Experimental increase in food supply influences the outcome of within-family conflicts in Tengmalm’s owl

Within a family there are conflicts of interest between parents and offspring, and between male and female parents, over the supply of parental care. The observed pattern of parental care is the outcome of negotiations within the family, and may be influenced by environmental factors such as food abundance. We experimentally increased food supply to ten Tengmalm’s owl (Aegolius funereus) nests from hatching to fledging, mimicking natural cached prey. Ten un-supplemented nests served as controls. Parents and offspring were fitted with radio-tags. Food provisioning by parents was measured both in the (1) mid- and (2) late nestling stage and in the (3) early and (4) late post-fledging stage. In response to food supplementation, both males and females reduced food provisioning, but the effect was more pronounced in females. Females generally contributed much less to food provisioning than males, and food supplementation increased the difference between the sexes. Mass loss during the brooding stage was substantially lower for supplemented than for control females. Food supplementation did not improve offspring survival, and had no effect on body measurements of nestlings. In conclusion, parents of both sexes used the increased food supply to reduce the costs of caring for their current offspring, but females responded more strongly than males.     

Males are faster foragers than females: intersexual differences of foraging behaviour in the Apennine chamois

Availability of food resources and individual characteristics can influence foraging behaviour, which can differ between males and females, leading to different patterns of food/habitat selection. In dimorphic species, females are usually more selective in food choice, show greater bite rates and spend more time foraging than males. We evaluated sexual differences in foraging behaviour in Apennine chamois Rupicapra pyrenaica ornata, during the warm season, before the rut. Both sexes selected nutritious vegetation patches and spent a comparable amount of time feeding. However, males had a significantly greater feeding intensity (bite rate) and a lower search effort for feeding (step rate), as well as they spent more time lying down than females. Females selected foraging sites closer to refuge areas than males. In chamois, sexual size dimorphism is seasonal, being negligible in winter–spring, but increasing to 30–40 % in autumn. Our results suggest that males enhance their energy and mass gain by increasing their food intake rate during the warm season, to face the costs of the mating season (November). Conversely, females seem to prioritize a fine-scale selection of vegetation and the protection of offspring. A great food intake rate of males in the warm season could have developed as a behavioural adaptation leading herbivores to the evolutionary transition from year-round monomorphism to permanent dimorphism, through seasonal dimorphism.     

Nestling provisioning in polygynous great reed warblers (Acrocephalus arundinaceus): do males bring larger prey to compensate for fewer nest visits?

Most studies of social polygyny in birds have examined male provisioning on the basis of the number of feeding visits. This may be misleading if males compensate for infrequent visits by bringing larger prey at each visit. We investigated nestling provisioning in the socially polygynous great reed warbler, Acrocephalus arundinaceus, in south Central Sweden in 1996–1997. We collected data on rate of feeding visits, prey size and the amount of biomass delivered by males and females. Males had lower rates of feeding visits and provided smaller prey to nestlings in secondary than in monogamous and primary nests. Secondary females had higher rates of feeding visits and brought larger prey than monogamous and primary females. These results confirm that secondary females face a potential cost of polygyny through a lower rate of male feeding, and that this cost was reinforced by the significantly lower male provisioning rate (biomass h^–1) at secondary nests. Secondary females compensated for the lack of male assistance by increasing their rate of feeding and bringing larger prey. As a result, offspring in nests of secondary females received as much food as did those in nests of primary females. Prey load size increased with the parent’s proportion of feeding visits, suggesting that parents use different feeding strategies depending on their amount of responsibility for nestling provisioning. We suggest that parents which take the main responsibility for nestling feeding have to forage further away from the nest, and based on optimal-foraging theory, they should then on average bring larger prey to their nest. Received: 4 April 1999 / Received in revised form: 12 October 1999 / Accepted: 23 October 1999     

Carer provisioning rules in an obligate cooperative breeder: prey type, size and delivery rate

