Breeding Success Of Females Research Articles

Pattern and variability in the breeding system of Atlantic salmon (Salmo salar), with comparisons to other salmonids

The breeding system of Atlantic salmon (Salmo salar) is shaped both by natural selection for offspring production and by sexual selection for access to mating opportunities. These evolutionary forces operate with differing intensities in the two sexes to shape their breeding behaviour and tactics. Female breeding success is largely dependent on egg production, access to breeding territories, and nest quality and survival. By contrast, male breeding success is largely determined by access to ovipositing females. As such, the breeding system of Atlantic salmon is similar to that of other members of the subfamily Salmoninae. However, early male maturity, a common pattern within the Salmoninae, reaches its greatest expression in both terms of frequency and magnitude of the mature male size difference in Atlantic salmon. Despite generalities, spawning populations of Atlantic salmon are not static, as they exhibit spatial and temporal variability in demography (e.g., spawner density, sex ratio, age at maturity, and body size). Events, both natural and anthropogenic (e.g., exploitation, habitat alteration, and climatic changes), affect this variability and ultimately shape the breeding system.

Reproductive strategies of Atlantic salmon: ecology and evolution

Atlantic salmon (Salmo solar, Salmonidae) show a diversity of life history, behavioural and morphological adaptations for reproduction which have evolved as an outcome of competition to maximize reproductive success. Reproductive traits of females have been shaped principally by natural selection for offspring production and survival, those of males by sexual selection for access to matings. Female Atlantic salmon invest approximately six times more energy in offspring production (i.e. gonads) than males and face an important trade-off between number and size of eggs to produce that will maximize the number of surviving offspring. Timing of breeding and the construction of nests appear adapted to increase offspring survival. The most important determinant of female breeding success is body size because it affords high fecundity, access to breeding territories and decreased probability of nest destruction. Asynchronous female spawning and the male ability to spawn rapidly and repeatedly results in male-biased operational sex ratios that generate intense male competition for mates. This has likely been responsible for the evolution of elaborate male secondary sexual characters associated with fighting and status signalling. Furthermore, it has given rise, through frequency-dependent selection, to two alternative male breeding phenotypes: (1) large, anadromous males; and (2) small, mature male parr. Anadromous males invest heavily in behavioural activity on the spawning grounds, searching and fighting for mates and courting them, with body size being an important determinant of their breeding success. This behavioural activity carries a heavy cost, as anadromous males have significantly reduced survival relative to females. In contrast, mature male parr invest proportionally more in testes for sperm competition and attempt to ‘sneak’ access to matings. While this behaviour also carries costs in terms of subsequent growth and survival, male parr are more likely to breed again, either prior to or following a migration to sea, than anadromous males. While knowledge about the breeding of Atlantic salmon is detailed, we are only beginning to understand the ultimate causes and/or functional significances of their reproductive strategies. Predictive models of the life history variation are developing, focusing on the need for empirical study and testing of life history and reproductive patterns.

The Role of Territory Choice, Mate Choice and Arrival Date on Breeding Success in the Savi's Warbler Locustella luscinioides

We investigated how territory quality, settlement date and morphometry affected several components of yearly breeding success of a Swiss population of Savi's Warblers Locustella luscinioides. Territories occupied by males differed from unoccupied sites of similar size and location by having higher and denser reeds, a more extensive straw litter, and a thicker cover of dead sedge leaves. Territories with these characteristics were the ones first chosen by males upon spring arrival. These males, however, did not differ in morphometry from those that arrived later. Availability of suitable nesting sites, rather than food availability, appears to be an important choice criterion for territories. Early arriving males had higher breeding success than late males because of a higher mating success and more successful clutches. The positive correlation between male breeding success and territory quality was thus mediated through their common dependence on occupancy date. Female breeding success decreased with the date of first-clutch laying, mainly because late-nesting females fledged fewer broods. Breeding success in either sex did not correlate with morphometry. Our results provide clear support for territory choice by males, but not for mate or territory choice by females, and show the crucial role played by individual settlement date on many aspects of the breeding cycle of both sexes. We propose a lottery model of mate choice: arriving females obtain the best available territories even without choosing mates or territories; since males occupy territories sequentially and in order of decreasing quality, the few unpaired males available at any moment also occupy the best available territories.

Breeding Success of Hatchery and Wild Coho Salmon (Oncorhynchus Kisutch) in Competition.

