Unpaired Females Research Articles

Sexual dimorphism in the effects of mating on the in vitro release of LHRH from the ferret mediobasal hypothalamus

A sexually dimorphic pattern in the secretion of luteinizing hormone (LH) has previously been shown to occur in response to mating in an induced ovulating species, the ferret, with mating augmenting the secretion of LH in females but not in males. The aim of this study was to determine whether this dimorphic pattern in the postcoital secretion of LH reflects a dimorphic effect of mating on the neural release of luteinizing hormone releasing hormone (LHRH). The effect of mating on the in vitro release of LHRH from mediobasal hypothalami (MBH) collected from breeding male and female ferrets was studied. Luteinizing hormone releasing hormone release and content were significantly reduced in tissues from estrous females sacrificed 0.25 h after mating compared to unpaired estrous females and estrous females sacrificed 1 or 2.6 h after the mating stimulus. By contrast, the release of LHRH from MBH fragments and LHRH tissue content were equivalent in breeding males that were sacrificed 0.25 h after mating and in breeding males that were left unpaired. These data suggest that the postcoital surge of LH in the female ferret is preceded by a release of LHRH that initially depletes neuronal terminals within the MBH, whereas LHRH release, like pituitary LH secretion, is minimally affected by mating in males.

Sex difference in the effect of mating on c-fos expression in luteinizing hormone-releasing hormone neurons of the ferret forebrain.

The pulsatile secretion of LH was previously found to rise in female ferrets after receipt of an intromission, whereas in males that achieved an intromission, both LH and testosterone secretion were either reduced or unchanged. We sought to determine whether this sexually dimorphic pattern of LH secretion reflects a sex difference in the effect of mating on the activity of forebrain neurons that secrete LHRH. Immunocytochemical methods were used to localize the nuclear protein product of the immediate early gene, c-fos, as an index of increased neuronal activity after mating. Nuclear FOS immunoreactivity (FOS-IR) was monitored in LHRH-IR neurons as well as other non-LHRH forebrain neurons. In confirmation of previous reports, LHRH-IR perikarya in ferrets of both sexes were located medially along the base of the brain at rostral, medial, and caudal levels of the preoptic-hypothalamic continuum. In each of these regions a significantly higher percentage of LHRH-IR neurons was colabeled with nuclear FOS-IR in mated than in unpaired females. By contrast, an equivalent low percentage of LHRH-IR neurons was colabeled with FOS-IR in mated and unpaired male ferrets. Significantly more FOS-IR neurons (not colabeled with LHRH) were detected in the bed nucleus of the stria terminalis, the medial preoptic area, the dorsal-medial hypothalamus, and the medial amygdala (MA) of mated vs. unpaired females. By contrast, mating significantly augmented FOS-IR only in the MA of male ferrets. The results suggest that the sexually dimorphic pattern of LH secretion that occurs in ferrets after mating reflects a selective activation of LHRH neurons in the female forebrain. This sex-specific increase in the responsiveness of LHRH neurons to mating may depend on input from a limbic circuit which includes the medial amygdala, bed nucleus of the stria terminalis, and medial preoptic area.

Copulatory behaviour in a colony of lesser kestrels: sperm competition and mixed reproductive strategies

Copulation attempts were observed at a colony of lesser kestrels, Falco naumanni , in southern Spain in 1989 and 1990. Of 1397 observed copulation attempts, 6·7% were extra-pair copulations. Ten of the 13 focal pairs showed a seasonally bimodal pattern of daily copulatory rates. On average, the peaks occurred 65 and 5 days before the laying of the first egg. The mean number of copulations per female per clutch was 326·2. The three other pairs, formed by one yearling and an older individual, had a single maximum at an average of 15 days before laying. Frequent copulations in the lesser kestrel might have several functions: the early peak would be almost certainly outside the females' fertile period and would be related to pair bonding and sexual stimulation of the pair members. The peak closer to egg laying could be related to sperm competition since most extra-pair copulation attempts occurred close to the date of lay. Male lesser kestrels followed a mixed reproductive strategy, but the success of the extra-pair copulation attempts depended on the females (the larger sex) and their pairing status. Unpaired females tended to accept copulations with already paired males, whereas paired females rejected extra-pair copulations. Both sexes follow a strategy that tends to advance their respective laying dates and hence improve their breeding success.

