Male Decoys Research Articles

Free‐ranging male northern map turtles use public information when interacting with potential mates

AbstractSperm competition is prevalent in animals, and many adaptations have evolved to reduce its risk. Males can reduce the risk of sperm competition by using public information when interacting with potential mates. Specifically, males can reduce sperm competition by avoiding females already affiliated with rival males. We tested this hypothesis in a population of wild northern map turtles (Graptemys geographica), a gregarious species with seemingly prevalent sperm competition. We used 3D‐printed decoys and underwater videography to record the response of free‐ranging males to female decoys affiliated or not with rivals. More visits were made by males to female decoys when rivals were present, suggesting a form of eavesdropping during mate selection. Males were more likely to interact with the female decoy, however, when rivals were absent, suggesting that they behave to reduce sperm competition. Moreover, the types of interactions differed between the accompanied and the unaccompanied female decoys, indicating an audience effect during male–female interactions. Finally, males interacted more with the male decoys than with the female decoys in the treatment with rivals, indicating a yet unclear form of male–male interaction. Collectively, our results suggest that free‐ranging male northern map turtles use public information for mate selection and to reduce sperm competition.

Incubating parents serve as visual cues to predators in Kentish plover (Charadrius alexandrinus).

Ground-nesting birds face many challenges to reproduce successfully, with nest predation being the main cause of reproductive failure. Visual predators such as corvids and egg-eating raptors, are among the most common causes of nest failure; thus, parental strategies that reduce the risk of visual nest predation should be favored by selection. To date, most research has focused on egg crypsis without considering adult crypsis, although in natural circumstances the eggs are covered by an incubating parent most of the time. Here we use a ground-nesting shorebird, the Kentish plover (Charadrius alexandrinus) as model species to experimentally test whether decoy parents influence nest predation. Using artificial nests with a male decoy, a female decoy or no decoy, we found that the presence of a decoy increased nest predation (N = 107 nests, p < 0.001). However, no difference was found in predation rates between nests with a male versus female decoy (p > 0.05). Additionally, we found that nests in densely vegetated habitats experienced higher survival compared to nests placed in sparsely vegetated habitats. Nest camera images, predator tracks and marks left on eggs identified the brown-necked raven (Corvus ruficollis) as the main visual nest predator. Our study suggests that the presence of incubating parents may enhance nest detectability to visual predators. However, parents may reduce the predation risk by placing a nest in sites where they are covered by vegetation. Our findings highlight the importance of nest site selection not only regarding egg crypsis but also considering incubating adult camouflage.

Simulated leks increase the capture of female Sirex noctilio in the absence of host volatiles

Male Sirex noctilio Fabricius (Hymenoptera: Siricidae) form leks in the upper canopy where mating occurs and have an orange abdominal segment not present in females. This study used field trapping studies in South Africa to examine the impact of stimuli (primarily visual) associated with male S. noctilio on the capture of conspecific females. Although we observed that traps baited with dead males (simulated leks) captured more females than unbaited traps, traps baited with a synthetic blend of host volatiles captured more female S. noctilio than traps baited with dead males. Additionally, dead males provided no additive effect on trap catch when combined with the host volatile lure regardless of the simulated lek size or if males were alive or dead. These results are the first to experimentally confirm an effect of a conspecific, and potentially mating related, visual stimulus on the behavior of S. noctilio. There appears to be limited potential for improvement of existing S. noctilio trapping methods by the addition of male decoys as deployed in this study.

Density dependence and habitat quality modulate the intensity of display territory defence in an exploded lekking species

We evaluated the effect of conspecific abundance and habitat quality of leks on the territorial behaviour of males in an exploded lekking species, the Little Bustard (Tetrax tetrax). The hypothesis that males more intensely defend territories with higher conspecific abundance and better habitat quality was evaluated experimentally analysing the agonistic response of experimental males to male decoys placed on their displaying areas. Decoy experiments showed that the intensity of display territory defence by little bustard males is density dependent. The time experimental males took to return to their display sites after decoy placement decreased with abundance of both males and females. The strength of their final response was positively associated to local male and female abundance in the vicinity of their display sites. Habitat quality also influenced males’ display territory defence since the intensity of male response increased with the degree of natural vegetation cover. Habitat quality was particularly relevant in explaining variation of experimental males’ snort call rate, which decreased with the degree in plough cover and increased with the number of fields in the lekking area. Snort call rate decreased with the level of male aggregation and was lowest in males exhibiting the strongest aggressive response to decoys. These results add new evidence for the density dependence of species’ breeding territorial behaviour, supporting density-dependent models of lek formation and reinforcing the role of resources defence in exploded lek mating systems.

