treated here as a proxy for risk perception. We followed squirrel monkey troops for 3 years in Suriname, South America, and accounted for environmental and social variables associated with potential predator encounters. We utilized logistic regression models applied to a robust and long-term data set to reveal relationships among factors affecting squirrel monkey anti-predator responses. Our analyses revealed that height, season, type of predator stimulus, and mixed-species associations with capuchin monkeys (Sapajus apella) were highly related to intensity of squirrel monkey anti-predator responses. Moreover, our analyses revealed that squirrel monkeys overestimate the immediate threat of predation when individuals have incomplete information regarding the potential predator.

Mixed-species associations can arise without heterospecific attraction

Despite widespread research on the interaction rules that drive group-living behavior in animals, little is known about the spatial self-organization of individuals in heterospecific groups. This has led to significant challenges in teasing apart the various mechanisms thought to underpin multispecies groups. One potentially useful approach for gaining an understanding of this process is to identify the rules that best predict the observed distribution of individuals across these groups. In order to gain an insight into the decision-making process that might generate patterns of heterospecific associations, we collected data on the number and distribution of nests in breeding colonies that contained 3 species of weaverbird. We found no evidence of segregation by species, either within or between colonies. Using agent-based simulations of males applying different rules of attraction and repulsion to conspecifics or heterospecifics, we found that the best-fitting rule contained no heterospecific attraction. In this rule, individuals picked colonies based on an optimal distribution of conspecific nests. Given that nests are an important sexual signal in weavers, our findings suggest that this rule is biologically relevant: Males are seeking an optimal trade-off between attracting females via lekking and competing for mates if too many conspecific nests are present. © The Author 2014. (Less)

Behavioural responses of wintering black-faced spoonbills (Platalea minor) to disturbance

Context Behavioural responses can be used to understand the impacts of disturbance on animals and to develop management strategies, and there is considerable conservation interest in quantifying the effects of disturbances on wild animals. Aims We seek to formulate a management plan for the endangered black-faced spoonbills (Platalea minor) in a non-breeding ground, on the basis of their behavioural responses to different types of stimulus categorised by threat level (threatening vs non-threatening) and human involvement (anthropogenic vs natural). Methods We documented 16 stimuli from 379 disturbance events through continuous observation on 31 daily selected focal individuals, and estimated flight distances caused by human approach under different conditions at a non-breeding site in Korea. Key results The spoonbills showed the strongest behavioural response to the non-threatening anthropogenic stimuli, and human approach followed by motor vehicles caused longer responses per event than did any other type of disturbing stimulus. Flight distance caused by human approach varied depending on conditions; inactive spoonbills in mixed-species associations started to flee at the greatest distance (197.4 m, with a 50% probability), whereas the spoonbills in a single conspecific flock were the most tolerant of human approaches, regardless of their activeness (61.0–61.7 m, with a 50% probability). Conclusions Human approach had been identified as the most important disturbing stimulus that should be controlled as a priority. We also suggest that black-faced spoonbills benefit from the presence of other waterbirds and exploit them as an early warning system. Implications The tolerance of sympatric species as well as the behavioural response of target species should be considered when a buffer area for wildlife management, particularly against human disturbance, is planned. How the site has been used and what the species composition is are still important components for the design of safe refuges and roosts.

Rare Mixed-Species Associations Between Sperm Whales and Risso's and Northern Right Whale Dolphins Off the Southern California Bight: Kleptoparasitism and Social Parasitism?

Rare Mixed-Species Associations Between Sperm Whales and Risso's and Northern Right Whale Dolphins Off the Southern California Bight: Kleptoparasitism and Social Parasitism?

Anti-predator behaviour of Sahamalaza sportive lemurs, Lepilemur sahamalazensis, at diurnal sleeping sites