The divergence of hatchery fish in traits important for reproductive success has raised concerns about their ability to rehabilitate wild populations, and the threat that their inevitable straying poses to biological diversity through introgression. We therefore undertook a study of the breeding competition and success of sea-ranched hatchery fish placed in direct competition with wild fish. Experiments using wild and hatchery coho salmo (Oncorhynchus kisutch) were conducted within a controlled stream channel, allowing selective manipulation of breeding competition and density. Hatchery fish, particularly males, were competitively inferior to wild fish, being less aggressive and more submissive. As a consequence, hatchery males were denied access to ovipositing females; they partook in fewer spawnings, held more distal positions in spawning hierarchies, and attained only an estimated 62% of the breeding success of wild males. By contrast, competition did not appear to inhibit hatchery females as overtly as males. Hatchery and wild females exhibited similar levels of aggressive behavior, however hatchery females did suffer greater delays in the onset of breeding, failed to spawn larger proportions of their eggs, and lost more eggs to nest destruction by other females. They averaged an estimated 82% of the breeding success of wild females. There was thus a sex bias in the breeding disadvantage of hatchery fish, with males suffering more than females. Furthermore, the breeding disadvantage was density dependent with the relative success of hatchery to wild fish declining with increasing density. These results imply that hatchery fish have restricted abilities to rehabilitate wild populations, and may pose ecological and genetic threats to the conservation of wild populations.

Nesting Synchrony by Female Red‐Winged Blackbirds: Effects on Predation and Breeding Success

I analyzed some of the factors affecting female breeding success in an eastern population of Red—winged Blackbirds (Agelaius phoeniceus). Over three seasons, 37% of all nests produced at least one fledgling. Over 77% of nests that failed were destroyed by predators. Nests in all 3 yr were clumped in time, with 50% of the nests being started in just 16% of the season. Temporal proximity of other nests (number of other nests started within ±2 d) was positively associated with the number of chicks fledged. The probability of avoiding predation increased with number of temporal neighbors even with date of first egg and female age controlled in a multivariate analysis. Clutch size was associated only with settlement date and not number of temporal neighbors nor date of first egg. Breeding earlier by females resulted in higher total success for some measures in some seasons, but early nests failed sooner and more frequently. Returning females bred earlier than new females, and even though their first nests were more likely to fail, they typically renested within the peak of nesting or, if their first nest succeeded, sometimes started a second brood before the end of the season. New females to the study are started nests late, but tended to do so within the peak of nesting thereby improving their changes of success. These results highlight the importance of considering demography in studies of the ecological causes of synchronous breeding and the timing of avian breeding seasons.

Variable Mating System of a Sedentary Tropical Duck: The White-Cheeked Pintail (Anas bahamensis bahamensis)

I studied the breeding chronology, courtship activities, pair-bond relation-ships, and parental-care behavior of a sedentary population of White-cheeked Pintails (Anas bahamensis bahamensis) in the Bahamas from 1985 to 1987. The timing and duration of breeding seasons was variable and associated with variation in the onset and amount of winter and spring rainfall. Year-around courtship, mate switches, courtship of brood females, and the formation of extrapair liaisons all reflected intense and continuous competition for quality mates. Most White-cheeked Pintails paired monogamously, but a low level of polygyny occurred regularly: each year, 4 to 9% of paired males had two mates during the breeding season, despite a strongly skewed sex ratio in favor of males (1.45:1). Polygynous males were particularly effective at guarding their mates during the breeding season, an important de¬terminant of female breeding success. The term "male-quality polygyny" is proposed to characterize this form of polygyny. Both long-term pair bonds and mate changes between years were recorded: I0 of 23 marked pairs (43%) stayed together for two or more breeding seasons, while 13 pairs (57%) divorced. Mate retention in the second year was not related to breeding success in the first year. Only females provided parental care, but some males continued to escort and guard their mate for at least part of the brood-rearing period. Although highly variable, male attendance declined with both hatch date and duckling age. Some birds associated as pairs year around, and several pairs stayed together during the wing molt. The sedentary lifestyle in this subtropical climate and the potential for variable and extended breeding seasons appear to be the key ecological factors influencing the complex and variable mating system of this species.

Recruitment of mates and deceptive behavior by male Tengmalm's owls

Tengmalm's owl Aegolius funereus is a hole-nesting polygynous species in which female nomadism is a reaction to cyclic lows of staple prey. For 2 years during a peak in vole abundance, I examined recruitment of mates and male singing behavior in a local population. Females and about half of the breeding males seemed to arrive successively throughout the breeding period of the first year. In contrast, a majority of the breeding males and likely also females were already on the breeding grounds at the beginning of the breeding season in the second year. Before breeding in the first peak year, males were singing at up to five different nest holes. All early breeding males (67% of total number) continued to sing at secondary nest holes after attracting and installing primary females. Secondary singing locations were never closer than 300 m from primary females' nest holes, although closer locations were used for singing before mating. All bigynous males with nests within the study area were also singing at tertiary nest holes, but none successfully attracted a third female. Polyterritoriality was confirmed by a late male settling between primary and secondary nest holes of a bigynous male. Polyterritorial behavior and reduced breeding success of secondary females are in line with the deception model. Females apparently could not discriminate between paired and unpaired singing males. Females made short visits to different males before mating. During courtship, the number and quality of nest holes defended by males may have been of particular importance to female choice.