The effect of intruders on territorial Palestine Sunbirds (Nectarinia osea) during the pre-egg laying period

In monogamous species, conspecific aggression is a primary tactic to deter intruders. Alternative reproductive strategies, such as extra-pair copulations or intra-specific brood parasitism that peak at the pre-egg laying period and are practised by intruders, pose a significant threat to the monogamous pair. Here, we addressed the question whether male and female Palestine Sunbirds (Nectarinia osea) demonstrate different aggression levels toward conspecific intruders during the pre-egg laying period. We further asked whether indirect measures of certainty of paternity affect male aggression toward such intruders. For that, we measured the aggression of both partners toward intruders while using stuffed birds. We conducted two experiments on a monogamous and territorial Palestine Sunbird population. In the first experiment, we exposed breeding pairs to a conspecific or a heterospecific intruder and measured their aggression toward it. During the second experiment, we exposed breeding pairs to an intruder, before and after temporarily removing the local male. The local male demonstrated higher aggression than the local female toward a male decoy. Toward a female decoy, the local male demonstrated lower aggression than the local female. Following their release, the removed males did not change their aggression toward the male decoy, but rather they decreased their aggression toward the female decoy. These results suggest that the indirect male’s certainty of paternity may be an important factor of the male’s interests when he faces a risk of potential intra-specific brood parasitism. Furthermore, these findings suggest that each gender is more sensitive to its similar intruder gender, and acts according to its interests in terms of its aggression toward the intruder.

Differential maturation of sexual traits: revealing sex while reducing male and female aggressiveness

Sexually mature young males of many dichromatic species begin breeding while expressing female-like or incomplete adult phenotypes, which contradicts sexual selection theory. It is thought that by delaying the acquisition of adult appearance, young males may avoid aggression from other males, but reduce mating opportunities, resulting in trade-offs between these 2 life-history traits. Thus, an optimal young male phenotype should allow mating while reducing aggressiveness. Differential character maturation (i.e., the acquisition of adult appearance does not occur in all traits at the same time) has been described in bird species, but has largely been neglected in the study of delayed plumage maturation. Using the common kestrel Falco tinnunculus, we tested the prediction that maturation of different adult traits depends on the aggressiveness elicited by both male and female individuals and that young male phenotypes should be a combination of traits triggering low levels of aggressiveness while still revealing sex. We explored the pattern of delayed plumage maturation observed in 4 different adult male traits (head, rump, tail, and back plumages) in young breeding males captured over 8 years. We then analyzed the intensity of agonistic responses against these traits in breeding males and females by presenting young male decoys showing these mature characters. We found that those characters eliciting high aggressiveness in males (head and rump) were partly expressed in young males and only 1 (tail) of the 2 traits (tail and back) eliciting high aggressiveness in females was poorly expressed. Our study suggests that conspecific aggressiveness of both males and females can play a significant role in determining patterns of delayed plumage maturation.

Adaptive significance of permanent female mimicry in a bird of prey

Permanent female mimicry, in which adult males express a female phenotype, is known only from two bird species. A likely benefit of female mimicry is reduced intrasexual competition, allowing female-like males to access breeding resources while avoiding costly fights with typical territorial males. We tested this hypothesis in a population of marsh harriers Circus aeruginosus in which approximately 40 per cent of sexually mature males exhibit a permanent, i.e. lifelong, female plumage phenotype. Using simulated territorial intrusions, we measured aggressive responses of breeding males towards conspecific decoys of females, female-like males and typical males. We show that aggressive responses varied with both the type of decoys and the type of defending male. Typical males were aggressive towards typical male decoys more than they were towards female-like male decoys; female-like male decoys were attacked at a rate similar to that of female decoys. By contrast, female-like males tolerated male decoys (both typical and female-like) and directed their aggression towards female decoys. Thus, agonistic responses were intrasexual in typical males but intersexual in female-like males, indicating that the latter not only look like females but also behave like them when defending breeding resources. When intrasexual aggression is high, permanent female mimicry is arguably adaptive and could be seen as a permanent 'non-aggression pact' with other males.