Providing food to developing offspring is beneficial for offspring but costly for carers. Understanding patterns of provisioning thus yields important insights into how selection shapes (allo-) parental care strategies. Broadly, offspring development will be influenced by three components of provisioning (prey type, size and delivery rate). However, all three variables are rarely considered simultaneously, leading to suggestions that the results of many studies are misleading. Additionally, few studies have examined the provisioning strategies of breeders and non-breeding helpers in obligate cooperative breeders, wherein reproduction without help is typically unsuccessful. We investigated these components of provisioning in obligately cooperative chestnut-crowned babblers (Pomatostomus ruficeps). Prey type was associated with size, and delivery rate was the best predictor of the overall amount of food provided by carers. As broods aged, breeders and helpers similarly modified the relative proportion of different prey provided and increased both prey size and delivery rate. Breeding females contributed less prey than male breeders and adult helpers, and were the only carers to load-lighten by reducing their provisioning rates in the presence of additional carers. While our results suggest that breeders and helpers follow broadly comparable provisioning rules, they are also consistent with the idea that, in obligately cooperative species, breeding females benefit more from conserving resources for future reproduction than do helpers which have a low probability of breeding independently.     

Polygyny and male parental care in Savannah sparrows: effects on female fitness

Summary To determine the effects of male mating status on female fitness, we compared the reproductive success, survival, and future fecundity of female Savannah sparrows (Passerculus sandwichensis) mated to monogamous vs. polygynous males in a 5-year study on Kent Island, New Brunswick, Canada. The proportion of males with more than one mate varied from 15 to 43% between years and sites. Polygynous and monogamous males fledged young of equal size in every year of the study. Females who shared paternal care with other females laid as many eggs per clutch and clutches per season as monogamously mated females. In most years polygynously mated females showed no delay in laying a second clutch, and they suffered no reduction in fecundity the following year. Recruitment of a female's offspring into the breeding population was generally independent of her mating status. Fitness costs of being mated to a polygynous male were only apparent in one year of the study, during which females mated to polygynous males had higher over-winter mortality than those mated to monogamous males. That same year, young raised by polygynous males were only one-third as likely to survive to reproductive maturity (as inferred by returns) as those raised by monogamous males. A male's mating status had no effect on his own survivorship. A male's mating status did not necessarily reflect his contributions to raising nestlings, which may partially explain why monogamously and polygynously mated females had equal fitness. At 35 nests the proportion of food deliveries brought by individual males varied from 0 to 75%; on average, males brought fewer than 30% of all food deliveries. Yet parental care by polygynous males was no less than that of monogamous males, at least at the nests of their primary females. Secondary females tended to receive less male assistance during the nestling stage, but their reproductive success was indistinguishable from that of primary females. Females feeding young without male assistance made as many food deliveries/h as did pairs in which males brought at least 30% of all food deliveries. Unassisted females did not suffer diminished fledging success or produce smaller fledglings. The benefits of polygyny for male Savannah sparrows are clear: polygynous males recruit more surviving offspring into the breeding population than monogamous males. The fitness of females, on the other hand, appears to be unaffected by whether their mate was monogamous or polygynous except in occasional years. Polygyny may be maintained in this population by the constraints of a female-biased sex ratio, the inability of females to predict a male's paternal care based on his morphology or behavior, the poor correlation between a male's mating status and his assistance at the nest, and inconsistent natural selection against mating with a polygynous male. Correspondence to: N.T. Wheelwright     

Female-coerced monogamy in burying beetles

The reproductive interests of the sexes often do not coincide, and this fundamental conflict is believed to underlie a variety of sex-specific behavioral adaptations. Sexual conflict in burying beetles arises when a male and female secure a carcass that can support more offspring than a single female can produce. In such a situation, any male attracting a second female sires more surviving offspring than he would by remaining monogamous, whereas the female's reproductive success decreases if a rival female is attracted to the carcass. Monogamously paired males on large carcasses do in fact attempt to attract additional females by means of pheromone emission, whereas males on small carcasses do not. Females physically interfere with male polygynous signaling using various behavioral tactics. We demonstrate that such interference leads to a significant decrease in the amount of time that males spend signaling, according females a means by which to impose monogamy on their mates.     