Copulatory behaviour in a colony of lesser kestrels: sperm competition and mixed reproductive strategies

Copulation attempts were observed at a colony of lesser kestrels, Falco naumanni, in southern Spain in 1989 and 1990. Of 1397 observed copulation attempts, 6·7% were extra-pair copulations. Ten of the 13 focal pairs showed a seasonally bimodal pattern of daily copulatory rates. On average, the peaks occurred 65 and 5 days before the laying of the first egg. The mean number of copulations per female per clutch was 326·2. The three other pairs, formed by one yearling and an older individual, had a single maximum at an average of 15 days before laying. Frequent copulations in the lesser kestrel might have several functions: the early peak would be almost certainly outside the females' fertile period and would be related to pair bonding and sexual stimulation of the pair members. The peak closer to egg laying could be related to sperm competition since most extra-pair copulation attempts occurred close to the date of lay. Male lesser kestrels followed a mixed reproductive strategy, but the success of the extra-pair copulation attempts depended on the females (the larger sex) and their pairing status. Unpaired females tended to accept copulations with already paired males, whereas paired females rejected extra-pair copulations. Both sexes follow a strategy that tends to advance their respective laying dates and hence improve their breeding success.

Experimental Evidence for the Function of Mate Replacement and Infanticide by Males in a North-Temperate Population of House Wrens

Male birds who replace other males on territories may kill, care for, or act indifferently towards offspring of the previous resident. In short-lived species, selection should favor adoption or indifference over infanticide only when the resident female is the only mate the replacement male is likely to obtain, and when this female will desert the male if her offspring are killed. We tested this hypothesis by removing 29 male House Wrens (Troglodytes aedon) from territories in a population in which unpaired females are readily available as mates. Ten of 16 males that were subsequently observed on experimental territories removed offspring from nests as predicted. At least four resident females bred with these males. All infanticidal males paired within 72 hr of replacement and most subsequently fledged young of their own. An eleventh replacement male fed young after eggs hatched but disappeared before nestlings fledged. The five remaining males appeared indifferent to resident offspring. Unlike infanticidal males, however, indifferent males did not remain on territories continuously, court resident females, or attempt to attract a mate to the territory. No indifferent male bred on the territory. Our observations suggest that selection should strongly favor a "replacement-then-infanticide" strategy among unsettled male House Wrens in this population. However, for reasons yet unknown, some males fail to complete this sequence of behaviors.

Courtship and Aggression in Canvasbacks: Influence of Sex and Pair-Bonding

Time-activity budgets, courtship, and aggression of paired and unpaired Canvasbacks (Aythya valisineria) were studied on the upper Mississippi River and on Long Point Bay, Lake Erie, during spring migration. Of Canvasbacks present, 25-28% were female of which 17-27% were paired. Paired Canvasbacks spent more time in foraging aggression and initiated and won more encounters than did unpaired individuals. Paired females engaged in more courtship-related threats, chases, and neck-stretches than did unpaired females. Paired females exceeded paired males in foraging aggression and in courtship behaviors such as neck-stretches and chases, with the trend in all chases reversing just before or upon arrival at nesting sites. Thus in the early stages of pairing, females rather than males appeared to assume the primary role in foraging aggression and repelling courtship advances of other males.