The role of androgen receptors in regulating territorial aggression in male song sparrows

This paper examines the role that androgen receptors (ARs) play in modulating aggressive behavior in male song sparrows, Melospiza melodia morphna. Song sparrows are seasonally breeding, territorial birds that maintain year-round territories with male–female pair bonds formed during the spring breeding season. Plasma testosterone levels peak as territories are established and mates acquired. In late summer, testosterone levels fall and remain basal during the non-breeding season. We examined the role of ARs in regulating territorial aggression in captive song sparrows under short- and long-day conditions as well as just prior to, and at the start of the breading season in freely living birds using the nonsteroidal antiandrogen flutamide to block AR function. Birds were implanted with either empty or drug filled silastic implants for 18 to 42 days and then challenged with a novel male decoy to assess the individual birds level of male–male aggression. Freely living birds remained on their home territory and underwent a simulated territorial intrusion, whereas laboratory-held birds were assessed using a laboratory simulated territorial intrusion and remained in their home cage. Experimental treatment of male song sparrows decreased aggressive behavior during the pre-breeding life history substage (March–April) in freely living birds as well as in laboratory-held birds under long-day (16L:8D) conditions. During the early breeding substage (April–May) there was no measurable effect of flutamide treatment on aggressive behavior, nor was there a difference in behavior in the (8L:16D) laboratory birds. This demonstrates that ARs are an important component of the neuroendocrine control of aggressive behavior. Given that flutamide only affected aggression during the pre-breeding substage and in LD birds, the results suggest that AR dependent control of aggressive behavior changes as song sparrow life history states change.

Experimental evaluation of live cage-traps for black-billed magpies Pica pica management in Spain

Black-billed magpies (Pica pica) are considered as a nest predator of game and non-game birds in Europe. In rural areas of Spain magpie control is commonly used as a management tool in small game hunting estates. Cage-traps with a magpie as a decoy are the legal method most commonly used for controlling magpies in Spain although its performance has not yet been experimentally tested. We evaluated the selectivity, efficiency, and the effect of different factors on capture rate of these traps for magpie control and determine the effect of magpie removal on magpie density. Only four out of 197 captures corresponded to non-target species, which were released unharmed. Since the release of non-target captures depends on the daily checking of the trap and the trapper commitment, in order to guarantee the efficiency and selectivity of this method traps should be revised daily by full time, qualified trappers. The efficiency of this method is high during the breeding season, reducing magpie density in the area where the control is performed. Highest capture rates were obtained in the first days after cage-traps setting. Neither the gender nor the origin (local or foreign) of the decoy significantly affected the capture rate. Among male decoys, experimentally increased testosterone levels did not increase capture rates. According to our results, the tested cage-traps with a living decoy could be employed as an efficient and selective method for magpie population management in Spain, when used by full time, qualified trappers.

Food availability during migratory stopover affects testis growth and reproductive behaviour in a migratory passerine

Long-distance migratory passerines initiate testicular recrudescence during spring migration to meet the demands of timely reproduction upon immediate arrival on the breeding grounds. The degree of testicular development is known to depend on environmental factors like stopover habitat quality; reproductive performance may be strongly impacted by testicular maturation upon arrival on the breeding grounds. We investigated the effect of stopover food availability on subsequent reproductive performance in garden warblers ( Sylvia borin). Spring migration was simulated by repeated food deprivation and re-feeding to imitate the alternation of flight and stopover periods. During the two final stopover periods, males were either kept under ad libitum food (ad libitum males) or under limited food conditions (limited males). After simulated arrival in the breeding area, manipulation of previous stopover food availability resulted in significantly slower testicular recrudescence ( p < 0.001) and decreased plasma testosterone ( p < 0.01) in limited males compared to ad libitum males. Body mass change was not significantly different between the two groups ( p = 0.38). Limited males also exhibited reduced performance in reproductive behaviours employed in territorial and sexual contexts. Limited males had a longer ‘freezing’ interval ( p < 0.05) and decreased activity ( p < 0.01) when challenged with a live male decoy. In direct confrontation between limited and ad libitum males in the presence of a female, limited males exhibited significantly fewer behavioural traits in sexual context, i.e. directed to the female ( p < 0.001). Therefore, we suggest that conditions encountered during previous migratory stopover may affect subsequent annual reproductive success by influencing key reproductive behaviours.