The Relationships among Polygyny, Male Parental Care, and Female Breeding Success in the Indigo Bunting

The reproductive success of males and females in a polygynous, territorial bird species is affected by the ability of the male to provide care to some or all of the young and by the benefits of pairing polygynously (Orians 1969, Patterson et al. 1980, Muldal et al. 1986). The benefits to males of attracting a second mate will be reduced if the pursuit of additional females affects a male's ability to care for present young (Maynard Smith 1977). For female reproductive success the benefits of nesting on the territory of an already-mated male will also depend in part on that male's ability to care for the female's young. To understand the evolution of polygyny, it is critical to know the effect of male parental care on the reproductive success of both males and females and how polygyny determines the levels of male parental care. In many species polygynous males feed the young infrequently in some or all of their broods (reviewed by Verner and Willson 1969, Wittenberger 1981). Usually, the young in these neglected broods suffer increased mortality either before or after fledging (Patterson 1979, Muldal et al. 1986). In other populations males show-no parental care (Orians 1961, Payne 1969, Patterson et al. 1980), but presumably the number of surviving young would be increased if males helped feed the young even in these species. Indigo Bunting (Passerina cyanea) males are occasionally polygynous (Carey and Nolan 1979, Payne 1982) and reportedly provide little help with the care of the offspring (Verner and Willson 1969, Carey and Nolan 1979). In breeding Indigo Buntings studied in detail by Carey and Nolan (1975,1979), males showed little tendency to feed young. This lack of male care made the effects of polygyny on female reproductive success appear negligible. In a separate population with a similar frequency of polygyny (Payne 1982), however, I found that some males feed nestlings and over a third feed fledglings (Westneat 1988). This difference between populations led me to study the effect of male parental care on male and female reproductive success and to investigate the role polygyny might play in determining the levels of male parental care in this second population. I collected data as part of a larger study on the mating behavior and parental care of Indigo Buntings (Westneat 1987a, b, 1988). The study was conducted in May-August 1983-1985 along an active railroad line 5 km northeast of Niles in Cass Co., Michigan.

Bigyny in Tengmalm's Owl Aegolius funereus: Effect of Mating Strategy on Breeding Success

We examined the frequency of bigyny among Tengmalm's Owls in nest boxes, and the breeding success of males and females of different mating status. In two vole peak years 9% and 14%, respectively, of the males trapped were bigynous. On average bigynous males reared more fledglings and monogamous ones (7.8 vs 4.2 in 1982 and 9.5 vs 5.1 in 1984). All bigynous males breeding more than one year were site tenacious and tended to produce more fledglings when bigynous than other bigynous males (8.8 vs 7.3 in 1982 and 10.8 vs 8.7 in 1984). Primary females laid more eggs than monogamous females breeding at the same time. Because of lower nestling survival secondary females produced fewer fledglings than did primary females (2.8 vs 5.1 in 1982 and 3.3 vs 6.2 in 1984) and monogamous females. In total, individual bigynous males did not bring more prey to their primary and secondary females than did monogamous males to their mates. During the laying period they fed secondary and primary females equally well but tended to favour the primary ones later on. Evidence of polyterritorial behaviour is in line with the deception hypothesis.

Demographic characteristics of an unexploited population of fox squirrels (Sciurus niger)

Demographic changes in an unexploited population of fox squirrels (Sciurus niger) were studied between 1971 and 1978 on 45.2 ha of upland forest in east-central Illinois. Known numbers of squirrels varied between 69 and 142. Adult squirrels (>15 months old) comprised >50% of the known population during the study, with annual survival exceeding 60%. Young-of-the-year made up <20% of the livetrapped population. Increasing populations were characterized by increases (P < 0.05) in numbers of yearling–adult males and young-of-the-year of both sexes. Recruitment depended on both immigration and on production of young by resident females. Recruitment success (proportion of new squirrels resident at least 6 months after initial capture) of immigrants was best for adults and yearlings and poorest for subadults. Adult female breeding success was highest in the winter and correlated negatively with the number of females conceiving the previous breeding period. The size and composition of the seed crop of trees in autumn had no effect (P > 0.05) on recruitment, conception, survival rates, or changes in overall density when at least some winter-storable tree seed was available. In most years, adult females were dispersed evenly, but adult males and young of both sexes showed an aggregated or random dispersion. Breeding rates for yearling females, recruitment success of immigrants and of juveniles born on the study area, and overall changes in squirrel densities between trapping periods correlated negatively (P < 0.05) with the number of adult females but not with the number of males or younger females. Interpretation of dispersion of adult females and the relationship of numbers of adult females to population change suggest that fox squirrels stabilize their numbers in the absence of obvious environmental stresses through aggressive interactions between adult females and resident young and immigrants of all ages.