Causes of assortative mating in arthropods

Assortative mating by size, one of the most common mating patterns in natural populations, may be explained by three hypotheses: (1) mate choice, whereby large males, large females or large individuals of both sexes choose large mates because they benefit reproductively and are differentially capable of exercising choice; (2) mate availability, whereby large females or large individuals of both sexes are differentially available for mating because of size-differential variation in mating probability or timing that is unrelated to mate choice; and (3) mating constraints, whereby relative size differences between males and females cause physical or energetic difficulties with courtship, pairing or mating. Analysis of the causes of assortative mating in well-studied arthropod species provides some support for each hypothesis in one or more species. However, comparative analysis of many species indicates that assortative mating is most closely associated with the pattern of large size of paired males and females relative to unpaired males and females and the occurrence of long pairing durations and male fighting. These comparative data therefore implicate two hypotheses for the causes of assortative mating in most arthropods: male choice of large females combined with male-male competition, and increased availability of large females combined with male-male competition. The female availability hypothesis may be more plausible because male choice and male-male competition are apparently antagonistic processes whose effects usually do not coincide. Distinguishing between hypotheses based on mate choice, mate availability and mating constraints is crucial to understanding the roles of natural selection and sexual selection in causing assortative mating.

Wing fluttering display by incubating male Wilson's phalaropes

Wilson's phalarope (Phalaropus tricolor) exhibits extreme sex-role reversal. Males provide nearly exclusive parental care for eggs and chicks, whereas females compete directly for mates in scramble competition. Males on incubation recess are often harassed by unpaired females. Males flying to nests to resume incubation performed a distinctive flight display when followed by females. Conversely, unaccompanied males returning to nests rarely performed the display. We hypothesize that this display signals to females a male's unavailability as a prospective mate.

Intraspecific nest parasitism in the European starling Sturnus vulgaris

Biochemical genetic markers were used along with conventional methods (abnormal laying sequence/clutch size, unusual egg shape/pigmentation) to identify intraspecific nest parasitism at two British nestbox colonies of the European starling. Between 11 and 37% of first clutches were parasitized during 1977–1979. Parasitic females probably comprised all of the following categories: (1) paired females contesting a nestbox occupied by another pair; (2) previously paired females who had laid a clutch but had been unsuccessful; (3) unpaired females who had copulated with males that already had a mate and nest site; and (4) ‘professional’ nest parasites who distributed at lest some of their eggs in one or more nests other than their own. Although parasitized nests had higher clutch sizes, parasitism led to fewer host young fledging per egg laid, mainly through the eviction of eggs and subsequent nest desertion. Number of parasitic young fledged per egg laid was highest when eggs were laid synchronously with the host, when host clutches were larger, or a smaller number of parasite eggs were added to a nest, thus favouring parasites that distribute their eggs amongst a number of nests. A greater pressure on nest sites may have accounted for the higher levels of parasitism at the Aberdeen colony and for the greater number of parasite eggs laid in a nest. Although most parasitic female starlings appeared to be much less successful than non-parasitic ones, nest parasitism in the starling might evolve directly when one or more of the following advantages are present. (1) There are no constraints on the number of eggs a female may lay but there are constraints on the number of young she may feed adequately. (2) Female survival is increased by having fewer or no eggs/young to care for. (3) Current feeding conditions favour the survival of more young than would be produced by the most common clutch size. Intraspecific nest parasitism is considered to be a first stage in the evolution of interspecific nest parasitism.