Female aggressive response and hormonal correlates—an intrusion experiment in a free-living passerine

The optimal mating system is rarely the same for males and females—whereas males usually benefit from attracting additional females to the territory, this could incur costs for the resident female. Females should therefore prevent prospecting females from settling on the territory. We studied the male and female behavioral and hormonal responses to simulated female territorial intrusions in free-living bluethroats during the pre-laying period. In the study population, polygyny occurs with potential fitness costs for the resident female. We recorded different aspects of aggressive behavior before and after presentation of a live female decoy and playback of female song. These behaviors were compared with a set of intrusions using a male decoy. At the end of a trial, the birds were captured, and blood samples were analyzed for androstenedione, testosterone, estradiol and corticosterone. During the pre-intrusion period, none of the females were observed. Females generally responded strongly to the female decoy by increased flight rate, vocalizations, and by conspicuous perching. Nearly half of the males displayed to the female decoy but never while the resident female was present. We suggest that resident female aggression in bluethroats prevents courtship by her mate and signals her mating status to the female intruder. Female aggression could therefore prevent additional females to settle on the territory and shape the mating system. Females that responded with song had higher levels of estradiol. These findings suggest that estradiol may support aggression in breeding female birds.

Males of a single-brooded tropical bird species do not show increases in testosterone during social challenges

The challenge hypothesis poses that in socially monogamous vertebrates, males increase circulating testosterone in response to aggressive challenges to promote intense and persistent aggression. However, in bird species that raise only a single brood during short breeding seasons as well as those with essential male parental care, males lack the well-documented testosterone response to social challenges. We tested male behavioral and hormonal responses to social challenges in a neotropical bird species, the buff-breasted wren ( Thryothorus leucotis), which is single-brooded with extensive male parental care, but in contrast to most species studied to date, has a long breeding season. We presented live female, male, and paired decoys with song playback for 30 min during pre-breeding and breeding periods. Males responded aggressively to all intruders, but male decoys elicited somewhat weaker responses overall. Responses to female decoys were most intense during pre-breeding, whereas pair decoys elicited stronger responses at breeding. Plasma testosterone concentrations did not differ between challenged and unchallenged males, or among males exposed to different decoys or during different seasons. Plasma corticosterone in pre-breeding males was higher in challenged than unchallenged males and varied positively with the duration of social challenge. Circulating dehydroepiandrosterone concentrations were similar in challenged and unchallenged males, but correlated positively with the proportion of time males spent in close proximity to the decoy. Both testosterone and corticosterone results support recent findings, suggesting that brood number and essential male care, but not breeding-season length, may be important determinants of male hormonal responsiveness during aggressive interactions.

Delayed plumage maturation in Eurasian kestrels: female mimicry, subordination signalling or both?

Delayed plumage maturation (DPM) of young males in sexually dichromatic birds has been explained under the female mimicry hypothesis (FMH) or the subordination signalling hypothesis (SbSH), among other adaptive ideas proposed. In this study, we tested in the Eurasian kestrel, Falco tinnunculus, the predicted sexual deception and aggressiveness under the FMH and SbSH to explain DPM in birds. We analysed aggressive and courtship behaviour of breeding males and females in the presence of adult male, adult female and 1-year-old male natural decoys. We designed two different treatments: one presenting un-moulted 1-year-old males and a second presenting partly moulted 1-year-old males, as observed in nature. Males attacked adult male decoys more frequently than 1-year-old male and female decoys in both moulted and un-moulted treatments and no sexual displays were observed. Females were more aggressive towards female and 1-year-old male decoys as compared to male decoys in the un-moulted treatment. In the moulted treatment, females were more aggressive towards female decoys as compared to both male decoys. In addition, females solicited copulas from moulted, but not from un-moulted, 1-year-old male decoys. Our results suggest that FMH can be a mechanism which explains DPM in un-moulted 1-year-old males. However, when they show moulted plumage (more than 80% in the studied population during the spring), the SbSH seems to be the mechanism that better explains sexual and agonistic behaviour from adults towards 1-year-old males.