In response to predation pressure by raptors, snakes, and carnivores, primates employ anti-predator behaviours such as avoiding areas of high predation risk, cryptic behaviour and camouflage, vigilance and group formation (including mixedspecies associations), and eavesdropping on other species’ alarm calls. After detecting a predator, primates can produce alarm calls, show predator-specific escape strategies or even mob the predator. It remains unclear how solitary nocturnal primates respond to diurnal predation pressure while they sleep or rest. The aim of this study was to investigate the diurnal anti-predator behaviour of the nocturnal and solitary Sahamalaza sportive lemur, Lepilemur sahamalazensis, which regularly rests in exposed locations. We observed the responses of 32 Sahamalaza sportive lemurs to playbacks of territorial calls of an aerial predator (Madagascar harrier hawk), mating calls of a terrestrial predator (fossa), and the contact calls of a medium-sized bird (crested coua) as a control, at different diurnal sleeping sites. Lemurs never showed a flight response after replays of predator or control calls, but regularly froze after harrier hawk calls. Lemurs scanned the sky immediately after playback of harrier hawk calls, and the ground or trees after fossa calls. Lemur vigilance increased significantly after both predator calls. After crested coua calls the animals became significantly less vigilant, suggesting that contact calls of this bird serve as indicators of predator absence. We found no response differences between different types of sleeping sites. Our results show that resting Sahamalaza sportive lemurs recognise predator vocalisations as indicators of increased predation risk, discern vocalizations of different predators, and employ anti-predator behaviours specific for different predator classes. Their behavioural responses while resting or sleeping are comparable to those of active primates, and their response rate of 80% shows that this solitary and nocturnal primate is constantly aware of its environment.

Social organisation of thornbill-dominated mixed-species flocks using social network analysis

Mixed-species associations are a widespread phenomenon, comprising interacting heterospecific individuals which gain predator, foraging or social benefits. Avian flocks have traditionally been classified as monolithic species units, with species-wide functional roles, such as nuclear, active, passive, or follower. It has also been suggested that flocks are mutualistic interactions, where niches of participating species converge. However the species-level perspective has limited previous studies, because both interactions and benefits occur at the level of the individual. Social network analysis provides a set of tools for quantitative assessment of individual participation. We used mark-resighting methods to develop networks of nodes (colour-marked individuals) and edges (their interactions within flocks). We found that variation in flock participation across individuals within species, especially in the buff-rumped thornbill, encompassed virtually the entire range of variation across all individuals in the entire set of species. For example, female, but not male, buff-rumped thornbills had high network betweenness, indicating that they interact with multiple flocks, likely as part of a female-specific dispersal strategy. Finally, we provide new evidence that mixed-species flocking is mutualistic, by quantifying an active shift in individual foraging niches towards those of their individual associates, with implications for trade-off between costs and benefits to individuals derived from participating in mixed-species flocks. This study is, to our knowledge, the first instance of a heterospecific social network built on pairwise interactions.

Under my wing: lesser kestrels and jackdaws derive reciprocal benefits in mixed-species colonies

In mixed-species assemblages, antipredator benefits for a timid species nesting close to a more pugnacious one are often reported. Advantages for the protected species are usually manifested in terms of higher reproductive success than conspecifics nesting remote to the protector species. Whether the protector species also accrues any benefit remains untested, and the species-specific behavioral traits underlying enhanced reproductive output in mixed-species associations remain poorly documented. We studied associations between lesser kestrels (Falco naumanni) and jackdaws (Corvus monedula) nesting in rural buildings in the Gela Plain (Italy). We tested for interspecific interactions of jackdaws and lesser kestrels by comparing single- and mixed-species colonies in terms of: 1) vigilance, nest attendance, and intra- and interspecific interference and 2) defensive responses shown by each species when confronted with potential predators or competitors. In both species, nesting in mixed associations was accompanied by decreased vigilance. Diminished vigilance, however, was associated with decreased defensive effort for lesser kestrels, whereas jackdaws increased alarm calling, but only in small colonies. Our results reveal a reciprocal influence on behavior of lesser kestrels and jackdaws nesting within the same colony, whereby both species experience decreased energetic expenditures associated with vigilance, but only lesser kestrels appear to benefit via reduced nest defense effort. We discuss our results in light of the possible asymmetrical information transfer triggered by a common set of nest predators that favors the exchange of interspecific information regarding predator detection and defense.