Body Composition of Female Mallards in Winter in Relation to Annual Cycle Events

Body mass and carcass composition were determined from 267 female Mallards (Anasplatyrhynchos) collected during the winters of 1980-1981, 1981-1982, and 19821983 in the Mingo Basin of southeastern Missouri. Changes in carcass composition were related to annual events, such as migration, molt, and pairing. Body and carcass masses were similar between adults and immatures within status groups. The first females arriving in the Mingo Basin in fall were mostly adults engaged in the prealternate molt; body (1,010 g) and lipid (65 g) masses of these females in mid-prealternate molt were low compared to females collected at other times during winter. Females gained body and lipid mass following the completion of the prealternate molt and most became paired in November and December. Paired females had greater lipid masses than unpaired females throughout fall and winter. Body and carcass masses of nonmolting females remained constant over winter. Females initiated the prebasic molt with relatively large body (1,118 g) and lipid (168 g) masses. Lipid mass declined curvilinearly through the middle of the prebasic molt but then increased to 188 g and 219 g by late prebasic molt and premigratory stages, respectively. Adults proceeded through annual events earlier than immatures and both immatures and adults proceeded through annual events earlier in the wet winter of 1982-1983 than in the drier winters of 1980-1981 and 1981-1982. Mallards complete annual events earlier than most other ducks and female Mallards that complete annual events early in winter may have, at least historically, realized survival and reproductive potential advantages, especially following wet winters.

Effect of Pairing In vitro on the Glutathione Level of Male Schistosoma mansoni

The effect of in vitro incubation on the level of the intracellular nucleophile, glutathione (GSH), in adult Schistosoma mansoni was investigated. The GSH levels of freshly collected adult male and female parasites were 8.5 +/- 2.5 and 2.7 +/- 0.7 nmol/10 worms, respectively, as determined by an enzymatic assay. Twenty-four-hour incubation of unpaired males in RPMI-1640 medium at 37 C resulted in a 1.7-fold increase (P less than 0.001) in GSH level that remained elevated for at least 7 days. The increase was dependent on exogenous L-cystine, suggesting that it was due to biosynthesis of GSH. Biosynthesis in male S. mansoni was confirmed by isolating [3H] GSH from parasites incubated in medium containing L-[3H] cystine or [3H] glycine. In contrast to unpaired males, the GSH level of paired males as well as that of unpaired or paired females did not increase after 24 hr in vitro. When males that had been incubated unpaired for 24 hr were allowed to couple in vitro with freshly collected females, their GSH level fell to that of continuously paired males. These observations provide evidence that in vitro female schistosomes can influence the physiology of the male.

Molt in the Annual Cycle of Canvasbacks

Abstract We studied the intensity of molt of capital, down, and contour plumages of male and female Canvasbacks (Aythya valisineria) from late fall through winter, spring migration, and nesting. Molt intensities in both juvenile and adult males were highest among arriving fall migrants and during prewinter fat deposition, and molt in juvenile males increased during early spring migration. Among females, capital molt was high among arriving migrants in fall, and molt of all plumage types increased during spring migration and courtship. Paired females tended to molt more heavily than unpaired females during spring migration, but many females initiated intensive prebasic molt before pairing. Molt in females peaked in spring migration and early breeding, concurrently with accelerated fat storage and ovarian growth. Capital and contour molt in females decreased during laying and essentially ceased during incubation, perhaps to mitigate energetic constraints during these periods. Prewinter nutrient storage appeared independent of molt intensity, except for body protein among juveniles. During spring migration, molt intensities were greater in adult females with higher levels of body fat and protein, indicating that molt did not necessarily precede or interfere with nutrient storage. We suggest that outside the laying, incubation, and wing molt periods, the energetic costs of molt in Canvasbacks are not sufficient to mandate temporal separation of molt from nutrient storage, migration, courtship, or early stages of rapid ovarian growth.

The distribution and daily movement of stone crabs (menippe mercenaria) in an intertidal oyster habitat on the northwest coast of Florida

The distribution and daily movement of stone crabs inhabiting a semi‐artificial, intertidal, oyster habitat was monitored from May to December, 1983, on the northwest coast of Florida. The population structure was characterized by an abundance of adult males and juveniles of both sexes in the summer followed by an influx of adult females during the early fall mating season. Adult heterosexual pairs were sighted exclusively in September and October. Adult males in heterosexual pairs were significantly larger than unpaired adult males. Paired and unpaired females did not differ in size nor were the sizes of paired individuals correlated. Nearest neighbor analyses indicated adult males were distributed randomly during the pre‐mating, mating and post‐mating seasons. Tag‐recapture data indicated there were no differences in frequency or distance of movement between the sexes or size classes. The higher frequency of pairing for large males suggests non‐random mating occurred; however, a behavioral mechanism exp...