Sexual competition explains patterns of individual investment in territorial aggression in western bluebird winter groups

In central California, western bluebirds, Sialia mexicana, live year-round on their breeding territories, feeding extensively on mistletoe berries through the winter. In spring, they usually breed as pairs and occasionally have helpers, whereas in winter they live in groups of up to 13 birds including family members and unrelated immigrants. Individuals of both sexes and all ages defend the winter territory against conspecifics. We presented caged male and female decoys to free-living groups of western bluebirds to test predictions of two hypotheses for adaptive variation in individual investment in winter territorial defence: kin value (kin selection) and reproductive competition (future breeding success). We found that individual investment in defence did not increase with kin value (the number of offspring equivalents that an individual had in the group) but varied as predicted by reproductive competition. Females spent more time on cages of female decoys than they did on cages of male decoys, but they did not differ in their frequency of aggressive displays (hops, wing flicks) towards intruders of either sex. In contrast, males spent equivalent periods on the cages of decoys of both sexes, but they hopped more frequently on cages of males. Although the form of aggression differed for males and females, aggression for both tended to be more pronounced towards same-sex conspecifics. The persistent effect of group size as a negative covariate indicates that group living enables individuals to reduce time spent in active territory defence. Members of large groups were able to reduce their investment while maintaining the same level of response to intruders as did members of smaller groups. Patterns of response to caged intruders provide novel support for the hypothesis that sexual competition is important in territorial defence outside the breeding season.

Paternity assurance responses to first-year and adult male territorial intrusions in a courtship-feeding raptor

Extrapair paternity occurs in many bird species and males use mate guarding, aggression or frequent copulations to reduce the risk of losing paternity. Because paternity assurance behaviour is likely to be costly, it should be adjusted to varying levels of perceived risk. We evaluated experimentally the adjustment of paternity assurance behaviour to risk in a raptor in which courtship feeding occurs: the Montagu's harrier, Circus pygargus. We used decoys with or without prey to simulate territorial intrusions by adult or first-year males. Inexperienced first-year males should represent a lesser risk than adult males and decoys with prey should represent a greater risk than those without prey, because courtship food could be traded for extrapair copulations. Aggression was less intense towards first-year than adult males during the female's fertile period. However, when decoys were presented with prey, aggression was similar towards first-years and adults. Males increased their short-term copulation rate in response to male decoys, particularly those presented with prey. Males also increased copulation duration, but only in response to adult male decoys. Our results show that male Montagu's harriers can fine-tune their paternity assurance behaviour to varying levels of risk, in this case the type of territorial intrusion. We also discuss the implications of our findings for the adaptive significance of delayed maturation in plumage colour in this species.

Sex pheromone of the tsetse species, Glossina austeni: isolation and identification of natural hydrocarbons, and bioassay of synthesized compounds

Copulatory responses of male Glossina austeni (Newstead) (Diptera: Glossinidae), that were elicited after contact with frozen female tsetse, were not observed after solvent washing of cuticular lipids. Chromatographic analysis of extracts from laboratory-reared and field-collected G. austeni females yielded natural hydrocarbons that were highly stimulatory to males. Most of this activity was produced by compounds in the alkene fraction. Gas chromatograms (GC) contained five natural alkenes; these were separated by preparative GC for bioassays conducted in Tanzania. The two major alkenes were identified using gas chromatography-mass spectrometry (GC-MS) to be 13,17-dimethyltritriacont-1-ene and 13,17-dimethylpentatriacont-1-ene, after the samples had undergone derivatization using dimethyl disulphide and saturation with deuterium. These alkenes and natural alkanes were quantified from G. austeni of both sexes from laboratory and field samples to confirm that their presence was consistent in this species. Trials of synthetic samples resulted in the order of biological activity for the stereoisomers of 13,17-dimethyltritriacont-1-ene as follows: S,R-33:1 > R,S- 33:1 > S,S-33:1 > R,R-33:1. Dose-response data showed an ED(50) at 5 microg per treated, solvent-washed male decoy. Of the four stereoisomers of 13,17-dimethylpentatriacont-1-ene, R,R-35:1 showed the most activity. This is the first report of alkene-induced sexual activity in males of the genus Glossina.

Effects of acute treatment with 8-OH-DPAT and fluoxetine on aggressive behaviour in male song sparrows (Melospiza melodia morphna).