Species interactions are disrupted by habitat degradation in the highly threatened Tumbesian region of Ecuador

Interactions between species in groups are often ignored in studies of the effects of anthropogenic change on species' persistence, and yet, given their global ubiquity, mixed-species groups have the potential to be models for community ecology. In this paper we examine the impacts of rural community activities that are pervasive in tropical regions on mixed-species flocks of birds in a unique coastal tropical dry forest ecosystem, with the aim of advancing both our understanding of mixed-species associations and how best to conserve them. We examined the effects of small-scale clearing of trees and livestock grazing on mixed flocks in two vegetation types in the highly threatened Tumbesian region of Ecuador. Because the fitness benefits of flocking come from a reduced risk of predation or enhanced feeding efficiencies, or both, habitat degradation could lead to changes in the benefits of flocking and thus flock characteristics. We predicted that, in more disturbed vegetation, flocks would have fewer species and individuals and that species would show lower flocking propensities and feeding efficiencies. Based on observations of 431 flocks, we found that these predictions were supported in tropical dry forest but not always in arid scrub vegetation. Less disturbed tropical dry forest had greater mean species richness per flock than more disturbed tropical dry forest, but mean species richness per flock did not differ between more and less disturbed arid scrub. Flock species richness and composition differed across the disturbance levels in tropical dry forest, but not in arid scrub. Many species had their highest flocking propensities and greatest foraging efficiencies in less disturbed, compared to more disturbed, vegetation. We show that existing park management, which allows community development and livestock grazing within national park borders, leads to degradation of the vegetation and disrupts species' interactions in tropical dry forest. We provide specific conservation recommendations to mitigate these disruptive influences. Examining intraspecific associations across diverse communities is of the utmost importance to both answering basic theoretical questions in community ecology and conserving species diversity in the face of anthropogenic landscape change.

What drives island-associated tropical dolphins to form mixed-species associations in the southwest Indian Ocean?

Mixed-species associations are temporary aggregations of individuals of different species involved in similar activities. Such associations form for foraging, protection against predators, and social advantage. Mixedspecies groups in delphinids are frequent in the wild. We aimed to understand the ecological significance of mixed-species group formation by 2 tropical delphinids, the spinner dolphin (Stenella longirostris) and the pantropical spotted dolphin (Stenella attenuata), in waters surrounding the island of Mayotte in the southwestern Indian Ocean. We used sighting data collected year-round from 2004 to 2009. We encountered a total of 67 mixed-species groups (comprising 21% of all groups observed) of spinner and pantropical spotted dolphins around Mayotte. No daily or seasonal variability in the occurrence of associations was detected. Behavioral activities of single- and mixed-species groups differed significantly. Foraging was observed only in single-species groups of pantropical spotted dolphins. Mixed-species groups were larger than single-species groups. When in association, spinner dolphins used deeper waters than while in single-species groups. No evidence of association for social advantage was observed. We suggest that spinner dolphins associate with spotted dolphins for protection against predators when transiting between resting areas.

Interspecific primate associations in Amazonian flooded and unflooded forests

Stable associations between two or more primate species are a prominent feature of neotropical forest vertebrate communities and many studies have addressed their prevalence, and their costs and benefits. However, little is known about the influence of different habitat types on the frequency, seasonality, and composition of mixed-species groups in Amazonian forest primates. Here we examine the features of interspecific primate groups in a large mosaic of flooded (várzea and igapó) and unflooded (terra firme) forest in central Amazonia. In total, 12 primate species occurred in the study area, nine of which were observed in mixed-species associations. Primates were more than twice as likely to form associations in várzea forest than in terra firme forest. Squirrel monkeys were most frequently found in mixed-species groups in all forest types, most commonly in association with brown capuchins. Another frequent member of interspecific associations was the buffy saki, which often formed mixed-species groups with tamarins or brown capuchins. There was no seasonality in the frequency of associations in terra firme forest whereas associations in várzea forest were twice as frequent during the late-dry and early-wet seasons than in the late-wet and early-dry seasons. Interspecific primate associations were common in all forest types, but the degrees to which different species associate varied between these environments. We suggest that the temporal variation of várzea forest associations is connected with seasonal changes in habitat structure and resource abundance. However, more work is needed to pinpoint the underlying causes of mixed-species associations in all forest types and their strong seasonality in várzea forest.