Establishment and organization of territories in the dragonfly Sympetrum rubicundulum (Odonata: Libellulidae)

Territory formation by males of the dragonfly Sympetrum rubicundulum was studied at a shallow temporary pond in Pennsylvania. Most males arrived at the pond between 1100 and 1200 hours and sought unoccupied sites in which to land, which they immediately began defending against conspecific males. Incoming males were never observed to challenge previous residents for possession of territories. Most conflicts between males occurred in the mid-afternoon and involved occupants of adjacent territories; residents experienced a clear ownership advantage in these conflicts. Escalated contests for possession of territory sites did not occur, perhaps because unoccupied sites were continually becoming available due to territory abandonment. Certain regions of the pond were consistently favoured by both sexes and most mating occurred in them: the spatial distributions of copulating pairs, ovipositing females and unpaired females were in close agreement with the spatial distribution of territorial males. High density sites were also the first sites occupied in the morning by incoming males. Although most of the pond and the surrounding land was thickly vegetated, territorial males and ovipositing females were both significantly clumped along the water's edge. The spatial distribution of males may have resulted from a balance of factors: peripheral sites were better than central ones for encountering incoming females, but areas far from the water's edge were unsuitable for female oviposition.

Influence of mating on surface nutrient exchange in schistosomes

In schistosomes, the mating process influences male-female transfer and gender-specific exchange of nutrients. The paired male schistosome provides glucose to the female partner. Male-to-female intertegumental transfer of(14)C-labeled glucose,(14)C-labeled 3-O-methylglucose, [(14)C]2-deoxyglucose and 2-fluorodeoxyglucose has been demonstrated in schistosomes. This phenomenon has been studied extensively inSchistosoma mansoni, and confirmed inSchistosoma japonicum, as well asS. haematobium, using radioactive pulsing methods. Male schistosomes contain significantly greater quantities (nmol/mg worm water) of glucose than do females. The transfer of glucose is apparently not an energy-dependent process, but occurs along this concentration gradient. Most, if not all, of the glucose utilized by the female is transferred from the male partner via tegumentary-facilitated diffusion mechanisms, free diffusion, or some combination of these two components. Unpaired male schistosomes contain greater quantities of glycogen than do comparable paired schistosomes, indicating that the presence of a female in the gynecophoral canal depletes the reserves of the male partner; this is additional indirect evidence for male-to-female transfer of glucose. Tegumentary surface uptake of acidic amino acids has been compared in paired and separated male and female schistosomes. InS. mansoni, a saturable carrier-mediated mechanism has been defined which operates only in unpaired male and unpaired female teguments. In contrast, the uptake of aspartate and glutamate is not seen in paired worms of this species. Tegumental uptake of acidic amino acids is not observed in paired or unpaired male or femaleS. japonicum. However, inS. haematobium, significant quantities of aspartate are taken up by both paired and unpaired male schistosomes. Measurable aspartate uptake is seen in paired femaleS. haematobium, but in the separated female, there is minimal uptake of this acidic amino acid. Thus the permeability of the teguments of human schistosome species to acidic amino acids is modified in response to the paired state inS. mansoni andS. haematobium, but these characteristics are not shared byS. japonicum.

Polygyny in Barrow's Goldeneye

Polygyny in Barrow's Goldeneye

Effects of male courtship and gonadal steroids on pair formation, egg‐laying, and serum LH in Canvasback ducks (Aythya valisineria)