The role of serotonin in modulating male aggressive behaviour was investigated in male song sparrows, Melospiza melodia morphna, using two different serotonergic drugs, fluoxetine and 8-OH-DPAT. Fluoxetine is a selective serotonin reuptake inhibitor of the neuronal reuptake pump increasing synaptic concentrations of serotonin, and 8-OH-DPAT is a specific serotonin (5-HT1A) receptor agonist. The serotonergic control of aggression in passerines has not been previously investigated. We examined these behaviours within a controlled setting using a laboratory simulated territorial intrusion, with a hierarchical scale to quantify male-male aggressive behaviour. Utilizing this scale, we quantified the extent of male aggressive behaviour in two experiments. In experiment 1, song sparrows were given 100 micro l, s.c. injections of either fluoxetine (10 mg/kg) or 8-OH-DPAT (1 mg/kg). Experiment 2 was a dose-response study using three doses of 8-OH-DPAT (0.1, 1 and 10 mg/kg). In both studies, aggressive behaviour was measured 1 h after injection for 10 min in response to the presence of a novel male decoy combined with playback of conspecific song. Both drugs significantly reduced male aggressive behaviour, and 8-OH-DPAT did so in a dose-dependent manner. The effect of the two drugs upon general activity was also measured using infra-red perch hop detectors. Activity levels were not effected by either fluoxetine or 8-OH-DPAT at all of the respective doses, indicating that the reduction in aggressive behaviour was specific. These results demonstrate that, in a passerine species, the serotonergic system negatively regulates male-male aggressive behaviour. These results further demonstrate that aggression can be effectively studied in a laboratory setting and natural aggressive responses can be elicited using this method.

Effects of Testosterone on Song, Aggression, and Nestling Feeding Behavior in Male Great Tits, Parus major

As outlined in the trade-off hypothesis of testosterone (T) secretion, fluctuations in T during the breeding season might reflect how males allocate their time and energy to competitive behaviors for mates and territories, associated with high T levels, and parental activities, associated with low T levels. In the present study, great tit, Parus major, males were implanted with T-filled or empty silastic capsules at the start of the breeding season and the behavior of these two male categories was compared during the entire breeding season. As a measure of competitive behavior we looked at song behavior and territorial responsiveness to a male decoy, during the three main stages of the breeding period (the egg-laying, incubation, and nestling stages). As a measure of parental care we looked at feeding behavior during the nestling stage. Our results only partly supported the trade-off hypothesis. T implants increased plasma androgen levels and enhanced spontaneous song activity and the production of aggressive vocalizations in response to a decoy. However, our results suggest that the degree of physical aggression might be less than fully coupled with T. First, approach to the decoy was not affected by the treatment. Second, although T levels are known to vary from high during egg laying to low while feeding young, control and T-treated males spent similar amounts of time close to the decoy in the three breeding stages. Our results thus suggest that vocal and physical aggression might be regulated differently in the great tit. Furthermore, in contrast with most other studies on temperate bird species but in agreement with a previous study on the great tit, T treatment did not affect male feeding rates. As the dose of T we used was lower than that typically used in other studies, we cannot at present completely exclude the possibility that the latter result reflects this lower dose of T rather than the species used.

Pheromone-mediated copulatory responses of the screwworm fly,Cochliomyia hominivorax

A bioassay based on male copulatory responses occurring on contact with dead decoy insects was used to confirm the existence of a sex pheromone in the screwworm fly,Cochliomyia hominivorax. Males responded to female but not male decoys. Mated and virgin females were equally stimulatory. Activity was abolished when females were washed with hexane but partially restored by treatment with crude hexane extract of females. Responses decreased when extracts were diluted and when the number of females extracted per milliliter of hexane was decreased from 20 to 1 in the preparation of extracts concentrated to 0.4 female/μl. Sexually mature female decoys of the 009 strain, the most laboratory-adapted of three strains examined in intrastrain tests, produced few copulatory attempts compared with those of Aricruz or DE-9 strains. However, newly emerged 009 as well as Aricruz females elicited responses from about 80% of sexually mature males. Those of the DE-9 strain stimulated fewer than 1%. The observation that 009 females were maximally stimulatory before becoming receptive to mating suggests that these strain differences resulted from laboratory colonization.