Foraging benefits and limited niche overlap promote a mixed species association between two solitary species of spider

Mixed species associations are of general importance because of the diversity of ecological relations they can represent (e.g., mutualisms, commensalisms, exploitative relationships). We test for a non-random association between two normally solitary species of spider, the black widow ( Latrodectus mactans ) and the orchard spider ( Leucage venusta ). We use field observations comparing solitary and associated individuals of both species to elucidate what effect the association has on prey capture. We then evaluate what influence the association has on prey consumption, as inferred from change in body mass after a reciprocal removal experiment. From our field census we confirm that these species associate non-randomly in wild populations; furthermore, we failed to detect a difference in the number or kinds of prey which hit solitary and associated webs of either species, suggesting the association is not merely an aggregation to microhabitats which afford higher prey availability. Our observational data on prey capture indicate the association serves to increase the capture efficiency of Le. venusta , in part through ricocheted prey, but we failed to detect a prey capture effect for La. mactans . The results of our reciprocal removal experiment suggest the relationship is a commensalism: benefiting Le. venusta and leaving La. mactans unaffected.

Interspecific associations of Cape ground squirrels with two mongoose species: benefit or cost?

Mixed-species associations have been described in many vertebrate species, but few behavioral studies have investigated associations between species from different mammalian orders. Cape ground squirrels (Xerus inauris) are highly social rodents that inhabit burrows with two species of mongoose, but the benefits of these interspecific associations to ground squirrels remain unresolved. We compared the behavior of squirrels while solitary, with conspecifics, and in the presence of suricates (Suricata suricatta) and yellow mongooses (Cynictis pencillatus). Squirrels spent less time alert and more time feeding when suricates were present, but increased vigilance in the presence of yellow mongooses. In a series of mobbing trials with a puff adder (Bitis arietans), a common predator of all three species, Cape ground squirrels were the most active in mobbing the snake. Our results suggest that Cape ground squirrels benefit from associating with suricates, but not necessarily with yellow mongooses. Both mongoose species benefit from the burrowing activities of the squirrels for thermoregulation and escape from predators, and a suite of other organisms may similarly benefit from the habitat modifications by Cape ground squirrels, suggesting they could be considered ecosystem engineers of the arid and semi-arid regions of southern Africa. Thus, the association between Cape ground squirrels and suricates appears mutually beneficial, whereas yellow mongooses may merely be commensals of the squirrels.

Cooperation and competition in two forest monkeys

Putty-nosed monkeys, Cercopithecus nictitans stampflii, occur at various sites in West Africa, particularly in the transition zone between rainforest and savannah. The species is sometimes seen in primary rainforest, although at a curiously low density compared with that of other monkey species. We conducted a 24-month field study in the tropical rainforest of Tao ¨ National Park, Ivory Coast, and found that putty-nosed monkeys require an ecological niche almost identical to that of the Diana monkeys, Cercopithecus diana diana. Moreover, the niche breadth of putty-nosed monkeys was significantly decreased in the presence of Diana monkeys, suggesting that feeding competition with Diana monkeys kept putty-nosed monkeys from successfully colonizing a rainforest habitat. However, contrary to the interspecies competition hypothesis, groups of both species almost completely overlapped in home ranges and formed near-permanent mixed-species associations, rather than avoiding each other. We hypothesized that Diana monkeys tolerated immigrating putty-nosed monkeys and formed mixed-species groups with them, despite high levels of competition, because of their merit in predation defense. Direct observations and a series of field experiments confirmed that male putty-nosed monkeys play a vital role in defense against crowned eagles, suggesting that puttynosed monkeys obtain access to feeding trees by offering antipredation benefits to Diana monkeys. We discuss these findings in light of biological market theory. Key words: alarm calls, biological markets, feeding ecology, mutualism, niche overlap, predation, semantic. [Behav Ecol 15:400‐411 (2004)]

Mixed-species associations of Diana and Campbell's monkeys: the costs and benefits of a forest phenomenon

AbstractOne of the most striking behavioural patterns of many forest primates concerns their tendency to live in semi-permanent mixed-species groups. Functional investigations have ascertained that individuals obtain some antipredator benefits without paying the costs of intra-species resource competition. Despite these advances, very little is known about the subtle mechanisms that keep mixed species groups together on a daily basis. Our results showed that in the Diana-Campbell's monkey association both species benefited from each other in diverse and idiosyncratic ways. In the presence of Campbell's monkeys the conspicuous Diana monkeys were more likely to descend into the lower forest strata, increased their foraging behaviour, and individuals became less vigilant. The cryptic Campbell's monkeys, in turn, were able to use the higher forest strata and exposed areas more often, spread out over larger areas, were more likely to travel, and engaged in more conspicuous vocal behaviour when associated with Diana monkeys. These data suggested that both species benefited from each other in ways that went beyond passive group-size related antipredator benefits, such as a dilution effect and increased chances of predator detection. Instead, the increased safety of the mixed species group allowed individuals to exploit their ecological niche more broadly, to forage more efficiently, and to engage in more social behaviour, suggesting that the benefits of mixed species groups are much more varied and diverse than currently thought.