Female territoriality and the function of scent-marking in a monogamous antelope (Oreotragus oreotragus)

In monogamous species, females often choose between males according to the quality of the territories they defend, but the extent to which females themselves contribute to territory defence is frequently underestimated. Here we test for differences in male and female roles during paired scent-marking bouts, a key component of territorial defence, in a monogamous antelope. In two populations (Kenya, Zimbabwe) of klipspringer, Oreotragus oreotragus, both males and females usually scent-marked at the same site, but there were significant differences between sexes in terms of investment within bouts. Females initiated most bouts, thus dictating the marking strategy of the pair. Males initiated relatively few bouts, but deposited more scent marks per bout than females and were usually the last to scent-mark before leaving the site; they marked on the same branches as the female and thus overmarked her scent. Both sexes deposited more marks during paired than solo visits. Immediately preceding and following scent-marking bouts, males approached females and females left males more often than expected. Female scent-marking rates were higher when they were receptive than at other times, and this increase was matched by elevated marking rates of males. Females may increase marking rates when they are receptive in order to test the quality of their mate or to incite male competition. However, these ideas are unlikely to explain female scent-marking behaviour outside the mating season, which appears to be related primarily to territorial defence. We suggest that these differences in investment in scent-marking bouts are consistent with predictions that females may be autonomously territorial and that overmarking of female scent by males is a form of mate-guarding. Received: 17 November 1999 / Received in revised form: 24 February 2000 / Accepted: 13 March 2000     

Compensating for the costs of polygyny in hen harriers Circus cyaneus

In polygynous species, the adults are faced with a dilemma during chick rearing. Males must decide how to distribute food between their females and food allocation patterns are often highly unequal. In turn, the females that receive less food from males have to decide how much time to invest in additional hunting. If they spend more time hunting, then they leave their young exposed to weather and predators. However, if they stay at the nest, they increase the risk of their chicks starving. One way that birds may compensate for reduced provisioning is by increasing the size of prey caught. We tested this hypothesis by comparing prey deliveries to nests of hen harriers, Circus cyaneus, with females of different breeding status. As expected, male harriers delivered less food items to the nests of polygynous females, and especially their secondary, or β females. However, both sexes were able to compensate by delivering larger items and there was no difference in the overall mass of food delivered to nests. Moreover, females spent a similar amount of time at the nest, irrespective of status, and there were no overall differences in breeding success. Our results show that polygynous female harriers can compensate for the costs of polygyny, but we suggest that their ability to do so will vary according to the abundance of both large prey and predators.     

Brood-provisioning strategies in Dawson's burrowing bee, Amegilla dawsoni (Hymenoptera: Anthophorini)

Males of Dawson's burrowing bee are dimorphic in size. Although large (major) males defeat smaller ones in competition for emerging females and therefore are more likely to mate, majors are greatly outnumbered by half-sized (minor) males. Nesting females might produce many minor males, despite their low reproductive value, because female behaviour is governed by a mixed evolutionarily stable strategy (ESS), in which case the ratio of majors to minors should not be affected by changes in female condition. In contrast, a conditional-strategy hypothesis predicts that older, wing-worn or stressed females unable to forage efficiently should be especially likely to produce minor offspring, which require less brood provisions. To test these alternative hypotheses, we manipulated the condition of nesting female bees by the addition of weights and the removal of their wing margins. These manipulations, done early in the flight season, failed to increase the production of minor males, a result consistent with the mixed-ESS hypothesis. However, unmanipulated females were far more likely to produce minor males if they were small or if they were nesting late in the season, when foraging conditions had deteriorated, findings that are consistent with a conditional provisioning strategy. Thus it appears that the abundance of minor males is the result of a conditional provisioning strategy of nesting females, which may be superimposed on a fixed tendency to produce large offspring early in the season and small ones later.     

Territory defence in tropical birds: are females as aggressive as males?

Much of our knowledge concerning the functions of territorial behaviour and how territories are defended by individuals comes from research on birds. The vast majority of this work has focused on temperate zone breeding territoriality in which territories are defended most obviously by males. Our understanding of the female role in territory defence is limited because they are less conspicuous and much harder to observe. We studied sex roles in territory maintenance and defence in a duetting, resident neotropical passerine, the white-bellied antbird (Myrmeciza longipes). This species maintains territories and pair bonds year round and both sexes sing and actively participate in territory defence. We performed a series of playback experiments throughout the dry (non-breeding) and wet (breeding) seasons. We exposed territorial pairs to three types of stimuli including: (1) single sex, male only songs, (2) single sex, female only songs, and (3) both sex songs/duets. Contrary to findings for most other tropical species, individuals defended their territories with equal levels of aggression regardless of stimuli. Furthermore, sex roles were very different, with males responding more aggressively than females to all stimuli throughout both seasons. Both males and females consistently responded more aggressively to territorial intrusions during the dry season than during the wet season, likely because food abundance is low in the dry season and territory value is high. Our analysis of duetting behaviour suggests that duets do not serve a significant role in mate guarding, or territory defence.     