The effects of social stimuli on the reproductive physiology of female birds have been widely studied. This study examined the endocrine and behavioural responses of female Canvasbacks (Aythya valisineria) to changes in the behavioural and endocrine states of the male, as signalled by his courtship displays. Male Canvasbacks exposed to female conspecifics exhibited a variety of courtship displays which increased significantly with testosterone administration. However, the testosterone‐increased courtship displays of males did not increase female receptivity or serum LH, or stimulate reproductive development or egg‐laying in unpaired females placed with them. Females confined in cubicles within a pen that held breeding pairs and a free‐swimming flock had higher serum LH levels and laid more eggs than did females in cubicles in another pen without breeding pairs or a free‐swimming flock. This suggests that social factors, such as vocalizations from a flock and breeding pairs, may increase serum LH and egg‐laying in unpaired females. Oestradiol implants in female Canvasbacks did not elevate receptivity to courting drakes, nor did they stimulate pair formation, nesting activity, or egg‐laying behaviour. Testosterone implants in drakes and oestradiol implants in females decreased serum LH in each group. These results show that increasing the amount of courtship is not adequate to stimulate pairing behaviour or increases in reproductive hormones. Thus, our results suggest that female receptivity to male courtship may be a key determinant of pair formation and consequent synchronization of the behaviour patterns between members of a pair of wild birds. Our results may further suggest a dissociation between effects of courtship and pair formation in wild Canvasbacks.

Requirements for a Mixed Reproductive Strategy in Avian Species

Trivers (1972) suggested it would be adaptive for males of species that invest considerably in parental care to pursue a mixed behavioral strategy of helping a single female raise offspring while taking advantage of opportunities to fertilize eggs of other females. A working assumption of influential studies since Trivers has been that, in avian species, males forcefully fertilize females, resulting in a successful mixed strategy for the males. In many avian species it appears that males are unable to forcefully fertilize females. Therefore, the male mixed behavioral strategy requires a complementary female mixed strategy. Females that might willingly copulate with nonmate males include paired females involved in sophisticated cuckolds, newly bonded females, unpaired females attempting to obtain mates, or unpaired females attempting to reproduce with the help of nonmates. We studied male and female behavior in our marked study population of herring gulls to determine if both sexes pursue mixed behavioral strategies. Thirty-five percent of paired herring gull males copulated with willing nonmate females, but these females were unpaired marked females that solicited from the males. Presumably, the paired males also would have copulated with nonmate paired females if they had had the opportunity. Paired females did not attempt to copulate with nonmate males despite ample opportunity. Differences between the behavior of paired and unpaired females suggests that females pursue a mixed behavioral strategy depending on their pairing status. This difference between paired and unpaired females' behavior illustrates confidence of paternity. Parental effort provided to a female by a male is mutually dependent upon confidence of paternity. This relationship has generally been ignored by influential studies of MRS that focus on EPC attempts and mate guarding, the inferred consequence of successful EPC. We found that male herring gulls increased their time on the territory during the fertile period. Although increased attendance could be used to infer that mate guarding was necessary, other data, including no increase in intrusions and no change in males' aggressiveness during the fertile period, and the fact that females were unattended for 20.2% of the fertile period, suggest that mate guarding may be relatively unimportant in herring gulls. Males were able to leave mates alone because of confidence of paternity provided by the female in exchange for parental effort in the form of courtship feeding. If the male EPC behavior in our study, and in other studies, is adaptive (i.e., increases the reproductive success of participating males and females) there must be situations in which EPC leads to successful rearing of offspring. Studies of female behavior will permit discovery of these situations.

Behavioral and Ecological Factors Contributing to American Black Duck-Mallard Hybridization