Interdemic variation in mixed-species association patterns: common diurnal primates of Kibale National Park, Uganda

We used interdemic variation in the tendency to form mixed-species groups to examine the costs and benefits of association among the primates of Kibale National Park, Uganda. A year-long survey of six sites revealed that the amount of time that the five common diurnal primates [red colobus (Procolobus tephrosceles), black-and-white colobus (Colobus guereza), redtail monkeys (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), and grey-cheeked mangabeys (Lophocebus albigena)] spent in mixed-species groups varied dramatically among sites. In many cases, the proportion of time that species associated was positively related to their densities. By using detailed behavioral observations of redtail monkeys and red colobus made over 4 years (2660 h) at four sites, we were able to reject the null hypothesis that associations occur by chance for only one of four sites. However, a correlative approach exploring the costs and benefits of association suggests that ecological variables do influence association patterns. We found that redtail monkeys and red colobus overlapped in diet (19.2% of their foraging effort) and traveled further when in mixed-species groups than when alone. Having demonstrated this, we examined the applicability of the ecological constraints model for predicting the proportion of the time spent in mixed-species groups based on food availability. For this analysis we concentrated on red colobus from the site with 35 months of observation and demonstrated that their tendency to be in mixed- species groups was related to food availability. We used two methods to examine if mixed-species associations function to decrease predation risk. First, chimpanzees are known to prey heavily on red colobus, but rarely kill other primates. The time red colobus spent in mixed-species groups was correlated to chimpanzee density, but it was not for the other monkey species, suggesting that mixed-species groups serve to decrease predation risk. Second, when red colobus groups contain more infants and are presumably at the greatest risk of predation, they form mixed-species groups most often. These results demonstrate that the costs and benefits of mixed-species associations vary dramatically over small spatial and temporal scales. If such variation is generally the case, then studies conducted at different locations or different times could easily highlight the importance of difference selective agents in favoring mixed-species associations.

Mutual and Parasitic Mixed-feeding Associations in Waterfowl: A Food-addition Experiment.

To examine the effects of food availability on the organization of mixed-species feeding associations in Tundra Swans (Cygnus columbianus), Common Pochards (Aythya ferina) and Northern Pintails (Anas acuta), we observed their social and feeding behavior before and after the addition of artificial food (rice grains). The feeding associations occurred when the swans paddled their legs underwater to forage for sinking food. Pochards dived under paddling swans and Pintails foraged around the feeding associations of the two species. Before the addition of food, Tundra Swans showed the largest increase in dipping frequency in the three-species associations because more sinking food was stirred up by the feeding actions of Pochards. Pochards were also found to dive more frequently and for shorter periods in the three-species associations than solitary. Pintails in the mixed-species associations obtained more food by using three feeding techniques and by shortening the distance moved while dabbling. After the addition of rice grains, association size and aggressive interactions increased. Because frequent diving by many Pochards disturbed the feeding swans, the swans in the three-species associations had the lowest dipping frequency. However, Pochards and Pintails gathered around feeding swans despite the swans' aggressiveness and they still achieved greater feeding frequencies in the three-species associations than solitary. Therefore, the functional significance of mixed-feeding associations before the addition of food was based on mutualism but, after rice grains were provided, it was based on parasitism in which the hosts were Tundra Swans and the parasites were Pochards and Pintails. The most abundant participants in the three-species associations were Pochards and Tundra Swans were mainly aggressive toward them. Adding food led to an increase in the number of Pochards, which was costly for Tundra Swans. We concluded that the organization of mixed-species associations depended on the number of Pochards joining the feeding association.