An experimental study of paternal behavior in red-winged blackbirds

Summary The effect of brood size and female nesting status on male parental behavior was investigated in red-winged blackbirds Agelaius phoeniceus using brood size manipulation experiments. Male redwings allocated parental effort on the basis of brood size and nestling age. Males began assisting females only at nests with at least three offspring older than three days. Female nesting status had no singificant influence on male parental care. When females were unable to meet a brood's demand for food, males assisted females with nestling feeding. Females did not reduce the amount of food delivered to nestlings when males assisted. The amount of food brought to nestlings by the male was additional to the amount of food provided by the female. Male assistance increased fledgling success. When female provisioning was sufficient to meet a brood's demand for food males did not assist. The value of male parental care varied inversely with the ability of the female to meet nestling food demands. The ability of unassisted females to provide sufficient food and to raise a brood of nestlings successfully appeared to be influenced by resource abundance.     

Sexual conflict over sperm ejection in monogamous pairs of kittiwakes<Emphasis Type="Italic"> Rissa tridactyla</Emphasis>

Socially monogamous partners suffer conflicting interests concerning various aspects of reproduction such as parental care, copulation and fertilization. Female black-legged kittiwakes commonly eject their mates' sperm immediately following copulations. Because sperm ejection reduces male sperm competitiveness and paternity assurance, males and females have conflicting interests as regards sperm ejection. Males whose mates ejected their sperm at least once remained longer on their mates' backs after the last insemination which apparently prevented the females from ejecting sperm. These results suggest that compelling females to retain their sperm may be a previously unidentified tactic employed by males to assure their paternity. Females tried to prevent their mates from witnessing sperm ejection by ejecting sperm after their mates departed from the nest. Females were more likely to eject sperm when they terminated the copulations by unbalancing the male. The conflict over sperm ejection was related to the ability of the females to end the copulations which covaried with the body mass of their mates. These findings suggest that conflicts in monogamous pairs also exist over the disposition of sperm.Communicated by C.R. Brown     

Sex ratio and the sexual conflict about brood care in a biparental mouthbrooder

The pay-off of deserting and leaving a mate to care for the offspring alone is generally assumed to depend mainly on the availability of alternative mating partners and on the potential spawning rate of males and females. Eretmodus cyanostictus is a monogamous mouthbrooding cichlid in which the clutch is successively incubated first by the female and then by the male. It has been suggested that parents are constrained to monogamy due to low remating probabilities for both sexes. We tested this hypothesis by varying the sex ratio experimentally. Mate desertion by either sex was not significantly higher when additional potential mates were present (males: 8.3%, females: 0%) than when there were no other same-sex conspecifics present (males: 0%, females: 0%). Males lost their mate to a male intruder during their incubation in 26.7% of cases. Pair members were more active and showed more aggression when same-sex conspecifics were present. Behavioural differences between treatments were strongest during the incubation period of a given sex. If no desertion takes place, sexual conflict may be expressed also on a second level, the amount of parental care each parent provides. Indeed, males took the offspring later when additional females were present, although male incubation time did not differ between treatments. A hitherto undescribed display behaviour of females was clear evidence of a conflict about the timing of shift of young. In conclusion, offering alternative mating opportunities did not strongly favour mate desertion in E. cyanostictus. It rather revealed a conflict between mates about when to shift the young.Communicated by M. Abrahams     

Trophic segregation between sexes in the Black Skimmer revealed through the analysis of stable isotopes