Investigation of courtship and pair formation of a wintering population of American black ducks (Anas rubripes) and mallards (A. platyrhynchos) near Ottawa, Ontario, indicated that initially drakes of both species exclusively courted and paired intraspecifically. After all female mallards had paired, the remaining mallard drakes joined black duck courtship groups. Of the 33 unpaired black duck females remaining at this time, only 27% formed intraspecific pairs, whereas 73% selected mallard drakes as mates, despite there being an excess of black duck drakes. Based on these results, a scenario for black duck-mallard hybridization is proposed. It involves ecological factors including the male-biased sex ratio in northern wintering populations, artificial feeding, and roost-site limitation. Other behavioral aspects, such as the earlier pair formation in mallards and the superiority exhibited by mallard drakes when competing for black duck females are discussed. J. WILDL. MANAGE. 48(3):846-852 One factor essential to a species' genetic integrity is its continued reproductive isolation from congenerics through geographic, habitat, or behavioral means (Lack 1971). Two species of dabbling duck, the American black duck and the mallard, are similar both morphologically and behaviorally, and their hybrids appear to be as viable and as fertile as the parental species (Johnsgard 1961). However, geographic range and evolved habitat differences have until recently deterred hybridization between these two species. Prior to 1900, the black duck and mallard were geographically isolated. Heusmann (1974) examined the evolutionary history of the black duck and hypothesized that it evolved during the Pleistocene glacial period when a population along the eastern coast of North America became isolated from western mallard populations. Habitat differences further segregated these populations as the black duck began breeding in the forested regions of the east and the mallard nested in grassland habitats of the west. In the present century, however, land use changes, including the deforestation of the eastern region, have caused a decline in the available breeding habitat of black ducks while simultaneously providing favorable habitat for nesting mallards. Johnsgard (1967) concluded that these environmental modifications were facilitating the colonization, by mallards, of extensive eastern areas formerly occupied by the black duck. The invasion of the mallard into black duck breeding territory has also resulted in the convergence of these two species on wintering grounds. Since the majority of pair bonding in both species occurs throughout the winter (Stotts and Davis 1960, Lebret 1961) and the courtship displays that lead to pair formation are similar (Johnsgard 1960), interspecific pairing can readily occur where these two species winter together. As a result of black duck hybridization with the much larger mallard gene pool and the constant reduction of its nesting habitat, it is believed by many that the black duck as a pure species will eventually disappear. The objectives of this study were to examine courtship activity and pair formation in 846 J. Wildl. Manage. 48(3):1984 This content downloaded from 157.55.39.110 on Sat, 24 Sep 2016 06:00:00 UTC All use subject to http://about.jstor.org/terms BLACK DUCK-MALLARD HYBRIDIZATION * Brodsky and Weatherhead 847 black ducks and mallards wintering together at the northern limit of their range and to determine the factors contributing to interspecific pairing between these two species. We thank H. G. Merriam for suggesting the suitability of the Manotick waterfowl population for this study, C. A. Barlow for comments on an earlier draft of this manuscript, and J. Sharkey-Thomas for feeding the ducks. Financial support was provided by the Nat. Sci. and Eng. Counc. of Can. and Carleton Univ.

Function of Manx Shearwater Calls in Mate Attraction

AbstractPlayback experiments were used to study the role of vocalization in mate attraction by Manx shearwaters, a nocturnal seabird. Pre-breeding birds were exposed to test calls of male and female shearwaters calling from the range of normal locations: above the colony as in a flyover call, on the ground in the colony, or from within the nesting burrow. Both female and male test calls increased the frequency of male calling but only male calls increased the frequency of female calling. These findings suggest that male calls 1) facilitated competing responses from other males that are attempting to attract females to their burrows and 2) stimulated calling in available females that require more directional information to find the advertising male. Male calling location, inside or outside the burrow, probably reflected a compromise between being audible to females and being susceptible to predation. Because the call was more widely broadcast, male test calls played from outside the burrow caused more females to land in the colony than the same call played from inside the burrow. However in a simultaneous choice test between a burrow and an outside male test call, males calling from inside did not lose females to the adjacent outside male. Thus calling from inside the burrow in an area where another male is calling outside maximized female attraction while minimizing risk of predation. The male call played from inside the burrow attracted as many females as a pair call from inside but the response of females to the pair was less sustained, indicating that duetting pairs-inhibited the response of unpaired females. Male and female calls emanating from above the colony suppressed the rate of female flyover calls suggesting that during this test condition other birds stopped calling in an attempt to orient to the source of the test call.