Hand specialization, sympatry, and mixed-species associations in callitrichines

Hand morphology in callitrichines (i.e., tamarins, marmosets, and Goeldi's monkey) is correlated with positional and foraging behaviors. This study examines hand shape in callitrichines using an allometric approach. It addresses a series of questions relating hand anatomy, insect foraging behavior, and resource partitioning in callitrichines. The main questions are: 1. Do the hands ofLeontopithecusdiffer in shape from all other callitrichine taxa allowing it to perform highly manipulative prey foraging behaviors? 2. Are the hands ofSaguinus fuscicollisadapted to manipulative foraging, and are they functionally similar toLeontopithecus’ hands? 3. Is hand morphology inS. fuscicollismore similar to the hand morphology of sympatric tamarin species with whom it does not form mixed species troops (S. nigricollisandS. tripartitus) than to those sympatric tamarin species with whom it does form mixed troops (S. mystax, S. labiatus, andS. imperator)? Measurements of hand length (HL), width (HW), and thickness (HT) were taken from 1350 museum specimens of callitrichines (Callithrix, Cebuella, Leontopithecus, Saguinus, andCallimico), squirrel monkeys (Saimiri), and owl monkeys (Aotus). The analysis indicates that hand shape covaries with foraging strategy. Specifically, the hands ofLeontopithecusare adapted for manipulative foraging and are relatively longer and more slender than the hands of other callitrichines. A similar pattern is observed in the hands ofS. fuscicollis, S. tripartitusandS. nigricollis. These latter species, however, differ significantly in shape from all other tamarin species. Large differences in hand morphology are observed among tamarin species that form mixed-species troops. These anatomical differences may permit resource partitioning and coexistence among these closely related taxa. Hand shape, expressed as log HLGM (logged hand length divided by the geometric mean of all measurements), is a good predictor of manipulative and non-manipulative prey foraging techniques employed by callitrichines.

Tamarin Mixed-Species Groups: An Evaluation of a Combined Captive and Field Approach

The geographical distributions of the tamarins Saguinus mystax, S. imperator and S. lahiatus do not overlap, but each of these species is sympatric with S. fuscicollis, and mixed-species associations are formed. One way to examine the costs and benefits of association is to compare mixed-species tamarin groups and single-species tamarin groups in the field. However, the stability and permanency of Saguinus associations mainly preclude such opportunities and comparisons between single- and mixed-species groups from different sites are confounded by a number of factors. However, predictions derived from field observations can be tested in captivity under controlled experimental conditions. In this paper, we evaluate the usefulness of testing hypotheses in captivity and describe an experimental study of vigilance in which the behaviour of single-species groups and mixed-species groups of S. fuscicollis weddelli and S. I. lahiatus was compared in order to determine the influence of each species on the other. The results demonstrate that the patterns of vigilance are both different and complementary. As such, they provide support for improved detection of predators as being one advantage of forming mixed-species groups. The discussion focuses on how the combined approach of systematic testing of hypotheses in captivity, together with field studies, can be employed to examine the advantages and disadvantages of Saguinus associations.

Standing Crops of Brook Trout in Adirondack Waters before and after Removal of Non-trout Species

Ponds and lakes in the Adirondack Mountain region of New York State that contain brook trout Salvelinus fontinalis are generally infertile and trout production is low when brook trout have to compete with non-trout species. Standing crop estimates for seven waters with mixed-species associations ranged from 50 to 100 lb/acre; suckers Catostomus spp. were a major component of each assemblage, Brook trout standing crops were under 1 lb/acre in all but one water and were particularly low where yellow perch Perca flavescens were present. Following reclamation, during which major competing species were removed, brook trout were restocked. Subsequent estimates of brook trout standing crops ranged from 5 to 16 lb/acre, and good trout fishing prevailed where previously there had been little or none.

Sand‐Colored Nighthawks Parasitize the Antipredator Behavior of Three Nesting Bird Species

On beaches along the Manu River of southeastern Peru, four bird species (Black Skimmer, Rynchops niger; Large—billed Tern, Phaetusa simplex; Yellow—billed Tern, Sterna superciliaris; Sand—colored Nighthawk, Chordeiles rupestris) nest in mixed—species associations. Costs and benefits of the association were distributed asymmetrically among the species. Nighthawks never actively defended their nests against predators. Terns and skimmers defended their nests by aggressively mobbing predators, and indirectly defended nearby or adjacent nests of nighthawks. As a result, nighthawks had greater hatching success when nesting near terns and skimmers. Terns and skimmers spent more time engaged in antipredator behaviors and being vigilant, but less time in parental care, and experienced lowered hatching success when among large nighthawk groups. In addition, both tern species fledged significantly fewer young per successful nest when nesting with many nighthawks. Results of this study demonstrate the first known parasitic relationship among members of a mixed—species nesting group.