The degree of individual or gender variation when exploiting food resources is an important aspect in the study of foraging ecology within a population. Previous information on non-breeding skimmers obtained through conventional methodologies suggested sex-related differences in prey species. In this study, stable isotope techniques were used to investigate the intraspecific segregation in diet and foraging habits of the Black Skimmer (Rynchops niger intercedens) at Mar Chiquita Coastal Lagoon (37°40′S, 57°22′W), Argentina. These results were compared with contemporary data on the trophic composition obtained by conventional methodologies. Blood samples were taken from birds captured with mist-nets during their non-breeding season. The isotopic signatures of skimmers showed a diet mainly composed of marine prey with some degree of estuarine fish intake. When comparing diet between sexes, males showed enrichment in ¹⁵N compared to females, while no differences were observed in ¹³C. The use of mixing models revealed differences in the relative composition of prey in the diet of male and female skimmers. This study highlights stable isotope analysis as a valuable tool to test inter-individual differences and sexual segregation in trophic ecology of Black Skimmers as compared to conventional methodologies. The results show a trophic segregation in the Black Skimmer during the non-breeding season that can be explained by differences in prey species and larger prey sizes of male skimmers. Our findings have significant implications for conservation since any environmental change occurring at wintering areas might have profound effects on several avian life-history traits, and could be different for males and females due to trophic segregation.     

Different tactics,one goal: initial reproductive investments of males and females in a small Arctic seabird

Despite a great number of studies on extra-pair paternity in birds, the actual roles of males and females in extra-pair contacts is poorly understood, as detailed behavioural studies comparing the reproductive performance of the two sexes prior to egg laying are relatively scarce. Here, we investigated mating behaviour (copulations and aggressive interactions), time budget and body condition (size-adjusted body mass and baseline corticosterone level) in the little auk (Alle alle), a monogamous and highly colonial, Arctic seabird. We performed the study in a large breeding colony of the little auk in Hornsund (Spitsbergen). We found that the males frequently attempted extra-pair copulations (EPCs), although these contacts were almost always unsuccessful, mostly because of the females’ rejection behaviour. These results clearly indicate that genetic monogamy is maintained through female control. Nevertheless, males tried to protect their paternity by staying in close proximity to their females and aggressively intervening when their mates became involved in EPCs. Compared to females, males also spent more time in the colony guarding nest sites. Despite the apparent sex differences in the time budget and frequency of aggressive interactions, body condition was similar in the two sexes, indicating comparable parental investments during the mating period.     

Multi-locus DNA fingerprinting supports genetic monogamy in Florida scrub-jays

Extensive behavioural and pedigree data on a colour-marked population of Florida scrub-jays (Aphelocoma coerulescens) suggested that this cooperatively breeding species is monogamous, with extremely rare exceptions in which males have two mates. We used multi-locus DNA fingerprinting to test these observations by determining genetic parentage. Despite restricted dispersal and high relatedness between behavioural parents and non-breeding members of the group, DNA fingerprints provided sufficient variability to determine parentage unambiguously in almost all cases. We found no evidence of extra-pair fertilisation of females or egg dumping, and confirmed a suspected case of polygyny in which a mother and daughter laid and incubated in the same nest. Our results confirm that detailed behavioural data allow accurate assignment of genetic parentage in this species. In Florida scrub-jays, large territory size may limit opportunities for cuckoldry, and persistent intense competition for limited breeding space may lead to low variance in the quality of established male breeders. These factors would reduce both the opportunity for, and benefits of engaging in extra-pair fertilisations. Delayed dispersal and cooperative breeding in this species have not evolved as avenues for direct reproduction by unpaired individuals. Received: 25 May 1998 / Accepted after revision: 26 June 1998     

Site fidelity and patterns of short- and long-term movement in the brilliant-thighed poison frog <Emphasis Type="Italic">Allobates femoralis</Emphasis> (Aromobatidae)

We studied movement and site fidelity of males and females of the territorial frog Allobates femoralis (Aromobatidae) in a population in the Nature Reserve “Les Nouragues” in French Guiana, South America. Observations during 3 months in 2006 ascertained intra-seasonal site fidelity for males and females. Males actively defend large multi-purpose territories whereas females retreat to small resting sites from where they commute to neighbouring males for courtship and mating. Female short-term movement corroborates the previous assumption of a polygynous or promiscuous resource-defence mating system. Year-to-year recaptures from 2005 until 2008 revealed distinct patterns of inter-annual movement for males and regional site fidelity for females. Males abandon their territories and have to re-negotiate them when reproduction starts again at the end of the dry season. Females are not subject to intra- or inter-sexual territorial competition and as a result move significantly less between reproductive seasons than males. Male long-term movement reflects spatial structure and prevailing social interactions and is a reliable indicator for tadpole deposition sites. The combined effects of intra- and inter-seasonal movement promote the diversity of mates for both sexes.     