Physical size, condition, and demography of chamois (Rupicapra rupicapra L.) in the Avoca River region, Canterbury, New Zealand

Abstract The physical and demographic characteristics of chamois in the Avoca region are evaluated from 306 animals shot and autopsied between 1975 and 1978. These data are compared with published and unpublished information for chamois populations in Westland and Canterbury. Avoca chamois were large-framed, but weighed less than Westland chamois. The weight difference suggests better habitat condition and food resources for Westland animals, but the large skeletal size of Avoca chamois is unexplained. High rates of juvenile mortality were caused by acute bacterial-pneumonia infections (Pasteurella). These deaths and other losses by natural causes were offset by the good breeding success of adult females so that stable population numbers were maintained.

SELECTION ON BILL CHARACTERS IN A POPULATION OF DARWIN'S FINCHES: GEOSPIZA CONIROSTRIS ON ISLA GENOVESA, GALÁPAGOS.

Lande and Arnold's (1983) technique for measuring selection on correlated quantitative traits was used to identify the targets of selection and to reveal the direction of selection on three bill dimensions, during different stages of the life cycle in a population of Darwin's finches, Geospiza conirostris, on Isla Genovesa, Galápagos. There was a tendency towards disruptive selection during dry conditions, arising from differential survival. In terms of longevity and breeding success of females, the direction of selection was to increase bill length. For males competing for territories, selection acted to increase bill depth and bill length. The effects of male-male interactions were separated from those of female choice. Male-male interactions selected for deep and long bills, whereas females chose their mates on the basis of a male's territory position and plumage coloration. The results reveal three factors constraining changes in bill dimensions: a tendency for the mean of a dimension to shift in one direction is counteracted by selection in the opposite direction on 1) another, positively correlated, bill dimension, 2) the same dimension in the other sex, and 3) the same dimension at another stage of the life cycle. If these factors are overcome by strong directional selection at one stage of the life cycle and relaxation at another, there can be an evolutionary response because the bill dimensions in this population are known to be heritable. The results complement those found in studies of G. fortis on another island and strengthen the view that these populations of Darwin's finches are frequently subjected to natural selection.

Song Activity and Territory Quality in the Corn Bunting Miliaria calandra; With Comments on Mate Selection

Song activity, length of song period and nesting success were studied in the promiscuous Corn Bunting Miliaria calandra in order to evaluate the significance of song. The quality of territories was assessed through duration of territory occupation and the habitat composition of territories. The Corn Bunting starts territorial behaviour in high quality territores in winter. The large size dimorphism is suggested as a result of social dominance in winter flocks. Early occupation of territories may be a necessary cost in order to obtain the best territory. Territories were aggregated and high quality territories were situated in distribution centres near each other. These territories were small, and still they probably held a large amount of resources. The song activity of the males is suggested to function as a means of female attraction. Annual and diel song activity was high. Many females were recorded in territories belonging to males with a high song activity, and the breeding success of females nesting in these territories was high. Successful males continued to sing late in the breeding season allowing attraction of renesting females and evaluation of breeding success of males by females. Males only invest in territorial behaviour and copulations whereas the breeding activities are performed nearly exclusively by the female.

Territory and Social Organization in a Population of Dunnocks Prunella modularis

A small population of Dunnocks Prunella modularis in a semi-natural habitat in southern England was studied for 51/2 years. The social organization during the breeding season was complex. Males outnumbered females in the proportion 1.25:1. Some territories were shared by two males, one of which was dominant. All such territories were in high-quality habitat. Dominant males did most of the mating; in a few cases subordinate males mated with females before the laying of a second or later clutch. In addition, one quarter of the males were bigamous. Bigamous males fed the young of both females. There was no significant difference in the breeding success of females associated with one or two males. In five years with mild winters females survived slightly better than males; in one very severe winter males survived normally but all colour-ringed females disappeared. The data suggest that males are more at risk than females during the breeding season, but that females, because they are subordinate to males at feeding places, are more at risk during periods of food shortage. Cutaneous disease was rather common in the population, and affected males more than females. Different patterns of male and female mortality may result in temporally and spatially fluctuating sex ratios in the breeding population, and this may affect pairing relationships; but it is suggested that the main underlying factor involved in the Dunnock's unusual social organization is its feeding ecology, linked with the independence of male and female territories. Foraging on small, well-dispersed food items on the ground in thick cover, Dunocks probably cannot effectively exclude intruders from their territories. Males may thus tolerate an intruder which accepts subordinate status. It may be a better strategy for a male without a territory to accept subordinate status in a territory of high quality (i. e. in habitat where females are densest), with the possibility of eventually taking over the territory, rather than to take up a territory of poorer quality (where females are sparse or may be absent).