Maternal characteristics and the production and recruitment of sons in the eastern kingbird (<Emphasis Type="BoldItalic">Tyrannus tyrannus</Emphasis>)

Sex allocation theory predicts that if variance in reproductive success differs between the sexes, females who are able to produce high-quality young should bias offspring sex ratio towards the sex with the higher potential reproductive success. We tested the hypothesis that high-quality (i.e., heavy) female eastern kingbirds (Tyrannus tyrannus) that bred early in the breeding season would produce male-biased clutches. A significant opportunity for sexual selection also exists in this socially monogamous but cryptically polygamous species, and we predicted that successful extra-pair (EP) sires would be associated with an excess of male offspring. Although population brood sex ratio did not differ from parity, it increased significantly with female body mass and declined with female breeding date, but was independent of the morphology and display (song) behavior (correlates of reproductive success) of social males and EP sires. Male offspring were significantly heavier than female offspring at fledging. Moreover, the probability that male offspring were resighted in subsequent years declined with breeding date, and was greater in replacement clutches, but lower when clutch size was large. Probability of resighting female offspring varied annually, but was independent of all other variables. Given that variance in reproductive success of male kingbirds is much greater than that of females, and that male offspring are more expensive to produce and have a higher probability of recruitment if fledged early in the season, our results support predictions of sex allocation theory: high-quality (heavy) females breeding when conditions were optimal for male recruitment produced an excess of sons.     

Sperm storage reflects within- and extra-pair mating opportunities in a cooperatively breeding bird

In passerine birds, storage and maturation of sperm takes place in the cloacal protuberance (CP), an external swelling of the reproductive organ. The considerable variation in CP size among species is presumed to be a consequence of varying levels of sperm-competition, but whether individual variation in CP size within a species also reflects sperm competition is not well established. Here, we study temporal variation in male CP size in relation to within-pair and extra-pair mating opportunities and cuckoldry risk in purple-crowned fairy-wrens Malurus coronatus. This is a socially monogamous cooperatively breeding passerine that can breed year-round and has low levels of extra-pair paternity (in 6?% of broods). We show that male CP size sharply increased a few weeks before, and rapidly regressed after his partner laid eggs, consistent with a cost of its maintenance and/or sperm production. Surprisingly, despite low levels of extra-pair paternity, CP size of non-breeding and pre-breeding males was positively correlated with the number of breeding females in the population, suggesting that CP size is sensitive to extra-pair mating opportunities. However, CP sizes do not seem to reflect cuckoldry risk: CP size of dominant males was unaffected by the presence of a subordinate that was unrelated to the dominant female, although those subordinates occasionally sire offspring, and had a larger CP than subordinates living with their mother. Our results suggest that, even in a species with low levels of extra-pair paternity, individual investment in sperm storage reflects both within-pair and, albeit to lesser extent, extra-pair mating opportunities.     

Food and the deceptive acquisition of mates by polygynous male harriers

Summary Female northern harriers Circus cyaneus are polygynous, marsh-nesting raptors, whose mate choices are enigmatic. I determined the mate choice cues employed by females by correlating the order in which males were chosen with characters that 1) significantly influenced reproductive success; 2) were assessable prior to settlement; and 3) varied between breeding situations. Only nest sites and male provisioning performance met all these conditions: wet nest sites significantly (P<0.05) increased nest survival and high provisioning rates significantly (P<0.01) enhanced brood survival. The order in which females settled was strongly correlated with provisioning performance in both years (r _s -0.65, and r _s -0.84), but not with nest site quality. Females thus apperared to choose males principally on provisioning performance. Despite using the same cue, however, females choosing mated males reared only 0.28 young for every nestling raised by concurrently settling monogamous females (the lowest ratio recorded for any avian species). Extrapolation of male courtship provisioning patterns from clutch sizes and laying date indicated that females received similar proportions prior to and during egg-laying, but that males later preferentially fed females. Secondary females therefore chose mated males on the basis of a temporally changing and unreliable cue. The polyterritoriality, cooperative harems, skewed sex ratio, and sexy-son hypotheses were all inadequate in explaining polygyny in harriers. Female choice of mated males among harriers is best explained by the deceitful provisioning of food by males.